Theoretical And Experimental Investigation Of Pt-based Alloy Catalyst Electrocatalytic Methanol Oxidation For Methanol Fuel Cell

Direct methanol fuel cells(DMFCs),an ideal clean energy,have wide applications because of their high energy conversion efficiency,environmental friendliness,convenient storage and transportation.Precious metal Pt has been demonstrated as superior performance anodic catalysts for DMFCs due to its high catalytic activity.However,the broad application of DMFCs has been impeding because of the expensive cost and scarcity of Pt.Moreover,the reaction intermediates(such as CO)adsorbed on the Pt active sites,which lead to a decrease in the number of active sites available for methanol catalytic oxidation and thereby contribute to slow kinetics and poor catalytic activity and durability.Therefore,it is very important to looking for and designing the catalysts with high Pt utilization,high catalytic activity,stability and anti-CO poisoning.In this paper,we use periodic DFT calculations to investigate the decomposition mechanisms of methanol on PtAu(111)and PtPd(111)surfaces,and to explore the essential cause of the difference ability of anti-CO poisoning and the electrocatalytic activity for methanol oxidation reaction(MOR),then verify the theoretical calculation results through the experimental method.Finally,PtCo alloy nanowire catalysts with different atomic proportions were prepared by soft template method and used in the catalytic oxidation of methanol.The research will provide some theoretical and experimental guidance for the rational design of a fuel cell catalyst.This paper will study from the following five parts:In Chapter 1,we presented a brief introduction about the working principle,development status and reaction mechanism of the direct methanol fuel cell.Besides,the preparation technologies of the catalyst were also introduced.Furthermore,we also elaborated the significance of our work.In Chapter 2,a detailed introduction of the theoretical calculation method,density functional theory(DFT)and transition state theory(TST)mainly applied in this paper,were presented.Besides,we introduced the physical characterization and electrochemical characterization methods used in the catalyst during the experiment process.In Chapter 3,the decomposition mechanism of methanol on PtAu(111)surface was systematically studied by performing periodic density functional theory(DFT).In the study of most stable adsorption configurations of intermediates,we consider two pathways: CO path and non-CO path.Through the possible intermediate and reaction transition state,we finally confirm the most likely two pathway: CH3 OH ? CH2 OH ? CHOH ? CHO ? CO(CO pathway)and CHO ? HCOOH ? COOH ? CO2(non-CO pathway).By comparison of CO and non-CO pathways,we found that the decomposition of methanol on the PtAu(111)surface occurs mainly through the non-CO pathway.However,calculated results show that methanol tends to produce more CO before the formation of CO2.The energy barrier of CO generate in CO pathway is only 0.21 eV,and the elimination of CO needs to overcome an energy barrier of 0.74 eV.Therefore,we predict that the occurrence of non-CO pathway does not completely inhibit the production of CO,there may be present deposition of CO on PtAu(111)surface and thereby contribute to poisoning of the catalyst and leading to poor catalytic activity and poor stability.In Chapter 4,self-consistent periodic density functional theory(DFT)was employed to investigation the mechanisms of methanol decomposition on PtPd(111)surface.Then,by the comparison of the mechanisms of methanol decomposition on PtPd(111)and PtAu(111)surfaces,we explore the essential difference of the ability of anti-CO poisoning and the electrocatalytic activity for methanol electrooxidation.Finally,theoretical calculation results were verified through the experimental method.The theoretical calculation results show that the main pathway of methanol decomposition on PtPd(111)is CH3 OH ? CH2 OH ? CHOH ? CHO ? CO(CO pathway).The energy barrier of the rate-determining step of methanol decomposition on PtPd(111)was lower than that on PtA(111),indicating that the electrocatalytic activity of the PtPd catalyst is higher than that of the PtAu catalyst.The decomposition of methanol on PtAu(111)occurs mainly through the non-CO pathway,and the barrier of CHO decomposes to generate CO is 0.21 eV,indicating that the occurrence of non-CO pathway does not completely inhibit the production of CO,there are still some CO deposition on the surface.The decomposition of methanol on the PtPd(111)surface occurs mainly through the CO pathway,and the energy barrier of generating CO is 0.26 eV,indicating that there will be a lot of CO deposition during the methanol oxidation process.Therefore,the CO-poisoning tolerance of the PtAu catalyst is significantly better than that of the PtPd catalyst..The experimental results show that the PtPd catalysts have a higher MOR activity and stability than PdAu catalyst.In Chapter 5,PtCo,PtCo2,PtCo3 and Pt3 Co alloy nanowires catalysts were synthesized by soft template method and a comparative investigation of the electrocatalytic activity and stability in the methanol oxidation process.The results show that PtCo2/C nanowires catalysts exhibit enhanced electrocatalytic activity in methanol oxidation reaction compared with PtCo,PtCo3 and Pt3 Co electrocatalyst.Based on the hydrodynamic method,the electron transfer coefficient(?)and the diffusion coefficient(D0)of the electrode surface solution in PtCo,PtCo2,PtCo3 and Pt3 Co electrocatalytic methanol oxidation process were analyzed and calculated by linear sweep voltammetry curve.The results show that ? and D0 of PtCo2 alloy nanowires are the largest.Therefore,from the perspective of dynamics,we conclude that the enhanced performance of the PtCo2 alloy nanowires catalyst could be attributed to ? and D0.