Catalytic Layer Construction Based On Pt-based Nanowires For Proton Exchange Membrane Fuel Cell

The low-cost,high-performance and stable catalysts and Membrane electrode catalytic layer are the biggest obstacles in the commercialization of Proton Exchange Membrane Fuel Cells(PEMFCs).At present,the structural regulation and catalysts composition,surface-specific structures,and structure-activity relationship have important significance for reducing the cost to improve the Pt utilization and catalytic performance.Compared with supported Pt particle,self-supported ultra-thin Pt-based catalyst has attracted more attention,owning to its unique structural advantages.The conductive,and continuous thin film scaffold acts as an efficient electronic pathway independent of carbon support,providing long-term stability by preventing Pt degradation.The thinner catalytic layer enable fast charge transfer due to a shorter conduction path.In addition,the open adjustable pore structure can promote substance transfer kinetics.To achieve high performance and structure stability,a high efficient three phase interface was constructed in the membrane electrode.In view of the challenges in the structural design for efficient electrocatalysts,a series of one-dimensional nanostructured catalysts with high active area and stable structure were synthesized through simple one-step reduction method.With deep study of vacancy defects,we constructed defects by de-alloying process to increase surface roughness and adjust Pt electronic structure,thus optimize the adsorption ability between surface structure and intermediates,improving the catalyst activity and stability to meet the follow-up membrane electrode.The key parameters of the cathodic catalyst layer were co-optimized by finite element analysis,which provided the most important and convenient guidance for the structural design of the ultrathin catalyst layer(UTCL).The specific research content of this paper is summarized as follows:(1)We developed a simple one-step synthesis strategy for fabrication one-dimensional Pt-based nanowires(NWs)without surfactants in a large scale.We synthesized a series of Pt Cu NWs with ultra-fine size and controllable composition by regulating the ratio of Pt/Cu atoms.Density Functional Thoery(DFT)calculation showed that introducing Cu vacancy defects and Cu doping could significantly enhance the activity of Pt Cu NWs for Methanol Oxidation Reaction(MOR),showing a high performance of 2230 m A/mg _Pt for MOR.There shows a volcanic trend on MOR for Cu doping Pt Cu NWs.Pt Cu NWs with Cu vacancy defects were prepared by electrochemical dealloying.DFT results showed that the introduction of Cu vacancy defects enhanced the adsorption of Pt atoms to OH*intermediates,while weakened the adsorption of CO*intermediates.The selectivity of Cu vacancy to MOR reaction path was systematically studied.As well as,the(Oxygen Reduction Reaction,ORR)performance presents an 8.4-folds activity compare to Pt/C.The self-supported UTCL electrode constructed by one-dimensional Pt Cu NWs shows two-times performance and excellent stability than commercial Pt/C on Direct Methanol Fuel Cells(DMFCs).These results provide important ideas for the regulation active sites for MOR and ORR,and the structure-activity relationship between metal vacancy defects and MOR.(2)The regulation of precursors can be extended to prepare catalysts with other component structures,further enriching and extending the above surfactant-free synthesis method.In order to maximize Pt utilization,ultrafine ternary Pt Pd Cu NWs catalyst was successfully prepared.With abundant grain boundary defects and optimized Pt electronic structure,the catalyst has excellent bi-functional electrocatalytic activity for Ethanol Oxidation Reaction(EOR)and ORR.The surface highly disperse Cu structure was constructed by chemical acid etching strategy.Cu,as an oxygenophilic metal,could absorb OH_ads to remove CO_ads,thus reducing the CO poisoning of electrocatalyst.Pt site effectively adsorbed CH_x intermediate to promote the cleavage of C-C bond of ethanol molecule.In addition,the presence of Pd atom can lead to the compression strain on Pt surface to modify the electronic structure of Pt.As well as,Pd has a high reduction potential,which can protect the insoluble active site of Pt and improve the stability of EOR and ORR.As UTCL,the performance of the membrane electrode with 1.2 mg _Pt/cm² Pt load is about 3.9 times higher than that of the commercial Pt/C with 2 mg _Pt/cm².In general,the ternary alloy catalyst is of great significance for the development of efficient Direct Ethanol Fuel Cells(DEFCs)electrocatalyst.(3)The decoupling relationship among electron conduction,proton transport and oxygen transport in UTCL was successfully studied for the first time,and the proportion of each was quantitatively obtained under certain conditions for optimizing the electrode structure to maximize the utilization of catalyst.We synthesized Pt Pd Cu NWs by regulating the atomic ratio as an efficient ORR electrocatalyst.The UTCL structure was constructed by CCM(catalyst coated membrane)spraying method,and the variation of electron,proton and oxygen transport resistance in the catalytic layer with the thickness of the catalytic layer was studied.By coupling the relation of electron,proton and oxygen transport,the optimized catalytic layer thickness and corresponding Pt load were obtained,and then the Debye-Length controlled proton transport distance of 450 nm in UTCL was quantified.Based on the decoupling relationship,we added ionomer and pore-forming agent to build additional proton and oxygen transport channels,builting an excellent three-phase interface structure for electrochemical reaction,and achieving a high performance of 1225 m W/cm².Finally,the performance and stability of the optimized Pt-based UTCL electrode are better than that of commercial carbon supported Pt catalyst.It shows great application potential of high activity and high stability in PEMFC.This study provides a rich experimental basis and platform for reducing and optimizing the ohmic loss and mass transfer loss of Pt-based electrodes.(4)In this chapter,COMSOL Multiphysics commercial software is used to finite element simulate the quantitative effects of different key parameters such as the state of cathode water on the catalytic layer thickness and performance parameters of PEMFC by using the macroscopic two-dimensional homogeneous model.The catalytic layer in the model consists of three independent homogeneous phases,including pores,conductive electrodes,and ionomer skeleton.We considered the gas diffusion and flow in the diffusion layer and cathode catalyst layer,and also liquid water in gas diffusion layer and cathode catalyst layer pore,using the change of water phase and the formation of water in the cathode reaction to describe the changes for two phase of water,thus,it can describe completely the existence of water in PEMFC.Through the modeling process of protons,electrons,electrochemical reaction,the gas transport process within the porous media and solid complex physical and chemical process,we analyzed the PEMFC performance,oxygen concentration,electron and proton current distribution along the direction of the cathode catalyst layer thickness,The relationship between the thickness of the catalytic layer and the three phase interface structure in the catalytic layer was studied.The influence of various parameters on the performance of the cathodic catalytic layer was systematically analyzed.It is concluded that the optimal thickness of the cathode-catalytic layer is 450 nm.