Cobalt-Engineered Multimetallic Phosphides with Switchable HER–OER Activity for Durable Anion Exchange Membrane Water Electrolysis

Multimetallic transition-metal phosphides (TMPs) have been widely studied for alkaline water electrolysis, but the element specific reconstruction of their metal centers under operating conditions remains unclear, which is important for realizing durable and high-performance anion exchange membrane water electrolysis (AEMWE). Herein, we identify the main roles of Co, Ni, and Fe in TMP nanoparticles (NPs) with a hexagonal M₂P (M = metal) structure for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Spectroscopic analyses reveal dynamic surface reconstruction leading to catalytic activation. Co-centered compositional variation emerged as a key factor influencing bifunctional activity in the multimetallic Co-Ni-Fe-P system, in line with composition-dependent electronic structure modulation revealed by computational analysis. The optimized Co-Ni-Fe-P NP catalysts achieved overpotentials of 159 and 223 mV at 10 mA cm⁻² for the HER and OER, respectively. In AEMWE single cells, this catalyst displayed current densities of 5.73 A cm⁻² (bifunctional) and 11.43 A cm⁻² (anode only) at 2.0 V, maintaining durability for 500 h at 1.0 A cm⁻². This performance rivals that of state-of-the-art platinum group metal (PGM) and non-PGM catalysts. This study correlates metal-center evolution with composition–activity relationships, providing insight for developing durable non-PGM catalysts for practical green hydrogen production.