Research on highly efficient catalysts for electrochemical hydrogen evolution reaction (HER) remains a challenge. In this work, we successfully wrap copper (Cu) inside of copper phosphide (Cu3P) nanoparticle to form a copper/copper phosphide (Cu/Cu3P) core/shell structure attached on carbon nanotubes (CNTs) for enhanced HER activity in acid. The average size of the core/shell particles is around 25 nm, with about 5 nm of Cu3P as the outer layer. The catalytic activity of the core/shell structure is significantly promoted compared to the metallic Cu and Cu3P pure phases nanoparticles on CNTs, requiring overpotentials of 84 and 161 mV to achieve 10 and 100 mA cm(-2) of current density, respectively. The core/shell structure also presents high HER durability and stability, with the polarization curve overlapped after 5000 cycles of CVs and steady current density at 25 mA cm(-2) for as long as 10 h. To account for the promoted HER performance, the Cu/Cu3P structure is fully investigated by physical and electrochemical characterizations and density functional theory (DFT) calculations. The DFT results depict that the neutralized the adsorption Gibbs free energy of hydrogen atoms (Delta G(H)*) is induced by the electronic interactions between metallic Cu and phosphide phase. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.