Reconstructing active sites in Ni-Co double hydroxides to enhance electrocatalytic efficiency for nitrate reduction to ammonia

Nitrate-to-ammonia conversion presents an effective method to remediate nitrate pollution while transforming waste into a valuable product and has recently garnered significant attention. Beyond the extensively studied Cu-based catalysts, Co has also garnered significant attention. Identifying the real active sites and elucidating the mechanisms are urgently needed for its development in nitrate reduction. Co₃O₄, particularly its Co³⁺ sites, is an established active phase for nitrate reduction and has been extensively studied. However, unlike the deliberate construction of the Co₃O₄ phase or introducing doping to expose more Co³⁺ in the previous studies, it was found in this work that the active species above could be generated in Ni-Co double hydroxides in the context of nitrate reduction. The in situ generated Co₃O₄, especially the spontaneously more exposed octahedrally coordinated Co³⁺, can significantly facilitate the crucial adsorption of NO₃⁻ and thus the following reaction. Furthermore, incorporated Ni sites accelerate nitrate reduction kinetics by promoting hydrogenation, facilitated by their H*-generating capability. This enhanced catalytic activity yields a superior NH₃ production rate of 7.05 mmol h⁻¹ cm⁻². Besides, a new and more efficient approach for nitrate remediation that focuses on the nitrate sources was proposed and verified through experimentation.