Yolk-shell composite oxides with binuclear Co(II) sites toward low-overpotential nitrate reduction to ammonia

Electroreduction of nitrate to ammonia, a potential route for producing ammonia, is still confronted with a great challenge on developing efficient electrocatalysts. In this work, a reduced yolk-shell Co-based oxides (R-Co 3 O 4 ) electrocatalyst with oxygen vacancy-rich Co 3 O 4 and CoO was designed for nitrate reduction . This catalyst presented a nearly 100 % Faradaic efficiency (FE) of ammonia in a low potential from −0.05 to −0.1 V vs. RHE, and a maximum ammonia production rate of 7.25 mg h −1 mg cat. -1 at −0.05 V vs. RHE in 0.1 M KOH + 1000 ppm KNO 3 electrolyte. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) pattern proved that the content of Co 2+ was dramatically increased in R-Co 3 O 4 . The kinetic correlation experiments and Operando Raman spectra preliminarily manifest that the potential limiting step is the reduction of adsorbed *NO 3 . The density functional theory (DFT) calculations consistently demonstrated that on both oxygen-rich Co 3 O 4 and CoO, the adsorption of key intermediates (*NO 3 , *NO and *NH) are greatly enhanced on the bridge site between two Co 2+ , boosting the nitrate reduction at low over-potential. This work is helpful to design the catalytic material with rich and low-valence sites towards electroreduction of nitrate to ammonia.