Screening Ir-Free Trimetallic Alloys with Consideration of Metal Electronegativity and Oxophilicity toward Ammonia Electrooxidation
Zhengwei Zhang, Hao Shen, Yongying Wang, Zeguang Dong, Tieyu Hu, Zhongti Sun, Juan Yang, Yi Li
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, Pt 3 PdNi 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, Pt 3 Pd, and Pt 3 RuNi controls and much closer to commercial PtIr/C. Further carbon support selection has resulted in the optimal Pt 3 PdNi 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 *NH 2 to *NH, resulting in enhanced catalytic activity for the AOR. Moreover, the hydrazine electrooxidation experiments indicate that NH 3 adsorption and activation before N–N dimerization is kinetically sluggish.