Investigation of ethanol oxidation mechanism in fuel cell operating conditions
Final Report Abstract
The ethanol electro oxidation at gas diffusion electrodes (GDE) made of different catalysts, Pt/C, PtRu(1:1)/C and RSn(7:3)/C and acidic media membrane electrode assembly (MEA), was studied by on-line differential electrochemical mass spectrometry (DEMS) in a wide temperature range (30 - 90 °C) as a function of the anode potential, the fuel concentration and catalyst loading. The CO2 current efficiency (CCE) of the ethanol oxidation reaction (EOR) exhibits a maximum at about 0.6 V and decreases rapidly with further increasing potentials. The CCE for the EOR goes down with increase in concentration of ethanol. CCE for ethanol oxidation reaction shows a strong increase with increasing catalyst loading. The CCE increases with increasing temperature, exceeding 75% at 90 °C, 0.1 M ethanol, and 5 mg/cm2 Pt catalyst loading. PtSn/C shows high CCE, like Pt/C. But PtRu/C exhibits very small CCE. Of the intermediates, acetaldehyde is quite active for further oxidation. But acetic acid is fairly resistant against further oxidation. Our results indicate that the C-C bond scission observed for the EOR with CCE in excess of 50% has to proceed in parallel with ethanol oxidation to either acetaldehyde or acetic acid, and not sequentially from acetic acid further on, as acetic acid can not be oxidized any further. The DEMS measurements were also performed with all solid alkaline MEA which indicate that the in case of alkaline media ethanol undergoes significantly more complete electro oxidation to CO2 than in case of acidic MEA with same Pt anode. The CO2 current efficiency can be compared for acidic and alkaline MEA with similar electrochemical active area on the anode side. It is important to compare keeping similar ECA, as CO2 current efficiency for ethanol oxidation reaction increases with increasing electrochemically active area available in the catalyst layer. This has been found earlier in experiment with acidic MEAs. The CCE estimated in case of alkaline MEA with Pt anode is around 55% at 0.8V/DHE, 60 °C and 0.1 M EtOH. But in same conditions acidic MEA shows only 3% CCE. This indicates that the C-C bond scission rates are much higher in alkaline media. But how exactly the mechanism of ethanol oxidation in alkaline media is, is not clearly known. The fact that CO2 will form carbonates in presence of aqueous alkaline electrolyte make it difficult to study ethanol oxidation in FTIR or model DEMS systems. The polymer alkaline electrolyte as used in this study for making alkaline MEAs, provides an important opportunity to observe CO2 produced during ethanol oxidation reaction using DEMS system.
Publications
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"DEMS on ethanol oxidation in alkaline membrane electrode assembly". Fuel cells, 5 (2007) 417
V. Rao, Hariyanto, C. Cremers, U. Stimming
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"Electro oxidation of ethanol at gas diffusion electrodes: a DEMS study". Journal of The Electrochemical Society, 154 (2007)11
V. Rao, C. Cremers, U. Stimming
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"Electrochemical Characterization of Direct Alcohol Fuel Cells using in-situ Differential Electrochemical Mass Spectrometry". Dissertation, Physik Department, TU München, 2008
Vineet Rao
