Physics E19 Interfaces and Energy Conversion ZAE BAYERN Bavarian Centre for Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies Towards an Efficient Conversion of Ethanol in Low Temperature Fuel Cells: Ethanol Oxidation on Pt/Sn Catalysts and on Alkaline Medium Membrane Electrode Assemblies Vineet Rao 1, Carsten Cremers 3, Rainer Bußar 1,2 and Ulrich Stimming 1,2 1 Technische Universität München (TUM), Department of Physics E19, James-Franck-Str.1, D Garching, Germany 2 Bavarian Center for Applied Energy Research (ZAE Bayern), Walther-Meißner-Str. 6, D Garching, Germany 3 New address: Fraunhofer Inst Chem Technol, Dept Appl Electrochem, Pfinztal, Germany. DPG Frühjahrstagung 2009, Arbeitskreis Energie (AKE)

Physics E19 Interfaces and Energy Conversion ZAE BAYERN Bavarian Centre for Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies Motivation for Direct Fuel Cells (Direct FCs) The production of hydrogen from fossil fuels, such as natural gas, is connected with considerable losses in the overall efficiency of fuel cell systems; As yet, there is no widespread infrastructure for the distribution and storage of hydrogen; The energy density of hydrogen is lower than e.g. methanol or ethanol with respect to volume and weight; Ethanol is available as a renewable fuel from biomass; Direct fuel cell systems contain fewer components.

Physics E19 Interfaces and Energy Conversion ZAE BAYERN Bavarian Centre for Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies Aspects of Efficiency and Energy Density Ethanol is connected with a higher thermodynamic conversion efficiency η as compared to hydrogen; The energy density of ethanol is higher to the one of hydrogen. Ethanol is less toxic than methanol: ‘as save as bear’ (as Bavarians say)

Physics E19 Interfaces and Energy Conversion ZAE BAYERN Bavarian Centre for Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies CO 2 current efficiency for ethanol oxidation as a function of Potential, Temperature and Concentration; CO 2 current efficiency dependent on intrinsic nature of catalyst experiments with Pt, PtSn and PtRu; CO 2 current efficiency dependent on the catalyst loading and thus catalyst layer thickness:concept of resident time and active area; (CO 2 current efficiency on alkaline membrane electrode assemblies.) Outline of the presentation

Physics E19 Interfaces and Energy Conversion ZAE BAYERN Bavarian Centre for Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies CH 3 --CH 2 OH CH 3 --CHO CH 3 --COOHCH 3 --COOC 2 H 5 CO 2.CH ad.CO ad C 2 H 5 OH CH 4 Ethanol Oxidation Scheme (m/z=44, m/z=22 double charged ions) (m/z=15) (m/z=29, base peak) Esterification (m/z=43, base peak) (m/z=61) DEMS set-up

Physics E19 Interfaces and Energy Conversion ZAE BAYERN Bavarian Centre for Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies DEMS on anodic ethanol oxidation – influence of temperature, potential and concentration on CO 2 current efficiency (CCE) CO 2 current efficiency increases significantly with increasing temperature, decreases for anode potentials > 0.5 – 0.6V and decreases with increasing concentration. V. Rao, C. Cremers, U. Stimming Journal of The Electrochemical Society, 154 (2007) 11. This figure shows CO 2 current efficiency vs. potential for different temperatures. MEA with Nafion 117 membane. The anode feed is 0.1 M EtOH at 5 ml / minute.The approximate error limit is : ±10 %. 5 mg / cm 2 metal loading using 40 % Pt / C.

Physics E19 Interfaces and Energy Conversion ZAE BAYERN Bavarian Centre for Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies This figure shows CV and MSCV for m / z = 22, 29,15 and 61.The anode feed is 1 M EtOH at 5 ml/minute at 30 0 C.scan rate is 1 mV / s. This figure shows CV and MSCV for m / z = 22, 29 and 15.The anode feed is 0.1 M EtOH at 5 ml/minute at 30 0 C.scan rate is 5 mV / s. CO 2 CH 4 CH 3 -CHO Ester CH 3 -CHO CH 4

Physics E19 Interfaces and Energy Conversion ZAE BAYERN Bavarian Centre for Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies Effect of catalyst layer thickness or catalyst loading Role of resident time and active surface area Loading increases

Physics E19 Interfaces and Energy Conversion ZAE BAYERN Bavarian Centre for Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies Fuel cell: Convective + diffusive system C 2 H 5 OH+H 2 OH2H2 V. Rao, C. Cremers, U. Stimming Journal of The Electrochemical Society, 154 (2007) 11.

