Amorphous/Crystalline Heterostructure Nickel–Cobalt Oxides with Rich Oxygen Vacancies for Electrocatalytic Production of Benzoic Acid Coupled with Nitrate Reduction

Selective four-electron mild electrochemical oxidation of benzyl alcohol to high-value-added benzoic acid is regarded as a green alternative to conventional synthesis methods under moderate conditions. Herein, we synthesize oxygen vacancy-rich amorphous/crystalline heterostructure spinel-type oxides NiCo₂O₄ as a bifunctional electrocatalyst by a pulse voltammetry electrochemical treatment process, which can be utilized for the anode benzyl alcohol oxidation (AOR) and cathode nitrate reduction reaction (NO₃RR), respectively. The designed NiCo₂O_4-x-25/NF delivers only 1.18 V vs RHE at 10 mA cm^-2, and Faraday efficiency of benzoic acid of ∼100%. The systematic studies reveal that the amorphous structure of NiCo₂O_4-x-25/NF generates abundant oxygen vacancies, promoting the rapid generation of active sites and intermediate species adsorption. Meanwhile, the presence of the crystalline structure accelerates electron transfer and maintains structural stability, thereby improving overall performance. Impressively, it exhibits excellent electrocatalytic performance for AOR coupled with NO₃RR in an integrated electrolyzer, achieving a current density of 100 mA cm^-2 at an applied low cell voltage of 1.38 V and maintaining superior catalytic stability, withstanding continuous electrolysis for 144 h. This work provides a facile synthesis approach for an amorphous/crystalline heterostructure with high electrocatalytic performance, holding great potential for paired electrosynthesis of value-added chemicals.