Investigation Of Preparation And Mass Transport Enhancement Of The Membrane Electrode Assembly For Polyelectrolyte Fuel Cells

The polyelectrolyte fuel cells(PEFCs)have the characteristics of high power density,low operating temperature,and no pollutant emissions,so it can effectively alleviate the fossil energy crisis and applied to the fields of new energy vehicles and portable electronic equipments.After nearly two decades,PEFCs have developed rapidly.Especially in recent years,due to a lot of research on polyelectrolyte membranes and catalysts,the performance of PEFCs has been greatly improved.However,stable mass transport is still the biggest barrier restricting the development of PEFCs.Herein,we have improved the operational stability of PEFCs by constructing efficient mass transport channels in membrane electrode assembly(MEAs).The relationship between polyelectrolyte architecture,MEAs morphology and fuel cell performances has been investigated.Overall,multiple strategies aim to enhance the mass transport in MEAs have been proposed for PEFCs.Firstly,the gas and water transport in the catalyst layer has been improved by the ionomer cross-linking immobilization strategy.Then we propose a 3D interfacial design via in-situ cross-linking polymerization for efficient and stable mass transport,inspired by the interface modification based on covalent bond cross-linking in the field of interface chemistry.Finally,we design and prepare a cross-linked proton exchange membrane(PEM)for fast mass transport.The specific work includes four aspects:(1)In order to build a stable "three-phase" mass transport interface in the catalytic layer,we propose a novel approach involves ionomer cross-linking immobilization for the fabrication of durable catalyst layers in alkaline polyelectrolyte membrane fuel cells(APEFCs).Pt/C NPs catalysts are employed alongside a PPO-based quaternary ammonium ionomer(containing terminal styrenic side-chain groups)to form porous catalyst layers.Following thermally initiated cross-linking of the terminal vinyl groups,an interconnected ionomer network forms conductive shells around the Pt/C aggregates.An initial demonstration of an H2/O2 APEFC containing the new CBQPPO@Pt/C cathode shows that high peak power densities can be achieved(1.37 W cm-2 with additional 0.1 MPa back-pressurization).(2)Based on the ionomer cross-linking immobilization strategy,the fixed crosslinking group and the freely movable group are introduced into the polymer side chain together to improve the rigidity of the polymer and control the hydrophobicity of the catalytic layer.This route can enlarge the water balance area in MEAs during operating and prevent the flooding.When QPPO-CBN-2.75 is adopted,the water balance area will be increased and the humidity sensitivity of the MEAs will be reduced.(3)In chapter four,we present a three-dimensionally(3D)interfacial zipping design that substantially reduces the mass transport resistance through the MEA.Most importantly,this interface containing quaternary ammonium cations will provide ionically conductive interconnections and preventing interfacial delamination during wet/dry cycles.Ex-situ evaluation of interfacial bonding strength and in-situ durability tests demonstrate that the 3D-zipped interface strategy prevents interfacial delamination without sacrificing the fuel cell performance of the MEAs.An H2/O2 APEFC containing 3D-Zipped Interface Layer(3D-ZIL)shows high peak power densities(1.5 W cm-2 at 70? with 100%RH).Besides,the ZIL-MEA-based APEFC can withstand 120 h while maintaining its initial performance at 0.6 A cm-2(4)The work in the last chapter is different from the previous three chapters.We present a low-cost cross-linked proton exchange membrane SCFPAE-X.Among them,although the SCFPAE-20 membrane has a high ion exchange capacity(IEC),it can still maintain better mechanical properties,so the proton conductivity is high.When the platinum loading in cathode is very low(0.15 mgPt cm-2),the peak power density of SCFPAE-20 can still reach 0.9 W cm-2 with 100%RH and unpressured gases applied.Based on the designing of polyelectrolyte architecture,the above researches promote the transport of gas,water and ions in the APEFCs by controlling the MEAs morphology,thereby improving the fuel cell performance.