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Bilateral energy conference

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Materials development for solar fuel production and energy conversion

In the quest for carbon neutral energy generation, research and development efforts have been intensified considerably in recent years. Major efforts relate to carbon dioxide-free fuel production that includes the generation of hydrogen or the photochemical reduction of CO2 to higher hydrocarbons. In this context oxygen evolution from water has to be included. The regenerative fuel cycle is closed by the conversion of hydrogen in fuel cells.

Besides the control of the interfacial processes and specific device architecture, a grand challenge is the development of materials that allow efficient and robust operation under solar illumination at reactive interfaces. Such efforts are constrained by cost and scarcity considerations of materials and cell components. Therefore, research focuses on the search for Earth abundant catalysts and light absorbers that have the potential for large-scale terrestrial application.

The symposium should bring together scientists and engineers working in these fields. The broad context should enable an interdisciplinary exchange between the participants and hopefully establish new collaborations. In an era of sustainability, a special focus will be related to materialsrobustness and durability. Latest developments on well-established materials and their modification as well as the application of new, yet unex¬ploited ones, shall both be presented.

Hot topics to be covered by the symposium:

The symposium is focused on earth abundant catalysts and light absorbers for solar water splitting (OER and HER catalysts as well as complete systems), energy consumption in PEM-FC and catalytic conversion of CO2.

  • Novel materials and composites for photoconversion of solar illumination
  • Heterogeneous and homogeneous catalysts for HER, OER and CO2 reduction
  • Micro- and nanoarchitectures for light-induced water splitting
  • The reactive solution interface - stabilization strategies (surface functionalization, ALD)
  • Theoretical aspects - materials by inverse design
  • Advanced modeling of HER and OER - the role of the electrolyte
  • Monolithic integrated systems for photoelectrochemical water splitting
  • Metal-oxide basedORR catalysts for fuel cells
  • Recent advances in ORR on molecular structures
  • Corrosion resistant support materials
  • Membranes - fuel cells and solar fuel generators
  • Distinct catalysts architecture for enhanced multi-electrontransfer processes

List of invited speakers (confirmed):

  • Alexis Bell (Univ. California, Berkeley)
  • Kazunari Domen (Univ. Tokyo)
  • Gregory N. Parsons (NCSU, Raleigh)
  • Ib Chorkendorff (DTU, Lyngby)
  • Roel van de Krol (HZB, Berlin)
  • Albert Polman (AMOLF, Amsterdam)
  • Akihide Iwase (Tokyo Univ. Sci.)
  • Raffaella Buonsanti (Lawrence Nat. Lab.)
  • Jean-Pol Dodelet (INRS-EMT, Varennes)
  • Ken-ichiro Ota (Yokohama Nat. Univ.)
  • Matthias Arenz (Univ. of Copenhagen)
  • Jan Rossmeisl (DTU, Lyngby)
  • Marcel Risch (MIT, Cambridge)
  • Ruud Kortlever (Leiden Univ.)
  • Klaus Lips (HZB, Berlin)

Sponsors:

  hzb_logo_cmyk
  sentech

 

Symposium organizers:

 

Dieter Schmeißer (CO2 and OER catalysis)
University of Cottbus, Chair of Applied Physics and Sensors
Konrad-Wachsmann-Allee 17
03046 Cottbus
Germany
Phone +49 355 69 3073
fax + 49 355 69 3931
dsch@tu-cottbus.de

Hans-Joachim Lewerenz (Solar Fuels)
Joint Center for Artificial Photosynthesis, California Institute of Technology
1200 E. California Blvd
Pasadena, CA 91125
USA
Phone: +1 626 395 4149
lewerenz@caltech.edu

Ulrike I. Kramm (Catalysts for PEM-FC)
University of Cottbus, Chair of Applied Physics and Sensors
Konrad-Wachsmann-Allee 17
03046 Cottbus
Germany
Phone +49 355 69 2972
Fax + 49-355 69 3931
kramm@tu-cottbus.de 

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09:00 Coffee Break    
09:30 Welcome - Organizers    
 
WATERSPLITTING I : Ulrike I. Kramm
09:40
Authors : Alexis T. Bell
Affiliations : Joint Center for Artificial Photosynthesis Lawrence Berkeley Laboratory Berkeley, CA 94720, USA

Resume : Hydrogen produced by the photoelectrochemical splitting of water can be used for fuel-cell powered vehicles and for the removal of oxygen during the conversion of biomass to liquid transportation fuels. The attractive feature of both options is the avoidance of CO2 emission from fossil fuels occurring when hydrogen is produced by the reforming of methane. However, the development of devices for the photoelectrochemical splitting of water is challenged by the absence of efficient catalysts based on earth-abundant elements for the oxidation of water (i.e, the OER). This talk will discuss our current understanding of the relationships between the activity of OER catalysts and their structure and composition derived from recent experimental evidence obtained from in situ Raman and x-ray absorption (XPS, XANES, and EXAFS) studies combined with theoretical investigations based on quantum chemistry. The experimental studies reveal that the structure and composition of metal oxides involving Co, Ni, and Fe are dependent on the pH of the electrolyte and the applied potential and that the overpotential for the OER is a function of the catalyst composition and structure in its working state. A further finding of our theoretical studies is that there is an inherent minimum in the overpotential accessable using metal oxides and that new structural and compositional motifs need to be explored in order to achieve more active electrocatalysts.

Z.Z.1.1
10:25
Authors : Hen Dotan(1), Ofer Kfir(2), Elad Sharlin(1), Oshri Blank(1), Moran Gross(1), Irina Dumchin(1), Guy Ankonina(3), Avner Rothschild(1)
Affiliations : (1) Department of Materials Science & Engineering, Technion – Israel Institute of Technology, Haifa, Israel; (2) Physics Department, Technion – Israel Institute of Technology, Haifa, Israel; (3) Photovoltaics Laboratory, Technion – Israel Institute of Technology, Haifa, Israel

Resume : Solar-powered water photoelectrolysis is a promising route to produce clean hydrogen fuel from abundant and renewable resources: water and sunlight. Iron oxide (hematite) is one of the most promising photoanode candidates because of the unique combination of light absorption up to 600 nm, stability in aqueous solutions, abundance and low cost. However, its poor transport properties and short lifetime of minority charge carriers (holes) lead to massive bulk recombination that has hindered the development of efficient hematite photoanodes. Here we show a novel approach to overcome the mismatch between short collection length of the holes and long absorption length by resonant light trapping in ultrathin films. This is achieved with a simple architecture wherein the photoelectrode is designed as a quarter-wave antireflection coating on a reflective substrate. Model calculations predicts that for a perfect reflector 71% of the solar spectrum, up to 600 nm, can be absorbed in ultrathin films (< 50 nm) wherein most of the photogenerated holes can reach the surface and oxidize water before recombination takes place. The rest of the light is back-reflected, and it can be easily harvested placing two photoanodes facing each other in a V-shaped cell. Water photo-oxidation current densities as high as 4 mA/cm^2 were obtained using a 90deg V-shaped cell comprising 26 nm thick Ti-doped hematite films on a silver-gold alloy coated substrates [Dotan et al., Nature Materials 12, 158, 2013].

Z.Z.1.2
10:50
Authors : Moran Gross, Avner Rothschild
Affiliations : Department of Materials Science and Engineering, Technion, Haifa, Israel

Resume : Hematite photoanodes convert solar power to hydrogen fuel by water photoelectrolysis. To overcome the challenge arising from the mismatch between short collection length of minority charge carriers and long light absorption length, different photon management schemes are examined. Encouraging results showing enhanced absorption in hematite thin films decorated with Au nanoparticles (NPs) were reported, but the gain in the water photo-oxidation currents falls short of the optical gain in light absorption. Furthermore, the root cause for the apparent photocurrent gain is unclear. This work examines the effect of Au NPs on the optical and photoelectrochemical properties of hematite thin films deposited by pulsed laser deposition on FTO-coated glass substrates. Au NPs of different diameters were obtained by dewetting of Au films. The Au NPs were embedded below the hematite films or placed on top of them. The embedded configuration displayed enhanced photocurrent by as much as 30%, under simulated solar radiation, with respect to the Au-free films. The enhancement was observed close to the photocurrent onset potential, fading out at higher potentials. Based on these findings we believe this enhancement is mostly a catalytic rather than plasmonic effect. The on-top configuration displayed corrosion characteristics that are atypical for hematite photoanodes. Similar characteristics were observed in pristine Au electrodes, indicating that they arise from Au oxidation.

Z.Z.1.3
11:10
Authors : Alejandra Ramirez, Philipp Hillebrand, Matthias May, Diana Stellmach, Peter Bogdanoff and Sebastian Fiechter
Affiliations : Helmholtz-Zentrum Berlin fuer Materialien und Energie GmbH, Institute for Solar Fuels, Hahn-Meitner-Platz 1, 14109 Berlin, Germany

Resume : The direct conversion of sunlight into chemical ener¬gy using an artificial membrane, which is able to split water into hydrogen and oxygen, is consi¬dered as paramount and ambitious research goal. One possible solution is the conversion of sunlight into chemical energy via photonic excitation of a thin film photovoltaic structure immersed in water, which is combined with corrosion-stable layers at front and back contact to catalyse water splitting. Besides novel semiconduc¬ting materials, noble metal-free catalysts are needed for this purpose. In this contribution, different MnxOy phases were prepared as thin films to elucidate the structure – function relationship. For this, amorphous Mn(O,OH)x films, anodically deposited on F:SnO2/glass and annealed at different temperatures to form crystalline Mn2O3 and Mn3O4, were tested as oxygen evolving catalysts. Electrochemical mass spectroscopy showed that the anodic currents of the crystalline films correlated well with the onset of oxygen evolution at current density of 19 mA/cm2 and an overpotential of 170 mV for Mn2O3 at pH 14. Compared to MnOx and Mn3O4, this oxide exhibited the smallest overpotential at highest current densities. X-ray photoelectron spectroscopy revealed that after electrochemical treatment the surfaces of Mn2O3 and Mn3O4 electrodes exhibited oxidation of Mn II and Mn III towards Mn IV under oxygen evolving conditions. The results indicate that besides a high surface roughness, important factors for the catalytic activity are reversible oxidation states, a large variety of Mn-O bond lengths and a high concentration of oxygen point defects. Thus, compared to amorphous MnOx and Mn3O4, crystalline α-Mn2O3 films is the most efficient catalyst for water oxidation.

Z.Z.1.4
11:30 small break    
11:40
Authors : J. Ziegler, F. Yang, B. Kaiser, W. Jaegermann, F. Urbain, F. Finger, U. Rau,
Affiliations : Institut für Materialwissenschaften, Technische Universität Darmstadt, Jovanka-Bontschits-Str. 2, 64287 Darmstadt, Germany;Institut für Materialwissenschaften, Technische Universität Darmstadt, Jovanka-Bontschits-Str. 2, 64287 Darmstadt, Germany;Institut für Materialwissenschaften, Technische Universität Darmstadt, Jovanka-Bontschits-Str. 2, 64287 Darmstadt, Germany;Institut für Materialwissenschaften, Technische Universität Darmstadt, Jovanka-Bontschits-Str. 2, 64287 Darmstadt, Germany;Forschungszentrum Jülich GmbH, Institut für Energie- und Klimaforschung, 52425 Jülich, Germany;Forschungszentrum Jülich GmbH, Institut für Energie- und Klimaforschung, 52425 Jülich, Germany;Forschungszentrum Jülich GmbH, Institut für Energie- und Klimaforschung, 52425 Jülich, Germany

Resume : A photo electrochemical cell build for water electrolysis requires a photo voltage of at least 1.8 V regarding overpotentials. Wide band gap semiconductors suffer from a limited spectral yield and efficiency. Therefore, we utilize amorphous silicon (a-Si) tandem cells as photocathodes for the hydrogen evolution reaction. The a-Si/a-Si tandem cells provide an optimized photovoltage of 1.8 V and achieve an efficiency of 9 % in a photovoltaic arrangement. In a PEC-arrangement the photocurrent is preserved, but the total efficiency visible in a 3-electrode measurement is reduced due to slow HER. Therefore, we deposited Pt onto the surface by sputtering. The tandem cells are then combined with a ruthenium oxide counter electrode in a 2-electrode arrangement and are capable of splitting water without external bias. Efficiencies above 5 % can be achieved under short circuit conditions. In order to identify the relevant loss processes additional 3-electrode measurements were performed for each involved half-cell. To improve the stability of the tandem cells we introduce buffer layers, which possess a sufficient mechanical stability on the tandem cells. The buffer layer should provide a good electrical coupling of the tandem cell to the HER catalyst. The properties of the buffer layers are characterized by electrochemical methods and by XPS.

Z.Z.1.5
12:05
Authors : A. Azarpira, M. Lublow, P. Bogdanoff, A. Fischer, C.A. Kaufmann, M. Krüger, Th. Schedel-Niedrig
Affiliations : A. Azarpira; P. Bogdanoff; C.A. Kaufmann; M. Krüger; Th. Schedel-Niedrig: Helmholtz-Zentrum Berlin fuer Materialien und Energie GmbH, Institute Solar Fuels, Berlin, Germany M. Lublow; A. Fischer: Technical University Berlin, Chemistry Department, Berlin, Germany.

Resume : Chalcopyrite semiconductors are highly efficient light absorbers, but their application as photocathodes for hydrogen evolution is hindered by severe photocorrosion. To address this, a novel type of composite thin film heterojunctions based on Pt-doped titanium dioxide and chalcopyrite is presented as highly efficient photocathodes for solar hydrogen evolution. The underlying concept relies on the utilization of phase-pure anatase titanium dioxide thin films, deposited onto visible-light absorbing chalcopyrite thin films, both as transparent conducting oxide and anti-corrosive layer. It will be shown that a direct deposition of pristine and Pt-doped titanium dioxide can be achieved on p-type semiconductor Cu(In,Ga)Se2 thin films by using ion layer gas reaction (ILGAR®) as deposition technique. Key parameters of the ILGAR® process, such as temperature, carrier gas flow rate and concentration of precursors were systematically varied to determine the optimum photoelectrocatalytic device performance. To enhance the conductivity of the TiO2 thin film, Pt-ion doping under varying concentrations was performed. Remarkably, a very high incident-photon-to-current-efficiency (IPCE) of more than 80% could be achieved between wavelengths of 300 nm and 1000 nm. The best Pt-TiO2/Cu(In,Ga)Se2 heterojunction electrodes reveal an onset potential for hydrogen evolution at 0.25 (NHE), a photocurrent density of ~10 mA/cm2 at the standard potential and operated remarkably stable for several hours.

Z.Z.1.6
12:25 Lunch break    
 
CO2 reduction : Dieter Schmeißer
14:10
Authors : Ruud Kortlever, Klaas Jan Schouten, Federico Calle Vallejo, Jing Shen, Marc Koper
Affiliations : Leiden Institute of Chemistry, Leiden University, Leiden, The Netherlands

Resume : This talk will summarize the mechanisms of the electrochemical CO2 reduction on a variety of metal surface, including copper and palladium. We will discuss the formation of C-C bonds on copper electrodes and the formation of formic acid at low overpotential on palladium-alloy electrodes.

Z.Z.3.1
15:10
Authors : Alexander Bonk, Michal Gorbar, Andreas Züttel, Aldo Steinfeld, Ulrich F. Vogt
Affiliations : Alexander Bonk 1,2; Michal Gorbar 1; Andreas Züttel 1; Aldo Steinfeld 3; Ulrich F. Vogt 1,2 1 Empa, Swiss Federal Laboratories for Materials Science and Technology, Labora-tory for Hydrogen & Energy, 8600 Dübendorf, Switzerland 2 University of Freiburg, Department of Crystallography, D-79098 Freiburg i. Brsg. 3 Department of Mechanical and Process Engineering, ETH Zurich, CH-8092 Zurich

Resume : Solar thermochemical RedOx cycles based on metal oxides can be used to reduce water and CO2 to form H2 and CO, respectively. Due to its excellent thermodynamic and kinetic properties, cerium dioxide is one of the most promising non-volatile metal oxides in this context. In a 2-step solar thermochemical cycle, above 1400°C ceria will release oxygen from its lattice, to form nonstoichiometric Ceria (CeO2-δ). In the next step, oxygen vacancies react with CO2 and/or H2O to form CO and H2, respec-tively at around 900°C. Stoichiometric CeO2 is regained, closing the RedOx cycle. In this study we developed chemically homogeneous, as well as mechanically stable porous ceramics for solar driven thermochemical cycles. The control of thermodynamics, kinetics and the transport of concentrated solar en-ergy to the reactant is essential to design highly efficient ceria based structures. For this purpose, ceria was doped with isovalent cations. Samples have been investigat-ed regarding their RedOx potential, phase purity as well as their sintering behaviour at high temperatures using thermogravimetric analysis, X-ray diffraction and electron microscopy.

