Ethanol electro-oxidation is known to proceed through a series of elementary reactions that involve free and adsorbed species, like acetyl, hydroxyl, and carbon monoxide, and lead to a mixture of oxidation products such as acetaldehyde, acetic acid, carbon dioxide and methane. The major electro-oxidation products of ethanol on Pt electrodes are acetaldehyde and acetic acid, instead of carbon dioxide, making incomplete oxidation of ethanol one of the main challenges for the development of Direct Ethanol Fuel Cells (DEFCs). As recently shown by the authors [1], acetaldehyde plays an important role because it is not a truly final product of the reaction, but a free intermediate species that can be oxidized to acetic acid or, even further, to carbon dioxide. The kinetic model presented in [1] showed its capability to be adapted, via genetic algorithm optimization, to different binary Pt-based catalyst compositions, reproducing the anode overpotential and product selectivity in a wide range of operation conditions. Moreover, the consideration of several intermediate species enabled improved modeling of the crossover flux and the resulting mixed potentials induced at the cathode side.

In this work, we present a 1D + 1D model for liquid-feed DEFCs. One-dimensional convective transport along the channel is coupled to a one-dimensional model that accounts for species transport through the MEA, as well as electrochemical kinetics, including the mixed potential due to ethanol/acetaldehyde crossover. The complex kinetics of the multi-step ethanol oxidation reaction is described with the model proposed in [1]. A simple along-the-channel convective model is used to describe the downstream variation of the concentrations of the different species. This provides, in particular, the variation of the local current density. The results show that the current density distribution is highly dominated by the decrease of the ethanol concentration down the anode channel, together with the change of the production/consumption rate of free species. The variation of the ethanol and acetaldehyde concentration impacts the anode reaction rates and the ethanol/acetaldehyde crossover flux, which affects the cathode reaction rates.

References

[1] J. Sánchez-Monreal, P. A. García-Salaberri, M. Vera, Mathematical Modeling of Direct Ethanol Fuel Cells Using a Multi-Step Chemical Kinetic Mechanism, ECS Trans. 72 (2016) 1-16.