Screening Ir-Free Trimetallic Alloys with Consideration of Metal Electronegativity and Oxophilicity toward Ammonia Electrooxidation

Abstract

Ir-contained alloys represent the state-of-the-art ammonia oxidation reaction (AOR) electrocatalysts for direct ammonia fuel cells, but they are greatly impeded by their high cost. Here, we rationally design and synthesize Ir-free trimetallic alloys with the consideration of metal electronegativity and oxophilicity that govern the reactivity of the alloy surface. By introducing a metal (i.e., Pd) with an electronegativity like Ir and oxophilic metals (i.e., Mn, Fe, Co, or Ni) into Pt, we have screened a high-performance Ir-free trimetallic electrocatalyst system. Among others, Pt₃PdNi was experimentally selected as an optimal AOR electrocatalyst, showing an onset potential of ∼0.45 V versus the reversible hydrogen electrode, lower than those of Pt, Pt₃Pd, and Pt₃RuNi controls and much closer to commercial PtIr/C. Further carbon support selection has resulted in the optimal Pt₃PdNi deposited onto carboxyl-functionalized carbon black displaying the highest peak current density of 252.9 A g_Pt^–1. Density functional theory calculations further demonstrated that PdNi atoms in Pt decrease the reaction energy barrier of electrochemical dehydrogenation of *NH₂ to *NH, resulting in enhanced catalytic activity for the AOR. Moreover, the hydrazine electrooxidation experiments indicate that NH₃ adsorption and activation before N–N dimerization is kinetically sluggish.