Exploring the Reaction Mechanism of Efficient Electrochemical Nitrate Reduction to Ammonia over Carbon Cloth-Supported α-Co(OH) ₂ Nanosheets

Electrochemical nitrate reduction reaction (eNO₃RR) with Co-based catalysts provides a sustainable and efficient approach to wastewater purification and nitrogen recovery. However, their inevitable and uncontrollable reconstruction during the eNO₃RR leads to uncertain actual active sites and a confused understanding of the mechanism. Herein, the active sites and structural evolution of α-Co(OH)₂/CC during the eNO₃RR process were investigated via in situ and ex situ characterizations. The results show that α-Co(OH)₂ nanosheets are converted to β-Co(OH)₂ in a strongly alkaline electrolyte and then act as active sites to adsorb NO₃^- and spontaneously react to generate CoOOH. Whereas CoOOH is swiftly reduced to β-Co(OH)₂ at negative cathodic potential, realizing a nitrate-electrically mediated cyclic evolution process and synergistically catalyzing the eNO₃RR. Consequently, α-Co(OH)₂/CC achieves a Faradaic efficiency as high as 92% at -0.1 V vs RHE and a high NH₃ yield rate of 338 mmol·g_cat^-1·h^-1 under -0.6 V vs RHE. This work offers novel insight into the structural evolution and catalytic behavior of Co-based catalysts in eNO₃RR.