Structurally disordered CoS_x-Co(OH)₂ heterointerface for boosting alkaline hydrogen evolution reaction

The kinetics of the hydrogen evolution reaction (HER) process in alkaline media are inherently slower than in acidic media, resulting in a significant need for high-performance, durable, and cost-effective electrocatalysts. Structurally disordered heterostructure nanomaterials bring together the advantages of disordered and composite materials. Accordingly, this study demonstrates a structurally disordered cobalt sulfide/hydroxide (CoS_x-Co(OH)₂) heterostructure framework with an intimate interface between two distinct phases, establishing a highly active environment. The heterostructure catalyst is in situ coated on a three-dimensional (3D) microporous Cu foam (CF) skeleton using a one-step electrodeposition approach at a high cathodic current. The intimate, binder-free electrical connection between active components and highly conductive 3D Cu scaffold dramatically decreases interfacial resistance and promotes rapid electron penetration throughout the catalyst system. Consequently, the CoS_x-Co(OH)₂/CF catalyst acquires a current density of 10 mA cm^-2 at an overpotential of 88 mV vs. RHE in a 1 M KOH, markedly surpassing Co(OH)₂. Besides, it displays a favorable Tafel slope of 81.1 mV dec^-1, low charge-transfer resistance, an enlarged electrochemically active surface area, and exceptional durability for 1000 cycles and 500 h of continuous operation. In addition to half-cell tests, a compact zero-gap alkaline electrolyzer, utilizing the CoS_x-Co(OH)₂/CF cathode and NiFe LDH anode, exhibits stable operation at 500 mA cm^-2 at 60 °C for ⁓240 h, indicating the practical feasibility. Density functional theory (DFT) calculation results suggest that the heterointerface generates a robust built-in electric field, facilitating the electron transport from CoS_x to Co(OH)₂, downshifting the Co d-band center, and enhancing the Gibbs free energy of hydrogen adsorption (ΔGH*). The resultant "adsorb-dissociate-desorb" synergy, where water dissociation is promoted on Co(OH)₂ and H* conversion is strengthened on CoS_x, verifies the expedited kinetics of alkaline HER. Therefore, this synergistic integration offers a versatile platform for regulating active sites accessibility, combining high catalytic reactivity and exceptional durability throughout economical, non-precious metal schemes.