Multiphase synergy in a/c-Ru2P@Ru/C heterostructure enables efficient alkaline seawater electrolysis at industrial current density

“Time-amount” precise phosphidation successfully constructs amorphous/crystalline/metallic three-phase heterojunction. • Multiphase heterojunction functional synergy facilitates the spatial division of labor for HER pathway at the nanoscale. • Ru→Ru 2 P→a-Ru 2 P relay electron transfer mechanism ensures its excellent seawater HER at industrial current density. Sluggish water dissociation kinetics and stability issues caused by chloride ions pose severe challenges for hydrogen evolution reaction (HER) catalysts in alkaline seawater environments. This study proposes a design philosophy of “multiphase heterojunction functional synergy”. Through a precise microwave phosphidation strategy, a three-phase heterojunction catalyst composed of amorphous Ru 2 P, crystalline Ru 2 P, and metallic Ru (a/c-Ru 2 P@Ru/C) was successfully synthesized. Systematic modulation enabled controllable synthesis of the catalyst's microstructure (amorphous/crystalline phase ratio) and macroscopic composition. This catalyst demonstrates outstanding comprehensive performance in alkaline seawater media: it exhibits a low overpotential of only 36 mV at a current density of 10 mA cm −2 , maintains stable operation for over 200 h at a high current density of 500 mA cm −2 , and shows leading cell voltage for overall water splitting. A series of characterizations indicate that the successful formation of a three-phase heterostructure in a/c-Ru 2 P@Ru/C induces interfacial electron transfer from Ru to phosphides, thereby optimizing the electronic structure of active sites. This multiphase synergy achieves a “spatial division of labor” in the HER reaction pathway: electron-rich amorphous regions promote water dissociation, while electron-modulated metallic sites accelerate hydrogen desorption. Concurrently, the robust heterointerface ensures long-term stability under harsh seawater conditions.