Improved ammonia synthesis and energy output from zinc-nitrate batteries by spin-state regulation in perovskite oxides

Abstract

Electrocatalytic nitrate reduction to ammonia (eNRA) is a promising route toward environmental sustainability and clean energy. However, its efficiency is often limited by the slow conversion of intermediates due to spin-forbidden processes. Here, we introduce a novel A-site high-entropy strategy to develop a new perovskite oxide (La0.2Pr0.2Nd0.2Ba0.2Sr0.2)CoO3-delta (LPNBSC) for eNRA. The LPNBSC possesses a higher concentration of high-spin (HS) cobalt-active centers, resulting from an increased concentration of [CoO5] structural motifs compared to conventional LaCoO3. Consequently, this material exhibits a significantly improved electrocatalytic performance toward ammonia (NH3) production, resulting in a 3-fold increase in yield rate (129 mu mol h-1 mgcat. -1) and a 2-fold increase in Faradaic efficiency (FE, 76%) compared to LaCoO3 at the optimal potential. Furthermore, the LPNBSC-based Zn-nitrate battery reaches a maximum FE of 82% and an NH3 yield rate of 57 mu mol h-1 cm-2. Density functional theory calculations reveal that A-site high-entropy management in perovskites facilitates nitrate activation and potentially optimizes the thermodynamic rate-determining step of the eNRA process, namely, *HNO3 + H+ + e- -> *NO2 + H2O. This work presents an efficient concept for modulating the spin state of the B-site metal in perovskites and offers valuable insights for the design of high-performance eNRA catalysts.