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 (CoOh) rather than tetrahedral Co (CoTd) sites. Moreover, CoOhO₆ is electrochemically transformed to CoOhO₅ along with the formation of O vacancies (Ov) during the process of NO₃⁻RR. Both experimental and theoretic results reveal that in situ generated CoOhO₅-Ov configuration is the genuine active site for the NO₃⁻RR. To further enhance the activity of CoOh sites, we replace inert CoTd with different contents of Cu²⁺ cations, and a volcano-shape correlation between NO₃⁻RR activity and electronic structures of CoOh is observed. Impressively, in 1.0 M KOH, (Cu0.6Co0.4)Co₂O₄ with optimized CoOh 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 CoOh sites.