Three molecular catalysts based on mononuclear nickel(II) complexes with square planar geometries, [BzPy]₂[Ni(mnt)₂] (1), [BzPy]₂[Ni(i-mnt)₂] (2), and [BzPy]₂[Ni(tdas)₂] (3) (BzPy = benzyl pyridinium) are synthesized by the reaction of NiCl₂∙6H₂O, [BzPy]Br, and Na₂(mnt)/Na₂(i-mnt)/Na₂(tdas) (mnt = 1,2-dicyanoethylene-1,2-dithiolate for (1), i-mnt = 2,2-dicyanoethylene-1,1-dithiolate for (2), and tdas = 1,2,5-thiadiazole-3,4-dithiolate for (3)), respectively. The structures and compositions of these three catalysts are characterized by XRD, elemental analysis, FT-IR, and ESI-MS. The electrochemical properties and the corresponding catalytic activities of these three catalysts are studied by cyclic voltammetry. The controlled-potential electrolysis with gas chromatography analysis confirms the hydrogen production with a turnover frequency (TOF) of 116.89, 165.51, and 189.16 moles of H₂per mole of catalyst per hour at a potential of − 0.99 V (versus SHE) in acetonitrile solutions containing the catalysts, respectively. In a neutral buffer solution, these three molecular catalysts exhibit a TOF of 411.85, 488.76, and 555.06 mol of H₂per mole of catalyst per hour at a potential of − 0.49 V (versus SHE), respectively, indicating that Complex3constitutes the better active catalyst than Complexes1and2. For fundamental understanding, a catalytic HER mechanism is also proposed.Graphical abstract