Physics E19 Interfaces and Energy Conversion Bavarian Centre

Physics E19 Interfaces and Energy Conversion Bavarian Centre for Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies ZAE BAYERN 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 Buar 1,2 and Ulrich Stimming1,2 1 Technische Universitt Mnchen (TUM) , Department of Physics E19, James-Franck-Str.1, D-85748 Garching, Germany 2 Bavarian Center for Applied Energy Research (ZAE Bayern), Walther-Meiner-Str. 6, D-85748 Garching, Germany 3 New address: Fraunhofer Inst Chem Technol, Dept Appl Electrochem, Pfinztal, Germany. DPG Frhjahrstagung 2009, Arbeitskreis Energie (AKE) Physics E19 Interfaces and Energy Conversion Bavarian Centre for Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies Motivation for Direct Fuel Cells (Direct FCs) ZAE BAYERN 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 Bavarian Centre for Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies Aspects of Efficiency and Energy Density ZAE BAYERN 105,0% 100,0% h = D G0/D H0 95,0% 90,0% 85,0% 80,0% 75,0% 70,0% 65,0% 60,0% Ethanol is connected with a higher thermodynamic conversion efficiency 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 Bavarian Centre for Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies ZAE BAYERN 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.) Physics E19 Interfaces and Energy Conversion Bavarian Centre for Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies Ethanol Oxidation Scheme ZAE BAYERN teflon disc with holes microporous membrane CH3--CH2OH to MS vacuum detection zylinder CH3--CHO (m/z=29, base peak) o-ring anode outlet .CHad .COad DEMS set-up C2H5OH CH3--COOH CH3--COOC2H5 Esterification CH4 (m/z=15)

CO2 (m/z=43, base peak) (m/z=61) (m/z=44, m/z=22 double charged ions) Physics E19 Interfaces and Energy Conversion Bavarian Centre for Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies ZAE BAYERN DEMS on anodic ethanol oxidation influence of temperature, potential and concentration on CO2 current efficiency (CCE) 1M Ethanol 0.1M Ethanol 0.01M Ethanol 0.7 0.6 o CO2 current efficiency T = 60 C 0.5 0.4 0.3 0.2 0.1 0.0 0.4 0.5 0.6 0.7 0.8 0.9

1.0 potential /V vs. RHE 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 / cm 2 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. 200 150 100 50 0 Ii / pA 400 600 IF / mA 200 800 1000 1200 0 CO2 Ii / pA 0 200 400 600 800 1000 1200 m/z = 29 -3,925

7,4 600 800 1000 1200 m/z = 15 CH4 4 0 200 400 600 m/z = 61 Ester 0.28 0 200 400 600 800 1000 1200 potential(mV/RHE) 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. CH3-CHO 7,2 4,8 800 1000 1200 200 400 600 800 1000 1200 m/z= 29

0 0 Ii / pA Ii / pA 400 Ii / pA Ii / pA 200 CO2 m/z= 22 0 0 0 200 400 600 800 1000 1200 -3,926 CH3-CHO 10 8 ZAE BAYERN 0 m/z = 22 -0.8 20 Division 1: Technology for Energy Systems and Renewable Energies 50 0

-0.4 Bavarian Centre for Applied Energy Research Ii / nA IF / mA Physics E19 Interfaces and Energy Conversion 200 400 600 800 1000 1200 m/z= 15 4,2 CH4 3,6 0 200 400 600 800 1000 1200 potential(mV/RHE) 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. Physics E19 Interfaces and Energy Conversion Bavarian Centre for Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies ZAE BAYERN Effect of catalyst layer thickness or catalyst loading Loading increases 2 o 0.20mg/cm 2 0.25mg/cm 2

0.80mg/cm 2 2.45mg/cm 2 4.20mg/cm 2 8.00 mg/cm CO2 efficiency at 90 C 0.1 M EtOH, 5 ml/min flow rate 40%Pt/C 0.8 0.7 0.5 0.4 0.3 0.2 CO2 current efficiency 40% Pt/C o at 90 C, 0.1MEtOH, 0.6V/RHE 0,6 CO2 current efficiency CO2 current efficiency 0.6 0,8 0,4 0,2 0.1 0.0 0,0 0.4 0.5

0.6 0.7 potential /V vs. RHE 0.8 0.9 0 1 2 3 4 5 6 2 Platinum loading(mg/cm ) Role of resident time and active surface area 7 8 9 Physics E19 Interfaces and Energy Conversion Bavarian Centre for Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies ZAE BAYERN Fuel cell: Convective + diffusive system C2H5OH+H2O V. Rao, C. Cremers, U. Stimming Journal of The Electrochemical Society, 154 (2007) 11.

H2 Physics E19 Interfaces and Energy Conversion Bavarian Centre for Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies ZAE BAYERN 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. Physics E19 Interfaces and Energy Conversion Bavarian Centre for Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies Anodic ethanol oxidation Effect of chemical composition of catalyst ZAE BAYERN Faradic currents for ethanol oxidation are 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. Physics E19 Interfaces and Energy Conversion Bavarian Centre for Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies

ZAE BAYERN Acetic acid electro-oxidation on Pt and 20wt%PtSn(7:3)/C 2 4.3mg/cm Pt 2 2 mg/cm 20% PtSn(7:3)/C 12 10 8 6 4 current(mA) 2 0 -2 -4 -6 -8 -10 -12 o 70 C 0.1M Acetic Acid scan rate:5mV/s -14 -16 -18 0 200 400 600 800 1000 1200 Potential /mV vs. RHE

Acetic acid is resistant to electro-oxidation on Pt This rules out acetic acid as an intermediate for CO2 formation Physics E19 Interfaces and Energy Conversion Bavarian Centre for Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies ZAE BAYERN Acetaldehyde electro-oxidation 100 0.1M acetaldehyde o 90 C,5ml/minute 5mV/s 2 40%Pt/C,8mg/cm Pt CO2 current efficiency 0,9 0 3.0 0 200 400 600 800 1000 1200 2.5 2.0 Im/z =22 (pA)

0.1M acetaldehyde o 90 C,5ml/minute 2 40%Pt/C,8mg/cm Pt 0,8 50 1.5 CO2 current efficiency current(mA) 150 0,7 0,6 0,5 0,4 0,3 1.0 0,2 0.5 0.0 0,1 0 200 400 600 potential /mV vs.RHE 800 1000 1200 0,5

0,6 0,7 0,8 0,9 potential(V) Faradaic current and CO2 current efficiency for acetaldehyde electrooxidation are high enough to justify acetaldehyde as an intermediate for EOR Physics E19 Interfaces and Energy Conversion Bavarian Centre for Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies ZAE BAYERN 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= 90C, 0.1M EtOH, 0.1MAcetaldehyde Physics E19 Interfaces and Energy Conversion Bavarian Centre for Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies Conclusions / Summary ZAE BAYERN

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.) Physics E19 Interfaces and Energy Conversion Bavarian Centre for Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies ZAE BAYERN Planned activities Anode 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. Cathode Anode PtRu/C PtSn/C PtSn/C PtRu/C or catalyst layer Variation of: sequenz of layers catalyst loading structured catalyst layer with PtRu and PtSn Physics E19 Interfaces and Energy Conversion Bavarian Centre for Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies ZAE BAYERN Vielen Dank fr 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 Frhjahrstagung 2009, Arbeitskreis Energie (AKE)

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