Pulsed Laser-Twisted Spinel-to-Rocksalt High-Entropy 3d-Metal Oxides for Selective Ammonia Electrosynthesis

Abstract

The electrochemical synthesis of ammonia (NH3) via the nitrate reduction reaction (eNO3RR) intends an efficient replacement to the Haber-Bosch technique, operating under ambient conditions. Nitrate-based voltaic cells present a multifunctional system by simultaneously removing wastewater pollutants, producing NH3, and generating energy. Herein, high-entropy spinel oxide (HE-SPO) derived from divalent (Mn, Fe, Co, Ni, and Cu) 3d transition metals are transformed into single-phase (MnFeCoNiCu)O high-entropy rock-salt oxides (HE-RSO) via pulsed laser irradiation in liquids, achieving high-entropy phase twisting with structural stabilization. The HE-RSO electrocatalyst demonstrated exceptional eNO3RR-to-NH3 conversion, with an NH3 production rate of 15.34 mg h-1 cm-2 at -0.4 V versus RHE and a Faradaic efficiency of 92%. In situ Raman spectroscopy revealed Co and Cu as dual-active sites, facilitating the N-end mechanism for eNO3RR, which is further validated via density functional theory calculations. Leveraging this high-efficiency eNO3RR-to-NH3 system, the HE-RSO catalyst is integrated into a Zn-nitrate battery, reaching a high output voltage of 1.22 V and a power density of 1.75 mW cm-2. This study highlights the pulsed laser process as a new avenue for high-entropy structural stabilization and underscores the potential of HE-RSO for sustainable NH3 production and integrated energy applications.