Phase Boundary Engineering of Co 2 P‐CoP Branched Nanoparticles Enhances Cobalt Oxidation for Oxygen Evolution Electrocatalysis

ABSTRACT The boundaries between two different crystal phases contain atoms with unique electronic structures and coordination numbers that can significantly influence catalytic performance. Cobalt phosphide adopts Co 2 P and CoP crystal phases, and both are active for oxygen evolution reaction (OER), which offers the opportunity to improve catalytic activity through the creation of phase boundaries. Here we show that mixed‐phase Co 2 P‐CoP branched nanoparticles enriched with boundaries between the Co 2 P and CoP phases can be synthesized by controlled phosphidation of Co branched nanoparticles. We found that the slow transformation from Co 2 P to CoP is key to achieving Co 2 P‐CoP phase boundaries. These nanoparticles exhibit excellent OER performance with an overpotential of 240 mV that is 81 mV lower than that of a commercial RuO 2 standard, and is >3.5 times more active than the Co 2 P and CoP pure‐phase counterparts. Density functional theory calculations reveal that there is a partially positive charge stabilized on the Co atoms at the crystal phase boundaries that leads to enhanced OER activity. These results highlight the effectiveness of utilizing crystal phase boundaries in nanomaterials as a strategy for enhancing catalytic performance.