Dispersing catalytically active metals as single atoms on supports offer an efficient pathway to minimize amount of precious metals. Although platinum (Pt) is highly active for HER, it is highly desirable to find ways to improve the HER performance and also keeping them stable during catalytic reactions while minimizing the Pt loading. Herein, we report a cathode polarization technique to disperse isolated single Pt atoms on beta-Mo2C as a catalyst for HER. The isolated Pt atoms partially occupy Mo sites in Mo2C lattice by forming Pt-Mo shells, which maximize the utilization ratio of platinum noble metals. The single atoms catalyst Mo2C/CFP-Act even exhibited 1.9 and 1.1 times higher current density (@ potential of 0.4 V vs. RHE) than that of Pt NPs catalysts (Mo2C/CFP-Pt) after mass normalization in both acidic and alkali solution. Furthermore, DFT calculations demonstrate that Mo2C/CFP-Act exhibits favorable Delta G(H*) for the adsorption and desorption of hydrogen. The high HER activity of the Mo2C/CFP-Act catalyst is related to the Mo-Pt center located in Mo2C matrix, where the electronic structure of the Mo-Pt centers more efficient in donating electron to the sigma* antiorbital of the H3O+ molecule. This study sheds new light on the HER catalysis mechanism of isolated metal atoms based on fundamental understanding in molecular governing factors. (C) 2020 Elsevier Inc. All rights reserved.