Mathematical Modeling of Direct Ethanol Fuel Cells Using a Multi-Step Chemical Kinetic Mechanism
Authors: Sanchez-Monreal Juan , García-Salaberri Pablo A. , Vera Marcos 
Published: February 2020

Direct Ethanol Fuel Cells (DEFCs) offer a clear alternative to hydrogen and/or methanol fuel cells due to the high energy density of ethanol, a renewable fuel that can be readily produced by the fermentation of biomass and which is much less toxic than methanol. Nevertheless, due to the complex kinetics of the ethanol oxidation reaction (EOR), complete conversion to CO2 is hardly achieved and other side products are typically found during DEFC operation [1]. Several multi-step reaction schemes have been proposed for the EOR. 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 and carbon dioxide. In fact, the major oxidation products of ethanol on Pt electrodes are acetaldehyde and acetic acid instead of CO2, making incomplete oxidation of ethanol one of the main challenges for the development of DEFCs. It has been found that the main reason for low CO2 selectivity is related to the C-C bond cleavage due to the blocking effect of the surface oxidant [2]. Binary catalysts, such as Pt-Sn and Pt-Ru, exhibit a larger activity for the EOR compared to pure Pt electrodes. In this case, the blockage of active sites is partially mitigated via a bifunctional mechanism that allows the absorption of hydroxyl groups at lower potentials on the secondary metal, thus favouring further oxidation of Pt-adsorbates blocking active catalyst sites [3].

In this work, a 1D along-the-channel + 1D across-the-channel DEFC model is proposed. The complex kinetics of the multi-step ethanol oxidation reaction is described using the reaction mechanism proposed by Meyer et al. [4], which considers free and adsorbed intermediate species on a binary catalytic layer based on Pt. The adsorbed intermediate species are modeled using coverage factors that account for the blockage of the active reaction sites on the catalyst surface. The reactions rates are described by Butler-Volmer equations, accounting for the effect of the mixed potential caused by ethanol crossover. By adjusting the reaction constants, several catalyst types can be modeled and the their selectivities can be reproduced. Due to the low concentrations of carbon dioxide and methane (main reaction products of C-C bond cleavage) produced under typical operating conditions, a short kinetic mechanism is also proposed. The short mechanism stops at the acetic acid formation step, completely ignoring the formation of carbon dioxide, methane, and the accompanying intermediate adsorbed species. This simplification leads to analytical expressions for the coverage factors of acetyl and OH, thereby reducing the computational cost without significant impact in the predicted cell performance.

References

[1] G. Li and P. G. Pickup, J. Power Sources, 161, 256 (2006).

[2] R. Kavanagh, X.-M. Cao, W.-F. Lin, C. Hardacre and P. Hu, Angew. Chem. Int. Ed., 51, 1572 (2012).

[3] R.M. Antoniassi, A. Oliveira Neto, M. Linardi, E.V. Spinacé, Int. J. Hydrogen Energ., 38, 12069 (2013).

[4] M. Meyer, J. Melke and D. Gerteisen, Electrochim. Acta, 56, 4299 (2011).