Constructing Hydrangea-Like Iron-Cobalt Phosphides via Boron-Assisted Strategy as an Efficient Catalyst for Water Splitting at High Current Density

Abstract

Finding suitable bifunctional catalysts for industrial hydrogen production is the key to fully building a hydrogen energy society. In this study, we present a novel approach to modifying the surface morphology of electrodeposited cobalt phosphide (CoP). Specifically, we have developed a method to create a hydrangea-like structure of bimetallic cobalt-iron phosphide (B-CoFeP@CoP) through ion-exchange and NaBH4-assisted strategies. This catalyst exhibited excellent bifunctional catalytic capability at high current densities, achieving a current density of 500 mA cm⁻² at a small overpotential (387 mV for OER and 252 mV for HER). When assembled into an OWS electrolyzer, this catalyst showed a fairly low cell voltage (≈1.88 V) at 500 mA cm⁻² current density., Furthermore, B-CoFeP@CoP shows ceaseless durability over 120 h in both freshwater and seawater with almost no change in the cell voltage. A combined experimental and theoretical study identified that the unique hydrangea-like structure provided a larger electrochemically active surface area and more effective active sites. Further analysis indicates that during the OER process, phosphides ensure that bimetallic active sites adsorb more OOH ^* intermediates and further DFT calculations showed that B-Fe₂P and B-Co₂P acted as active centers for dissociation of H₂O and desorption of H₂, respectively, to synergistically catalyze the HER process.