Bifunctional OER/NRR Catalysts Based on a Thin-Layered Co₃O_4–x/GO Sandwich Structure

Due to ample low-coordinated surface atoms, a distorted lattice endows thin-layered transition metal oxides with a flexible electronic structure, making them the ideal candidates for overall ammonia synthesis. This work proposes a novel and facile method for the controllable synthesis of thin-layered Co₃O₄ catalysts with graphene as a conductive matrix to further enhance the overall N₂ fixation performance. X-ray photoelectron spectroscopy (XPS) and synchrotron radiation X-ray absorption spectroscopy (XAS) demonstrate that the sandwiched Co₃O_4–x/GO catalysts enable exposure of more coordination unsaturated active sites, resulting in numerous oxygen vacancies. With a higher conductivity and distorted crystalline structure, excellent electrochemical NRR activity is realized with a NH₃ production rate of 5.19 mmol g^–1 h^–1 and a Faradaic efficiency of 10.68% at −0.4 V vs reversible hydrogen electrode (RHE). The density functional theory (DFT) calculation demonstrates that introducing oxygen vacancies in thin-layered cobalt oxides could result in an increased density of states (DOS) near the Fermi level, which would accelerate the NRR rate-determining step. Charge transfer could be accelerated through a weak Co 3d–N 2p σ hybrid bond with a lower energy level. No obvious performance decay could be found after six cycles. Furthermore, the sandwiched Co₃O_4–x/GO catalyst exhibits a low overpotential of 280 mV@10 mA cm^–2 and an outstanding durability for the anode OER, even better than those of the benchmark RuO₂. Such an inexpensive sandwiched transition metal oxide catalyst shows great potential in the field of overall N₂ fixation.