Developing atomically precise catalysts for hydrogen evolution reaction (HER) will help understand the catalytic reaction mechanisms for guiding the synthesis of new HER catalysts. A Cu-based metal-organic framework (Cu-II-MOF) consisting of trinuclear mixed-ligand units was prepared under hydrothermal conditions, and the structure, morphology, and elemental composition were characterized by (single crystal) XRD, TEM, SEM, XPS, BET, TGA and ATR-FTIR. Doping of 9.2 wt% Cu-II-MOF powder into the carbon paste electrode caused a roughly 440 mV positive shift of the HER potential at 10 mA cm(-2) in 1.0 M H2SO4. The Cu-II-MOF showed a high working stability due to the dual-ligand-protected Cu active centers. The intrinsic catalytic mechanism was investigated by Tafel slope analysis, solid state electrochemistry and electrochemical impedance spectroscopy. The Cu-II ions in the Cu-II-MOF are successively reduced through two single-electron steps over a potential range of 0.7 to 0.1 V vs. RHE to form single-atom Cu-0 active centers, on which adsorbed H atoms can be generated with a positive onset potential of about 100 mV vs. RHE through a following chemical redox reaction. The mechanistic findings point out the necessity and feasibility of developing bicomponent or bifunctional MOF-based catalysts to efficiently accelerate both the first and second electron transfer steps of HER. (C) 2019 Elsevier Ltd. All rights reserved.