Towards an Efficient Conversion of Ethanol in Low Temperature Fuel Cells:

1. 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 Rao1, Carsten Cremers3, Rainer Bußar1,2and Ulrich Stimming1,2 1 Technische Universität München (TUM) , Department of Physics E19, James-Franck-Str.1, D-85748 Garching, Germany 2 Bavarian Center for Applied Energy Research (ZAE Bayern), Walther-Meißner-Str. 6, D-85748 Garching, Germany 3 New address: Fraunhofer Inst Chem Technol, Dept Appl Electrochem, Pfinztal, Germany. DPG Frühjahrstagung 2009, Arbeitskreis Energie (AKE)

2. 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.

3. 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)

4. Outline of the presentation • CO2 current efficiency for ethanol oxidation as a • function of Potential, Temperature and Concentration; • CO2 current efficiency dependent on intrinsic nature of catalyst • experiments with Pt, PtSn and PtRu; • CO2 current efficiency dependent on the catalyst loading and • thus catalyst layer thickness:concept of resident time and active area; • (CO2 current efficiency on alkaline membrane electrode assemblies.)

5. CH3--CH2OH CH3--CHO .CHad .COad C2H5OH CH3--COOH CH3--COOC2H5 CH4 CO2 Ethanol Oxidation Scheme (m/z=29, base peak) DEMS set-up Esterification (m/z=43, base peak) (m/z=61) (m/z=44, m/z=22 double charged ions) (m/z=15)

6. DEMS on anodic ethanol oxidation – influence of temperature, potential and concentration on CO2 current efficiency (CCE) This figure shows CO2 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 / cm2 metal loading using 40 % Pt / C. CO2 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.

7. CO2 CO2 CH3-CHO CH3-CHO CH4 CH4 Ester 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 0C.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 0C.scan rate is 5 mV / s.

8. Effect of catalyst layer thickness or catalyst loading Loading increases Role of resident time and active surface area

9. C2H5OH+H2O H2 Fuel cell: Convective + diffusive system V. Rao, C. Cremers, U. Stimming Journal of The Electrochemical Society, 154 (2007) 11.

10. Effect of catalyst layer thickness or catalyst loading Resident time: Average time spent by the reactant molecules in the reactor Active surface area: area where electrochemical reactions can take place V. Rao, C. Cremers, U. Stimming Journal of The Electrochemical Society, 154 (2007) 11.

11. Anodic ethanol oxidation – Effect of chemical composition of catalyst Faradic currents for ethanol oxidationare similar at PtSn/C and PtRu/C At PtRu/C practically no CO2 is formed! V. Rao, C. Cremers, U. Stimming Journal of The Electrochemical Society, 154 (2007) 11.

12. 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 CO2 formation

13. Acetaldehyde electro-oxidation Faradaic current and CO2 current efficiency for acetaldehyde electro-oxidation are high enough to justify acetaldehyde as an intermediate for EOR

14. Discussion about mechanism of EtOH oxidation CH3-CH2-OH CH3-CHO CHads ,COads negligible 14% CH3-COOH CO2 75% 86% 8mg/cm2 Pt,40%Pt/C, T= 90°C, 0.1M EtOH,0.1MAcetaldehyde

15. Conclusions / Summary • CO2 current efficiency for ethanol oxidation reaction (EOR) depends strongly on potential, temperature and concentration; • Catalyst layer thickness and electrochemical active area also affects CO2 current efficiency strongly; • Intrinsic nature of catalyst is important: PtRu(1:1) exhibits low CO2 formation (CO2-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.)

16. 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 Anode Cathode PtRu/C PtSn/C PtSn/C PtRu/C Variation of: sequenz of layers catalyst loading or ‚structured‘ catalyst layer with PtRu and PtSn catalyst layer

17. Vielen Dank für Ihr Interesse! Acknowledgements • 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 DPG Frühjahrstagung 2009, Arbeitskreis Energie (AKE)