1. Physics E19 Interfaces and Energy Conversion ZAE BAYERN Bavarian Centre of Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies Study of the Ethanol and Ethylen Glycol Electrooxidation by Fuel Cell Differential Electrochemical Mass Spectroscopy (FC-DEMS) V. Rao 1, C. Cremers 2, U. Stimming 1,2 1 Technische Universität München, Physik-Department E19 James-Frank-Str., 85748 Garching 2 ZAE Bayern, Abteilung 1 Walther-Meißner-Str. 6, 85748 Garching DPG Frühjahrstagung 2006, Arbeitskreis Energie, München, 20. März 2006

2. Physics E19 Interfaces and Energy Conversion ZAE BAYERN Bavarian Centre of Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies Motivation for studying Ethanol Oxidation Reaction (EOR) Much less toxic than methanol: ``as safe as beer´´ High energy density (10 kWh/kg) Easily available from renewable resources Ethanol Oxidation Reaction (EOR) using FC-DEMS † To study the completeness of EOR To understand the differences between fuel cell and model electrode conditions To study the mechanism of EOR for different catalysts such as Pt, PtSn, PtRu, PtRh † FC-DEMS = Differential Electrochemical Mass Spectroscopy at Fuel Cells

3. Physics E19 Interfaces and Energy Conversion ZAE BAYERN Bavarian Centre of Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies CO 2 current efficiency for EtOH and EG oxidation as a function of potential, temperature, fuel concentration Electro-oxidation of the probable intermediates acetic acid and acetaldehyde Dependency of CO 2 current efficiency on the kind of catalyst: experiments with Pt, PtSn and PtRu based catalysts Dependency of CO 2 current efficiency on the catalyst loading and thus catalyst layer thickness: concept of residence time and active area Outline

4. Physics E19 Interfaces and Energy Conversion ZAE BAYERN Bavarian Centre of Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies Commonly Accepted Ethanol Oxidation Reaction Scheme

5. Physics E19 Interfaces and Energy Conversion ZAE BAYERN Bavarian Centre of Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies DEMS sensor for fuel cell gas diffusion electrodes DEMS set-up DEMS measurement of the oxidation of ethanol using 1.0 M (left) or 0.1M (right) solution of ethanol as fuel

6. Physics E19 Interfaces and Energy Conversion ZAE BAYERN Bavarian Centre of Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies CO 2 current efficiency as a function of potential and temperature CO 2 current efficiency - Increases significantly with increasing temperature - decreases for anode potentials > 0.5 – 0.6V The first observation indicates a temperature activated process The second observation may be explained by increasing coverage of Pt with oxygen species Catalysts 40% Pt/C (E-Tek) with a loading of 5 mg/cm 2 ; Anode feed 0.1 M Ethanol at 5 ml/min; The approximate error limit is  10%.

7. Physics E19 Interfaces and Energy Conversion ZAE BAYERN Bavarian Centre of Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies Apparent activation energies Arrhenius plots for - the Faradic current (upper) - the CO 2 m/z = 22 MS-signal (lower) Catalysts 40% Pt/C, 5 mg/cm2 Anode feed 0.1 M Ethanol, 5 ml/min Anode potential 0.6 V (RHE) The apparent activation energy for the CO 2 formation is much higher than that found for the oxidation of adsorbed CO. CO oxidation does not appear to be the rate determining step.

8. Physics E19 Interfaces and Energy Conversion ZAE BAYERN Bavarian Centre of Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies CO 2 current efficiency as a function of concentration The maximum CO 2 current efficiency decreases with increasing concentration of ethanol Possible explanation: Intermediate products face higher competition for re-adsorption and thus further oxidation Reaction orders calculated from first two points are depicted on the figure; temperature is fixed at 30 o C; anode potential is set to 0.6 V vs RHE

9. Physics E19 Interfaces and Energy Conversion ZAE BAYERN Bavarian Centre of Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies Electro-oxidation of intermediates: Acetaldehyde Electro-oxidation of acetaldehyde yields high CO 2 current efficiencies fairly high faradic currents

10. Physics E19 Interfaces and Energy Conversion ZAE BAYERN Bavarian Centre of Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies Electro-oxidation of intermediates: Acetic acid Acetic acid is highly resistant to oxidation at Pt and quite resistant to oxidation at PtSn/C This rules out acetic acid as a major pathway for CO 2 formation

11. Physics E19 Interfaces and Energy Conversion ZAE BAYERN Bavarian Centre of Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies Updated reaction pathway scheme CH 3 --CH 2 OH CH 3 --CHO CH 3 --COOH ad H 3 C--COOC 2 H 5 CO 2.CH ad.CO ad C 2 H 5 OH CH 4 X

12. Physics E19 Interfaces and Energy Conversion ZAE BAYERN Bavarian Centre of Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies Dependency of the CO 2 current efficiency on the kind of catalyst used Faradic currents for ethanol oxidation are similar at PtSn/C and PtRu/C At PtRu/C practically no CO 2 is formed!

13. Physics E19 Interfaces and Energy Conversion ZAE BAYERN Bavarian Centre of Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies Effect of catalyst loading / catalyst layer thickness CO 2 current efficiency increases with increasing Pt loading. This corresponds to: - Increased active surface area; - Increased electrode thickness and thus increased residence time.

14. Physics E19 Interfaces and Energy Conversion ZAE BAYERN Bavarian Centre of Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies Effect of surface area with different types of catalyst For the supported catalysts Pt/C and PtSn/C, CO 2 current efficiency appeared to be correlated with the CO stripping charge in the same way. The unsupported catalyst behaves differently. Perhaps due to the much thinner catalyst layer.

15. Physics E19 Interfaces and Energy Conversion ZAE BAYERN Bavarian Centre of Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies First tests on the ethylene glycol electro-oxidation The anode feed is 0.1 M EG at 5 ml / minute. The approximate error limit is: ±10 %. 5 mg / cm 2 metal loading using 40 % Pt / C. Arrhenius plots Anode potential set to 0.6V(RHE)

16. Physics E19 Interfaces and Energy Conversion ZAE BAYERN Bavarian Centre of Applied Energy Research Division 1: Technology for Energy Systems and Renewable Energies Conclusions 1)CO 2 current efficiencies for the EOR depend strongly on the potential, temperature and concentration; 2)CO 2 current efficiencies for EGOR do not depend on the potential, unlike EOR; 3)Catalyst layer thickness and electrochemical active area also affect the CO 2 current efficiencies strongly; 4)The kind of catalyst used is important: PtRu(1:1) exhibits very low CO 2 formation; 5)Fuel cell behaves like a chemical reactor: residence time dependence.