Controllable synthesis of CoN₃ catalysts derived from Co/Zn-ZIF-67 for electrocatalytic oxygen reduction in acidic electrolytes

Metal nitrides have attracted significant attention due to their noble metal-like electron features; however, their applications are still limited by numerous predicaments in their synthesis owing to their large bond enthalpy and high ionization potential, which is generally implemented under extra-high pressure and temperature. Herein, the controllable synthesis of CoN₃ nanoparticles embedded in graphite carbon was successfully achieved through the in situ pyrolysis of a Co/Zn-ZIF-67 (ZIF, zeolitic imidazolate framework) precursor (Co/Zn molar ratio ranging from 5/95 to 9/91 in Zn-ZIF-67 crystals). During the pyrolysis, the Co/Zn-ZIF-67 precursor was first converted into Co nanoparticles (NPs) embedded in N-doped porous carbon (Co@NC), accompanied by the release of NH₃ from the decomposition of the ZIF structure. The abundant micropores formed by the evaporation of Zn and large surface area of Co@NC facilitate the contact between NH₃ molecules and Co, generating CoN₃ species. Importantly, when the CoN₃@NC-7-1000 sample was evaluated as an electrocatalyst for the oxygen reduction reaction (ORR), it exhibited high performance with a positive half-wave potential (0.72 V vs. RHE) and a high current density (5.40 mA cm^-2) in the 0.5 M H₂SO₄ electrolyte. According to the density functional theory (DFT) calculation, the exposed (220) facet of CoN₃ with a low energy barrier can benefit the adsorption of O₂ molecules.