Interfacial Co–Ni Dual‐Site Synergy in Confined Co ₉ S ₈ –Ni ₃ S ₂

Although heazlewoodite‐type nickel sulfide (Ni ₃ S ₂ ) exhibits promising hydrogen evolution reaction (HER) activity, its application in overall water splitting remains limited by sluggish oxygen evolution reaction (OER) kinetics associated with insufficient water activation. Herein, Co ₉ S ₈ –Ni ₃ S ₂ heterostructures confined in nitrogen‐rich spongy carbons are constructed to regulate interfacial electronic properties and unlock Co–Ni catalytic synergy for efficient water electrolysis. A melamine sponge is employed as a 3D scaffold to direct the spatial anchoring of Co ²⁺ and Ni ²⁺ precursors, followed by in situ sulfidation and carbonization to form a conductive porous carbon framework with confined sulfide heterointerfaces. The resulting heterointerfaces trigger significant spin polarization and electron redistribution from Ni ₃ S ₂ to Co ₉ S ₈ through interfacial Co–S–Ni configurations, thus activating the Co–Ni dual sites for enhanced water adsorption and dissociation. The catalyst exhibits outstanding bifunctional OER/HER activity and long‐term stability in alkaline media, while an alkaline electrolyzer assembled with the catalyst requires only 1.515 V to deliver 10 mA cm ⁻² . Moreover, in situ and ex situ analyses reveal dynamic reconstruction of the Co–Ni sites under OER conditions, continuously generating additional active centers and sustaining high catalytic activity. This work paves a new way for designing earth‐abundant and high‐performance heterostructured electrocatalysts toward green hydrogen production.