Preparation And Investigation Of Molybdenum Composite For Electrocatalytic Hydrogen Evolution

Hydrogen energy is an environmentally benign secondary energy source,and electrochemical water splitting is a promising hydrogen production technology.Nowadays,noble metal materials are outstanding water splitting catalysts,but the scarce resources and high expenses seriously impeded their application.Therefore,the investigation of low-cost and efficient non-noble metal catalysts is crucial.This research is based on the theory,design and synthesis of high-efficiency and high-stability molybdenum-based compounds and their composites,and systematically studies the performance of hydrogen evolution.The thermodynamic stability and hydrogen evolution activity of different molybdenum phosphide and molybdenum carbide（MoP,Mo₂C）faces were systematically studied by the density functional theory（DFT）calculations and the projector augmented-wave（PAW）technique.The calculation of the hydrogen adsorption free energy reveals the surface orientation changes and the Co doping effects of MoP and Mo2 C on hydrogen evolution reaction（HER）performance,which lays a good theoretical foundation for the structural design and performance control of related materials.Among them,the P-terminated（100）surface of MoP material is relatively stable,（111）and（110）surfaces have excellent HER activity,and Co doping enhances the HER activity of the（001）and（101）surfaces.The（111）,（101）and（001）surfaces of Mo2 C are relatively stable,and the HER performance of（101）surface is the best.Co doping has little effect on the HER performance of C sites,but can greatly improve the performance of Mo sites.MoP microstructures were synthesized by a temperature-programmed reduction method.The MoP/GC electrocatalysts were synthesized through a hydrothermal process.Thanks to the unique carbon coating structure and interaction with MoP,the activity and stability of the MoP/GC electrocatalysts have been significantly improved.In 0.5 mol·L^-1 H₂SO₄,MoP/GC exhibits a low onset overpotential of 47 m V,a Tafel slope of 54.0 m V·dec-1 and strong catalytic stability.The effect of different sintering temperature and carbon source cont ents on the performance of MoP/GC catalysts for hydrogen evolution was investigated.Taking into account both activity and stability of catalysts,the optimum sintering temperature and content of carbon source are determined for MoP/GC catalysts.Based on theoretical calculation results,the hierarchical CoP₃/NiMoO₄ nanosheets on Ni foam（CoP₃/NiMoO₄-NF）were synthesized by hydrothermal,annealing and phosphorization treatment as electrodes for overall water splitting.The effects of different hydrothermal temperature/time and Co/Mo molar ratios on the performance of CoP₃/NiMoO₄-NF electrodes were investigated.Taking into account both activity and stability of the electrodes,the optimum hydrothermal temperature/time and Co/Mo molar ratio were determined for CoP₃/NiMoO₄-NF electrodes.The CoP₃/NiMoO₄-NF（Co/Mo = 1/1）electrocatalyst reveals a low overpotential of 92 m V for HER and 347 m V for OER at 10 m A·cm^-2 in 1.0 mol·L^-1 KOH.Especially,the CoP₃/NiMoO₄-NF-2 exhibits exceptional performance for overall water splitting which presents a low cell voltage of 1.57 V at 10 m A ·cm-2,and outstanding durability which could maintain over 12 h.The reasons for the improvement of the CoP₃/NiMoO₄-NF catalytic performance are as follows: The high phosphorous content of CoP₃ offers numerous active sites that promotes the electrocatalytic activity;The hierarchical heterostructures consisting of the NiMoO₄ nanosheet arrays and flower-like CoP₃ nanosheets provide a superior specific surface area,which make more exposure of the active sites and facilitate the electron transfer;The conductive Ni foam not only provides a skeleton to support the hierarchical CoP₃/NiMoO₄ nanosheets,but also facilitates the flow of electrons between the NF and the hierarchical CoP₃/NiMoO₄ nanosheets during HER and OER processes.Based on theoretical calculation results,the heterostructure Mo₂C/Mo₃Co₃ C bouquet-like nanowire arrays on Ni foam（Mo₂C/Mo₃Co₃C-NF）were synthesized by hydrothermal and chemical vapor deposition process as electrodes for hydrogen evolution reaction.The effect of different hydrothermal/sintering temperature and Co/Mo molar ratios on the HER performance of Mo₂C/Mo₃Co₃C-NF electrodes were investigated.Particularly,the addition of Co to the molybdenum carbide adjusts the M-H bond strength and surface electronic structure,which improves the activity of HER.Taking into account both activity and stability of the electrodes,the optimum hydrothermal/sintering temperature and Co/Mo molar ratio were determined for Mo₂C/Mo₃Co₃C-NF electrodes.Electrochemical tests show that the Mo₂C/Mo₃Co₃C-NF（Co/Mo = 2/4）electrode has a high electrochemical active area,which is about 49 times of that of NF.As expected,the Mo₂C/Mo₃Co₃C-NF electrode exhibits a low onset overpotential of 24 m V and achieves an overpotential of 87 mV at the current density of 10 m A·cm^-2 in 1 M KOH.The Tafel slope is 50.7 m V·dec^-1,and after 5,000 cycles of accelerated aging test,there is almost no change in hydrogen evolution performance.