Fabrication And Electrochemical Properties Of Tungsten-based Catalysts

With the limited storage and consumption of fossil energy,the environmental pollution caused by fossil energy is becoming more and more serious.Hydrogen energy has the advantages of good combustion performance,easy storage,and rich resources,so it has become the first choice for the development of sustainable energy.Electrolysis of water is a promising way of hydrogen production,which has the advantages of high efficiency and non-pollution.In the process of water electrolysis,it is necessary to employ high activity Pt and other noble metals catalysts to drive the cathodic hydrogen evolution reaction.In addition,IrO2 is an excellent catalyst with high efficiency and stability in the process of oxygen evolution reaction.Therefore,from the perspective of energy saving and cost reduction,it is an effective way to develop catalysts that can replace precious metals.However,the process of hydrogen production is accompanied by nitrogen reduction.Ammonia generated by nitrogen reduction is an important energy medium for agriculture and industry.Therefore,we can improve the performance of hydrogen evolution or nitrogen reduction by regulating catalysts.This research is also of great significance.According to the literature background research,the hydrogen evolution catalysts of electrolytic water are mainly concentrated in transition metal compounds,and the oxygen evolution/oxygen reduction catalysts are mainly concentrated in perovskite,spinel and layered oxyhydrogen compounds.The activity of the catalysts was improved by designing morphology,constructing interface/heterogeneous engineering,exploiting vacancy/defect engineering and doping heteroatoms.Therefore,by exploring these related electrochemical catalytic behaviors,we designed different catalysts from HER,NRR,and oxygen catalysis(OER,ORR)to discuss the improvement of the activity.It is mainly divided into the following three parts:1.W2C microspheres with the hollow structure were prepared by the solvothermal and CH4/H2 carbonization method.The phase change of the WO3 precursor in the process of carbonization was explored with different carbonization temperatures as an entry point.By carbonizing hollow WO3 microspheres at different temperatures,we found that there is a biological phase transition in the carbonization process.At different temperatures,the phase results of WO3 are complete different after carbonization.When the temperature was below 650?,it is mainly the reduction of WO3 by H2.On the contrary,CH4 gradually participates in the reaction and begins to carbonize,and the degree of carbonization increased with the increasing temperature.Eventually,the optimal HER catalyst was W2C,which had the lowest overpotential(153 mV),the smallest Tafel slope(67.8 mV dec-1),and low charge transfer resistance.The overpotential and impedance showed a negligible decay and related morphology come out slight deformation after 20000 cycles,which indicated that the catalyst was suitable as a cathode catalyst for water electrolysis and had certain commercial application value.On the basis of the previous step,Mo2C/W2C heterojunction catalysts were prepared by hydrothermal and CH4/H2 carbonization methods.The influence of the structure and composition of carbides on their electrochemical properties were investigated with different precursors.A series of characterization tests were carried out to study the changes in morphology and electrochemical properties.The precursor of MoO3/WO3 heterojunction was carbonized to obtain Mo2C/W2C.It was found that Mo2C/W2C possessed an obvious heterogeneous interface by TEM,which proved that Mo2C/W2C is a heterogeneous structure rather than a mixture phase.Furthermore,the overpotential of Mo2C/W2C in 0.5 M H2SO4 and 1.0 M KOH electrolyte was 140 mV and 132 mV respectively when the current density was 10 mA cm-2,and the performance of the catalyst decreased slightly after 5000 cycles.The performance of Mo2C/W2C in 1.0 m KOH is better than Pt/C at high current density,which indicates that Mo2C/W2C has higher activity and is a better choice for electrocatalytic hydrogen evolution catalyst.2.In the process of cathodic catalysis,HER and NRR reactions occur simultaneously,but they are competitive.WS2/WO2 heterostructure was constructed by modifying WS2 nanosheets for HER.The defects at the interface of the WS2/WO2 hetero structure were constructed by introducing WO2 into WS2/WO2,and the active sites at the edge were reduced,so as to inhibit HER and improve NRR activity.The XRD diffraction peaks of WS2/WO2,WS2,and WO2 are in coincidence with standard PDF cards,which proved the existence of the WS2/WO2 phase.The existence of WS2/WO2 heterostructure interface can be verified through the FTIM characterization analysis,and the disordered state of the lattice at the interface can be observed,and EDS mapping also showed that the three elements were uniformly distributed.More importantly,WS2/WO2 expressed excellent NRR activity.The existence of NH4+was detected with indole phenol blue indicator and the content of NH4+in the electrolyte was determined by UV.Compared with the three catalysts,the WS2/WO2 heterojunction showed a high NH4+yield of 8.53 ?gNH3 h-1 mg-1 and FE is as high as 13.5%.Also,the 15N2 experiment method and NMR calculation method were used to verify the source and data validity of N in NH4+.The relationship between structure and activity was analyzed by DFT theory.Based on the first principles,NRR analysis of the three structures showed that WS2/WO2 can form a stable adsorption structure with N2,and the activation energy of hydrogenation is low,so as to NH3 is easy to desorb and diffuse from the surface,which is conducive to the NRR reaction.The high NRR activity of the WS2/WO2 heterojunction catalyst was confirmed by theoretical calculation and experimental results.3.Based on the application of WS2 in the field of electrocatalysis,it had a good activity in HER.However,due to its limitations in oxygen catalysis,we introduced Co3O4 into WS2 nanosheets to explore more possibilities of catalysts.Different W-O bonds of WO3 were formed according to XPS data while Co3O4 was introduced,and the energy shift of S 2p decreased.Besides,it was found that the magnetic moment of Co3O4/WS2 was significantly increased by the measurement of magnetic susceptibility.Combined with the change of the valence state ratio of Co3+to Co2+in XPS,it was found that the content of Co3+was not only increased,but also the spin state changed accordingly from lower spin state to intermediate state or high spin state.The adjustment of electronic structure promoted the improvement of oxygen catalytic performance.The half-wave potential of Co3O4/WS2 was 810 mV,which was higher than that of Co3O4/C and WS2 catalysts.According to the performance of OER,the activity and stability of Co3O4/WS2 were improved greatly.The overpotential of Co3O4/WS2 was 330 mV when the current density was 10 mA cm-2,which was better than that of Co3O4/C(393 mV)and WS2(494 mV).Moreover,while the Co3O4/WS2 was applied to the zinc-air battery system,it also possessed an excellent activity,such as high-power density(174 mW cm-2),and decent charge-discharge stability.Therefore,this work offered very useful information for rational design of high-performance NRR electrocatalysts.