Manipulating spin-state conversion to promote asymmetric d–p orbital hybridization for high-efficiency nitrate electroreduction to ammonia

Abstract

The electrochemical nitrate reduction reaction (eNO₃⁻RR) presents a sustainable solution for water pollutant management and green ammonia (NH₃) synthesis. However, hindered by the spin-forbidden barrier, the sluggish hydrogenation kinetics of the key intermediate *NO severely limits the production of NH₃. Here, we reported for the first time the realization of a controllable transition of the inner Co spin-state from a low spin to a high spin in CuCo₂O₄ through the Mn doping-driven oxygen vacancy strategy (Mn–CuCo₂O_4−x). The elevated Co spin-state enhanced Co 3d (d_xz/d_yz/d_z²)–*NO 2p asymmetrical orbital hybridization, facilitating *NO intermediate adsorption and the subsequent hydrogenation. Thanks to the Cu–Co synergistic effect enhanced via spin-state modulation, the Mn–CuCo₂O_4−x/graphene oxide aerogels (GAs) exhibited an attractive NH₃ yield rate of 2.14 mg h⁻¹ cm⁻² with a dramatic NH₃ faradaic efficiency of 98.37% at an environmentally relevant NO₃⁻ level (10 mM NO₃⁻–N), far superior to that of Co₃O₄/GAs, CuCo₂O₄/GAs and as-reported catalysts. Moreover, the strong interfacial interaction between GAs and Mn–CuCo₂O_4−x suppresses structural reconstruction of Mn–CuCo₂O_4−x, endowing the hybrid with robust stability. Herein, we confirm that spin-state modulation can enhance the Cu–Co synergistic effect and reveal a universal strategy to optimize intermediate adsorption/conversion through the spin-state, opening up a new avenue for deep purification of water pollutants based on spin optimization and providing general principles for the rational design of catalytic materials.