Z.Z.3.3
15:30 Coffee break    
 
Large Scale Facilities : Dieter Schmeißer
16:00
Authors : Klaus Lips
Affiliations : Institute Silicon Photovoltaics Helmholtz-Zentrum Berlin für Materialien und Energie Kekuléstr. 7, 12489 Berlin, Germany

Resume : One of the main challenges for today’s global society is a reliable, cost-effective and environmentally-friendly supply of energy. According to many energy scenarios, renewable energies will carry the major load within a future sustainable energy system. Important roles in the scenarios play solar cells which convert sunlight directly into electricity. Technology development and mass production have pulled down the costs of photovoltaics (PV) during the past decades. However, in order to accommodate the necessary economic constraints to massively implement PV on a global scale, substantial cost reductions are further needed and the integration of PV in a supply system tackling the fluctuating availability of solar radiation is a must. This calls for economically suitable solutions for energy storage. To achieve these ambitious goals, a more knowledge-based approach to material research will become necessary with a fast and direct feedback between sophisticated analytics and state-of-the-art deposition systems which are capable to process complete devices and study their properties under in operando conditions. A promising approach is the coupling of synchrotron-based X-ray characterization techniques providing the unique possibility to map the electronic and chemical structure of thin layers, interface regions and surfaces with high lateral and in-depth resolution with a variety of deposition and post-treatment capabilities in one dedicated interconnected vacuum system. In a concerted effort, the Helmholtz-Zentrum Berlin für Materialien und Energie GmbH (HZB) and the Max Planck Society (MPG) are developing EMIL (Energy Materials In-situ Laboratory), a world-wide unique facility at the BESSY II synchrotron light source, dedicated to the in-situ and in-system X-ray analysis of materials and devices for photovoltaic (PV) applications and of (photo)catalytic (CAT) processes. EMIL will be taken into operation in 2015 and is designed such that it can serve up to three experimental end-stations that each can simultaneously access soft and hard X-rays in an energy range of 60 eV – 10 keV. The CAT end-station will focus on near ambient pressure hard X-ray photoelectron spectroscopy (NAP-HAXPES) while the two SISSY end-stations will provide a range of X-ray analysis techniques such as X-ray photoelectron spectroscopy (XPS and HAXPES) and -microscopy (XPEEM), as well as X-ray absorption (XAS) and emission/fluorescence (XES/XRF) spectroscopy to study the chemical and electronic structure of materials at a variety of depths at ultra-high vacuum conditions. In this presentation I will provide an overview of the analytic capabilities of the SISSY and CAT end-station, its connection to the EMIL thin-film deposition facilities and our plans for its utilization for photovoltaic and (photo)catalytic materials research. In particular, I will highlight the possibilities for future national and international collaborations at his unique research platform.

Z.Z.3.1
16:35
Authors : J. A. Haber, J. M. Gregoire , C. Xiang, S. Mitrovic, S. Suram, P. F. Newhouse, E. Soedarmadji, M. Marcin, K. Kan, D. Guevarra, N. Becerra, R. Jones, M. Pesenson, A. Shinde, L. Zhou, E. W. Cornell, J. Jin
Affiliations : Joint Center for Artificial Photosynthesis, California Institute of Technology, Pasadena, California 91125, USA. Engineering Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

Resume : The High Throughput Experimentation (HTE) project of the Joint Center for Artificial Photosynthesis (JCAP, http://solarfuelshub.org/) performs accelerated discovery of new earth-abundant photoabsorbers and electrocatalysts. Through collaboration within the DOE solar fuels hub and with the broader research community, the new materials will be utilized in devices that efficiently convert solar energy, water and carbon dioxide into fuels. JCAP-HTE builds high-throughput pipelines for the synthesis, screening and characterization of photoelectrochemical materials. In addition to a summary of these pipelines, we will describe several new screening instruments for high throughput (photo-)electrochemical measurements. These instruments are not only optimized for screening against solar fuels requirements, but also provide new tools for the broader combinatorial materials science community. We will also describe the high throughput discovery, follow-on verification, and device implementation of a new quaternary metal oxide catalyst. This rapid technology development from discovery to device implementation is a hallmark of the multi-faceted JCAP research effort.

Z.Z.3.2
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ORR on (Oxi-)nitrides : Jean-Pol Dodelet
09:25
Authors : Ken-ichiro Ota, Koichi Matsuzawa, Shigenori Mitsushima, Akimitsu Ishihara
Affiliations : Yokohama National University

Resume : Introduction Polymer electrolyte fuel cells (PEFCs) are expected to use for the home cogeneration system and fuel cell vehicles which will be ultimate clean car, due to their high power density and low operating temperature. The ENEFARMs (home co-generation system using 1 kW PEFC system) are operating more than 50,000 units in Japan. Fuel cell vehicles will be commercialized in 2015. The driving range of fuel cell vehicles is more than 500 km at one charge. The fuel cell vehicles can start even at – 30oC. However, a significant cost reduction is needed especially for fuel cell vehicles. And the estimated amount of Pt reserve is too small to supply for the huge number of fuel cell systems. In order to commercialize the fuel cell systems widely, the development of a non-precious metal cathode is strongly required. We think that new non-precious metal cathodes should have both high stability and high catalytic activity for the ORR. In particular, we believe that high stability in cathodic condition is essentially required for the cathode catalyst. Some transition metal oxides could be used for the cathode of PEFC. However, most of the transition metal oxides are not stable in the acidic and oxidative atmosphere. We started this study by searching stable materials in acid and in oxygen containing atmosphere. Group 4 and 5 metal oxides, which are well known as valve metals, are stable even in acidic and oxidative atmosphere. However, these oxides are generally insulator. In order to get some electrical conductivity, these oxides should be modified by the formation of the oxygen vacancy and/or the substitution of foreign atoms. We have reported that partially oxidized group 4 and 5 metal carbonitrides were stable in an acid solution and had a definite catalytic activity for the oxygen reduction reaction (ORR) (1-5). We have tried to apply group 4 and 5 metal oxide-based compounds to the cathode catalyst. In this paper we will report our recent results of group 4 and 4 transition metal oxide based cathode for PEFCs. Experimental Powders of metal compounds (carbonitride or metal complexes that contain nitrogen) were heat-treated at 800-1200oC under different flowing rate of the H2/N2 gas mixtures that containing small amount of oxygen to obtain specimens with different oxidation state. After heat treatment, the compounds changed to oxides that contained small amount of carbon and nitrogen. Heat treated powder was mixed with alcohol, carbon and Nafion. The mixture was dipped on a glassy carbon rod (5 mm diameter) and the working electrode was made. All electrochemical measurements were examined in 0.1 M H2SO4 at 30oC under atmospheric pressure using a conventional 3-electrode cell. The RHE was used for the reference in the same solution. Slow scan voltammetry (scan rate: 5 mVs-1) was performed under O2 and N2 atmosphere to obtain the current for the oxygen reduction reaction (ORR). The onset potential was defined as the electrode potential at the ORR current density of -0.2 μA cm-2. Results and Discussion The catalytic activity of our material strongly depended on the degree of oxidation (DOO) for these compounds. An appropriate oxidation is essential to have a definite catalytic activity for the ORR. The onset potentials of partially oxidized Zr and Ta compunds have reached 1.05 V vs RHE that is the onset potential of commercial Pt-C. This means that the activity of our oxide based cathode is almost same as that of Pt. Next step is the increase of the density of active sites on our catalyst for the improvement of current density.. In order to improve the current density especially at high potentials we have tried to use metal complexes that have nitrogen as starting materials. More than 1000 times improvements in catalytic activity have been obtained using these materials. By the single cell test we have obtained 3 mA/cm2 for ORR at room temperature. We also have obtained more than 0.1 A/cm2 using a 5 x 5 cm fuel cell at 80oC. Although these oxide catalysts are still some difference in the ORR activity from that of Pt/C catalyst, these materials have great potential for PEFC cathode, especially for the cost reduction that is the most important issue for the present PEFC technology.. Acknowledgement. The authors wish to thank to the New Energy and Industrial Technology Development Organization (NEDO) for their financial support. References [1] A.Ishihara, Y.Shibata, S.Mitsushima, K.Ota, J.Electrochem. Soc., 155, B400.(2008) [2] A.Ishihara, Y.Ohgi, K.Matsuzawa, S.Mitsushima, K.Ota, Electrochim. Acta, 55, 8005. (2010) [3] K.Ota, Y.Ohgi, K.D.Nam, K.Matsuzawa S. Mitsushima, A. Ishihara, J. Power Sources, 196, 5256-5263 (2011). [4] Y.Ohgi, A.Ishihara, K.Matsuzawa, S.Mitsushima, K.Ota, M.Matsumoto, H.Imai, J.Electrochem.Soc., 160, F162-F167 (2013) [5] A.Ishihara, M.Tamura, Y.Ohgi, M.Matsumoto, K.Matsuzawa, S.Mitsushima, H.Imai, K.Ota, J.Phys.Chem. C, 117, 18837-18844 (2013)

Z.Z.4.1
10:00
Authors : Marcel Risch, Yang Shao-Horn
Affiliations : Research Laboratory of Electronics, Massachusetts Institute of Technology, 77 Massachusetts Ave, MA-02139, Cambridge; Research Laboratory of Electronics Department of Mechanical Engineering Department of Material Science and Engineering Massachusetts Institute of Technology, 77 Massachusetts Ave, MA-02139, Cambridge

Resume : Mastering oxygen electrocatalysis is key to the development of efficient energy storage and conversion devices such direct solar and electrolytic water-splitting devices, fuel cells, and metal-air batteries. Oxides have shown high activity for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR).[1-5] However, the lack of fundamental understanding of the oxide surfaces limits mechanistic understanding at the molecular level of both reactions. Identifying a fundamental descriptor that links surface structure and chemistry to the catalytic activity can guide the search for highly active catalysts that are cost effective and abundant in nature. Crystalline thin films are ideally suited to accurately determine the ORR activity of oxides due to the well-defined surface morphology and orientation.[6] We showed recently that activity descriptors such as eg occupancy and O p-band center, correlate with both OER and ORR activity, albeit in different temperature regimes.[1,3,5,7] Using insight from descriptor approaches paves the road not only for the development of better ORR catalysts but also for bi-functional materials that excel at both OER and ORR. 1. Suntivich, et al. Science 334, 1383 (2011).
 2. Lee et al. J. Phys. Chem. Lett. 3, 399 (2012). 3. Grimaud et al. Nature Communications 4, 2439 (2013). 4. Grimaud et al. J. Phys. Chem. C 117, 25932 (2013). 5. Suntivich et al. Nature Chem. 3, 546 (2011).
 6. Stoerzinger et al. Energy & Environ. Sci. 6, 1582 (2013). 7. Lee et al. Energy & Environ. Sci. 4, 3966 (2011).


Z.Z.4.2
10:35
Authors : Jan Rossmeisl
Affiliations : Department of Physics, Technical University of Denmark Building 311, 2800 Lyngby

Resume : For energy conversion the oxygen reduction and oxygen evolution reactions are of extreme interest. For both reactions a huge over potential is needed to obtain a reasonable current, and in both cases this is mainly due to sluggish catalysis. I study the reasons for the over potential on the basis of density functional simulations and determine activity descriptors, which are easy to calculate and therefore suited for computational screening for more active catalyst materials. Universal trends in the activity for the oxygen reactions on metals and oxides are identified. I show examples on design of new catalysts which were suggested by the simulations and later in rotating disk experiments showed an increased activity. Furthermore, some directions to improve eletrocatalysis are suggested.

Z.Z.4.3
11:10
Authors : Prashanth W. Menezes,1 Arindam Indra,1 Diego González-Flores,2 Nastaran Ranjbar,1 Ivelina Zaharieva,1 Peter Strasser,1 Holger Dau,2 Matthias Driess 1
Affiliations : 1. Technische Universität Berlin Department of Chemistry: Metalorganics and Inorganic Materials Strasse des 17. Juni 135, Sekr. C2 10623 Berlin, Germany 2. Fachbereich Physik, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany

Resume : The development of low cost, naturally abundant, environmentally benign and highly active catalysts for the energy production, conversion and storage is of utmost importance and play a prime role in the renewable energy technologies. The discovery of such materials for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) could resolve the problems involved in the highly challenging reactions of water splitting, fuel cells and metal-air batteries. In the present work, nanochains of cobalt oxide were synthesized by relatively low temperature degradation of single source precursor cobalt oxalate dihydrate and then this material has been used for the first time as highly active multifunctional catalyst for electrochemical OER, chemical and photochemical water oxidation as well as for ORR. The cobalt oxide catalyst exhibited enhanced activities towards OER in both alkaline and neutral solution. In addition to the electrochemical OER experiments, chemical water oxidation and photochemical water oxidation in various buffer solutions were investigated and the detected catalytic rates were clearly higher than in any previously investigated cobalt oxides. The ORR was carried out in an alkaline medium and results obtained here outperformed the benchmark of precious metal catalysts in terms of catalytic activity, stability and durability. The employed single source precursor strategy for the synthesis of cobalt oxide can easily be scaled up commercially and could be rationally designed not only for the practical application in energy production and conversion but also in energy storage.

Z.Z.4.4
11:35 Lunch Break    
 
Watersplitting II (III/V-compounds) : Jan Rossmeisl
13:30
Authors : Takashi Hisatomi, Kazunari Domen
Affiliations : Department of Chemical System Engineering, The University of Tokyo

Resume : Metal (oxy)nitride photocatalysts have attracted much attention in terms of solar hydrogen production because of their band structures suitable for water splitting under visible light irradiation. (Ga1-xZnx)(N1-xOx) is capable of overall water splitting when being modified with hydrogen evolution cocatalysts. Ta3N5 modified with proper cocatalysts is applicable as an oxygen evolution photocatalyst in Z-scheme water splitting. Modification of (oxy)nitride photocatalysts with oxygen evolution cocatalysts is important for the water splitting reaction because they are thermodynamically less stable than oxides. The reactivity of photoexcited holes must be controlled kinetically so that water oxidation is facilitated while self-oxidation can be suppressed. Recently, it was found that coloading (Ga1-xZnx)(N1-xOx) with oxygen evolution cocatalysts such as Mn3O4, RuO2, and IrO2 not only improved the photocatalytic activity for overall water splitting but also the durability of the oxynitride. Similar effect was also observed for the TaON photocatalyst. Additionally, CoOx deposited via a heat treatment under an NH3 flow was found to work as an efficient oxygen evolution cocatalyst for various (oxy)nitrides such as LaTiO2N, Ta3N5, and BaNbO2N. It was revealed that the lifetime of photoexcited carriers could be extended significantly by loading oxygen evolution cocatalysts. In this talk, recent advancement in oxygen evolution cocatalysts and (oxy)nitride photocatalysts will be presented.

