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Alloys, especially Mo–Ni alloys, are excellent HER electrocatalysts in alkaline solution. The HER activity of the Mo–Ni alloys can be improved by grafting H2O dissociation promoters (metal oxides or hydroxides) to promote the dissociation of H2O in alkaline solution. However, most of the reported Mo–Ni alloys grafting with H2O dissociation promoters only have simple one-dimensional (1D) structures, which cannot further improve the catalytic performance. Compared with 1D nanomaterials, two-dimensional (2D) nanomaterials have higher specific surface area, electrical conductivity, more exposed active sites and mass transfer channels. Herein, a novel 2D nanosheets self-supporting composite electrocatalyst MoNi4/MoO2.5-NF derived from the NiMoO4 nanosheets was reported for the first time. Benefiting from the unique 2D nanosheets structure and the synergistic effect of each component, the catalyst shows high electrocatalytic HER activity in 1 M KOH solution. The MoNi4/MoO2.5-NF needs overpotential of 49 mV to reach 10 mA cm−2. The HER activity of MoNi4/MoO2.5-NF can be further improved through electrochemical-cycling activation (CV) process. After 10 CV cycles, the overpotential to reach 10 mA cm−2 of the catalyst decreased from 49 to 27 mV and even exceeded 20% Pt/C (32 mV). The improvement of HER performance is attributed to the increase in the electrochemically active surface area, the reduced impedance and Tafel slope after CV process. [ABSTRACT FROM AUTHOR]