Designing Efficient Nitrate Reduction Electrocatalysts by Identifying and Optimizing Active Sites of Co-Based Spinels

Cobalt-based spinel oxides (i.e., Co₃O₄) are emerging as low-cost and selective electrocatalysts for the electrochemical nitrate reduction reaction (NO₃^-RR) to ammonia (NH₃), although their activity is still unsatisfactory and the genuine active site is unclear. Here, we discover that the NO₃^-RR activity of Co₃O₄ is highly dependent on the geometric location of the Co site, and the NO₃^-RR prefers to occur at octahedral Co (Co_Oh) rather than tetrahedral Co (Co_Td) sites. Moreover, Co_OhO₆ is electrochemically transformed to Co_OhO₅ along with the formation of O vacancies (O_v) during the process of NO₃^-RR. Both experimental and theoretic results reveal that in situ generated Co_OhO₅-O_v configuration is the genuine active site for the NO₃^-RR. To further enhance the activity of Co_Oh sites, we replace inert Co_Td with different contents of Cu²⁺ cations, and a volcano-shape correlation between NO₃^-RR activity and electronic structures of Co_Oh is observed. Impressively, in 1.0 M KOH, (Cu_0.6Co_0.4)Co₂O₄ with optimized Co_Oh sites achieves a maximum NH₃ Faradaic efficiency of 96.5% with an ultrahigh NH₃ rate of 1.09 mmol h^-1 cm^-2 at -0.45 V vs reversible hydrogen electrode, outperforming most of other reported nonprecious metal-based electrocatalysts. Clearly, this work paves new pathways for boosting the NO₃^-RR activity of Co-based spinels by tuning local electronic structures of Co_Oh sites.