Exploring efficient earth-abundant electrocatalysts to replace platinum for hydrogen evolution reaction (HER) is of great significance for sustainable energy generation and use.1-4 MoS2 is a promising HER electrocatalyst. It has been experimentally and theoretically demonstrated that the catalytic activity correlates with the active edge sites rather than the exposed basal planes area. Thus, one feasible strategy toward enhanced HER performance is to increase the number of active edge sites. Herein, we develop a template-free method to synthesize ultrasmall MoS2 nanoparticles (~3 nm) decorating porous N-doped carbon network (MoS2@PNCN) via pyrolysis of the dicyandiamide/ammonium tetrathiomolybdate mixture. The novel 3D architecture of MoS2@PNCN with rich porosity has a continuous conducting network and provides full contact between the catalyst and the electrolyte. Due to the abundant active edge sites of MoS2, this catalyst exhibits high HER activity with a low onset potential of -30 mV, a small overpotential of 104 mV at a current density of 10 mA cm-2, and a Tafel slope of 58.6 mV dec-1. Meanwhile, the ultrasmall MoS2 nanoparticles protected by few-layered carbon shells show a superior stability even after 10 h of continuous operations. This study opens up a new path for synthesis of 3D porous nanomaterials for electrocatalysis and beyond.
 H. Wang, Y. Cao, C. Sun, G. Zou, J. Huang, X. Kuai, J. Zhao, L. Gao. Strongly coupled molybdenum carbide on carbon sheets as a bifunctional electrocatalyst for overall water splitting, ChemSusChem, 2017, 10, 3540-3546.
 H. Wang, Y. Cao, G. Zou, Q. Yi, J. Guo, L. Gao. High-performance hydrogen evolution electrocatalysat derived Ni3C nanoparticles embedded in a porous carbon network, ACS Applied Materials & Interfaces, 2017, 9, 60-64.
 H. Wang, Q. Yi, L. Gao, Y. Gao, T. Liu, Y. Jiang, Y. Sui, G. Zou. Hierarchically interconnected nitrogen-doped carbon nanosheets for efficient hydrogen evolution reaction, Nanoscale, 2017, DOI: 10.1039/C7NR06374A.
 H. Wang, L. Gao. Recent developments in electrochemical hydrogen evolution reaction, Current Opinions in Electrochemistry, 2017, DOI: 10.1016/j.coelec.2017.09.025.
We thank the funding by National Natural Science Foundation of China (grant U1401248) and the financial support from the program of China Scholarship Council (no. 201706920081).