Formation of mixed-phase NixB/Co3O4/Co(OH)2 and its application as a pre-catalyst for the oxygen evolution reaction

Abstract

Herein, a mixed-phase nickel boride/cobalt oxide/cobalt hydroxide, Ni_xB/Co₃O₄/Co(OH)₂, composite was formed to give a scalable and non-precious pre-catalyst for the oxygen evolution reaction (OER) in alkaline medium. The material was synthesised via a simple chemical reduction method, where amorphous Ni_xB particles were deposited onto cobalt oxide rods undergoing partial transformation into Co(OH)₂ sheets. This approach suppresses the agglomeration of Ni_xB particles, increasing surface accessibility. The optimised composite (Ni_xB/Co₃O₄-100 mg/Co(OH)₂) achieved a low overpotential of 370 mV at 100 mA cm⁻². It retained 99.2% of its activity after more than 90-h of continuous operation at 100 mA cm⁻² for the OER in 1.0 M KOH, indicating excellent stability. Electrochemical impedance analysis revealed a 20-fold increase in the electroactive surface area compared to the bare substrate. XPS analysis following stability testing under OER conditions confirmed a full surface reconstruction, with the loss of boron from the surface and formation of catalytically active NiOOH and CoOOH species, indicating that Ni_xB, Co₃O₄ and Co(OH)₂ act as precursors to the true active phase. This work highlights a practical strategy for designing robust, non-precious OER catalysts through the phase reconstruction of boride/oxide hybrids.