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 Co3O4-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 Co3O4-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.
Keywords: 2D materials; bifunctional OER/NRR catalysts; oxygen vacancies; sandwich structure; transition metal oxides.