Catalytic systems based on nanospheres usually show satisfied catalytic activities due to their huge specific surface areas for active sites immobilization. However, their catalytic activities are usually restricted by the absence of active sites inside the nanospheres and the inadequate communications between the interior and exterior of the nanospheres. Therefore, it can be anticipated that great improvement will be achieved if a porous three-dimensional catalytic system can be designed, through which the activity sites can be distribute interior and exterior. Herein, polydopamine (PDA) NPs with uniform sizes of 300 nm are prepared. Then, the Ru nanoclusters (Ru NCs) are uniformly generated inside and on the surface of the PDA NPs to afford the bi-nanospheres (Ru-PDA NPs). After annealing, the porous carbonized Ru-PDA NPs (Ru-cPDA-750 NPs) with electroactive nitrogen atoms are finally obtained, exhibiting small overpotential (42 mV at the current density of 10 mA cm(-2)), low value of Tafel slope (35 mV decade(-1)) and robust stability for hydrogen evolution reaction (HER), which are comparable to that of Pt/C catalyst. Moreover, the study on the effect of Ru NCs amount on electrocatalytic ability reveals that RucPDA NPs loaded with 0.08 wt% of Ru NCs exhibit the best HER catalytic performance compared with the Ru-cPDA NPs loaded with other amount of Ru NCs (0.04-0.16 wt%). (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.