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

Abstract

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-Co3O4) electrocatalyst with oxygen vacancy-rich Co3O4 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 mgcat.-1 at −0.05 V vs. RHE in 0.1 M KOH + 1000 ppm KNO3 electrolyte. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) pattern proved that the content of Co2+ was dramatically increased in R-Co3O4. The kinetic correlation experiments and Operando Raman spectra preliminarily manifest that the potential limiting step is the reduction of adsorbed *NO3. The density functional theory (DFT) calculations consistently demonstrated that on both oxygen-rich Co3O4 and CoO, the adsorption of key intermediates (*NO3, *NO and *NH) are greatly enhanced on the bridge site between two Co2+, 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.