A new catalyst (Ni-Pd(OH)(2)/C) for hydrogen evolution reaction (HER) was prepared by coelectrodeposition of Pd(OH)(2)/C nanoparticles and Ni on a Cu substrate in two steps. Furthermore, the effect of Mo ions in alkaline solution (1 M NaOH) on the electrocatalytic activity of Ni-Pd(OH)(2)/C nanocomposite was studied as an in-situ activator for the HER. The various electrochemical methods were employed to study the HER activity of the investigated new catalyst, including linear sweep voltammetry (LSV), the steady-state polarization Tafel curves, electrochemical impedance spectroscopy (EIS) and chronoamperometry (CA). The electrochemical measurements showed that the Ni-Pd(OH)(2)/C nanocomposite as a catalyst for the HER has an excellent catalytic activity with good stability in alkaline solution. Furthermore, the rate constants of the forward and backward reactions of Volmer and Heyrovsky steps were estimated using Tafel-impedance data and revealed that the proton discharge electrosorption or Volmer reaction (k(1)= (6.8 +/- 0.7) x 10(-8) mol cm(-2) s(-1)) was the rate determining step (RDS) of the HER on the surface of Ni-Pd(OH)(2)/C nanocomposite. Also, it was observed that the presence of Mo ions in alkaline solution could significantly increase the HER activity of Ni-Pd(OH)(2)/C nanocomposite. The comparison of RDS rate constant value with surface roughness (R-f) of Ni-Pd(OH)(2)/C catalyst showed that its high activity toward the HER originated from both increase in the surface roughness (similar to 20%) and increase in synergistic effect (similar to 80%). (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.