Mechanism of oxygen vacancy engineering CoOX/Fe3O4 regulated electrocatalytic reduction of nitrate to ammonia

To enhance the activity of the nitrate reduction reaction (NO 3 − RR), the development of oxygen vacancies electrocatalysts is a promising approach for improving the efficiency of ammonia synthesis. However, the mechanism by which oxygen vacancies regulate NO 3 − RR to ammonia remains poorly understood. In this study, a series of CoO X /Fe 3 O 4 composite catalysts derived from ZIF-67 containing oxygen vacancies (OVs) were synthesized to elucidate the role of OVs on the activity and selectivity of ammonia synthesis. Structural characterization revealed that the concentration of OVs in the catalysts increased with the addition of iron ions. Electrochemical experiments and theoretical calculations demonstrated that OVs promote interfacial electron transfer, alter the adsorption conformation of NO 3 * on the catalyst surface, and reduce the activation energy barrier of NO 3 *. Nonetheless, we observed that high concentrations of OVs exhibited a preference for the product NO 2 − at high potentials, which we attribute to the strong adsorption of NO* by the OVs, impeding the subsequent hydrogenation process. Additionally, electron paramagnetic resonance (EPR) and activated hydrogen (H*) quenching experiments indicated that the catalyst was unable to deliver substantial amounts of H* in the buffered electrolyte, resulting in low ammonia productivity. The ammonia Faraday current efficiency (FE) of CoO X /Fe 3 O 4 -90 in 0.1 M KOH and 0.1 M NO 3 − was 82.22 %, with an ammonia production rate of 1.09 mmol h −1  cm −2 .