Design And Synthesis Of Advanced Molybdenum Disulfide Based Electrocatalyst For Hydrogen Evolution Reaction

The consumption of fossil fuels continuously aggravates the problem of environmental pollution and causes tension in the international energy situation.Therefore,the development of clean and renewable energy to replace fossil fuels urgently needs to be put on the agenda.Hydrogen is one of the most ideal energy carriers in the future renewable energy because of its advantages of a wide range of sources,renewable cycles,high combustion heat value and zero pollution.It can be developed to deal with the increasingly serious energy and environmental crises.Among the currently known hydrogen production methods without fossil fuel,the hydrogen evolution reaction of electrolyzed water has the advantages of cleaner and more efficient.Extensive use of the abundant water resources on the earth to produce clean and pollution-free hydrogen energy has become a widely concerned technology to hydrogen production.However,the high cost of hydrogen production by electrolyzed water,the high cost and the scarcity of earth reserves of platinum-based noble metals catalyst for hydrogen evolution reaction with the most superior performance at present,have greatly restricted the further industrial development of hydrogen evolution reaction by electrolyzed water.One of the current breakthroughs lies in the development of highly active and stable non-noble metal electrocatalysts for hydrogen evolution reaction with abundant earth reserves and low prices.Molybdenum disulfide（MoS₂）is considered as the most likely non-noble metal catalyst to replace platinum because of its low cost and abundant reserves,which is close to the free energy of hydrogen adsorption of platinum and can exist stably in strong acid environment.Current research generally believes that the active site of MoS₂ is located at the edge,and due to the two-dimensional graphene-like structure and semiconductor properties,MoS₂ is still plagued by problems such as difficult exposure of the active site and weak conductivity,which limit the improvement of its electrocatalytic performance.Therefore,in view of the shortcomings of MoS₂,such as limited active sites and weak conductivity,this paper adopts a series of strategies to design and construct the catalyst based on the titanium black-molybdenum disulfide（Ti₄O₇-MoS₂）catalyst developed by the team in the early stage.The highly active MoS₂-based electrocatalyst for hydrogen evolution is optimized and prepared by increasing the number of catalytic active sites,expanding the effective reaction area,adjusting the phase structure,improving the charge injection and migration rate and improving the stability of the catalyst.The main research contents are as follows:（1）The carbon nitride-titanium black-molybdenum disulfide composite catalyst（C₃N₄-Ti₄O₇-MoS₂）was prepared by a simple hydrothermal synthesis method.The effects of the addition of carbon nitride（C₃N₄）on the structure and hydrogen evolution performance of electrocatalyst were investigated,and the interaction and synergistic effects among different species in the catalysts were discussed in detail.The research results show that after being compounded with C₃N₄ and Ti₄O₇,the composite catalyst obtains more active sites and interfacial charge redistribution,and has a moderate surface hydrogen adsorption kinetics and better stability without affecting the morphology of MoS₂.Therefore,it shows significantly improved electrocatalytic hydrogen evolution activity,i.e.,the overpotential is 251 mV at 10mA·cm^–2,and the Tafel slope is 54 mV·dec^–1,which is greater than that of the MoS₂.In addition,the interface effect after the introduction of C₃N₄ and Ti₄O₇ enables the composite catalyst to maintain stable catalytic performance after a long-term hydrogen evolution reaction.（2）The 1T-2H mixed phase MoS₂ was prepared in situ by a simple hydrothermal method,and carbon quantum dots（CQDs）and Ti₄O₇ were introduced into it to construct a carbon quantum dot-titanium black-molybdenum disulfide nanocomposite catalyst（CQDs-Ti₄O₇-MoS₂）.The promotion mechanism of the chemical bonds between different components on the hydrogen evolution performance of the composite catalyst was discussed in detail.The research results show that the CQDs of the surface oxygen-rich groups aggravated the relative displacement of the sulfur layer in MoS₂,and obtained the mixed phase MoS₂ with a larger proportion of 1T phase.The existence of 1T phase makes the composite catalyst exhibit better conductivity and catalysis performance.Moreover,the abundant oxygen-containing groups of CQDs combine with MoS₂ to form intermolecular chemical bonds,which promotes charge transfer between the interfaces and improves the weak conductivity of MoS₂.In addition,the introduction of CQDs also introduces more active sites into the inert basal plane of MoS₂,making the composite electrocatalyst exhibit superior electrocatalytic hydrogen production activity compared with bare MoS₂.The electrochemical test results show that the overpotential of the CQDs-Ti₄O₇-MoS₂ composite catalyst is 241 mV at 10 mA·cm^–2,the Tafel slope is 48mV·dec^–1,and the exchange current density reaches 5.31μA·cm^–2,which realizes a more efficient electrocatalytic hydrogen evolution process.Furthermore,CQDs and Ti₄O₇ effectively stabilizes the phase state of MoS₂,which make the composite catalyst excellent stability,showing that the degradation of hydrogen evolution performance was almost negligible after50 h continuous test and 10,000 CV cycle test.（3）The pretreatment hydrothermal method was used to prepare a more efficient composite electrocatalyst NCQDs-Ti₄O₇-MoS₂ through in situ modification of Ti₄O₇-MoS₂ with nitrogen-doped carbon quantum dots（NCQDs）.The promotion mechanism of the introduction of NCQDs on the hydrogen production performance of the composite catalyst was discussed in detail,and the multi-functional effect of NCQDs was explored.The results show that NCQDs in the system can regulate the structure of the catalyst,control the crystal size of the catalyst,and introduce more defects into the basal plane of MoS₂,which improve the efficiency of water electrolysis.In addition,CQDs and surface nitrogen sites can also tune the surface electronic structure of the catalyst,which act as an electron transport channel to accelerate the electron migration and mass transfer process.At the same time,the strong interaction between NCQDs and MoS₂ improves the tolerance of the catalyst.The electrochemical test results show that the onset potential of the composite catalyst is 162 mV,the overpotential is only 218 mV at 10mA·cm^–2,and the Tafel slope is 46 mV·dec^–1.Moreover,the composite catalyst can still maintain excellent hydrogen evolution stability after 10,000 CV cycles and 30 h durability test,indicating that the composite catalyst has excellent performance of electrocatalytic hydrogen evolution.