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·gcat–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.