Preparation And Investigation Of Imidazole-containing High-temperature Polymer Membrane Materials

High temperature proton exchange membrane fuel cells(HT-PEMFCs)have superior properties including simple water and thermal management,high tolerance to fuel impurities,e.g.CO,fast electrochemical reaction kinetics,etc.As a key material of HT-PEMFCs,proton exchange membranes(PEMs)should possess high proton conductivity under elevated temperature and low humidification and also should exhibit high thermal stability,chemical stability and mechanical strength.Developing high temperature PEMs with excellent physicochemical properties and high durability is an important issue for the progress of PEMFCs.The phosphoric acid doping level(ADL)of membrane determines its conductivity;the higher ADL,the higher conductivity of the membrane.However,the high ADLs may dramatically decrease the mechanical strength of membranes and thus remarkable affect the performance and life time of the fuel cell.Therefore,improvements on both conductivity and mechanical strength of the PEMs have become the key issues to further development of the HT-PEMFCs.In this thesis,a series of HT-PEMs was fabricated and modified based on polybenzimidazole(PBI)and Poly(2,6-dimethyl-1,4-phenylene oxide)(PPO).The physicochemical properties relating to application in fuel cells were investigated for the prepared PEMs.This thesis includes five chapters.A brief overview on the development of fuel cells and a comprehensive review of updated progress of HT-PEMs including PBI membranes and PPO membranes is given in chapter 1.All the experimental methods and techniques adopted in the present thesis are described in chapter 2.In Chapter 3,a series of silane cross-linked PBI membranes were fabricated from PBI and two different silane compounds,i.e.3-chloropropyltriethoxysilane(Pr-Si)and((chloromethyl)phenylethyl)trimethoxysilane(Ph-Si)(J&K Scientific,China).And the theoretical cross-linking degrees of the membrane were 5%,10%,30%,50%,respectively.Comprehensive characterizations and comparisons of these membranes were made.The results indicated that all the silane cross-linked PBI membranes exhibited high proton conductivity and improved mechanical strength.For silane cross-linked PBI membrane,the crosslinked silane network was achieved in the membrane by a hydrolysis process,which improved the phosphoric acid doping level.Therefore,the improved proton conductivity and mechanical strength were achieved by the membrane with a crosslinking structure.For instance,the proton conductivity of the PBI-30%Ph/13.6PA membrane was 0.13 S cm-1 at 180 ?,the mechanical strength was 4.7 MPa at room temperature(RT)and 1.4 MPa at 120 ?.Fuel cell tests of PBI-30%Ph/13.6PA membrane at 180? showed an open circuit voltage(OCV)of 0.92 V,a peak power density of 428 mW cm-2.In Chapter 4,two different compounds(1,3,5-tribromo-2,4,6-triethylbenzene(Bm3Br)and 1,3,5-tribromo-methylbenzene(B3Br))were used as crosslinkers to prepare a series of cross-linked PBI membranes.And the theoretical cross-linking degrees of the membranes were 7.5%,15%,30%,respectively.The results indicated that all the cross-linked PBI membranes exhibited excellent mechanical strength and chemical stability,which satisfy the requirements of the HT-PEMFCs.For instance,the mechanical strength of Bm3Br-7.5%/11.6PA membrane was 8-54 MPa at RT and 3.52 MPa at 120 ?;and the mechanical strength of B3Br-7.5%/11.1PA membrane was 9.2 MPa at RT and 3.88 MPa at 120 ?;the conductivity of Bm3Br-15%/12.8PA membrane was 0.114 S cm-1 at 180 ?.Fuel cell tests of Bm3Br-7.5%/11.6PA membrane at 180?showed an open circuit voltage(OCV)of 0.93 V,a peak power density of 335 mW cm-2.In chapter 5,the PEMs with interpenetrating networks were described in order to allow the membranes having both high proton conductivity and enhanced mechanical properties.In this chapter the PEMs with interpenetrating network BPPO-X%VIm-DVB-PS-Y%DMIm(X%?Y%represent the molar ratio of VIm and DMIm with-CH2Br in the BPPO)composed of BPPO and PS/VIm/DVB were prepared.Comprehensive characterizations and comparisons of these membranes were made.The results indicated that compared with BPPO-100%DMIm membrane,the interpenetrating network structure remarkable enhanced the mechanical properties of membranes.For instance,the mechanical strength of BPPO-100%DMIm membrane was 2.49 MPa at room temperature.However,the mechanical strength at RT of BPPO-70%VIm-DVB-PS-30%DMIm and BPPO-100%VIm-DVB-PS membranes was 5.04 MPa and 7.89 MPa,respectively.The conductivity at 180 ? was 0.049 S cm-1 for BPPO-100%VIm-DVB-PS,and 0.067 S cm-1 for BPPO-70%VIm-DVB-PS-30%DMIm membrane,respectively.