Physics E19 Interfaces and Energy Conversion ZAE BAYERN Bavarian Centre for Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies V. Rao, C. Cremers, U. Stimming Journal of The Electrochemical Society, 154 (2007) 11. Resident time: Average time spent by the reactant molecules in the reactor Active surface area: area where electrochemical reactions can take place Effect of catalyst layer thickness or catalyst loading

Physics E19 Interfaces and Energy Conversion ZAE BAYERN Bavarian Centre for Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies Anodic ethanol oxidation – Effect of chemical composition of catalyst Faradic currents for ethanol oxidation are similar at PtSn/C and PtRu/C At PtRu/C practically no CO 2 is formed! V. Rao, C. Cremers, U. Stimming Journal of The Electrochemical Society, 154 (2007) 11.

Physics E19 Interfaces and Energy Conversion ZAE BAYERN Bavarian Centre for Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies Acetic acid electro-oxidation on Pt and 20wt%PtSn(7:3)/C Acetic acid is resistant to electro-oxidation on Pt This rules out acetic acid as an intermediate for CO 2 formation

Physics E19 Interfaces and Energy Conversion ZAE BAYERN Bavarian Centre for Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies Acetaldehyde electro-oxidation Faradaic current and CO 2 current efficiency for acetaldehyde electro- oxidation are high enough to justify acetaldehyde as an intermediate for EOR

Physics E19 Interfaces and Energy Conversion ZAE BAYERN Bavarian Centre for Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies Discussion about mechanism of EtOH oxidation CH 3 -CH 2 -OH CH 3 -CHO CH 3 -COOH CH ads,CO ads CO 2 negligible 86%75% 14% 8mg/cm 2 Pt,40%Pt/C, T= 90°C, 0.1M EtOH, 0.1MAcetaldehyde

Physics E19 Interfaces and Energy Conversion ZAE BAYERN Bavarian Centre for Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies CO 2 current efficiency for ethanol oxidation reaction (EOR) depends strongly on potential, temperature and concentration; Catalyst layer thickness and electrochemical active area also affects CO 2 current efficiency strongly; Intrinsic nature of catalyst is important: PtRu(1:1) exhibits low CO 2 formation (CO 2 -efficiency); PtSn(7:1) catalysts shows more complete oxidation; In fuel cell active area and resident time is important for the completeness of oxidation; (Ethanol oxidation is more complete on alkaline membrane electrode assemblies.) Conclusions / Summary

Physics E19 Interfaces and Energy Conversion ZAE BAYERN Bavarian Centre for Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies Planned activities Identification of a potentially synergy between PtSn and PtRu and thus a structured catalyst layer Combination of supported PtRu and PtSn catalysts within a catalyst layer; Optimization of flow field geometry depending on catalyst layer structure. Anode Cathode catalyst layer ‚structured‘ catalyst layer with PtRu and PtSn PtRu/CPtSn/C or PtRu/CPtSn/C Variation of: sequenz of layers catalyst loading

Physics E19 Interfaces and Energy Conversion ZAE BAYERN Bavarian Centre for Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies We thank Prof. Dr. Gong-Quan Sun and Dr. Lei Cao, Dalian Institute of Chemical Physics (DICP) in Dalian, PR-China, for providing catalyst samples. We acknowledge financial support from Sino-German Center for Science Promotion, Beijing under contract GZ 211 (101/11) and German Research Foundation (DFG) under contract Sti 74/14-1 Acknowledgements Vielen Dank für Ihr Interesse! DPG Frühjahrstagung 2009, Arbeitskreis Energie (AKE)