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

Cobalt-based spinel oxides (i.e., Co ₃ O ₄ ) are emerging as low-cost and selective electrocatalysts for the electrochemical nitrate reduction reaction (NO ₃ ^- RR) to ammonia (NH ₃ ), although their activity is still unsatisfactory and the genuine active site is unclear. Here, we discover that the NO ₃ ^- RR activity of Co ₃ O ₄ is highly dependent on the geometric location of the Co site, and the NO ₃ ^- RR prefers to occur at octahedral Co (Co _Oh ) rather than tetrahedral Co (Co _Td ) sites. Moreover, Co _Oh O ₆ is electrochemically transformed to Co _Oh O ₅ along with the formation of O vacancies (O _v ) during the process of NO ₃ ^- RR. Both experimental and theoretic results reveal that in situ generated Co _Oh O ₅ -O _v configuration is the genuine active site for the NO ₃ ^- RR. To further enhance the activity of Co _Oh sites, we replace inert Co _Td with different contents of Cu ²⁺ cations, and a volcano-shape correlation between NO ₃ ^- RR activity and electronic structures of Co _Oh is observed. Impressively, in 1.0 M KOH, (Cu _0.6 Co _0.4 )Co ₂ O ₄ with optimized Co _Oh sites achieves a maximum NH ₃ Faradaic efficiency of 96.5% with an ultrahigh NH ₃ 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 NO ₃ ^- RR activity of Co-based spinels by tuning local electronic structures of Co _Oh sites.