Polycrystalline CoO x ‑Bo Hybrid as Proficient Electrocatalyst for Addressing Kinetically Sluggish Anodic Reaction in Water Splitting

Herein, we demonstrated that a polycrystalline cobalt oxide/borate (CoO x -Bo) hybrid catalyst prepared by coprecipitation followed a simple annealing process with a viable boron source of less hazardous ammonium borate, an efficient electrocatalyst for the oxygen evolution reaction (OER). The borate species in the crystalline cobalt oxide lattice provides a tunable polycrystalline morphology with a defect-rich lattice and numerous grain boundaries in the CoO x -Bo hybrid electrocatalyst, which significantly boosts the OER activity compared to the crystalline counterparts of Co3O4 and precious IrO2 in a harsh alkaline electrolyte (1 M KOH). The borate modulated CoO x -Bo achieves a 10 mA/cm2 geometrical current density for the OER with a very low overpotential (η) of 271 mV and small Tafel slope of 34 mV dec–1, in an inert glassy carbon (GC) support, while only requiring η10 of 267 and 32 mV dec–1 in a 3D nickel foam (NF) support at the same current density. The CoO x -Bo catalyst assembled in a two-electrode system with a standard Pt–C cathode only consumed 1.53 V potential bias and exhibited robust stability up to 150 h@10 mA/cm2. The CoO x -Bo is irreversibly oxidized to CoOOH active transformation via surface reconstruction during the OER condition. The cyclic voltammogram (CV) profiles, RRDE evaluation, and postcharacterization observation revealed the formation of a CoOOH active phase upon the long-term OER process and corresponding surface reconstruction. This research provides a new way to synthesize defect-rich, short-range ordered structures of polycrystalline materials with numerous grain boundaries and lays valuable experimental and postcharacterization foundations for the structure and properties of OER catalysts.