Polycrystalline CoO<sub><i>x</i></sub>‑Bo\nHybrid as Proficient Electrocatalyst for Addressing Kinetically Sluggish\nAnodic Reaction in Water Splitting

Herein, we demonstrated that a polycrystalline cobalt\noxide/borate\n(CoO<sub><i>x</i></sub>-Bo) hybrid catalyst prepared by\ncoprecipitation followed a simple annealing process with a viable\nboron source of less hazardous ammonium borate, an efficient electrocatalyst\nfor the oxygen evolution reaction (OER). The borate species in the\ncrystalline cobalt oxide lattice provides a tunable polycrystalline\nmorphology with a defect-rich lattice and numerous grain boundaries\nin the CoO<sub><i>x</i></sub>-Bo hybrid electrocatalyst,\nwhich significantly boosts the OER activity compared to the crystalline\ncounterparts of Co<sub>3</sub>O<sub>4</sub> and precious IrO<sub>2</sub> in a harsh alkaline electrolyte (1 M KOH). The borate modulated\nCoO<sub><i>x</i></sub>-Bo achieves a 10 mA/cm<sup>2</sup> geometrical current density for the OER with a very low overpotential\n(η) of 271 mV and small Tafel slope of 34 mV dec<sup>–1</sup>, in an inert glassy carbon (GC) support, while only requiring η<sub>10</sub> of 267 and 32 mV dec<sup>–1</sup> in a 3D nickel\nfoam (NF) support at the same current density. The CoO<sub><i>x</i></sub>-Bo catalyst assembled in a two-electrode system\nwith a standard Pt–C cathode only consumed 1.53 V potential\nbias and exhibited robust stability up to 150 h@10 mA/cm<sup>2</sup>. The CoO<sub><i>x</i></sub>-Bo is irreversibly oxidized\nto CoOOH active transformation via surface reconstruction during the\nOER condition. The cyclic voltammogram (CV) profiles, RRDE evaluation,\nand postcharacterization observation revealed the formation of a CoOOH\nactive phase upon the long-term OER process and corresponding surface\nreconstruction. This research provides a new way to synthesize defect-rich,\nshort-range ordered structures of polycrystalline materials with numerous\ngrain boundaries and lays valuable experimental and postcharacterization\nfoundations for the structure and properties of OER catalysts.