Electrochemical Insight into Na_xCoO₂ for the Oxygen Evolution Reaction and the Oxygen Reduction Reaction

Layered NaxCoO2 provides multiple degrees of freedom for manipulating its structure and physical properties by tuning the Na concentration, leading to specific functionalities including thermoelectricity, superconductivity, and potentiality in Li-/Na-ion batteries. However, the contribution of varied Na to charge transfer, electrocatalytic kinetics, and energetics in terms of the electrochemical interface reaction for the oxygen evolution reaction (OER) in water splitting and the oxygen reduction reaction (ORR) in fuel cells is not yet fully understood. This work reveals that varied Na concentrations indirectly affect the electrochemical OER or ORR activity by changing the Co-O bond in the constituent CoO6 octahedron of NaxCoO2. Tuning the Na concentration gives rise to the unique evolution of the electronic configuration and subsequently further enhances the Co-O bond's covalency, which results in promoting the catalytic kinetics of OER and ORR. As the Fermi level descends deeper into the O 2p orbitals with increasing Na extraction, the lattice oxygen becomes active in the proton-electron transfer process, which is reflected in the pH and oxygen-concentration dependence of the OER activity. Based on the characterization of its electrochemical properties, the high electrocatalytic activity of Na0.75CoO2, which exhibits competent OER activity superior to that of IrO2, is rationalized. Meanwhile, intrinsic Na0.75CoO2 reveals a half-wave potential of 0.74VRHE for ORR. The evolution of the structure and the electronic configuration of NaxCoO2 related to its electrochemical properties enables further improved NaxCoO2-based catalysts for efficient electrochemical OER and ORR.