(Invited) Challenges and Opportunities in Developing Anode Catalysts for Direct Ethanol and Ammonia Fuel Cells

Abstract

Direct liquid fuel cells (DLFC) are attractive alternatives of batteries and hydrogen fuel cells for applications where high energy density and easy to refill are critical. For example, in powering a 200W-1kWh drone, a DLFC system can weigh only 1/10 of a battery and refill is simpler than hydrogen and faster than battery recharging. Among liquid fuels, ethanol stands out for its high energy density and ammonia is carbon free. A drawback is the lack of anode catalysts for complete oxidation of ethanol and ammonia at low potentials with sufficient activity per mass of platinum group metals (PGM). Dr. Shimshon Gottesfeld is currently leading an ARPA-E project on “Direct ammonia hydroxide exchange membrane fuel cells”, and I am in the team to develop high performance catalysts for ammonia oxidation reaction (AOR) with low PGM contents. While I learned from reading his papers on a wide range of topics and talking with him at conferences in many years, working under his leadership made me further appreciate his contribution in advancing fuel cell technologies. Inspired by his sharp and open mind in choosing and solving technical issues and in pursuing deep fundamental understandings, I’d like to discuss the challenges in developing anode catalysts for ethanol and ammonia oxidation and to illustrate promising directions and opportunities based on new insights in reaction mechanisms and material/structure/performance correlations. The performances of ethanol oxidation reaction (EOR) and AOR in alkaline media on the best catalysts that we recently developed will be presented and compared. The results show that facile EOR is possible at ambient temperature when a direct 12-electron EOR pathway is activated. Analyses of in situ infrared reflection absorption spectra indicate that the C-C bond splits via ethanol dissociation and full oxidation completes without CO as poisoning intermediate. However, carbonate formation in alkaline media lowers pH and reduces EOR activity. Carbon-free ammonia circumvents this issue, but AOR activity needs enhancement from elevated temperature due to the high activation barrier for N-N bond formation. Most studies so far focused on Pt-based catalysts and the polarization curves were measured at ambient temperature. Our study of temperature-dependent AOR kinetics found that activity enhancement from 25 to 60 oC is in the order of Ir (5.5) > IrPt (4.2) > Pt (2.0).1 For both EOR and AOR, Ir is effective in promoting dehydrogenation, and thus, in lowering the onset potentials. Thus, Ir-based catalysts are more promising than Pt-based catalysts for AOR. I’ll also show that the C-C bond cleavage in EOR and the N-N bond formation in AOR prefer distinctly different atomic structures. The new insights point out opportunities and directions for future studies. Acknowledgements The author thanks Zhixiu Liang, Liang Song, Jingyi Chen, Shiqing Deng, and Eli Stavitski for their collaborations. The work was funded by the Advanced Research Projects Agency-Energy (ARPA-E), U.S. Department of Energy, under Award Number DE-AR0000805 and by the Division of Chemical Sciences, Geosciences and Biosciences Division, US Department of Energy under contract DE-SC0012704. References Song, L. et al. Temperature-Dependent Kinetics and Reaction Mechanism of Ammonia Oxidation on Pt, Ir, and PtIr Alloy Catalysts. J. Electrochem. Soc. 165, J3095–J3100 (2018).