Anion exchange membrane fuel cells (AEMFCs) are a promising alternative to the more established proton exchange membrane fuel cell (PEMFC) technology, with advantages mainly related to the reduction in expense of materials used. While Hydrogen is the most popular fuel for AEMFCs, ammonia’s high energy density together with its established transport and distribution technology, make it an attractive candidate as an alternative fuel for AEMFCs.

Although early attempts to utilize ammonia as a fuel in AEMFCs resulted in poor performance, recent studies of direct ammonia anion-exchange membrane fuel cells (in short - DAFCs), with advanced anion exchange membranes (AEMs) and electrocatalysts, have yielded new and promising performance measure values (see e.g. [1]) thus motivating further analyses and development of these systems. The two main challenges to be addressed when considering this technology are a slow ammonia oxidation reaction (AOR) and ammonia crossover.

In this contribution, we will present a computational analysis of a DAFC, aimed at probing performance and performance stability of this system. A special emphasis will be placed on the effect of ammonia crossover and the resultant parasitic reaction (involving ammonia oxidation in the cathode) on system behavior. Following an appropriate introduction, we will describe our modeling approach, properties, and parameters, where our one-dimensional and transient model of a DAFC, including degradation kinetics capabilities, is conceptually based on our previous studies of hydrogen-fueled AEMFCs (e.g. [2-4]). The computational domain consists of a membrane, anode and cathode gas diffusion layers, and anode and cathode catalyst layers. The model captures species transport and significant electrochemical reactions taking place in the cell. After detailing our modeling approach, we will present calculations aimed at validating our model against experimental data of a DAFC operated with KOH-free anode feed at high temperatures (100 and 120 °C). Following this, we will show and discuss results concerning the effect of ammonia crossover on DAFC performance and performance stability. The initial (un-degraded) cell performance analysis reveals (among other things) a dependence of the parasitic reaction rate through the cathode on the value of current density. More interestingly, the simulated results demonstrate (under certain conditions) a surprising and nontrivial impact of ammonia crossover on cell longevity.

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References

[1] D. Achrai, Y. Zhao, T. Wang, G. Tamir, R. Abbasi, B.P. Setzler, M. Page, Y. Yan, S. Gottesfeld, Journal of The Electrochemical Society. 167 (2020) 134518. https://doi.org/10.1149/1945-7111/abbdd1

[2] R. Dekel, I. G. Rasin, M. Page, S. Brandon, Journal of Power Sources, 375 (2018) 191-204. https://doi.org/10.1016/j.jpowsour.2017.07.012

[3] R. Dekel, I. G. Rasin, S. Brandon, Journal of power Sources, 420 (2019) 118–123. https://doi.org/10.1016/j.jpowsour.2019.02.069.

[4] K. Yassin, I. G. Rasin, S. Willdorf-Cohen, C. E. Diesendruck, S. Brandon, D. R. Dekel, Journal of Power Sources Advances, 11 (2021) 100066. https://doi.org/10.1016/j.powera.2021.100066.