Z.Z.5.1
14:05
Authors : Ela Nurlaela, Samy Ould-Chikh, Silvano del Gobbo, Eric Puzenat, Jean Marie Basset, and Kazuhiro Takanabe
Affiliations : Ela Nurlaela, Samy Ould-Chikh, Jean Marie Basset, Kazuhiro Takanabe: Division of Physical Sciences and Engineering, KAUST Catalysis Center (KCC); Silvano del Gobbo: Solar and Photovoltaic Engineering Center, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia; Eric Puzenat: IRCELYON, UMR CNRS 5256 Université de Lyon, 2 Avenue Albert Einstein, 69626 Villeurbanne Cedex, France

Resume : Distinct photocatalytic performances were found when Ta3N5 was synthesized from commercially available Ta2O5 or from home-made Ta2O5 starting from TaCl5 via sol-gel route. In the case of photocatalytic O2 evolution with Ag+ as a sacrificial reagent, Ta3N5 made from commercial Ta2O5 showed higher activity than Ta3N5 from sol-gel route. When the Ta3N5 photocatalysts were decorated with Pt nanoparticles in similar manner, the sol-gel route Ta3N5 show higher photocatalytic hydrogen evolution activity from 10% methanol aqueous solution than Ta3N5 from commercial Ta2O5. Detailed surface and bulk characterizations were applied to obtain fundamental insight to explain the resultant photocatalytic activities.The characterization techniques used to elucidate the bulk properties showed only negligible difference between these two photocatalysts. On the other hand, our deep surface characterization on the surface properties proved that the very thin outmost layer of Ta3N5 in a few nanometers consists of reduced state of tantalum, in contrast to the previous reports that Ta3N5 has an outmost layer in its oxidized form. The electron diffraction patterns revealed that a surface thin layer of 1-2 nm was identified as cubic TaN for both Ta3N5 synthesized from commercial and sol-gel Ta2O5 but to different extent. Electrochemical and Mott-Schottky analyses demonstrated that the TaN surface layer drastically affect the energetic picture at semiconductor-electrolyte interface, which can in turn affect the photocatalytic performance. In addition, this finding will open new direction on development of Ta3N5 photocatalyst particularly on surface properties and modification as the main focus on improving photocatalytic activity.

Z.Z.5.2
14:30
Authors : O. Supplie [1,2], MM. May [1,2], H. Stange [1], C. H?hn [1], H-J. Lewerenz [3], and T. Hannappel [1,4]
Affiliations : [1] Helmholtz-Zentrum Berlin f?r Materialien und Energie, Institute for Solar Fuels, Hahn-Meitner-Platz 1, 14109 Berlin, Germany; [2] Humboldt-Universit?t zu Berlin, Institut f?r Physik, Newtonstr. 15, 12489 Berlin, Germany; [3] California Institute of Technology, Joint Center for Artificial Photosynthesis, 1200 East California Boulevard, Pasadena, CA 91125, USA; [4] Technische Universit?t Ilmenau, Institut f?r Physik, Gustav-Kirchhoff-Str. 5, 98684 Ilmenau, Germany

Resume : Solar hydrogen generation is considered as key challenge for renewable fuel production and reliable energy storage. We estimate the band-alignment of III-V ternary compounds relative to the redox potential of water from available literature data and suggest a photochemical diode based on dilute nitride GaPN grown lattice-matched on Si(100)?a tandem configuration with bandgaps predicted to be close to theoretical optimum for light-induced water splitting [1]. Recently, we showed that the atomic order at GaP(100) surfaces has a significant impact on the interface formation with water[2]. The influence of N on the GaPN surface preparation during metalorganic vapor phase epitaxy (MOVPE) therefore is extremely important. We benchmark the in situ reflection anisotropy (RA) spectra of two different GaPN/Si(100) surfaces to photoelectron spectroscopy (PES) and low-energy electron diffraction (accessible after MOVPE-to-UHV transfer) which enables in situ control during surface preparation in VPE ambient. The Ga-rich GaPN/Si(100) surface may be prepared analogously to GaP(100) without significant N depletion in both bulk and surface layers (evidenced by PES and x-ray diffraction), while excess N impedes group-V-rich surface preparation. We attribute a feature in the RA spectra close to the E1 transition of GaP to N incorporation which allows to study GaPN growth in situ during MOVPE. [1] S. Hu et al., Energy Environ. Sci. 6, 2984 (2013) [2] MM. May et al., New J. Phys. 15, 103003 (2013)

Z.Z.5.3
14:50
Authors : Rudolph Martin1, Brigitte Bouchet-Fabre 2, Elisabeta Nienaltowska1, Marie.Christine Hugon 2, Tibériu Minéa 2
Affiliations : 1 LPGP, U-Psud -CNRS, Université Paris-Sud, F-91401 Cedex, France 2 LEDNA, NIMBE, CEA-CNRS, CEA-Saclay, F-91191 Gif sur Yvette Cedex

Resume : Tantalum nitride Ta-Nx ultra-thin films present a large variety of nanostructure depending of the nitrogen content and the deposition technique. This impacts directly their optical gap from zero for metallic Ta and TaN to 2.2eV for the semi conductive Ta3N5. That latter nanostructure is of special interest for solar production of hydrogen by water splitting or as a compound for photovoltaic multilayer system. We will show that Ta3N5 thin films may be elaborated using High Power Pulsed Magnetron Sputtering HIPPIMS deposition while it is not possible by conventional magnetron sputtering. HIPPIMS is of special interest for that materials production because it allows a larger incorporation of nitrogen inside the films and generates specific nanostructures. Therefore, the study is based on plasma diagnostic, nuclear analysis for concentration and density, grazing incidence X-ray scattering GIWAXS for the nanostructure, high resolved electron scanning microscopy SEM-Feg for the morphology and XPS for the local bonding. We will present our first results concerning the optical properties followed by Ellipsometry and reflexion-tranmission measurements.

Z.Z.5.4
15:10
Authors : Shaowen Cao, Yupeng Yuan, Lisha Yin and Can Xue*
Affiliations : School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore

Resume : Recently g-C3N4 has attracted great interest due to its low cost and visible-light activity. However, the fast electron-hole recombination and low activity for H2 evolution have restricted the use of g-C3N4 alone for solar fuels production. Herein, we present our recent work on development of decorated g-C3N4 with noble-metal-free cocatalysts, cobaloxime and NiS2.[1,2] Both co-catalysts can allow for effective receiving of excited electrons from g-C3N4 for H2 generation through efficient redox cycles. Further, we also demonstrated that the coupling of g-C3N4 coupling with other semiconductor structures, such as red phosphor, CdS, and In2O3, allows for creating heterojunctions to promote charge separation for efficient photocatalytic H2 generation and CO2 reduction.[3-5] Our studies demonstrate economic solar-to-fuels conversion platforms based on metal-free g-C3N4 photocatalysts and noble-metal-free coupling compounds and co-catalysts. 1. S. W. Cao, X. Liu, Y. Yuan, Z. Zhang, J. Fang, S. C. J. Loo,* J. Barber, T. C. Sum, C. Xue,* Phys. Chem. Chem. Phys. 2013, 15, 18363. 2. L. S. Yin, Y. Yuan, S. Cao, Z. Zhang, C. Xue*, RSC Adv. 2014, 4, 6127. 3. Y. P. Yuan, S. Cao, Y. Liao, L. Yin, C. Xue*, Appl. Catal. B: Environ. 2013, 140, 164. 4. S. W. Cao, Y. Yuan, J. Fang, F. Boey, J. Barber, S. C. J. Loo*, C. Xue*, Int. J. Hydrogen Energy 2013, 38, 1258. 5. S. W. Cao, X. Liu, Y. Yuan, Z. Zhang, J. Fang, S. C. J. Loo,* T. C. Sum, C. Xue*, Appl. Catal. B: Environ. 2014, 147, 940.

Z.Z.5.5
15:30 Coffee Break    
 
POSTER SESSION : xx
16:00
Authors : Chia-Yu Lin
Affiliations : Department of Chemical Engineering, National Cheng Kung University, Tainan City 70101, Taiwan

Resume : We report on an iron oxide electrode, which is prepared using simple and scalable chemical bath deposition (CBD) and perform as a highly efficient and robust electrocatalyst for water oxidation. By simply tuning the CBD conditions without post-treatment, iron oxides with different morphologies, including nanoparticles, nanorods, and nanoplates, and phases (e.g., alpha and gamma phases) are prepared. After detailed discussion on the growth mechanism of iron oxide, the effects of the morphology and phase of iron oxide on its electrocatalytic activity toward water oxidation, in terms of overpotential and turnover frequency, are thoroughly investigated.

Z.Z.P.1
16:00
Authors : M. Richter, D. Schmeißer
Affiliations : Brandenburg University of Technology Cottbus-Senftenberg, Applied Physics and Sensors, K.-Wachsmann-Allee 17, 03046 Cottbus, Germany

Resume : The electronic structure of the cobalt oxide based catalysts for OER is analyzed using synchrotron radiation photoelectron spectroscopy. The catalyst films are prepared by electrochemical deposition. X-ray photoelectron spectroscopy and resonant photoelectron spectroscopy is used to analyze the Co2p and O1s core levels, absorption edges and valence bands. Here we find a difference in the Co oxidation state as a function of film thickness (deposited charge). We discuss our resonant data in terms of the partial density of states of the valence and conduction band. For the individual Co3d states, we determine their configuration, their spin, and their energy level relative to the Fermi energy. At resonant excitation we find the Co2p partial DOS to exhibit sharp features next to the VBM for increased cobalt oxidation state instead for a broad emission at around 6eV below EFermi for a low oxidation state. We attribute such sharp features to the low spin (LS) configuration of Co3+. In contrast, our data prove the Co2+ ground state for thin pristine cobalt oxide films and demonstrate that it is exclusively in the Co3d7 high spin state. In addition, both cobalt oxide configurations have characteristic oxygen to Co charge transfer states in the band gap. We attribute the trivalent charge transfer state to be the active state for the oxygen evolution reaction. [1] M. Richter, M., D. Schmeißer, Applied Physics Letters 102, 253904 (2013)

Z.Z.P..5
16:00
Authors : G. Cacciato, F. Ruffino, M. Zimbone, V. Privitera, M. G. Grimaldi
Affiliations : G. Cacciato, F. Ruffino, M. Zimbone, M. G. Grimaldi Dipartimento di Fisica ed Astronomia-Università di Catania, via S. Sofia 64, 95123 Catania, Italy; G. Cacciato, F. Ruffino, M. Zimbone, V. Privitera, M. G. Grimaldi MATIS IMM-CNR, via S. Sofia 64, 95123 Catania, Italy;

Resume : Recent years have shown a renewed interest in the use of functional materials for conversion of solar to chemical energy by photocatalysis. Although these materials are, mostly, semiconductor oxides, it has been demonstrated that metal nanoparticles (NPs) improve their conversion efficiency due to their surface plasmon resonance properties. Au and Ag are of particular interest since exhibit resonant behavior in the visible range, allowing the harvesting of a large amount of the solar flux. Among semiconductor photocatalysts for water splitting, TiO2 has been investigated due to its abundance, stability, non-toxicity and high photocatalytic activity. So, composite materials fabricated by TiO2 thin films in connection with Au or Ag NPs gain attention as candidate for efficient photocatalysts devices. In this work, NPs are obtained after thermal dewetting of sputtered Au or Ag films on TiO2. We correlate the photocatalytic activity of the NPs-TiO2 composite film under UV and visible illumination with structural characteristics: crystal phase (anatase or rutile), density, size and plasmonic response of the Au and Ag NPs. Different designs are investigated: NPs simply deposited on the TiO2 surface or NPs either embedded in the TiO2 film. Structural and morphological analysis performed by XRD, SEM, AFM and RBS will be presented, so as plasmonic response evaluated by UV-visible spectroscopy. Finally, activity performance of the composite materials will be presented and discussed.

Z.Z.P.6
16:00
Authors : Lifei Xi, Sing Yang Chiam, Lydia Helena Wong, Yeng Ming Lam, Beata Kardynal
Affiliations : Peter Grünberg Institute, Semiconductor Nanoelectronics (PGI-9), Forschungszentrum Jülich GmbH, 52425 Jülich, Germany, Institute of Materials Research and Engineering (IMRE), A *STAR, 3 Research Link, 117602, Singapore, School of Materials Science and Engineering, Nanyang Technological University, 639798, Singapore, Institute of Materials in Electrical Engineering and Information Technology 2 (IWE2), RWTH, Aachen, D-52056 Aachen, Germany

Resume : In recent year, photoelectrochemical (PEC) cells offer the ability to convert solar energy to stored chemical energy through the splitting of water into molecular oxygen and hydrogen. Hematite (α-Fe2O3) has recently emerged as a promising photoanode material for the generation of dioxygen from water due to its favorable optical band gap (Eg: 2.2eV), excellent chemical stability in aqueous environments, ample abundance and low cost. Despite these advantages, hematite is limited by rather weak absorption and poor charge transport. This will prevent a trade-off between light absorption and carrier collection in flat substrates. Mesoporous conducting substrate with large surface area is a novel morphology for extra thin film (ETA) water oxidation, as the extra thin film which is coated on mesoporous conducting substrate favours good hole-collection at the semiconductor-electrolyte interface. In this study, the role of mesoporous conducting film thickness, hematite film thickness, the annealing temperature and the post surface treatment have been fully studied with XPS, TEM, EDX, XRD, UV-Vis, EIS and IPCE. Hematite on mesoporous substrates showed a higher Sn doping concentration compared to the flat substrate. Our study also showed that Sn incorporation into the hematite has dramatically improved the charge injection and charge transportation yields.

Z.Z.P.7
16:00
Authors : Yaowapa Treekamol (1), Mauricio Schieda (1), Iris Herrmann-Geppert (1,2), Thomas Klassen (1,2)
Affiliations : (1) Helmholtz-Zentrum Geesthacht Centre for Materials and Coastal Research, Max-Planck-Str. 1, 21502 Geesthacht; (2) Helmut-Schmidt University, Holstenhofweg 85, 22043 Hamburg

Resume : We present our experiments on the surface functionalization of TiO2 nanoparticles with silane coupling agents, in order to enhance its properties relevant to photoelectrochemical water oxidation. Photoelectrodes were prepared on FTO-coated glass substrates, and characterized in a photoelectrochemical cell using 0.5 M H2SO4 as electrolyte, and a platinum ring as counter electrode. Under simulated sunlight (AM1.5G), the electrodes prepared with functionalized TiO2 showed improved photocurrent densities for the oxygen evolution reaction (up to 0.14 mA cm-2 at 1V, compared to 0.04 mA cm-2 for pristine TiO2). The influence of electrode thickness was also studied, and the optimal thickness of modified TiO2 was found to be approximately 600 nm. The increase in photocatalytic activity for the electrodes prepared with functionalized particles can be explained by an enhanced dispersion in the casting solvent, resulting in more homogeneous films, as well as an improved contact between the particles and the substrate, resulting in more efficient electron collection. The electrodes were further characterized by electrochemical impedance spectroscopy, infrared, Raman and ultraviolet spectroscopy. The functionalized electrodes showed stability up to at least 1 hour under continuous illumination.

Z.Z.P.8
16:00
Authors : Anja Bieberle-Hütter 1, Irem Tanyeli 1, Reinoud Lavrijsen 2, Quanbao Ma 3, Robbert van de Kruijs 1, Erwin Zoethout 1, Jürgen Kohlhepp 2, Greg De Temmerman 1, Richard van de Sanden 1
Affiliations : 1 FOM-Institute DIFFER (Dutch Institute for Fundamental Energy Research), the Netherlands 2 Physics of Nanostructures and center for NanoMaterials (cNM), Department of Applied Physics, Eindhoven University of Technology (TU/e), the Netherlands 3 Inorganic Materials Chemistry, Chemical Engineering and Chemistry, Eindhoven University of Technology (TU/e), the Netherlands

Resume : Photo-electrochemical solar fuel conversion is a seminal method to convert solar energy into a storable fuel and contributes to finding sustainable energy solutions. However, the overall efficiency of photo-electrochemical solar fuel conversion is still low and the degradation of the electrodes is high. Increasing the surface area of the electrodes is a known and effective method to increase the catalytic active area and by this to increase the performance. Usually, nanostructures are fabricated from the bottom up, i.e. by means of growing nanotubes or nanowires. In this study, we use He plasmas with ion fluxes between 10^20 m-2 s-1 and 10^23 m-2 s-1 to nanostructure surfaces from the top down. The method was already proven on bulk materials where feature sizes below 100 nm were obtained. We have now transferred the technique to thin films deposited on typical solar fuel conversion substrates, i.e. F: SnO2 (FTO) on glass. The high processing temperature and delamination of the thin films from the substrate are major concerns when thin films are used compared to bulk material. We will show results of structural and microscopical characterization of successfully plasma nanostructured metal thin films of Fe, W, Ti, Mo on FTO-glass as well of the resulting oxides after post-annealing. Photo-electrochemical characterization results prove the feasibility of using these nanostructures to significantly improve the performance of current solar to fuel conversion materials.

