(Invited) Kinetic Characterization of Ir, Pt, and Irpt-Alloy Nanocatalysts for Ammonia Oxidation Reaction

Direct ammonia fuel cells (DAFC) is an attractive alternative of hydrogen fuel cells because of lower cost in storage and distribution of liquid fuel than hydrogen gas. A drawback is the sluggish kinetics of ammonia oxidation reaction (AOR). Based on previous DAFC studies and measurements using rotating disk electrodes (RDEs), Ir, Pt, and IrPt-alloy nanoparticles are considered as the most active AOR catalysts. However, many questions remain open in correlating RDE results obtained at ambient temperature with the catalysts’ performance in DAFC at elevated temperatures. This is due to low sustainable AOR currents at sufficiently low potentials at ambient temperature and complicated temperature effect on AOR kinetics. Here, we report an AOR study using gas diffusion electrode (GDE) in 1 M KOH solution at temperatures up to 60 o C. We kept ammonia concentration constant with time at temperatures higher than the boiling point of ammonia by saturating the solution with argon gas that bubbled through concentrated ammonia before entering the electrochemical cell. Three commercial nanocatalysts, Ir, Pt, and PtIr 1:1 alloy particles with 40 wt% metal on carbon support, were used to establish test protocol and to benchmark the platinum group metal (PGM) mass activity. The three catalysts exhibited distinct kinetic behaviors in terms of onset potential, maximum current, and activity enhancement factor with increasing temperature. Clearly characterized AOR kinetic behavior can be explained using the reaction pathways proposed based on previous DFT calculations. We’ll discuss in detail why Ir has lower onset potential, but lower peak current than Pt does, why temperature enhancement factor is higher for Ir than Pt, what are the inactive intermediates and how catalysts deactivated due to increased coverage of less active AOR intermediates at potentials above and below 0.6 V. New insights can help developing advanced AOR catalysts and making effective DAFCs. The method and concepts developed are generally applicable for studying oxidation of other liquid fuels, such as, methanol and ethanol as their oxidation reactions also involve multiple intermediates and pathways. Acknowledgements This research 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.