Synthesis Of Molybdenum-based Composites And The Study Of Their Lithium Storage Properties And Hydrogen Evolution Reaction Activities

With the increasing demand for energy and the depletion of traditional energy sources,coupled with the increasing environmental problems caused by traditional energy sources,it is an urgent need to develop and use clean and renewable new energy sources such as solar energy,wind energy,tidal energy,hydrogen energy and so on.Among them,solar energy,wind energy,and tidal energy can only provide intermittent energy output,thus high energy density energy conversion equipments are required to store it.Lithium-ion batteries are such ideal energy storage devices due to their high energy storage density and excellent cycle performance.As for the developing of hydrogen energy,the preparation methods of hydrogen gas is one of the main factors for restricting its wide use.The electrolysis of water through hydrogen evolution reaction?HER?is considered to be the next generation of efficient and clean hydrogen production technology.This paper focuses on the structure design and we have fabricated two molybdenum-based composites.The lithium storage properties as anodes and HER catalytic activities of these composites are studied respectively,as well as the advantages of our structural design.The main contents and innovations are as follows:1.We report a facile approach to prepare oriented MoS₂ nanoflakes on nitrogen-doped carbon nanosheets?MoS₂/NC?through heat treatment and hydrothermal reaction,by using DDA intercalated MoO₃?MoO₃/DDA?as precursor.XPS analyse confirmed the existence of C-S bond in MoS₂/NC,meaning that MoS₂ nanoflakes have a strong interfacial interaction with the NC matrix,which may result in good structural stability of MoS₂/NC electrode during cycles.When used as anodes of lithium ion battery,MoS₂/NC shows better electrochemical lithium storage performance than pure MoS₂.The reversible capacity of MoS₂/NC electrode after 100 cycles can reach to 722 mAh g^-1 at a current density of 100 mA g^-1.When the current density is increased by 20 times,it still has a reversible capacity of 554mAh g^-1,showing high rate performance.The enhanced electrochemical lithium storage performance of MoS₂/NC can be attributed to its unique structural design as follows:?1?The flexible NC substrate can not only relieve the stress of the electrode caused during lithiation/delithiation processes,but also provide a long-term continuous conductive network,guarantying the excellent lithium storage performance at high current densities.?2?The vertically aligned MoS₂ nanoflakes on the NC substrate is parallel to the transmission path of Li⁺,which is favorable for fast Li⁺diffusion inside MoS₂/NC electrode.What's more,the well-aligned MoS₂ nanoflakes with the proper interspace can alleviate the huge expansion during the insertion of lithium,resulting in high structure stability of MoS₂/NC electrode.?3?The strong interaction between MoS₂ nanoflakes and NC matrix further avoids the structural collapse of MoS₂/NC during long cycles.2.W-doped MoP nanosheets vertically grown on carbon cloth?W-MoP/CC?was designed and synthesized by a simple hydrothermal treatment and further phosphating process.Carbon cloth was used as raw material.W-MoP/CC exhibited excellent catalytic activity in the HER tests,in which the overpotential to achieve a current density of 10 mA cm^-2 and the Tafel slope in 0.5 M H₂SO₄ are 97 mV and 65.5 mV dec^-1,respectively.What's more,during a long time HER operation of 18 h,the catalytic performance of W-MoP/CC shows no significant reduction.The excellent HER activity of W-MoP/CC can be explained as follows:?1?The fine W-MoP nanosheets grown on the carbon cloth vertically can expose catalytically active sites as many as possible.?2?Doping of W can introduce defect sites into MoP,which are also catalytically active centers for HER.?3?W doping also increased the electrical conductivity of MoP,resulting in promoted electron transfer ability of W-MoP/CC during HER process.