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.