Rapid solid-state metathesis reactions for the formation of cobalt–iron monoboride solid-solutions and investigation of their water splitting electrocatalytic activity

Metal borides have received increased attention as potentially robust water splitting electrocatalysts. Some studies have reported synergistic electrocatalytic effects on hydrogen and/or oxygen evolution reactions (HER/OER) using mixed metal borides. This report describes the single-step, solvent-free, and rapid (few seconds) synthesis of a series of crystalline Co1−xFexB (x = 0–1) solid solutions in high isolated product yields (>80%) from exothermic, self-propagating solid-state metathesis (SSM) reactions between metal halides and elemental Mg/B reactants. Powder X-ray diffraction shows the Co1−xFexB products are single-phase with crystallite sizes near 60 nm. SEM/EDS and elemental analysis indicate products contain homogeneous Co/Fe distributions and form large micrometer-sized particle aggregates. The electrocatalytic HER with these well-structured crystalline Co1−xFexB materials in 1 M KOH shows increased HER activity at lower applied potentials as cobalt content increases. The OER activity of Co1−xFexB also generally shows improvement with increased cobalt content. Crystalline Co1−xFexB catalysts exhibit good long-term 24 h HER and OER stability in 1 M KOH. Post-electrochemistry Co1−xFexB analyses confirm the retention of product crystallinity after long term electrocatalysis.