Synthesis And Electrocatalytic Performance Study Of Carbon-supported Transition Metal Chalcogenides

Fuel cell is an energy conversion device that converts chemical energy into electrical energy,it has the advantages of higher specific energy,mild working conditions,less secondary pollution,and it has become the new generation of green renewable energy.However,the slow kinetics of oxygen reduction reaction(ORR)is a serious impediment to the commercialization of fuel cell.Pt-based nanomaterials are widely used as the state-of-the-art ORR catalysts;nevertheless,the prohibitive cost and scarcity,unsatisfying durability together with the poor methanol tolerance greatly impede their large-scale applications.On the basis of above problems,it absolutely challenges us to develop novel efficient and low costs alternatives as ORR catalysts.Among many non-precious metal catalysts,transition metal chalcogenides have attracted wide attention,due to their ease to synthesis,good electrocatalytic activity,and excellent resistance to methanol poisoning.In this thesis,based on the retional design of the interaction between the active site and the carrier,a series of carbon-supported transition metal chalcogenide catalysts were prepared by solvent thermal and high temperature carbonization method using cheap and readily available compounds as precursors.We explored the morphology,phase and microstructure of the catalyst and the catalytic activity,catalytic mechanism of the catalyst for oxygen reduction in depth.The main points in this dissertation are summarized as follows:(1)Sulfur and nitrogen co-doped reduced graphene oxide(S-N-r GO)was prepared by constant temperature reflux method.The oxygen reduction performance was tested in alkaline medium,and the effect of reaction temperature of S-N-r GO was investigated.The results showed that the S and N are doped into the skeleton of reduced graphene oxide by thiophene sulfur and pyridinium nitrogen,and the catalysts showed the best activity when the temperature reached 150?.Subsequently,Se-modified reduced graphene oxide was prepared by reduction method,and then used it as support to prepare Co Se₂/Se-r GO composite catalyst by solvent thermal method.The presence of anchored Se atoms favors the nucleation of cobalt to form the orthorhombic and hexagonal coexist Co Se₂.Due to the enhanced synergistic effect of active materials and supports,the catalysts performance for oxygen reduction reaction improved.When the Co Se₂ loading was 10%,the Co Se₂/Se-r GO composite catalyst had the best oxygen reduction activity,its half-wave potential reached 0.803 V(vs.RHE),which is only 14 m V lower than the commercial Pt/C catalyst,and it also showed a better stability and methanol tolerance ability than 20 wt%Pt/C.(2)Using 2-mercaptoimidazole as the sulfur source,the sulfhydryl group was coordinated with Co²⁺,after that the already coordinated 2-mercaptoimidazole was connected into a two-dimensional plane by 4,4-linked pyridine,then Co₉S₈/porous carbon composite catalyst was prepared by one-step pyrolysis.The composite catalyst had the best activity towards oxygen reduction reaction when cobalt sulfate was used as the chemical precursor and the heat treatment temperature reached 600°C.According to the tafel slope,the reaction rate of the composite catalyst is higher than commercial Pt/C.The electron transfer number of the oxygen reduction process was 3.72,which is close to 4,indicating that the oxygen reduction process of the catalyst was four electrons transfer process which is beneficial to the oxygen reduction.The composite catalyst also showed good stability and significantly better performance against methanol and silicate than commercial Pt/C.(3)The Co S/porous carbon composite catalyst was prepared by ion exchange combined with heat treatment method with ZIF67 as the precursor and thioacetamide as the sulfur source.The effects of sulfuration time and carbon material hybrid ZIF precursor to Co S/porous carbon was studied.The result showed that with the increase of sulfuration time,the size of cobalt sulfide particles gradually increased,and the disorder degree of carbon materials decreased first and then increased.At 10 min,the carbonaceous material had the lowest degree of defects,and the oxygen reduction activity of the composite catalyst was optimal.The addition of carbon nanotubes did not increase the specific surface area of the catalyst,but it greatly increased the cumulative pore volume and pore size which could promote the dispersion of cobalt sulfide,so that the composite catalyst exhibits the best oxygen reduction activity.The starting and half-wave potential were 0.926 V(vs.RHE)and 0.821 V(vs.RHE),respectively.The calculation of tafel slope showed that the reaction rate of cobalt sulfate was higher than commercial Pt/C,and the electron number of the oxygen transfer process was 3.68,which is close to 4,indicating that the oxygen reduction of the catalyst was a four-electron transfer reaction process,which is beneficial to the oxygen reduction reaction.(4)A series of Co_xM_yS-N/C(M=Cu,Ni,Fe,Mn,Sr)were prepared by high-temperature carbonization after sulfuration,in which Cu,Ni,Fe,Mn,and Sr are selected as the second metal,and ZIF67 is used as the precursor of MOF.It was showed that the addition of different metals infulence the morphology of the composite catalyst.With the addition of strontium nitrate,the particle size of the catalyst significantly reduced,the performance toward oxygen reduction reaction improved.After that,Fe_xZn_yS-N/C composite catalyst was prepared by the same method.The effect of reaction temperature was studied.As the temperature increased,the size of metal particles increased first and then decreased.When the reaction temperature reached 750°C,the start and half-wave potential of the composite catalyst were 0.909 V(vs.RHE)and 0.798 V(vs.RHE),respectively.Although there is still a certain gap compared with commercial Pt/C,the low cost and simple synthesis method of the catalyst provide a reference for the preparation of bi-metallic chalcogenides.