Copper is the prototype metal to produce hydrocarbons from CO2 electroreduction, however, rationale of the catalytic mechanism by copper nanoparticles has been severely impeded due to structural polydispersity. The recent efforts in the synthesis of nanosized copper clusters open up new opportunities for atomic-level understanding of the catalytic mechanism. Herein, we considered [Cu25H22(PH3)(12)]Cl cluster, bearing the highest ratio of hydride ligands, as a test model to examine its activity for CO2 electroreduction. Our DFT calculations showed that both the surface Cu and the hydride ligands are actively participated in the hydrogenation reactions and selectively reduce CO2 to HCOOH via the protonreduction channel and the hydride-proton channel, as opposed to the commonly produced CO on copper surface/nanoparticles. We also evaluated the competitive HER activity, and found that H-2 evolution requires higher barrier than HCOOH. Our studies highlight the important role of hydrides in directing the activity and selectivity for CO2 electroreduction. (C) 2020 Elsevier Inc. All rights reserved.