Hollow engineering of CoP with facilitated phase reconstruction for efficient electrocatalytic HMF oxidation

Co-based materials usually undergo in situ surface reconstruction during oxidation reactions, forming high-valent Co³⁺/Co⁴⁺ species as the true active sites. However, conventional bulk structures of Co-based materials hinder deep phase transformation, limiting the utilization of internal Co sites and suppressing catalytic efficiency. Here, we report the hollow engineering of cobalt phosphide (CoP) to facilitate exposure of Co sites and promote in situ transformation to Co³⁺/Co⁴⁺ active species for enhanced oxidation activity. Hollow CoP (H-CoP) is derived from ZIF-67 via controlled etching and phosphorization, with electrochemically active surface area 2.1 times that of conventional solid CoP (S-CoP). H-CoP achieves a current density of 10 mA cm⁻² at a lower potential (1.26 V <em>vs.</em> reversible hydrogen electrode, RHE) in 5-hydroxymethylfurfural oxidation reaction (HMFOR), with a HMF conversion of 99.5%, 2,5-furandicarboxylic acid yield of 98.6%, and Faraday efficiency of 97.5% at 1.45 V (<em>vs.</em> RHE), much superior to S-CoP. When applied as a bifunctional catalyst in the HMFOR coupled with hydrogen evolution reaction (HER) electrolyzer, H-CoP requires an ultralow voltage of 1.64 V to reach 10 mA cm⁻², with the cell voltage reduced by 190 mV compared to the conventional oxygen evolution reaction coupled with HER water splitting system.