Manipulating Spin-state Conversion to Promote Asymmetric d-p Orbital Hybridization for High-Efficiency Nitrate Electroreduction to Ammonia

Abstract

Electrochemical nitrate reduction reaction (eNO3-RR) presents a sustainable solution for water pollutant management and green ammonia (NH3) synthesis. However, hindered by 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 vacancies strategy (Mn-CuCo2O4-x). The elevated Co spin-state enhanced Co 3d (dxz/dyz/dz2)-*NO 2p asymmetrical orbital hybridization, facilitating *NO intermediates adsorption and the subsequent hydrogenation. Thanks to the Cu-Co synergistic effect enhanced via spin-state modulation, the Mn-CuCo2O4-x/graphene oxide aerogel (GAs) exhibited an attractive NH3 yield rate of 2.14 mg/h/cm2 with a dramatic NH3 Faradic efficiency of 98.37% at environmentally relevant NO3- level (10 mM NO3--N), far superior to the Co3O4/GAs, CuCo2O4/GAs and as-reported catalysts. Moreover, the strong interfacial interaction between GAs and Mn-CuCo2O4-x suppresses structural reconstruction of Mn-CuCo2O4-x, endowing the hybrid with robust stability. Herein, we confirm spin-state modulation can enhance the Cu-Co synergistic effect and reveal a universal strategy to optimize intermediate adsorption/conversion through 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.

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