Unsaturated crystalline water-induced structural transformation of cobalt phosphate for enhanced oxygen evolution

The coordination environment of Co center sites has a crucial influence on the oxygen evolution reaction (OER) performance of Co-based materials. However, the structure-performance relationship of Co-based OER electrocatalysts still remains unclear. Herein, cobalt phosphate with an unsaturated coordination structure was successfully prepared by partial removal of crystal water from Co₃(PO₄)₂·8H₂O while other features including chemical composition and crystal structure remain unchanged. The tetrahedrally coordinated cobalt phosphate obtained after electrochemical activation exhibits markedly superior OER activity, with a low overpotential of 327 ± 6 mV at 10 mA cm^-2, compared with octahedral-coordinated Co₃(PO₄)₂·8H₂O (381 ± 5 mV). In situ Raman spectra and methanol oxidation reaction experiments demonstrate that the unsaturated coordination environment facilitates the adsorption of surface intermediates and contributes to the formation of highly active CoOOH species under OER conditions.