Z.Z.P.9
16:00
Authors : Elisa Tordin, Dogukan Hazar Apaydin, Gottfried Aufischer, Stefanie Schlager, Engelbert Portenkirchner, Melanie Weichselbaumer, Niyazi Serdar Sariciftci
Affiliations : Linz Institute for Organic Solar Cells (LIOS), Physical Chemistry, Johannes Kepler University Altenbergerstrasse 69, A-4040 Linz, Austria

Resume : The electrochemical reduction of carbon dioxide has been subject of a great deal of interests in the past decades since it could be an efficient way to convert this greenhouse gas into valuable chemicals and fuel. This process is quite energetically inefficient since a high overpotential required to convert carbon dioxide to methanol or even methane. Therefore electrocatalytic reduction is quite a promising approach to overcome this problem as demonstrated by Bocarsly [1]1 and Lehn [2] who successfully reduced CO2 using pyridinium ions and [Re(bipy)(CO)3Cl] respectively as homogeneous electrocatalysts. Inspired by these achievements, we decided to move forward and covalently bind these electrocatalysts to a conductive polymer deposited on the working electrode in order to transform the homogeneous catalysts into heterogeneous catalysts, eliminating the need to recover them after the reaction. This aim can be achieved either by synthesizing monomers functionalized with pyridine or [Re(bipy)(CO)3Cl] [3] and, subsequently, electropolymerizing them on the working electrode or by direct functionalization of the polymers. We have succeeded in synthesizing the desired compounds and promising preliminary results concerning the electrochemical CO2 reduction have been observed. [1] A.B. Bocarsly et al; J. of Electr. Chem., 372, 1994, 145-150 [2] J.M. Lehn et al; J. Chem. Soc. Chem. Commun., 1984, 6, 328 [3] F.R. Keene et al; Aust. J. Chem., 1995, 48, 1425

Z.Z.P.10
16:00
Authors : M. Rioult, H. Magnan, D. Stanescu, A. Barbier
Affiliations : CEA-Saclay, DSM/IRAMIS/SPEC, F 91191, Gif sur Yvette Cedex, France

Resume : The transformation and storage of solar energy is a major challenge in the framework of renewable energy sources. The conversion into chemical energy stored in the form of hydrogen, through direct photoelectrochemical water splitting is a promising method. Hematite (α-Fe2O3) is a potential key photoanode material due to its optimal band gap, excellent chemical stability, abundance, non-toxicity and low cost. However, for practical applications hematite performances must be improved with respect to the absorption coefficient and holes diffusion length. Moreover, the roles of the long range crystalline order, quality and orientation remain largely unknown yet. In order to tackle these open questions we have studied the growth, the crystalline and electronic structures and the photoelectrochemical properties of epitaxial doped and undoped Fe2O3 films grown by oxygen plasma assisted molecular beam epitaxy on different substrates. This method allows obtaining perfectly single crystalline films with well-defined doping levels, defects and crystallographic structures well adapted for photoelectrochemical measurements [1]. We show that both crystalline structure (hematite, maghemite) and crystallographic orientation ((111), (001), polycrystalline)) strongly influence in a non-trivial way the photoelectrochemical properties. These results are discussed in relation with conductivity anisotropy and surface kinetic properties. [1] H. Magnan et al., Appl. Phys. Lett. 101, 133908 (2012)

Z.Z.P.11
16:00
Authors : A. Azarpira 1, M. Lublow 2, F. Yang 1, C. Merschjann 1, J. Pfrommer 2, A. Steigert 1, M. Lücke 1, A. Fischer 2, M. Driess 2, Th. Schedel-Niedrig 1
Affiliations : 1 Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Institute Solar Fuels 2 Technische Universität Berlin, Institut für Chemie

Resume : For the envisaged direct conversion of solar energy into storable fuels such as hydrogen, photoelectrocatalytic water splitting is likely to play a key role in the strategies for alternative fuel production. In photoelectrocatalytic water splitting, the two half-cell reactions of hydrogen and oxygen evolution (HER and OER) have to run at comparable rates in order to realize an optimal solar-to-fuels conversion efficiency. In this contribution thin film composites will be presented in this contribution used as novel photoelectrodes. The composite photoelectrodes are based on the device-grade silicon and chalcopyrite photovoltaic materials [1]. We will show that the composite photocathodes generate photovoltages up to 0.5 V, which results in a corresponding anodic shift of the photocurrent onset potential. Moreover, the best photocathode exhibits incident-photo-to-current efficiency up to 80% in the complete visible light range. Additionally, various robust and low-cost thin film transition-metal oxide electrodes prepared by electrophoretic coating are successfully applied as electrocatalysts [2]. The catalysts show low overpotentials for OER in the dark and reveal long-term stability with high current densities. A modified silicon photoanode coated with an ultra-thin NixOy layer displays an onset for OER shifted toward more cathodic values [2]. The overall objective of the R&D activities is the development of a monolithic tandem prototype device for visible-light driven water splitting. [1] F. Yang et al., J. Mat. Chem. A 1 (2013) 6407; ChemSusChem 5 (2012) 1227. [2] M. Lublow et al., Nature Materials (submitted 11/2013); J. Pfrommer et al., Angewandte Chemie (submitted 01/2014).

Z.Z.P.13
16:00
Authors : Gunawan Gunawan, Wilman Septina, Shigeru Ikeda, Takashi Harada, Michio Matsumura
Affiliations : Research Center for Solar Energy Chemistry, Osaka University

Resume : Copper chalcopyrite semiconductors include a wide range of compounds that are of interest for photoelectrochemical water splitting. In the present study, we investigated the chalcopyrite, especially CuInS2 (CIS) fabricated by electrodeposition and spray pyrolysis methods as photochatodes for water splitting. Thin film of CISed was formed by stack electrodeposition of copper and indium followed by sulfurization under H2S flow. CISed loaded with Pt (Pt-CISed) worked as photocathodes for H2 generation from an aqueous solution containing 0.1 M Na2SO4 (pH = 9). Introduction of an n-type CdS layer on the CuInS2 before the Pt loading (Pt-CdS/CISed) resulted appreciable improvements of H2 liberation efficiency and a higher photocurrent onset potential. Moreover, CISed film modified with In(OHx,Sy) (Pt- In(OHx,Sy)/CISed) also worked as an efficient photocathode with photocurrent as high as 14 mA cm-2 at 0 V (vs. RHE) and maximum applied bias photon-to-current efficiency of over 1.5% at 0.25 V (vs. RHE). In addition, thin film of CISsp and its Ga alloyed film (Ga:CISsp) were synthesized by spray pyrolysis modified by n-type CdS layer and loading of Pt. Both Pt-CdS/CISsp and Pt-CdS/Ga:CISsp worked as photocathodes with photocurrent as high as ca. 6 mA cm-2 at 0 V (vs. RHE) from an aqueous solution containing 0.1 M Na2SO4 (pH = 9). The Pt-CdS/Ga:CISsp photocathode with Ga/In ratio of ca. 0.25 had a ca. 0.1 V higher photocurrent onset potential than that of Pt-CdS/CISsp.

Z.Z.P.14
16:00
Authors : Sandra Hilaire, Markus Niederberger
Affiliations : ETH Zurich

Resume : WO3 nanoparticles were synthesized via a fast and efficient microwave-assisted nonaqueous sol-gel route within a few minutes. Different nanoparticle sizes could be obtained by varying the microwave irradiation time from 5 min to 90 min. Nanostructured WO3 photoanodes for photoelectrochemical hydrogen production were fabricated by doctor blading a slurry of the different WO3 nanoparticles onto a transparent conductive oxide glass substrate. After heat treatment at 600˚C for 45 minutes the films are porous, have a thickness of about 3 μm and are semi-transparent. Photocurrents of the different films were measured under 100 mW/cm2 AM 1.5 illumination in 1M H2SO4. Films prepared from the nanoparticles obtained after 10 min microwave irradiation, corresponding to an average crystal size of about 20 nm, were found to show the best performance. After electrochemical cycling of the films in the dark a maximum photocurrent of 2.5 mA/cm2 was obtained. This value could be increased to 5.2 mA/cm2 by addition of 0.5M MeOH to the electrolyte. Mechanisms to explain these observations will be discussed and further improvements will be outlined.

Z.Z.P.17
16:00
Authors : Néstor Guijarro, Mathieu Prévot, Kevin Sivula
Affiliations : Laboratory for Molecular Engineering of Optoelectronic Nanomaterials (LIMNO)

Resume : Cu2ZnSnS4 (CZTS) has attracted much attention as a promising material for solar energy conversion. However, a ubiquitous CdS thin film overlayer is required to promote the charge separation through a p-n heterojunction. In this work, CZTS thin film photocathodes, prepared by using a colloidal approach, were employed as a model system to examine the effect of different buffer layers on charge separation as measured by the photoelectrochemical performance. Routine CdS layers were compared with ultrathin CdSe and ZnSe (both of which have been generally disregarded to date). Photoelectrochemical measurements indicate a direct correlation between the CZTS (CB)-buffer (CB) energy gap and the yield of electron extraction. Additionally, surface modification with methylviologen demonstrated a synergistic effect in combination with the inorganic buffer layer to further boost the yield of charge extraction. Alternatively, a novel template-free low-temperature approach to fabricate high specific surface area CZTS films from nanocrystal inks is introduced. Photocurrent results with these films corroborate the premise that minimizing the minority carrier transit distance alone can drastically enhance the yield of charge separation. Overall, these findings provide compelling evidence that the performance of CZTS-based electrodes can be further improved by tuning the driving force imposed by the buffer layer for electron extraction, and by optimizing the CZTS/buffer layer surface area.

Z.Z.P.18
16:00
Authors : C.Toussaint, R.Cloots, C.Henrist
Affiliations : University of Liège - Chemistry Department - GREEnMat-LCIS

Resume : Solar energy is inexhaustible but variable during the day and the seasons. Photoelectrolysis of water (water splitting) convert this energy into chemicals like hydrogen to obtain an energy that can be stored and transported on demand. Hematite is a promising material for the photoanode in water splitting because of its high stability in water, cheapness, abundance and its band gap that allows the absorption of visible light (Eg: 2,1eV). Nevertheless, hematite has also some drawbacks including a short diffusion length of holes. We have implemented spin coating and templating to produce mesoporous hematite films. The nanostructuration can improve the performances in water splitting by reducing the diffusion length of holes and increasing the specific surface between the film and the electrolyte. A silica confinement scaffold allows to avoid the collapsing of mesoporosity at high temperature requested for dopant activation. To show the impact of the nanostructure, we have compared three films (mesoporous, collapsed and dense) in terms of hematite content (elemental analysis), nanostructure (electron microscopy), crystallinity (X-ray diffraction) and water splitting efficiency. We have also test two thermal treatments. This study allows discriminating between the effect of crystallization and grain boundaries impacting the charge transport, and effective interface with the electrolyte, through the preservation or not of open porosity.

Z.Z.P.19
16:00
Authors : K. Müller, F. Rachow, J. Israel, D. Schmeißer
Affiliations : Brandenburgische Technische Universität Cottbus-Senftenberg, Angewandte Physik-Sensorik, Konrad-Wachsmann-Allee 17, 03046 Cottbus, Germany

Resume : The “Power to Gas” approach is based on the catalytic conversion of CO2 with H2. At a moderate temperature of around 350°C, hydrogenation of CO2 into CH4 is possible by the Sabatier reaction CO2 + 4H2 ->CH4 +2H2O. This approach gives a basis for the reintegration of secondary products like CO2 into the energy supply. The CO2 is transferred into a reusable energy source, this offers a possibility of a chemical energy storage procedure when the produced CH4 is fed into the existing network of natural gas. As part of the BMBF project Geo Energy Research (GeoEn, grand No. 03G0767B), we study the Sabatier reaction in the context of the oxy-fuel process. Here, one important aspect is the stability of the performance of catalysts for Sabatiers reaction against typical contaminations of flue gas, generated in the power plant. Oxyfuel-CO2 carries SOx (3ppm) and NOx (15ppm) which are known to be deleterious for the performance of catalysts. We have tested mixtures of pure CO2 with increasing content of either SOx or NOx. We also studied synthetic mixtures of both, SOx and NOx with pure CO2. Finally, we used real processed oxy-fuel CO2. In all systems we find that the conversion rate remains above 80%, with ~100% selectivity within the first 24h. Also, the temperature window for the catalytic performance is not changed. In addition we report on important parameters for an upscaling from laboratory scale into technical application considering the flow density, temperature or catalyst.

Z.Z.P.20
16:00
Authors : Agnieszka Rzeszutek (1), Mauricio Schieda (1), Regina Just (1), Thomas Klassen (1,2), Iris Herrmann-Geppert (1,2)
Affiliations : (1) Helmholtz-Zentrum Geesthacht, Institute of Materials Research, Max-Planck-Str. 1, 21502 Geesthacht, Germany; (2) Helmut-Schmidt University, Holstenhofweg 85, 22013 Hamburg, Germany

Resume : Solar production of hydrogen is one of the most promising ways to utilize the renewable energy of the sun. Hydrogen generation by the photoelectrochemical water splitting process is limited by the water oxidation reaction (OER). Therefore, the increase of overall efficiency depends on the development of a photoanode with a large specific surface area, enhanced light capture and improved diffusion of gaseous products. In this work we report on the correlation between morphology and activity of TiO2 photoanodes for water splitting in terms of enhanced catalytic surface area, improved charge carrier dynamics and optimized light management. Two different approaches were applied to prepare structured electrodes. In the first approach a prestructured substrate (Ti foam) underwent thermal oxidation to fabricate the layer of catalyst. In the second approach the catalyst was structured to the form of inverse opals, using the sacrificial template method. The material was characterized by scanning electron microscopy (SEM), X-Ray diffraction (XRD) and Raman spectroscopy, and the photoelectrochemical performance was evaluated by cyclic voltammetry and incident photon to current efficiency (IPCE) under simulated (AM1.5G) sunlight. The application of prestructured substrates and structured catalysts (and the combination of both of them) will be further extended to other coating techniques (e.g. cold gas spraying technology) and photocatalysts (e.g. tungsten oxide), providing a basis for the preparation of structured electrodes with enhanced ability for visible light absorption.

Z.Z.P.22
16:00
Authors : Enrico Binetti 1,2, Zakaria El Koura2, Nainesh Patel2, Gianfranco Carotenuto1, Antonio Miotello2
Affiliations : 1 Institute for Composite and Biomedical Material - Italian National Research Council - I-38123 Povo, Trento (Italy) 2Dipartimento di Fisica, Università degli Studi di Trento, I-38123 Povo, Trento (Italy)

Resume : The photoelectrochemical water splitting requires the use of semiconductor materials having suitable energy-levels for water reduction/oxidation and high stability against photocorrosion. Possessing all the requirements and being a cost effective material, titanium dioxide has been widely investigated for the photoelectrochemical splitting of water [1]. However, its photocatalytic efficiency is limited by the low light absorption and the rapid recombination of photogenerated carriers. Several efforts have been devoted to improve its photocatalytic activity by including dopant atoms, both metals and non-metals, which lead to the formation of mid-gap electronic levels making it sensitive to the visible light and improving its conductivity [2]. In the present work TiO2 thin films, deposited by RF magnetron sputtering on glass/ITO substrates, have been doped with hydrogen by means of annealing in a high pressure hydrogen atmosphere. The obtained electrodes have been characterized by UV-Vis absorption spectroscopy, µ-Raman spectroscopy, Scanning Electron Microscopy, electrochemical and photoelectrochemical measurements. The results indicate not only an enhancement of the light absorption, but also an improvement of the catalytic activity of TiO2 when doped with hydrogen. 1. Chen, X., et al., Chem. Rev. 2010. 110(11): p. 6503-6570. 2. Jaiswal, R., et al., Appl. Cat. B: Environm., 2012. 126(0): p. 47-54.

