Mechanistic Insights into the Enhanced Ammonia Oxidation Activity of PtRh Electrodeposits

Ammonia has garnered significant attention as a promising hydrogen carrier due to its high volumetric energy density, milder storage conditions, and relatively mature infrastructure. The electrochemical ammonia oxidation reaction (AOR) can facilitate the release of hydrogen from ammonia at the point of use, enabling on-demand hydrogen production without the need for high pressure storage. However, current AOR catalysts exhibit high overpotentials and sluggish kinetics, and they are susceptible to poisoning by AOR byproducts. We report the AOR activity of electrodeposited bimetallic PtRh alloy nanostructured catalysts. The dilute Pt₉₂Rh₈ catalyst exhibits a lower overpotential (η = 0.41 V vs RHE) and higher activity than Pt alone (η = 0.48 V vs RHE). Valence-band X-ray photoelectron spectroscopy (XPS) showed that Rh shifts the d-band center of Pt toward the Fermi-level that tunes the adsorption energy of AOR intermediates. Furthermore, pre-, and post-AOR nitrogen and oxygen XPS of the samples provided insight into the nature of poisoning species on the Pt and alloyed surfaces. The Pt-only surface was found to be more oxidized than the Pt₉₂Rh₈ surface post-AOR, which suggests a surface active site blocking effect of the oxygenated species generated during AOR. In summary, this study offers new insights into the AOR mechanism and a design platform for the development of future Pt-based AOR electro-catalysts.