p–d Orbital Hybridization in Ag-based Electrocatalysts for Enhanced Nitrate-to-Ammonia Conversion

Abstract

Considering the substantial role of ammonia, developing highly efficient electrocatalysts for nitrate-to-ammonia conversion has attracted increasing interest. Herein, we proposed a feasible strategy of p–d orbital hybridization via doping p-block metals in an Ag host, which drastically promotes the performance of nitrate adsorption and disassociation. Typically, a Sn-doped Ag catalyst (SnAg) delivers a maximum Faradaic efficiency (FE) of 95.5±1.85 % for NH3 at −0.4 V vs. RHE and reaches the highest NH3 yield rate to 482.3±14.1 mg h−1 mgcat.−1. In a flow cell, the SnAg catalyst achieves a FE of 90.2 % at an ampere-level current density of 1.1 A cm−2 with an NH3 yield of 78.6 mg h−1 cm−2, during which NH3 can be further extracted to prepare struvite as high-quality fertilizer. A mechanistic study reveals that a strong p–d orbital hybridization effect in SnAg is beneficial for nitrite deoxygenation, a rate-determining step for NH3 synthesis, which as a general principle, can be further extended to Bi- and In-doped Ag catalysts. Moreover, when integrated into a Zn-nitrate battery, such a SnAg cathode contributes to a superior energy density of 639 Wh L−1, high power density of 18.1 mW cm−2, and continuous NH3 production.