Z.Z.P.26
16:00
Authors : Philipp Hillebrand, Peter Bogdanoff, Sebastian Fiechter
Affiliations : Helmholtz-Zentrum Berlin fuer Materialien und Energie GmbH, Institute for Solar Fuels, Hahn-Meitner-Platz 1, 14109 Berlin, Germany

Resume : The oxygen evolution reaction (OER) represents a bottle neck in the development of sustainable light-driven production of hydrogen from water using cheap and abundant electro-catalysts. Even though metal oxide catalysts have been used for decades, the mechanism of the OER has not been fully understood yet. We investigated cobalt and manganese oxide thin films in their behavior during OER using photoelectron spectroscopy (PES) and synchrotron radiation. In an in-line system we could perform cyclic voltammetry as well as potentiostatic measurements in the anodic range under well-defined inert atmospheric conditions and afterwards transfer the samples into the UHV system for PES studies, without breaking the atmosphere. The electrochemical experiments have been interrupted at several potentials by removing the electrolyte drop and XPS as well as UPS spectra have been recorded successively. Despite the fact that the cyclic voltammetry curves (CV) of amorphous CoOx layers - deposited potentiostatically on FTO glass – and crystalline α-Mn2O3 – prepared by reactive sputtering and galvanostatic deposition – differ significantly in the low potential range before the onset of the OER, the differences in the PES spectra are little. Nevertheless, we could find some shifts in the peaks that have been interpreted as a change in the oxidation state of the metal ions at the surface depending on the potential applied and associated with the appearance of new oxo-bondings.

Z.Z.P.27
16:00
Authors : N. Weidler, B. Kaiser, J. Ursul, W. Jaegermann
Affiliations : Institute of Material Science, Technische Universität Darmstadt , Jovanka-Bontschitz-Str. 2, 64287 Darmstadt

Resume : The interest in the field of renewable and green energies has increased considerably, which is due to the decreasing amount of available fossil fuels and to the emission of green house gases. But, energy from wind and sun has the big disadvantage that it is not available on a regular basis, i.e. it is necessary to have appropriate technologies to store the generated electricity. One possible alternative to store large amounts of energy is by employing suitable chemical compounds. Hydrogen produced by electrochemical water splitting could play a key role for energy storage. One of the major obstacles, which make the process uneconomical at this time, is the overvoltage loss for the oxygen evolution reaction (OER). To reduce the overvoltage standard catalysts e.g. RuO2 and IrO2 are used, which are highly priced and have a limited availability. Therefore, our work focuses on the development of a chemical vapor deposition (CVD) process to produce cheap cobalt based catalysts, which are also able to decrease the overvoltage of the OER. Catalysts are prepared using different precursors and electrode substrates (e.g. Titanium). The CVD deposition process must be optimized for several parameters like temperature, gas flow, etc. For chemical and structural characterization X-ray photoelectron spectroscopy (XPS) and high-resolution electron microscopy (HREM) are employed. Electrochemical characterization of the electrodes with different catalysts is done in a 3 electrode setup.

Z.Z.P.28
16:00
Authors : Dr. Khurram Saleem Joya(1,2) and Prof. Kazuhiro Takanabe(1)
Affiliations : (1) Division of Physical Sciences and Engineering, KAUST Catalysis Center (KCC), King Abdullah University of Science and Technology (KAUST), 4700 KAUST, Thuwal 23955-6900, Saudi Arabia (2) Leiden Institute of Chemistry, Gorlaeus Laboratory, Leiden University, P.O. Box 9502, 2300 RA, Leiden, The Netherlands

Resume : For solar fuel generation via water splitting reaction, development of stable and efficient electrocatalytic materials is highly demanded. In this pursuit, there is a lot of research activity related to artificial photosynthesis in order to develop a stand-alone solar to fuel conversion device, the “Artificial Leaf”. In combination with a suitable CO2 reduction catalytic system, the electrons and protons released from water oxidation can be reduced directly into liquid fuels. We here present simple electrochemical approach to make efficient and robust electrocatalysts on electrode surface in a neutral pH system that can be efficiently employed for water oxidation. The catalytic performance of these surface-assembled water oxidation catalysts (WOC) is remarkable over a wide range of pH (pH 6-11) with high oxygen evolution current densities.

Z.Z.P.29
16:00
Authors : Chittaranjan Das, Massimo Tallarida, and Dieter Schmeisser
Affiliations : Chair for applied physics and sensors BTU Cottbus-Senftenberg

Resume : Generation of hydrogen using solar power with the help of semiconductor photoelectrodes is an efficient and environmental friendly way to produce energy. Among the various semiconducting materials, Silicon could be one of the best choices for this application. However, certain problems like surface oxidation, charge transfer from Si surface to electrolyte, reflectance of the absorber are main issues for Si photocathodes. Surface oxidation and charge transfer can be optimized by passivation layers and by the use of catalyst on Si surface. Instead, the problem of surface reflectance can be avoided by structuring the silicon photoabsorber. Structuring of Si in the top down method is generally obtained by etching the substrate with metal catalyst on it, but this process needs several steps of sample preparation. The structuring can be also done using the electrochemical technique, without having any metal catalyst on the Si surface. In this work, we used the latter method and prepared microstructured Si photocathodes. These showed a better photo current as compared to bare Si in low pH electrolytes and an the onset potential shifted by 300mV positive in comparison to planar Si photocathodes.

Z.Z.P.30
16:00
Authors : Arindam Indra,1 Prashanth W. Menezes,1 Nastaran Ranjbar,1 Arno Bergmann,1 Ivelina Zaharieva,2 Peter Strasser,1 Holger Dau,2 Matthias Driess1
Affiliations : 1. Department of chemistry, Technische Universität Berlin, Strasse des 17 Juni 135, Sekr. C2, 10623 Berlin, Germany. 2. Fachbereich Physik, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany

Resume : Water oxidation and oxygen reduction can play the pivotal role of solving the problem of energy production and conversion in a sustainable and environmental friendly way. Also photochemical water oxidation in biomimetic fashion is considered as the most convenient way to convert solar energy into chemical energy. In nature, calcium manganese oxide cluster of photosystem II acts as the oxygen evolving center to oxidize water into oxygen. Over the years several manganese oxide based catalysts have been explored for the water oxidation in heterogeneous medium. Here we present a new route for the synthesis of the mixed valent manganese oxide from the low valent manganese monooxide nanoparticles by partial oxidation. This amorphous material was highly efficient in photochemical and electrochemical water oxidation with low overpotential and long run stability. Another approach was taken to synthesize amorphous mixed metal oxides by introducing simple solvothermal process with first row transition metals. These amorphous oxides were employed as the multifunctional catalysts for the photochemical and the electrochemical water oxidation as well as for the oxygen reduction reaction with high efficiency. In conclusion, designing of the amorphous oxides of cheap and abundant first row transition metals can show new directions in the synthesis of the potential materials for the energy storage and conversion in a convenient way.

Z.Z.P.31
16:00
Authors : Sophia B. Betzler , Andreas Wisnet, Christina Scheu
Affiliations : Sophia B. Betzler, Department of Chemistry & Center for NanoScience (CeNS), Ludwig-Maximilians-Universit?t M?nchen, 81377 Munich, Germany; Andreas Wisnet, Department of Chemistry & Center for NanoScience (CeNS), Ludwig-Maximilians-Universit?t M?nchen, 81377 Munich, Germany; Christina Scheu, , Max-Planck-Institut f?r Eisenforschung (MPIE), 40237 D?sseldorf, Germany

Resume : Applications in the field of photochemistry request nanostructured electrodes with large surface areas, high crystallinity and suitable properties. In this regard, new materials and morphologies are required, ideally achieved from environmentally-friendly low-cost synthesis strategies. Here we report on novel 3D hierarchical niobiumoxide superstructures, which can be derived from a template-free, one-step hydrothermal synthesis approach. The achieved hollow cubes are built up from a regular nanowire-network and offer both a large surface area and high crystallinity. Their formation mechanism was investigated by a combination of electron microscopic and spectroscopic methods, showing the transformation from amorphous hollow cubes to cubes built up from crystalline nanowires. Elaborate transmission electron microscopy (TEM) and site-specific thickness measurements using scanning TEM and electron energy-loss spectroscopy were applied for the investigation of the orthogonal nanowire-arrangement in the network, with a focus on the junctions. Two different types of junctions can be distinguished, a T-shaped type with no interference of the nanowires and nanowire-crossings were the nanowires partly interpenetrate at the overlap region. In addition to the structural characterization their photochemical applicability was analyzed, showing a band gap of 3.2 eV and photocatalytic activity.

Z.Z.P.33
16:00
Authors : D. H. Apaydin, E. D. Głowacki, E. Portenkirchner, N. S. Sariciftci
Affiliations : Linz Institute for Organic Solar Cells (LIOS), Institute of Physical Chemistry, Johannes Kepler University Linz, Austria

Resume : Limiting anthropogenic carbon dioxide emissions constitutes a major issue faced by scientists today. Technologies aim at capturing carbon dioxide, followed by sequestration or utilization. A key step for both sequestration and utilization approaches of carbon dioxide is controlled capture, storage and release. Here we report an efficient way of controlled capture and release of carbon dioxide using a cheap and abundant nontoxic industrial pigment, Quinacridone (QNC). Electrochemically-reduced electrodes having a structure of ITO/QNC (100nm) are capable of forming a stable QNC?carbonate salt. Carbon dioxide-loaded QNC films are stable in ambient conditions for hours and stored carbon dioxide can both be released by heating or by electrochemical oxidation. The amount of captured carbon dioxide was quantified by FT-IR. The uptake values for the thermal and electrochemical release processes were 2.28 mmol/g and 4.61 mmol/g respectively. These values are among the highest reported uptake efficiencies for electrochemical carbon dioxide capture. For comparison, the state-of-the-art aqueous amine industrial capture process has an uptake efficiency of ~8 mmol/g.

Z.Z.P.34
16:00
Authors : Yimeng Ma, Stephanie Pendlebury, Florian Le Formal and James Durrant
Affiliations : Department of Chemistry, Imperial College London, UK, SW7 2AZ

Resume : Bismuth vanadate (BiVO4) has attracted many research interests in solar water oxidation. Its narrow band gap allows the visible light harvesting in the solar spectrum, thus in theory achieving 7.5 mA/cm2 photocurrent. However, the recently research progress has been limited by the modest water oxidation efficiency, which is not high enough for a practical device. Transient absorption spectroscopy and photoelectrochemical measurements were conducted to investigate the charge carrier dynamics of water oxidation in undoped BiVO4 photoanodes. The density of long-lived holes was found to correlate with the width of the space charge layer as a function of applied bias, but not correlate with the photocurrent. This result indicates that there is a recombination loss in undoped BiVO4 photoanodes which kinetically competes against water oxidation. Using a simple kinetic model, the fraction of water oxidation holes was determined from the total density of long-lived holes, and found to correlate with the photocurrent density. The competing process was also determined to be the back electron/hole recombination, in agreement with chopped light transient photocurrent results. These results suggest that the water oxidation efficiency can be improved by blocking the back electron/hole recombination with surface accumulated holes in BiVO4 photoanodes. Further studies of doping effect are currently under investigation, in order to understand the charge carrier dynamics influenced by doping.

Z.Z.P.35
16:00
Authors : Florian Le Formal, Stephanie R. Pendlebury, James R. Durrant
Affiliations : Department of Chemistry, Imperial College London, South Kensington Campus, London, SW7 2AZ, United Kingdom

Resume : The kinetic competition between electron / hole recombination and water oxidation is a key consideration for the development of efficient photoanodes for solar driven water splitting. In our laboratory, we employ complementary techniques, transient absorption spectroscopy (TAS), transient photocurrents (TPC) and electrochemical impedance spectroscopy (EIS), to address this issue for one of the most widely studied photoanode systems: nanostructured hematite thin films. All three methods have evidenced the presence of a recombination process on the 10 ms – 1 s timescale. We are able to assign this recombination phase to recombination of bulk hematite electrons with long-lived holes accumulated at the semiconductor / electrolyte interface. Data from all techniques have been shown to be consistent with a simple kinetic model based on competition between the bias dependent recombination pathway and water oxidation by these long-lived holes, without including intra-band gap traps. This study particularly highlights the dual role of the space charge layer occurring on two different time scales, i.e. the initial photogenerated charge separation and the prevention of recombination across the depletion layer. Preventing this back electron / hole recombination requires the application of strong anodic bias compared to flat band, also observed in other semiconductor devices, and therefore needs to be accounted for in electrode design for PEC water splitting.

Z.Z.P.36
16:00
Authors : Ksenia Fominykh, Peter Zehetmaier, Kristina Peters, Johann M. Feckl, Thomas Bein, Dina Fattakhova-Rohlfing*
Affiliations : Department of Chemistry and Center for NanoScience (CeNS), Ludwig-Maximilians-University (LMU), Butenandtstr. 5-13 (E), 81377 Munich, Germany (*email: dina.fattakhova@cup.uni-muenchen.de)

Resume : Efficient electrochemical water splitting to hydrogen and oxygen is considered a promising technology to overcome our dependency on fossil fuels. Although hydrogen is the desired end product, the very slow kinetics and high overpotential of the oxygen generation are the limiting factors for efficient overall water splitting. Searching for catalytic materials for electrochemical oxygen generation is of great importance for improving the total efficiency of water splitting. We present a novel synthesis route for preparation of ultrasmall dispersible NiO nanocrystals, which show very high electrocatalytic activity in electrochemical water oxidation. Using a solvothermal reaction in tert-butanol, very small nickel oxide nanocrystals can be prepared with sizes tunable from 2.5 to 5 nm and a narrow particle size distribution. The crystals are perfectly dispersible in ethanol forming stable transparent colloidal dispersions. The nanocrystalline structure corresponds to phase-pure NiO with partially oxidized surface exhibiting Ni(III) states which could explain the efficient catalytic behavior in electrochemical water splitting. The nanoparticles demonstrate very high turn-over frequencies of 0.293 s 1 at an overpotential of 300 mV, even outperforming expensive rare earth iridium oxide catalysts. We believe that the unique features of these NiO nanocrystals provide great potential for preparation of numerous composite materials for (photo)electrochemical water splitting applications.

Z.Z.P.37
16:00
Authors : Sven Tengeler, Bernd Kaiser, Didier Chaussende, Wolfram Jaegermann
Affiliations : Institut für Materialwissenschaften and Graduate School of Energy Science and Engineering, Technische Universität Darmstadt , Jovanka-Bontschits-Str. 2, 64287 Darmstadt, Germany Laboratoire des Matériaux et du Génie Physique, Grenoble INP – CNRS, 3 parvis Louis Néel, BP257, 38016 Grenoble, France International Doctoral School in Functional Materials for Energy, Information Technology and Health, 351 cours de la liberation – 33405 Talence, France ; Institut für Materialwissenschaften and Graduate School of Energy Science and Engineering, Technische Universität Darmstadt , Jovanka-Bontschits-Str. 2, 64287 Darmstadt, Germany ; Laboratoire des Matériaux et du Génie Physique, Grenoble INP – CNRS, 3 parvis Louis Néel, BP257, 38016 Grenoble, France International Doctoral School in Functional Materials for Energy, Information Technology and Health, 351 cours de la liberation – 33405 Talence, France ; Institut für Materialwissenschaften and Graduate School of Energy Science and Engineering, Technische Universität Darmstadt , Jovanka-Bontschits-Str. 2, 64287 Darmstadt, Germany International Doctoral School in Functional Materials for Energy, Information Technology and Health, 351 cours de la liberation – 33405 Talence, France

Resume : A successful transition to renewable energies requires, due to their volatile nature, the means to store huge amounts of energy for extended durations. A promising approach is chemical energy storage. Here hydrogen plays a central role, either as pure compound or as raw material for hydrocarbon synthesis. At present industrial level water splitting is achieved by using a electrolyser unit (PEM or alkaline) in conjunction with a separate power source. While this arrangement has some advantages (flexibility and availability) the approach of photoelectrochemical water splitting should offer higher overall efficiency as well as lower overall device production costs. A suitable semiconductor electrode material for a self driven photoelectrochemical cell has to provide a photo voltage of about 2 eV. Furthermore it should be long term stable under reaction conditions, have a reasonably high solar absorption efficiency and it should be abundantly available. Cubic silicon carbide (3C-SiC) is a promising candidate, it is highly stable and has a bandgap of 2.3 eV which should be able to supply the desired voltage. The present work will focus on the evaluation of n-type 3C-SiC bulk samples. This includes research on the stability in the anodic/cathodic regime for varying electrolytes, comparison of the differences in behavior between single crystal and polycrystalline electrodes, determination of the photoeffect in dependence of surface treatment and flat band potential mapping.

