Diffusion-Regulated Cathodic Electrodeposition of Cr-Fe-Co-Ni Multicomponent Amorphous Electrocatalysts on a Nickel Foam for Integrated Alkaline Water Splitting

High-entropy and medium-entropy alloys are attractive electrocatalysts for alkaline water splitting, yet it remains difficult to fabricate compositionally uniform MEA coatings on porous nickel foams by conventional routes. Here, we address this challenge using a diffusion-regulated, potential-controlled electrodeposition strategy enabled by a microphase-separated 1-butyl-3-methylimidazolium chloride-H₂O mixture as the electrolytic solution, which restricts multi-ion mass transport and promotes synchronized co-deposition. By tuning the applied cathodic potential, we selectively obtain either an amorphous Cr-Fe-Co-Ni MEA coating or a Cr-Fe-Co-Ni hydroxide layer directly on a nickel foam within a single electrolyte bath. The electrodeposited Cr-Fe-Co-Ni exhibits a homogeneous amorphous structure with a coral-like porous morphology and a markedly increased electrochemically accessible surface area, leading to the efficient hydrogen evolution reaction activity (η₁₀ = 71.1 mV; η₁₀₀ = 193 mV). The Cr-Fe-Co-Ni hydroxide@nickel foam delivers the strong oxygen evolution reaction performance (η₁₀₀ = 220 mV). When integrated as a Cr-Fe-Co-Ni ǁ Cr-Fe-Co-Ni hydroxide electrolyzer, the device achieves low cell voltages (1.55 V at 10 mA cm⁻²; 1.84 V at 100 mA cm⁻²) and stable operation at 100 mA cm⁻² for 24 h. This work provides a simple and scalable route to integrated medium-entropy alloys/hydroxide electrodes for durable overall water splitting.