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[Display omitted] • CuNi@Cu with porous core–shell structure is obtained in one step. • The CuNi@Cu self-supported electrode can be scaled up for industrial applications. • CuNi@Cu exhibits high activity and stability in alkaline Ampere-level HER. • Bubbles are fine and easy to release due to the confinement and capillary effects. • CuNi@Cu exhibits a good application prospect in seawater electrolysis. Designing cheap, efficient, and stable electrocatalysts for hydrogen evolution reaction (HER) under ampere-level current density is essential for the industrial water electrolysis technique. However, the large H 2 bubbles produced at high current density decrease the activity and stability of the catalyst. Here, a self-supported electrode with porous yolk-shell CuNi@Cu electrocatalyst is obtained via an alternant electrodeposition and replacement reaction during the pulse electrodeposition process. The porous Cu shell can boost electrolyte diffusion and serve as an H 2 bubble fining sieve to enhance the activity and stability of alkaline HER at high current density with the low overpotential of 326 mV @1 A cm−2. The CuNi@Cu electrocatalyst can work steadily for 1000 h at 1 A cm−2 with negligible attenuation. The porous Cu shell can also repel cations in natural seawater owing to the positively charged surface. The spontaneous alternant reaction paves an avenue to design electrocatalysts with porous yolk-shell structures for large-scale industrial applications. [ABSTRACT FROM AUTHOR]