Study On The Electrocatalytic Performance Of Polyoxometalates Enhanced MoS_x

People’s daily life depends on a large amount of consumption of traditional fossil fuel.Hydrogen can be used as a substitute of traditional fossil fuel because of its non-toxic,being friendly to environment and high combustion calorific value.Among the technologies of hydrogen production,electrolysis of water is a relatively convenient and efficient hydrogen production method because of its simple process,high purity of product and fast production rate.The key to improve the efficiency of hydrogen production by electrolyzing water is to choose a nice electrocatalytic electrode.Among many electrode materials,molybdenum sulfide has attracted much attention because of its excellent electrocatalytic efficiency.At the same time,polyoxometalates have been found that can be used to enhance the electrocatalytic performance.In this thesis,ammonium tetrathiomolybdate（（NH4）2MoS4）was used as the precursor of MoS_x preparation,and SiW₁₂(H₄[Si(W₃O₁₀)₄]·xH₂O,SiW₁₂)which could maintain its structure in a wide range of pH value was selected to enhance the performance of MoSx.SiW12 enhanced MoSx electrocatalytic electrode was prepared on FTO conductive glass by periodic reverse pulse electrodeposition.The works is as follows:1.Explore the codeposition parameters of MoS_x and Si W₁₂In order to achieve the codeposition of MoS_x and SiW₁₂,the codeposition conditions of MoS_x and SiW₁₂2 were explored.The redox potential of（NH₄）₂MoS₄,SiW₁₂2 and their combination in the solution was measured by square wave voltammetry（SWV）.Finally,the potential of-1.1 V,which was negative than the redox potential of each solution and with less hydrogen evolution,was chosen as the low level potential of MoSx and SiW12 codeposition.The potential of zero charge of FTO conductive glass in（NH₄）₂MoS₄ and SiW₁₂2 solution was measured by AC voltammetry,and+0.1 V was chosen as the high level potential of codeposition.The prepared MoSx:SiW12 electrodes’SEM,XPS and XRD showed they can be successfully prepared by the above experimental methods.2.Preparation of MoS_x:SiW₁₂2 and study on the electrocatalytic performance of themFirstly,MoSx electrodes with different deposition time were prepared in a fixed concentration of（NH4）2MoS4 solution,and their polarization curves,EIS,Tafel diagram and interface capacitance in sulfuric acid standard solution were measured.The optimal deposition time of MoSx was 260 s.Then MoSx:SiW12 electrodes with different concentration of Si W₁₂2 were prepared at the optimum deposition time and their electrocatalytic hydrogen production performance was tested.The optimum doping concentration of SiW12 was 25μmol/L.Under the fixed deposition time and SiW12 concentration,the effect of different（NH4）2MoS4 concentration on the catalytic performance was studied,and the optimal concentration of（NH₄）₂MoS₄ was 2.5 mol/L.Under the condition of fixed deposition time,the relationship between the number of units and the catalytic performance was explored with（MoSx/SiW12）as the unit.The hydrogen evolution stability of MoSx electrode and MoS_x:SiW₁₂2 electrode were tested,in this experiment,the sample doped with Si W₁₂2 has better stability.These experimental results showed that SiW12 played the role of electronic bridge in electrocatalysis and can enhance the performance of electrocatalytic hydrogen evolution to a certain extent.3.Influence of interface modification of SiW12 on electrocatalytic performanceA layer of Si W₁₂2 modified layer was added between the FTO conductive glass and MoS_x:SiW₁₂,the thickness of MoS_x:Si W₁₂2 layer was changed and its electrocatalytic hydrogen evolution performance was tested.In this experiment,it was found that the catalytic efficiency was still improved under the condition of limited mass transfer,which indicated that the modified layer might play a role in enhancing electronic transport and reducing carrier recombination.Under the optimum deposition time of MoS_x:SiW₁₂,the best deposition time of SiW12 was found that was 50 s by changing the deposition time and testing their electrocatalytic performance.In order to study the mechanism of SiW12 interface modification,the Mott Schottky curve of MoS_x electrode,MoS_x:SiW₁₂2 electrode and SiW₁₂/MoS_x:SiW₁₂electrode were tested by AC voltammetry at different frequencies in sulfuric acid standard solution.It was found that the doping of SiW12 could make the electron enrichment area and the flat band potential of MoSx moving in the direction of positive potential,thus reducing the over potential of electrocatalysis.And the SiW₁₂2 modified layer could reduce the carrier recombination and enhance the electron transport.The stability of hydrogen evolution of SiW12/MoSx:SiW12 electrode was tested.It was found that the decrease of the stability caused by SiW12 modified layer due to its lack of adhesion.In this work,the electrocatalytic performance of Si W12 enhanced MoSx was studied in the aspects of doping and interface modification.It was found that SiW12 played a role in bridging electrons,enhancing electron transport and reducing carrier recombination in electrocatalysis,the shortcomings of SiW12 in interface bonding were pointed out.These works provided a reference for the enhancement of the electrocatalytic performance of MoSx and the application of polyoxometalates in electrocatalysis.