Z.Z.P.38
16:00
Authors : Dowon Bae*, Thomas Pedersen**, Brian Seger*, Ole Hansen**, Ib Chorkendorff*, Peter C.K. Vesborg*
Affiliations : *Department of Physics, Technical University of Denmark, 2800 Kgs. Lyngby; **Department of Micro- and Nanotechnology, Technical University of Denmark, 2800 Kgs. Lyngby

Resume : Si is a highly-studied material that has been demonstrated to perform photocathodic H2 evolution, but little work has been done studying its photocatalytic HER under back-side illumination - particularly given that the actual working condition for such photoelectodes is as a bottom cell of tandem water splitting device. In this work, the charge collection probability (CP(z)) was calculated using a simple back-illuminated Si model with pn-junction. The analytical calculation shows that photo current density (J) under the back-illumination increases with diffusion length-to-thickness ratio (Le/L), but only up to a certain point, beyond which the surface recombination rate (S) becomes the dominant factor. To verify model results, a bilateral light absorbing structure has been realized for experimental study by using p pn Si. Si thickness has been varied from 30 to 350 μm, and an Al2O3 passivation layer was introduced to verify the impact of Le/L and S, respectively. Compared to the 350-um-thick sample, the 50-um-thick one showed J increase by 2.7-fold due to the increased Le/L ratio. In addition, it was observed that the Al2O3 layer leads to further enhancement of J and Voc - close to the values reached under front-illumination. These results corroborate the modelling results, and it is evidenced in a comparison of CP(z) and measured internal quantum efficiency. The results of this work provide practical guidelines towards designing Si-based high efficient bottom cells.

Z.Z.P.39
16:00
Authors : Mehrdad Balandeh, Alessandra Tacca, Alessandro Mezzetti, Giorgio Divitini, Caterina Ducati, Laura Meda, Fabio Di Fonzo
Affiliations : Center for Nano Science and Technology - IIT@PoliMI, Via Pascoli 70/3, 20133 Milano (Italy); Eni S.p.A. Istituto ENI Donegani via G. Fauser, 4 - 28100 - Novara (NO) – Italy; Department of Materials Science & Metallurgy, University of Cambridge, Pembroke Street, CB2 3QZ Cambridge, UK

Resume : WO3 nanostructures are attractive photoelectrodes for water splitting with advantages of low cost, good chemical stability and relatively high efficiency due to WO3 visible light absorption. In this study, we propose quasi-1D hyperbranched WO3 nanostructures as a photoanode for high-efficiency photoelectrochemical water splitting. In this contribution, we will present our recent work on monoclinic-phase WO3 nanotrees are fabricated by means of Pulsed Laser Deposition (PLD) followed by annealing. The WO3 fabricated by PLD present a peculiar multiscale organization obtained by exploiting self assembly of gas-phase generated nanoparticles on a FTO coated glass. Overall, the novel photoanode resembles a forest composed of individual, high aspect-ratio, tree-like structures, assembled from crystalline nanoparticles. These novel structures are stable upon annealing in air at 500 °C, exhibiting monoclinic crystalline phase. The hierarchical quasi-1D nature of each tree represents an innovative compromise between nanorods/nanotubes (better electron transport) and the conventional isotropic nanoparticle photoanode (high surface area). These nanostructures have been employed as photoanode in photoelectrochemical (PEC) cell obtaining current density in excess of 2 mA/cm2 at 1 V bias potential versus RHE. The PEC performances are evaluated by linear sweep voltammagrams (LSV), transient amperometric i-t curves, incident photon to current conversion efficiency (IPCE) and electrochemical impedance spectroscopy (EIS) measurements.

Z.Z.P.40
16:00
Authors : Ivano E. Castelli, Kristian S. Thygesen, Karsten W. Jacobsen
Affiliations : Center for Atomic-scale Materials Design, Department of Physics, Technical University of Denmark

Resume : The conversion of solar light into electrons and holes which are used to split water into hydrogen and oxygen is one of the possible ways to address the world's pressing energy supply and storage problem. A material to be used as light harvester in a photoelectrochemical cell requires to be (i) chemical/structural stable under irradiation, and to have (ii) a band gap in the visible range with band edges well positioned with respect to the redox levels of water. In previous studies [1], we performed a computational screening for 20000 perovskites with focus on one-photon water splitting finding 20 new promising materials. Now, we extend the screening to a more general collection of materials already known to exist in nature (as described in the Materials Project database [2]). The descriptors for the screening are (i) the heat of formation, evaluated with respect to solid and dissolved phases using Pourbaix diagrams [3], (ii) the bandgap, calculated using the GLLB-SC functional [4], and (iii) the band edge positions, calculated using an empirical formula based on the electronegativities of the constituent atoms. Based on the screening, we suggest a handful of materials for further experimental investigation. [1] I.E. Castelli et al., Energy Environ. Sci., 5, 5814 (2012) and Energy Environ. Sci., 5, 9034 (2012). [2] https://www.materialsproject.org/. [3] I.E. Castelli et al., Topics in Catalysis (2013). [4] M. Kuisma et al., Phys. Rev. B 82, 115106 (2010).

Z.Z.P.41
16:00
Authors : C. Garlisi (a), A. Rizzo (a), D. Valerini (a), R.Rosato (a), M.L. Protopapa (a), S.Hernandez (b), L. Tapfer (a)
Affiliations : (a) ENEA - Italian National Agency for New Technologies, Energy and Sustainable Economic Development - Technical Unit for Brindisi Material Technologies, Laboratory of Materials Technology (UTTMATB-TEC), Brindisi Research Center, S.S. 7 Appia km. 706, 72100 Brindisi, Italy; (b) Istituto Italiano di Tecnologia, Center for Space Human Robotics, IIT@Polito, C.so Trento 21, 10129, Torino, Italy

Resume : A key ambition in research is to produce hydrogen using solar energy. The direct water splitting on a photocatalyst using sunlight would be a potential way to realize that at a large scale. Although there are several materials exhibiting this property under UV irradiation, the number of photocatalytic materials active under visible-light irradiation is still limited. Zr2ON2 looks promising because the presence of nitrogen in ZrO2 structure leads to a significant reduction in bandgap, making it suitable for visible photocatalytic processes. This work is focused on the study of the deposition parameters role (RF power, N2 partial pressure, O2 partial pressure) in sputtered ZrON films physical properties. The depositions were performed in argon-nitrogen-oxygen vapour atmosphere at a constant target power (200 W). Optical, morphological and structural properties have been investigated. The deposition conditions to obtain Zr2ON2 structure were achieved as confirmed by x-ray diffraction analysis. The optical properties have been investigated by performing optical reflectance and transmission measurements. The bandgap values obtained by Tauc plot show a progressive reduction as the content of oxygen inside the deposition chamber decrease, ranging from the value for pure sputtered zirconia to 2,9 eV in the absence of oxygen. Preliminary photoelectrochemical measurements have been performed in order to test this material as a photocatalyst for watersplitting.

Z.Z.P.42
16:00
Authors : Ji-Su Kim1, Byung-Kook Kim2, and Yeong-Cheol Kim1
Affiliations : 1School of Energy, Materials, and Chemical Engineering, KoreaTech, Cheonan, Korea; 2High Temperature Energy Materials Research Center, Korea Institute of Science and Technology, Seoul, Korea

Resume : We investigated the effect of triple phase boundary (TPB) of nickel (Ni) / yttria stabilized zirconia (YSZ) on water electrolysis using ab-initio thermodynamics calculations. Ni/YSZ TPB was constructed by attaching a cluster of 18 Ni atoms on an 18%-doped YSZ (111) surface. An extra oxygen vacancy (VO) near the surface was considered to mimic oxygen removal from water through YSZ under electrolysis mode. VO was stable, in order, at the sub-surface, TPB, and surface. VO at the surface and TPB reduced the energy barrier for water dissociation; VO at TPB further reduced the energy barrier for hydrogen molecule formation. From the calculated results, VO at TPB played a key role in water electrolysis. We will also elucidate VO migration mechanism and water dissociation reaction as a function of applied bias.

Z.Z.P.44
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Watersplitting III (OER) : Akihide Iwase
08:40
Authors : Roel van de Krol, Fatwa Abdi
Affiliations : Helmholtz-Zentrum Berlin, Institute for Solar Fuels, Germany; Delft University of Technology, The Netherlands

Resume : Multinary metal oxides are promising candidates for the conversion of solar energy to chemical fuels. They combine reasonable semiconducting properties with excellent chemical stability and low cost. With a bandgap of 2.4 eV, bismuth vanadate (BiVO4) is a promising member of this class that can be prepared by a simple spray pyrolysis technique. Photoelectrochemical measurements were used to identify which processes limit its performance. The slow surface reaction kinetics can be enhanced by applying a cobalt phosphate water oxidation co-catalyst, while the poor conductivity of the material can be improved by doping with tungsten. Intriguingly, W-doping improves the photocurrent, but also strongly reduces the carrier lifetime and the (already low) carrier mobility. The origin of this apparent discrepancy will be discussed. Further improvements were made by introducing a gradient in the dopant concentration. This results in a distributed n-n+ homojunction that enhances the charge separation. These improvements have resulted in photocurrents as high as 3.6 mA/cm2 at 1.23 V vs RHE under 1 sun illumination (AM1.5). We show that combination with an amorphous silicon tandem cell gives a stand-alone water splitting device with a solar-to-hydrogen (STH) efficiency of 4.9% [1]. Finally, we will briefly discuss the maximum realizable efficiencies with metal oxide / silicon heterojunction photoanodes. [1] F.F. Abdi et al., Nat. Commun. 4:2195, 1-7 (2013)

Z.Z.6.1
09:15
Authors : Andrew J. Logsdail1, John Buckeridge1, Aron Walsh2, C. Richard A. Catlow1, Ivan P. Parkin1, Alexey A. Sokol1, David O. Scanlon1.
Affiliations : 1 Department of Chemistry, University College London, London, United Kingdom. 2 Centre for Sustainable Chemical Technologies and Department of Chemistry, University of Bath, Bath, United Kingdom.

Resume : The discovery of the photolysis of water on the surface of TiO2 in 1972 launched four decades of intensive research on the underlying chemical and physical processes involved. Mixed phase samples of TiO2 have been shown to display much higher photoactivity than individual polymorphs, however the origin of this improved performance was never truly understood, in particular the fundamental band alignment between rutile and anatase. The general consensus for the past 20 years has been that the conduction band of anatase sits 0.2 eV above that of rutile, but this alignment motif did not explain the superior photocatalytic properties of the mixed phases. Instead, a complicated mechanism of interfacial trapping and charge transfer was suggested, albeit within the constraints of the assumed band alignment model. We have demonstrated that a type-II (staggered) natural band alignment of ~0.4 eV exists between anatase and rutile, with anatase possessing the higher electron affinity [1]. This decreases the effective band gap at the interface to ~2.8 eV, increasing its visible light activity. Our results help to explain the robust separation of photo-excited charge carriers between the two phases, and it is expected that our new rationalization of the anatase/rutile alignment will play a vital role in the future development of improved TiO2 based photocatalysts. [1] D. O. Scanlon et al., Nature Materials, 12, 798?801 (2013)

Z.Z.6.2
09:40
Authors : Michael Bernicke, Denis Bernsmeier, Erik Ortel, Ralph Kraehnert
Affiliations : Technische Universität Berlin, Berlin, Germany

Resume : Electrolysis applications like water splitting rely on efficient electro-catalysts. One state-of-the-art catalyst for the anodic side of PEM electrolysis cells are electrode coatings containing oxides of iridium. In order to reduce the content of noble metal a cheap “diluent” such as titanium can be used. Introducing defined porosity into catalytic coatings can improve the performance of IrO2 in Oxygen Evolution Reaction (OER). However, structure - performance relationships for IrO2 and Ir/TiOx catalysts in the OER remained so far largely unexplored. Control over pore size and pore connectivity of oxide coatings can be achieved when micelles of amphiphilic copolymers are employed as pore templates. We therefore synthesized templated mesoporous catalytic coatings via the Evaporation Induced Self-Assembling (EISA) approach. Dipcoated films were calcined under air flow at different temperatures between 350 and 625 °C. The material properties were analyzed by SEM, TEM, SAXS, XRD, BET and resistivity measurements. The OER activity was tested under acidic condition using a rotating disk setup. SEM images of mesoporous IrO2 catalysts calcined at 400 and 475 °C could be obtained. The catalyst films differed in pore morphology, crystallite sizes, electrical conductivity, electrochemically active surface area and OER performance. The effect of catalyst composition, crystallinity and the type of pore system on OER performance will be discussed in detail.

Z.Z.6.3
10:00 Coffee Break    
 
Watersplitting III (HER) : Akihide Iwase
10:30
Authors : Ib Chorkendorff
Affiliations : Department of Physics Technical University of Denmark

Resume : We have shown that bioinspired molecular clusters based on transition metal sulfides mimics nature's enzymes for hydrogen evolution when deposited on various supports [1, 2]. When these catalysts are deposited on p-type Si they can harvest the red part of the solar spectrum [3]. Such a system could constitute the cathode part of a tandem dream device where the red part of the spectrum is utilized for solar fuel evolution, while the blue part is reserved for the more difficult oxygen evolution. Silicon and another higher band gap material like for example GaP are, however, very prone to corrosion and we have recently investigated different strategies for protecting these semiconductors against corrosion using relative thick layer of TiO2 [4, 5]. This improvement in corrosion protection by deposition of TiO2will be discussed [6] in greater detail especially with respect to the anode. Finally we will also discuss the latest progress in developing new and better catalysts for the OER reaction since it is here the real challenge is to be found. 1. T. F. Jaramillo et al., Science 317, (2007) 100. 2. Y. Hou et al., Nature Materials 10, (2011) 434. 3. A. B. Laursen et al. ,Energy Environ. Sci., 5 (2012) 5577. 4. B. Seger, et al. ?Sustainable Hydrogen Production from a Molybdenum Sulfide Catalyst on protected n p -Silicon Photocathode. ? Angew. Chem. Int. Ed., 51 (2012) 9128. 5. B. Seger, et al. ? Using TiO2 as a Conductive Protective Layer for Photo-cathodic H2 evolution?, JACS 135 (2013) 1057. 6. B. Seger, et al. ?Silicon Protected with Atomic Layer Deposited TiO2 Part B- Conducting Versus Tunneling Through the TiO2 J. Mater. Chem. A, 2013, 1 (47), 15089 ? 15094.

Z.Z.7.1
11:05
Authors : Adina Morozan, Frederic Jaouen
Affiliations : University Montpellier 2 - CNRS Institut Charles Gerhardt Montpellier, UMR 5253

Resume : While the cathodic hydrogen evolution reaction (HER) is facile on Pt in acid medium, a large scale deployment of polymer electrolyte membrane (PEM) electrolyzers would require significant amounts of Pt. Replacement of Pt by non-precious metals (Co, Mo, W in most cases) has hitherto lead to an additional overvoltage of at least 100-150 mV at low current densities. Moreover, very few reports have looked into their properties in PEM electrolyzers. We will report on the activity for the HER of Mo-based catalysts involving a metal organic framework (MOF) as a platform for the dispersion of Mo as well as for the synthesis of an N-doped carbon support. The use of a nitrogen-rich MOF directs the synthesis toward molybdenum nitride during high temperature pyrolysis, with the Mo2N phase found after a pyrolysis in inert or reducing atmosphere. Catalysts were synthesized from MoII acetate, 1,10-phenanthroline and a Zn-based metal organic framework, with furfuryl alcohol optionally used for co-doping with oxygen. After pyrolysis in inert gas, the catalysts feature nanoparticles of γ-Mo2N with average size of 1.6 nm supported on a N-O co-doped carbon support. Optimized catalysts show, after in-situ electrochemical activation, an offset of only 40-60 mV compared to 70 wt. % Pt/C in RDE. Moreover, this high activity was also observed in PEM electrolyzer conditions with a negative shift vs. 0.5 mgPt cm-2 of only 100 mV at 100 mA cm-2.

