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Outstanding electrocatalysts for high‐efficiency water splitting demand not only the high intrinsic activity determined by the electronic structure but also a favorable mass transfer (electrolyte diffusion and bubble desorption) and strong structural stability. Here, a 3D core–shell electrocatalyst consisting of Co(OH)2 cavity array‐encapsulated NiMo alloy on the flexible carbon cloth substrate (Co(OH)2/NiMo CA@CC) is proposed. Density functional theory reveals that coupling NiMo with Co(OH)2 can better optimize the water adsorption/dissociation and hydrogen adsorption energies in hydrogen evolution reaction, and also accelerate the kinetics of oxygen evolution reaction. Based on this, the open porous structure of the outer Co(OH)2 cavity array further promotes the diffusion of the electrolyte into the heterogeneous interface between NiMo and Co(OH)2, significantly shortening the electron transfer pathways and exposing multiple active sites. In addition, the macroporous array structure accelerates the bubble evolution and desorption process, thus ensuring a rapid mass transfer. When served as bifunctional electrocatalysts toward alkaline overall water splitting, Co(OH)2/NiMo CA@CC delivers a current density of 10 mA cm−2 at a low cell voltage of 1.52 V. Results support the multiscale optimization of the surface/interface engineering induced by the macroporous array.

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