Advanced interface engineering of manganese–doped biphasic nickel-cobalt phosphide heterostructures enables high-efficiency overall water splitting in freshwater and seawater media

Green hydrogen fuel gas produced via electrocatalytic overall water splitting (OWS) is emerging as a promising solution to facilitate the transition progress from fossil fuels to sustainable renewable energy sources, effectively addressing the current challenges of energy shortage and environmental pollution. To improve OWS efficiency, we herein develop an affordable and high-performance bifunctional catalyst derived from a hierarchy of biphasic phosphides of manganese-doped nickel‑cobalt phosphide (Mn–Ni₂P/Co₂P) heterostructures with tunable and synergistic interfaces. The material displays a low required overpotential (η) of 83 mV for cathodic hydrogen evolution, and 258 mV for anodic oxygen evolution, to achieve a current density of 10 mA·cm⁻² in 1.0 M KOH alkaline medium. The Mn–Ni₂P/Co₂P_{(+, −)} couple is applied to configure an electrolyzer cell, which can effectively deliver a low voltage of 1.44 V in alkaline freshwater and 1.47 V in alkaline seawater at 75 °C to reach a current response of 10 mA·cm⁻² and maintain high activity after long-term continuous operation. Theoretical study indicates that the Mn incorporation into the phosphide heterostructures leads to the modification of structure and electronic properties, resulting in high charge transfer, increased electroactive surface area, and accelerated reaction kinetics for a highly efficient bifunctionality. Although minor surface oxidation and temperature sensitivity are observed during extended operation, these changes indicate intrinsic self-stabilization and suggest opportunities for further improvement through refined interface and surface design. This innovative engineering strategy paves the way to achieve high-performance and cost-effective electrocatalysts towards sustainable hydrogen production via OWS technologies.