Z.Z.7.2
11:30
Authors : Diana Stellmach,Peter Bogdanoff, Onno Gabriel, Bernd Stannowski, Rutger Schlatmann, Sebastian Fiechter
Affiliations : Helmholtz-Zentrum Berlin fuer Materialien und Energie Gmbh, Institute for Solar Fuels, Hahn-Meitner-Platz 1,14109 Berlin, Germany

Resume : Molybdenum disulphide is a cheap and earth abundant catalyst for electrochemical hydrogen evolution from water [1]. From DFT calculations it has been predicted that the edges of the particles crystallizing in a hexagonal layer structure are the active centers to catalyse hydrogen evolution. Recently, Kibsgaard et al. described the electrochemical deposition of MoS2 on a mesoporous 3D substrate with high activity [2]. In our approach we investigated the correlation of the structural and electrochemical properties of MoS2 nanoparticles. The catalysts were prepared by impregnating multi walled carbon nano¬tubes (MWCNT) with the same length, but different diameters. The MWCNT supported nanoparticles showed hexagonal symmetry as expected from the 2H-MoS2 phase. The ratio between edge and basal planes of the nanoparticles were identified by an electrochemical oxidation process and by Raman spectroscopy. To obtain inside into the morphology of the catalysts cross section transmission electron micrographs were performed elucidating the interaction between the layer particles and the MWCNTs. In our contribution the catalytic stability and activity will be discussed as a function of the sample morphology. [1] H. Tributsch and J.C. Bennettt, J. Electroanal. Chem, 81 (1977), 97-111 [2] J. Kibsgaard, Z. Chen, B. Reinicke and T. Jaramillo, Nature Materials, 11 (2012), 693-969

Z.Z.7.3
11:50
Authors : Joachim Klett, Rolf Schäfer, Bernhard Kaiser, Wolfram Jägermann
Affiliations : Eduard-Zintl-Institut für Anorganische und Physikalische Chemie, Technische Universität Darmstadt, Alarich-Weis-Strasse 8, 64287 Darmstadt, Germany; Eduard-Zintl-Institut für Anorganische und Physikalische Chemie, Technische Universität Darmstadt, Alarich-Weis-Strasse 8, 64287 Darmstadt, Germany; Institut für Material- und Geowissenschaften, Technische Universität Darmstadt, Jovanka-Bontschits-Strasse 2, 64287 Darmstadt, Germany; Institut für Material- und Geowissenschaften, Technische Universität Darmstadt, Jovanka-Bontschits-Strasse 2, 64287 Darmstadt, Germany

Resume : Platinum particles are one of the most important catalysts for fuel cells and electrolyzers. Typically the particle is small, having a high surface to volume ratio. When these particulate systems are immersed in the electrolyte they form interfaces not only with the electrolyte, but also with substrate. The contact properties become more complex, since three interfaces are formed at once. In our study we investigated Pt particles nucleated in gas phase and deposited on indium-tin-oxide (ITO) and Graphite surfaces by X-Ray photoelectron spectroscopy (XPS). The preparation was carried out under ultra-high vacuum conditions. Afterwards the deposited clusters were exposed to well defined amounts of water vapor. We observed a strong interaction between the ITO surface and water, that leads to a large change of the Fermi level position of the ITO surface. As a consequence the Pt particles show measurable charging. On the inert graphite surface this effect is nonexistent. We showed that for small particles the substrate material can have a direct influence on their electronic structure. Furthermore, the hydroxylation of ITO is suppressed if Pt was deposited beforehand. This is due to the catalytic activity of platinum leading to a preferred oxidation of ITO instead to the hydroxylation.

Z.Z.7.4
12:10 Lunch Break    
 
Protection schemes : Raffaella Buonsanti
14:00
Authors : Gregory N. Parsons1 Berç Kalanyan,1 Do Han Kim,1 Mark D. Losego,1 Kenneth Hanson,2 Leila Alibabaei,2 Aaron K. Vannucci,2 Thomas J. Meyer,2 Qing Peng,3 Jeffrey T. Glass
Affiliations : 1Dept. of Chemical and Biomolecular Engineering, NC State University, Raleigh NC 2Dept. of Chemistry, University of North Carolina Chapel Hill, Chapel Hill NC 3Dept. of Electrical and Computer Engineering, Duke University, Durham, NC

Resume : Recent advances in dye-sensitized photoelectrochemical synthesis cells (DSPECs) is pushing research toward new molecular materials and inorganic integration strategies that can produce efficient, stable and robust device designs for water-splitting and CO2 reduction. Photoanodes include a photoactive dye molecule and catalyst bound to a nanostructured metal oxide (e.g. mesoporous TiO2) that accepts and transport charge. Maintaining the integrity of the dye/oxide binding unit is especially challenging for water oxidation DSPECs where irradiation leads to dye desorption. Atomic layer deposition (ALD) is a sequential self-limiting reaction scheme that can precisely deposit inorganic and/or organic thin films that are highly conformal and uniform on 3D structures. Recently, our team discovered that when mesoporous TiO2 with surface-bound dye molecules is coated with a thin layer of metal oxide by ALD, the rate of dye desorption significantly decreases, even under highly oxidative conditions, with a relatively small impact on photoelectron generation. For example, by applying three ALD cycles of AlMe3-H2O (∼3 Å of Al2O3) to [Ru(bpy)2(4,4′-(PO3H2)-bpy)]2+ (RuP) dye on TiO2, the rate of dye desorption decreased significantly compared to similar electrodes without the ALD layer. Further ALD enhances stability but also increases emission and decreases injection yields from the metal-toligand charge-transfer (MLCT) excited state of RuP. We also translated this stabilization strategy to dye sensitized photovoltaic cells, and to DSPECs operating under highly oxidative pH conditions. We further find that ALD can produce highly conductive core/shell mesoporous oxides that decrease charge recombination and accelerate photoelectron transport through the device. We will describe the outstanding issues in ALD-integrated dye-sensitized devices and discuss in-situ analysis of the ALD reaction steps that promote dye stabilization.

Z.Z.8.1
14:35
Authors : Massimo Tallarida, Chittaranjan Das, Dieter Schmeisser
Affiliations : Brandenburg University of Technology, Applied Physics - Sensors, Konrad Wachsmann Allee, 17, 03046, Cottbus, Germany

Resume : Atomic layer deposition (ALD) is a chemical method to grow homogeneous thin films in an atomic controlled mode, which allows the conformal coating of complex structures with precise thickness and a high degree of purity. Recently, it was observed that oxides covered with few ALD cycles show a strong enhancement of functional properties. In the field of energy conversion and storage, it was found that the efficiency of photo-voltaic [1] and photo-electrochemical (PEC) [2] cells could be enhanced by applying a small number of ALD cycles. To determine the details of the observed efficiency enhancement in PEC cells we applied ALD to various photoabsorbers and investigated the modified oxides using Synchrotron Radiation Photoemission Spectroscopy (SR-PES) and X-ray Spectroscopy (XAS) in in-situ reactors [3]. We found that the very thin ALD films have a strong influence on the electronic properties and charge carrier dynamics of substrates. [1] Prasittichai, C.; Hupp, J. T. J. Phys. Chem. Lett. 2010, 1, 1611. [2] Le Formal, F.; Tetreault, N.; Cornuz, M.; Moehl, T.; Graetzel, M.; Sivula, K. Chem. Sci. 2011, 2, 737. [3] Tallarida, M.; Weisheit, M.; Kolanek, K.; Michling, M.; Engelmann, H. J.; Schmeisser, D. J. Nanopart. Res. 2011, 13, 5975?5983.

Z.Z.8.2
15:20 Coffee Break    
16:00 Plenary Lecture    
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Water splitting IV : Roel van de Krol
08:40
Authors : Akihide Iwase, Akihiko Kudo
Affiliations : Tokyo University of Science

Resume : Hydrogen is one of the clean energy sources. Photocatalytic water splitting is a promising and attractive technology for the clean hydrogen production. The systems for water spitting using photocatalyst can be divided into an electrode system and powdered system. For the electrode system, the photocatalyst materials were used as a photocatalyst electrode, which is well known as Honda-Fujishima effect. For powdered system, the photocatalyst materials were dispersed in the reactant solution. The powdered system was further divided into a single photoexcitation system and two-step photoexcitation (Z-scheme) system. In the present study, we have successfully developed visible-light-driven powdered photocatalyst and photoelectrochemical systems for solar water splitting Upon combining Ru-loaded SrTiO3:Rh for H2 evolution with BiVO4 for O2 evolution through ionic electron mediators, stoichiometric amounts of H2 and O2 evolved under visible light and simulated sunlight irradiation. We have also fabricated Z-scheme systems without electron mediator and with a solid electron mediator for water splitting. When the SrTiO3:Rh photocathode was combined with the BiVO4 photoanode to construct a photoelectrochemical cell, H2 and O2 steadily evolved in stoichiometric amounts under visible light and simulated sunlight irradiation. Thus, we have fabricated solar water splitting system using only oxide photocatalyst materials.

Z.Z.9.1
09:15
Authors : Mathieu S. Prévot, Néstor Guijarro, Kevin Sivula
Affiliations : Ecole Polytechnique Fédérale de Lausanne

Resume : Delafossite-type CuFeO2 is a promising material for direct solar water reduction given its known stability, CB-edge energy position, and band-gap of 1.5 eV, making it possible to reach STH efficiencies higher than 10% in a tandem device. Moreover its composition from earth abundant atoms accords with solar energy on a global scale. However, the inconvenience of the solid-state reactions typically used to produce delafossites has limited the facile preparation and the PEC investigations of thin-film CuFeO2 electrodes. To overcome these issues, we herein present a solution-based method to afford stable p-CuFeO2 thin-films. Our sol-gel route relies on cheap non-toxic abundant precursors, and involves easily scalable techniques. Prepared films were characterized by physical and electrochemical techniques, and were tested as photocathodes for water splitting. Initial photocurrents were stable over many days and in the 100 μA cm–2 range under AM 1.5G. The photocurrent onset at +0.8 V vs RHE further demonstrated its suitability for a tandem cell. Subsequently, we show that the performance can be drastically improved through doping with O and Mg to champion photocurrents of 1 mA cm–2. We finally describe further enhancement using hole-extraction underlayers and catalyst overlayers. Our results clearly reveal that p-type CuFeO2 is a promising material for solar water reduction, which should be able to reach high performance through careful tuning of its morphology and energy levels.

Z.Z.9.2
09:40
Authors : Rachel Morrish, Colin A. Wolden
Affiliations : Department of Chemical and Biological Engineering Colorado School of Mines Golden, Colorado, U. S. A.

Resume : WS2 thin films have shown potential as both photo-absorbers and hydrogen evolution catalysts. In addition to being earth abundant, tungsten disulfide is stable in acidic aqueous solution and exhibits an ideal band gap of 1.4 eV. Here we report on a low temperature plasma sulfurization process for preparation of WS2 layers from WO3. Conventional sulfurization methods rely on exposure to elemental sulfur vapors at temperatures > 800 °C. Our approach utilizes a plasma to dissociate H2S gas into atoms (S), metastables (S*), and ions (S+, S-) enabling high reactivity at relatively low temperatures compatible with low-cost glass substrates. Full conversion of 75 nm WO3 films to WS2 was achieved at 500 °C as confirmed by Raman, XPS, and calibrated EDAX compositional analysis. Studied over a temperature range of 350 – 550 °C, the plasma sulfurization process had an apparent activation energy of 63.6 +/- 1.9 kJ/mol. The prepared WS2 material exhibited n-type conductivity with an indirect transition at 1.38 eV and an absorption coefficient of ~ 4 × 104 cm-1 above the band gap. Preliminary measurements of the HER showed an overpotential of approximately 0.3 V.

Z.Z.9.3
10:00 Coffee Break    
10:30
Authors : David Lynch, Erin Creel, Raffaella Buonsanti
Affiliations : LBNL/JCAP, LBNL/JCAP/UC Berkeley Chemistry Department, LBNL/JCAP

Resume : Design of electrochemically stable and visible-light absorbing photoanodes for solar fuels applications poses new challenges for chemists and material scientists. Colloidal chemistry is a solution-phase approach to nanomaterial synthesis that, in addition to offer a high degree of control over size, shape and composition, permits synthesis of materials unaccessible in their bulk form. Colloidal nanocrystals can indeed be trapped in metastable crystalline phases thanks to the size-dependence of phase transformations (thermodynamics) and, above all, to the possibility of tuning the chemical potential of the reaction mixture by simply changing reactant concentration, temperature, precursors, surfactants (kinetics). In this context, we show that metastable monoclinic scheelite BiVO4-based light absorbers can be synthesized in nanocrystalline form. Recently, the chemistry to control dopant incorporation in nanocrystals has progressed, opening new opportunities. Dopant incorporation and location in the host lattice can be controlled by estabilishing a careful balance between the host and the dopant growth rates. We outline the critical role played by properly chosen chemistry to incorporate nitrogen and barium dopants in TiO2 and WO3 nanocrystals with the result of shifting their absorption in the visible range. Initial results of photoactivity and stability under visible light and in different pH conditions show the technological potentiality of these colloidal doped metal oxide as photoanodes for water splitting.

Z.Z.9.4
11:05
Authors : M. Lublow, M. Rohloff, A. Azarpira, Th. Schedel-Niedrig, I. Zaharieva, H. Dau, A. Fischer
Affiliations : M. Lublow; M. Rohloff; A. Fischer: Technical University Berlin, Department of Chemistry, Berlin, Germany. A. Azarpira; Th. Schedel-Niedrig: Helmholtz-Zentrum Berlin f?r Materialien und Energie, Institute Solar Fuels, Berlin, Germany. I. Zaharieva; H. Dau: Freie Universit?t Berlin, Physics Department, Berlin, Germany.

Resume : A novel metalorganic electrochemical approach is presented by which transition metal oxides can be formed on silicon under cathodic electrochemical conditions. The resulting amorphous films are observed to be efficient electrocatalytic layers with low overpotentials for the evolution of oxygen in alkaline electrolytes. Exemplifying results for nickel-, iron- and cobalt oxides, formed at the surface of n-type silicon supports will be presented. Depending on the deposited electrocatalyst, the most decisive parameters for realization of an efficient heterojunction for photo-assisted evolution of oxygen under visible light could be controlled, i.e.: i) light absorption behavior of the deposited oxide thin film influencing the overall sensitivity of the heterojunction to external illumination, ii) band equilibration at the silicon/oxide interface affecting both the distribution of the potential energy of light-induced minority charge carriers as well as overpotential values and iii) chemical/structural properties at the oxide/electrolyte interface determining the potential drop across the Helmholtz double-layer as well as eventual mass transport limitations. Along with SEM, EDX, XRD, TEM, XPS and XANES characterizations material formation will be discussed as well as structure-function relationships for electro- and photoelectrocatalytic properties will be deduced.

Z.Z.9.5
11:30
Authors : M. Kulmas (1), Y. Wu (2), R. Keding (1), M. Y. Bashouti (1), J. Bachmann (2), J.Ristein (3), S. Christiansen (1, 4)
Affiliations : 1 - Max-Planck-Institut for the Science of Light, 91058 Erlangen, Germany; 2 - FAU Erlangen-Nürnberg, Inorganic and Analytical Chemistry, 91058 Erlangen, Germany; 3 - FAU Erlangen-Nürnberg, Laser Physics, 91058 Erlangen, Germany, 4 - Helmholtz-Center Berlin, 14109 Berlin, Germany

Resume : The photosensitive composite structures, designed on nanostructured silicon and optimized for efficient energy conversion are introduced. The combination of TiO2, ZnO and their ratio is studied for water splitting application. The oxide semiconductors TiO2, ZnO are very promising materials for composite photoactive anode due to their stability in water and the photocatalytic properties. Atom layer deposition (ALD) process is able to control the shape, size, phase, composition and surface/interface energies of nanocrystalline domains TiO2/ZnO. For understanding of the mechanisms involved in the energy transfer between photons, chemical bonds, electrons and ions impedance voltammetry and lineal voltammetry were applied. The optical properties and microstructure of composite anode were optimized in relation to their photocatalytic H2 production by solar illumination. The characterization of those structures was carried out with different techniques such as SEM, EDX and their optical properties (A, T, PL, and CL) were measured.

