Electrooxidation of small organic molecules at elevated temperature and pressure: an online Differential Electrochemical Mass Spectrometry (DEMS) study

Abstract

The objective of the present work was to determine the kinetics of the electrooxidation of small organic molecules, in terms of the activity (Faradaic current) performed in a novel high temperature / high pressure thin-layer flow cell and selectivity (current efficiency for CO2 formation) obtained from the on-line differential electrochemical mass spectrometry (DEMS) by monitoring CO2 formation, on realistic carbon supported catalysts under well-defined, but nevertheless, fuel cell relevant reaction conditions (temperatures up to 100 oC, overpressure of 3 bar and continuous mass transport). The selectivity increasingly prevails at higher temperature, lower concentration and lower potentials (~ 46 % CO2 current efficiency at 100 °C, 0.1 M, 0.48 V). These trends result in a significantly higher apparent activation barrier for complete oxidation (68 ± 2 kJ mol¯¹ at 0.48 V, 0.1 M) vs. the overall ethanol oxidation reaction (42 ± 2 kJ mol¯¹ at 0.48 V, 0.1 M). The selectivity for acetaldehyde electrooxidation is ~ 80 % (at 0.5 V, 0.1 M, 100 °C) and the apparent activation barrier for complete oxidation is 39 ± 2 kJ mol¯¹ (at 0.6 V, 0.1 M) vs. overall acetaldehyde oxidation reaction (32 ± 2 kJ mol¯¹) and that for oxidation to acetic acid (27 ± 3 kJ mol¯¹). Analogous measurements on the electrooxidation of acetic acid show very low activities even at 100 °C, and a high apparent activation energy (173 ± 6 kJ mol¯¹). The electrooxidation of ethanol on Pt-based (Pt/C, PtRu black) and Pd-based (Pd/C, Pd/CeO2/C) catalysts in alkaline solution showed that the addition of Ru or CeO2 improves the tolerance at low potentials, while for higher potentials the activity of the monometallic catalysts is higher. The apparent activation energies, which are in the range of 11 - 32 kJ mol¯¹, vary significantly with potential. The results are important for further design and optimization of catalysts aiming at improvement of performance and durability relevant for direct ethanol fuel cells (DEFC)