Polycrystalline CoOx-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.