Z.Z.9.6
11:50
Authors : J. R. Galan-Mascaros, W. Y. Hernandez, B. Rodríguez-García, Sara Pintado, Sara Goberna-Ferron
Affiliations : Institute of Chemical Research of Catalonia (ICIQ); Catalan Institution for Research and Advanced Studies (ICREA),

Resume : Heterogeneous water oxidation catalysts (WOCs) possess many advantages regarding future applications since they are easier to process and more robust and stable in turnover conditions than their homogeneous counterparts. Regarding heterogeneous water oxidation, only metal oxides have shown most of the desired requirements of a technologically relevant WOC.[1] However, their long-term stability is still an open question since it heavily depends on the chemical environment, usually requiring alkaline media. We have recently discovered that coordination polymers of the Prussian blue family are active WOCs.[2] Transition metal hexacyanometallates, are able to promote water oxidation for days without significant appearance of fatigue with high turnover frequencies at neutral pH and ambient conditions. This family of coordination polymers represents a novel alternative to metal oxide WOCs, while adding the typical advantages of molecule-based materials: well-defined crystal structure, easy processing from solution, light-weight or transparency to visible light. In this communication we will report our latest results in this area. [1] H. Dau, C. Limberg, T. Reier, M. Risch, S. Roggan, P. Strasser. ChemCatChem 2010, 2, 
724; M. W. Kanan, Y. Surendranath, D. G. Nocera. Chem. Soc. Rev. 2008, 321, 109. [2] S. Pintado, S. Goberna-Ferron, E. C. Escudero-Adan, J. R. Galan-Mascaros. J. Am. Chem. Soc. 2013, 135, 13270–13273.

Z.Z.9.8
12:10 Awarding of the poster prizes    
12:20 Lunch Break    
 
Fe-N-C catalysts for ORR : Marcel Risch
14:00
Authors : Jean-Pol DODELET
Affiliations : INRS-Energie, Materiaux et Telecommunications

Resume : In recent years, the growing scarcity of easily accessible oil and the environmental concerns related to the exploitation of non-conventional oil fields led to intensified efforts to find alternatives for the gasoline-fuelled internal combustion engine currently used in most transportation applications. The proton-exchange membrane (PEM) fuel cell fueled by hydrogen from renewable sources constitutes an efficient and environmentally-harmless alternative to this gasoline-dependent technology, but has yet to overcome a number of challenges to reach full commercialization potential. One of these major hurdles is the excessive cost of the platinum-based catalysts currently used to boost the kinetics of the two half-reactions that occur inside the cell, i.e. the oxidation of H2 at its anode and the complementary O2 reduction reaction (ORR) at its cathode. Currently, Pt electrocatalysts account for about 1/3 of the fuel cell stack cost. As ORR is a much slower reaction, even on Pt, compared with the rate of H2 oxidation on the same metal, the Pt content at the cathode is usually 5 to 10 times that at the anode. Eliminating Pt from cathode catalysts has been challenging due to the limited number of catalyst candidates with decent prospects for high ORR activity, performance, and durability in acidic medium. Today, Fe/N/C-catalysts obtained from the pyrolysis of molecular precursors are the most promising non-platinum-group-metal (non-PGM) catalysts for the ORR in PEM fuel cells. For 2017, the revised US Department of Energy targets for non-PGM catalysts for ORR are a volumetric activity of ≥300 A cm-3 at 0.8ViR-free and a durability of 5000h. As far as their performance is concerned, these catalysts should also be able to replace Pt at the cathode of an 80 kWe transportation fuel cell system operating on direct hydrogen, keeping similar stack volume and weight. In Fe/N/C catalysts, the ORR-active metal is in ionic form. The choice of an iron ion at the heart of the main catalytic sites in Fe/N/C catalysts has been inspired by the occurrence of the same ion in performing biological oxido-reductases. Fe/N/C catalysts for PEM fuel cells have been produced according to many synthesis procedures, all of them using iron and nitrogen molecular precursors adsorbed on a carbon support before being pyrolyzed at high temperature. However, the most active and performing catalysts also use a molecular precursor to obtain the carbon support itself, which is produced at the same time as the active catalytic sites during the pyrolysis step. One of such Fe/N/C catalyst made with iron acetate as iron precursor and a zinc-methylimidazolate framework as N/C precursor will be analyzed in detail for the ORR activity of its Fe-based active sites, and the important parameters governing its performance. Stability results of this catalyst in fuel cell tests will also be presented and discussed.

Z.Z.11.1
14:35
Authors : Sreekuttan M. Unni, Sreekumar Kurungot
Affiliations : Physical Chemistry Division, CSIR-National Chemical Laboratory, Pune, India

Resume : Platinum and its alloys are routinely used in polymer electrolyte membrane fuel cells cathode due to high electrocatalytic activity towards dioxygen reduction. To replace Pt for reducing the cost and scalability of electrode materials, many non-precious (Fe or Co) or non-metal electrocatalyst are introduced. However, most of the existing catalyst endures low activity and performance degradation during long time use. Here, we report high temperature annealing of single walled carbon nanohorn (SWCNH) with melamine and iron precursor to create more active reaction centre for efficient oxygen adsorption and its reduction. High surface area (1300 m2/g) with micro pores (< 2 nm) and pyrrolic nitrogen coordinated iron create more active reaction site to improve the activity. Fe and N doped SWCNH annealed at 900 oC (FeNCNH-900) shows excellent oxygen reduction activity (ORR) through a 4-electron reduction pathway which is higher than Pt/C in 0.1M KOH solution. FeNCNH-900 shows potential fuel selectivity with excellent electrochemical stability and ORR activity is even increasing after 1000 potential cycling. With remarkable physico-chemical properties, this material can be a better electrode for devices such as solar cells and batteries.

Z.Z.11.2
15:00
Authors : Sebastian Brüller, Xinliang Feng, Klaus Müllen
Affiliations : Max Planck Institute for Polymer Research

Resume : Porphyrin based Fe/Co-N-C catalyst for efficient ORR Pt based catalysts exhibit the highest efficiency in the oxygen reduction reaction (ORR), the one half of the electrochemical reaction in fuel cells. However, the high costs as well as long term stability issues of Pt reduce its usage as catalyst. Therefore, lots of studies have been performed evaluating the use of non-precious metal catalysts (NMPC) as alternative to Pt. The performance of such catalysts is determined by the electronic nature of the active centre (metal coordination) and the morphology (porosity, pore size). Nitrogen coordinated metals have been discussed as most promising active sites. Herein, we demonstrate a strategy to obtain a homogenously distributed, nitrogen coordinated metal within a microporous carbon matrix. A comparison of a series of catalysts, based on metal containing polyporphyrins precursors, shows the beneficial effect of mixed metal catalysts. Co and Fe containing porphyrins were polymerized and pyrolyzed to obtain various microporous NMPCs. The morphology of the self-supporting catalyst was characterized by electron microscopy, EDX and nitrogen adsorption method. The different iron species were characterized both structurally and chemically by 57Fe M??bauer spectroscopy. Bimetallic catalysts exhibit improved performances in the ORR in acidic media.

Z.Z.11.3
15:20
Authors : Haiwei Liang, Xinliang Feng, Klaus Müllen
Affiliations : Max-Planck-Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany

Resume : Electrochemical reduction of oxygen is an important process for many energy conversion and storage technologies, including fuel cells, metal-air batteries, and electrolysers. Due to the high overpotential caused by the sluggish nature of oxygen reduction reaction (ORR), development of efficient ORR electrocatalysts is crucial for practical applications of these electrochemical devices. Although Pt and Pt-based alloys, up to now, are known as the most efficient catalysts for ORR, the high costs and scarce reserves of Pt significantly hinder its large-scale applications.1-2 Additional problem associated with Pt is its poor durability during long-term electrochemical process, as Pt-based catalysts suffer from nanoparticle migration, coalescence, and even detaching from support materials in both acidic and alkaline electrolytes.3 Accordingly, substantial efforts have been dedicated to searching for alternative ORR catalysts with low cost, high activity, and long-term durability.4-6 In particular, recent experimental observations and theoretical calculations both revealed that heteroatoms (e.g., nitrogen or/and phosphorus, boron)-doped carbon materials could serve as efficient metal-free electrocatalysts for ORR as the result of their unique electronic properties, which are derived from the heteroatom-induced charge transfer and delocalization.7-8 The addition of certain transition metals (e.g., Fe, Co) to the metal-free, nitrogen-doped carbon frameworks results in a nonprecious metal (NPM) catalyst system with improved ORR activity in both acidic and alkaline media. Two crucial factors govern the performance of metal-free and NPM catalysts, i.e., (i) elemental composition and the interactions between different components, which determine the intrinsic nature of active sites; and (ii) specific surface area and porous structure, which determine the accessible part of active sites and the transport properties of ORR-relevant species. Herein, we demonstrate a series of metal-free and NPM catalysts9 with well controlled porous structures that were prepared by using template synthesis as well as post-activation processes. Various precursors (e.g. Vitamin B12, polyaniline, poly(ophenylenediamine), sucrose) and inorganic templates (e.g. silica colloid, ordered mesoporous silica SBA-15, montmorillonite) were used to realize the mesoporous structure of metal-free and NPM catalysts. Different post-activation processes (including KOH, CO2, and NH3) were carried out to further increase the porosity of these materials. The prepared metal catalysts possess well-defined porous structures, a high Brunauer−Emmett−Teller (BET) surface area (up to 1300 m2/g), and nitrogen content (up to 9.5), thus resulting in an improved ORR performance in both acidic and alkaline media. Reference 1. Debe, M. K., Nature 2012, 486 (7401), 43-51. 2. Sealy, C., Mater. Today 2008, 11 (12), 65-68. 3. Yu, X. W.; Ye, S. Y., J. Power Sources 2007, 172 (1), 133-144. 4. Jasinski, R., Nature 1964, 201 (4925), 1212-1213. 5. Lefevre, M.; Proietti, E.; Jaouen, F.; Dodelet, J. P., Science 2009, 324 (5923), 71-74. 6. Wu, G.; More, K. L.; Johnston, C. M.; Zelenay, P., Science 2011, 332 (6028), 443-447. 7. Gong, K. P.; Du, F.; Xia, Z. H.; Durstock, M.; Dai, L. M., Science 2009, 323 (5915), 760-764. 8. Liu, R. L.; Wu, D. Q.; Feng, X. L.; Müllen, K., Angew. Chem. Int. Ed. 2010, 49 (14), 2565-2569. 9. Liang, H.-W.; Wei, W.; Wu, Z.-S.; Feng, X.; Müllen, K., J. Am. Chem. Soc. 2013, 135 (43), 16002-16005.

Z.Z.11.4
15:40 Coffee break    
 
ORR on Pt and its alloys : Marcel Risch
16:10
Authors : Matthias Arenz
Affiliations : University of Copenhagen

Resume : The main fundamental problems of polymer electrolyte membrane fuel cells (PEMFCs) to date are a low practical efficiency, due to the high overpotential for the oxygen reduction reaction (ORR), the high amount of noble metal catalyst in use, and the degradation of the catalyst in a fuel cell during operation. To clarify the processes and responsible factors for the degradation and the resulting loss of usable catalytic active material is one important step to achieve long term stability of PEMFCs. Up to now it is not clear which of the widely discussed mechanisms (Pt dissolution, particle migration and concomitant coalescence, particle detachement), is the main responsible process for degradation and/or to which extent the single mechanisms depend on the catalyst preparation and operation conditions. In this talk an overview of our activities in this field is presented. Especially we discuss the use of identical location microscopy (IL-TEM) and the systematic "tool-box" synthesis of carbon supported nanoparticle PEMFC catalysts.

Z.Z.12.1
16:45
Authors : Patricia Hernandez-Fernandez, Federico Masini, David N. McCarthy, Christian E. Strebel, Daniel Friebel, Davide Deiana, Paolo Malacrida, Anders Nierhoff, Anders Bodin, Jane H. Nielsen, Thomas W. Hansen, Anders Nilsson, Ifan E.L. Stephens, Ib Chorkendorff
Affiliations : Patricia Hernandez-Fernandez; Federico Masini; David N. McCarthy; Christian E. Strebel; Paolo Malacrida; Anders Nierhoff, Anders Bodin; Jane H. Nielsen; Ifan E.L. Stephens; Ib Chorkendorff Center for Individual Nanoparticle Functionality (CINF), Department of Physics, Kgs Lyngby DK-2800, Denmark Davide Deiana; Thomas W. Hansen Center for Electron Nanoscopy (CEN), Kgs Lyngby DK-2800, Denmark Daniel Friebel; Anders Nilsson SLAC National Accelerator Laboratory, 2575 Sand Hill Road, MS69, Menlo Park CA 94025, USA

Resume : Recently, Pt alloys have been indicated as viable materials to enhance the Pt activity for Oxygen Reduction Reaction (ORR) and abate the amount of expensive material in the ORR catalyst(1-3). A recent article from our laboratory.(3) revealed that polycrystalline extended surfaces of Pt3Y exhibit exceptionally high activity for the ORR. Moreover, their negative enthalpy of formation may provide kinetic stability against degradation by dealloying under fuel cell operation conditions. In this contribution, PtxY nanoparticles were produced by sputtering a Pt9Y in argon plasma and size selected using time-of-flight principles. Upon deposition on a glassy carbon Electrode, the nanoparticles were characterized by X-Ray Photoelectron Spectroscopy (XPS) and Ion Scattering Spectroscopy (ISS) then transferred to the electrochemical cell. We also confirm that the high activity of PtxY upon extended surfaces is reproduced in the more technologically relevant nanoparticulate form, with a maximum surface specific activity at 0.9 V RHE, of 13 mA/cm2, corresponding to a mass activity of 3 A/mg Pt. Spectroscopy and electro-microscopy allowed us to elucidate the microscopic reasons for the high activity. Importantly, post-electrochemistry studies with transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy reveal the depletion in Y of the outer atomic layers of the nanoparticles. Therefore, the PtxY NPs consist, after electrochemistry, of a PtxY core and a pure Pt shell. Extended X-Ray Absorption Fine Structure (EXAFS) analysis shows how alloying the two elements induce a reduction in the Pt-Pt distance in the PtxY NPs, and a compressive strain in the outer pure platinum shell. Consistent with the findings of Strasser et al.(4), this compressive strain is responsible for the attenuation of the strength of the Pt-OH bond on the NPs surface, which is responsible for the increase of the PtxY NPs activity for ORR. In conclusion, PtxY NPs were found to be an improved catalyst for ORR compared to Pt NPs. Furthermore, the microscopic reasons for such an improvement were determined. Future studies will focus on improving the activity and stability even further. References [1] V.R. Stamenkovic, B. Fowler, B.S. Mun, G. Wang, P.N. Ross, C.A. Lucas, N.M. Markovic, Science 315 (2007) 493-497. [2] I.E.L. Stephens, A.S. Bondarenko, U. Grønbjerg, J. Rossmeisl, I. Chorkendorff, Energy Environ. Sci. 5 (2012) 6744-6762. [3] J. Greeley, I.E.L. Stephens, A.S. Bondarenko, T.P. Johansson, H.A. Hansen, T.F. Jaramillo, J. Rossmeisl, I. Chorkendorff, J.K. Nørskov, Nat Chem 1 (2009) 552-556. [4] P. Strasser, et al., Nat Chem 2 (2010) 454-460.

Z.Z.12.2
17:10 Closing words (U.I. Kramm)    

No abstract for this day