Facet-Dependent Evolution of Active Components on Spinel Co₃O₄ for Electrochemical Ammonia Synthesis

Spinel cobalt oxides (Co₃O₄) have emerged as a promising class of catalysts for the electrochemical nitrate reduction reaction (eNO₃RR) to ammonia, offering advantages such as low cost, high activity, and selectivity. However, the specific role of crystallographic facets in determining the catalysts' performance remains elusive, impeding the development of efficient catalysts. In this study, we have synthesized various Co₃O₄ nanostructures with exposed facets of {100}, {111}, {110}, and {112}, aiming to investigate the dependence of the eNO₃RR activity on the crystallographic facets. Among the catalysts tested, Co₃O₄ {111} shows the best performance, achieving an ammonia Faradaic efficiency of 99.1 ± 1.8% with a yield rate of 35.2 ± 0.6 mg h^-1 cm^-2 at -0.6 V vs RHE. Experimental and theoretical results reveal a transformation process in which the active phases evolve from Co₃O₄ to Co₃O_4- with oxygen vacancy (O_v), followed by a Co₃O_4--O_v/Co(OH)₂ hybrid, and finally Co(OH)₂. This process is observed for all facets, but the formation of O_v and Co(OH)₂ is the most rapid on the (111) surface. The presence of O_v significantly reduces the free energy of the *NH₂ intermediate formation from 1.81 to -0.53 eV, and plentiful active sites on the densely reconstructed Co(OH)₂ make Co₃O₄ {111} an ideal catalyst for ammonia synthesis via eNO₃RR. This work provides insights into the understanding of the realistic active components, offers a strategy for developing highly efficient Co-based spinel catalysts for ammonia synthesis through tuning the exposed facets, and helps further advance the design and optimization of catalysts in the field of eNO₃RR.