Developing efficient and durable hydrogen evolution reaction (HER) electrocatalysts is critical for industrial and sustainable hydrogen production. Herein, a simple co-precipitation strategy is proposed to successfully construct catalysts with a Mott–Schottky heterojunction by coupling a transition-metal phosphate to the surface of stripped MXene thin-layer nanosheets (M3(PO4)2@MXene, M = Co, Ni, and Fe). The Co3(PO4)2@MXene with a unique tightly connected 2D/2D heterostructure and built-in electric field induces directional electron transfer at the interface, regulates the polarized structure of the active sites, and accelerates both mass and electron transport. Consequently, the optimized Co3(PO4)2@MXene demonstrates outstanding HER performance, achieving low overpotentials of 46 and 58.6 mV at 10 mA cm−2 in alkaline freshwater and seawater electrolytes, respectively. Moreover, the Co3(PO4)2@MXene heterojunction catalyst maintains stable operation at a high current density of 500 mA cm−2 for over 100 h in alkaline seawater electrolytes. More importantly, Co3(PO4)2@MXene can offer a low potential of 1.71 V at 500 mA cm−2 with stable operation for 50 h in a flow-type alkaline seawater electrolyser. This study provides a unique heterostructure in an electrocatalyst for an efficient HER and presents its potential application in seawater electrolysis.
