Mesoporous molybdenum carbide for greatly enhanced hydrogen evolution at high current density and its mechanism studies

Currently the catalysis of hydrogen evolution reaction(HER)is mainly focused on the inherent electrocatalytic activity at relatively lower current densities while scarce at high current densities.Nevertheless,the latter is highly demanding in efficient mass-production of hydrogen.A SiO_(2) nanospheres template-synthesis is used to prepare mesoporous molybdenum carbide nanocrystals-embedded nitrogen-doped carbon foams(mp-Mo_(2)C/NC).The material shows much more excellent catalytic activity than the non-etched Mo_(2)C/NC toward hydrogen evolution reaction(HER)in acidic medium.More interestingly mp-Mo_(2)C/NC still has larger overpotential than Pt/C at lower current densities,but possess remarkably smaller overpotential than the latter at higher current densities for much better electrocatalytic performance.An approach is developed to investigate the electrode kinetics by Tafel plots,especially with eliminating the diffusion effect,indicating that Pt/C and mp-Mo_(2)C/NC display different reaction mechanisms.At low current densities the former presents reversible reaction,while the latter shows mixed electrochemical polarization/reversible electrode process.In the region of higher current densities,the former becomes totally gas-diffusion controlled with large overpotential,while the latter can still retain an electrode polarization process for much lower overpotential at the same current density.Result endorses that the meso-porously structured mp-Mo_(2)C/NC plays a critical role in avoiding gas diffusion control-resulting large overpotential at high current densities.This work holds great potential for an inexpensive catalyst better than Pt/C in practical applications of mass-production hydrogen at high current densities,while clearly shedding fundamental lights on designs of rational HER catalysts for the uses at high current densities.

MoS2 nanosheet arrays supported on hierarchical porous carbon with enhanced lithium storage properties

MoS_2 nanosheet arrays supported on hierarchical nitrogen-doped porous carbon（MoS_2@C） have been synthesized by a facile hydrothermal approach combined with high-temperature calcination.The hierarchical nitrogen-doped porous carbon can serve as three-dimensional conductive frameworks to improve the electronic transport of semiconducting MoS_2.When evaluated as anode material for lithium-ion batteries,the MoS_2@C exhibit enhanced electrochemical performances compared with pure MoS_2 nanosheets,including high capacity（1305.5 mAhg^（-1） at lOOmAg^（-1））,excellent rate capability（438.4mAhg^（-1） at 1000mAg^（-1））.The reasons for the improved electrochemical performances are explored in terms of the high electronic conductivity and the facilitation of lithium ion transport arising from the hierarchical structures of MoS_2@C.