| Compared with traditional low temperature proton exchange membrane fuel cells,high temperature proton exchange membrane fuel cells(HT-PEMFCs)have the advantages of fast electrode reaction kinetics,simple hydrothermal management and strong anti-poisoning ability of catalysts,so they have received extensive attention from domestic and foreign researchers.Currently,the most widely used high temperature proton exchange membrane(HT-PEM)for HT-PEMFCs is phosphoric acid(PA)-doped commercial polybenzimidazole(PBI)membrane.Although HT-PEMFCs based on this membrane have excellent initial operational performance,the problems of poor stability and insufficient durability limit their further development.This is mainly due to the continuous leaching of PA in the PA-PBI membrane during HT-PEMFCs operation,resulting in a decrease in membrane conductivity.Therefore,it is of great significance to develop a new type of HT-PEM with high resistance to PA leaching.In addition,another key problem that has been widely ignored by researchers is that the leaching PA will enter the catalytic layer(CL),which will adversely affect the mass transport of O2 in the CL.Quantitative researches on this problem have rarely been performed.The density functional theory study shows that the main reason for the PA leaching of PA-PBI membrane lies in weak hydrogen bond interaction between PBI and PA molecules.Therefore,in order to improve the anti-PA leaching performance of HTPEM,a series of polyisatin-like quaternary ammonium(QA)cationic POXIA-QAs membranes with different side chain lengths are prepared by superacid-catalyzed polyhydroxy alkylation reaction.The strong ion-pair interaction between the QA cations at the end of the side chain and the phosphoric acid molecules enables the phosphoric acid retention of the POXIB-QA/PA membrane to reach 88.25%,which is significantly better than the commercial PBI membrane.Meanwhile,the rigid aromatic polymer skeleton endows the POXIA-QA membranes with excellent thermal and mechanical stability.For further increase the conductivity of HT-PEM,a series of polyfluorene-based QA cationic PBFx-QA membranes with bi-flexible alkyl side chains are designed and prepared.The end of each side chain is grafted with a QA cationic group,which enables the ion exchange capacity of the PBF2-QA membrane to reach 2.18 meq g-1 and the phosphoric acid doping level to reach 133.14 mol.While the conductivity of the membrane is greatly improved,it also further enhances the resistance of the membrane to phosphoric acid loss.HT-PEMFCs based on PBF1-QA membrane reach a peak power density of 1080.9 mW cm-2 at 180℃ with no significant drop in cell voltage during 10 h of operation.Although the problem of PA leaching has been ameliorated to some extent,it has not been completely eliminated.During long-term operation,the cell voltage decays significantly,especially in high current density region,which is due to the large amount of PA leaching from membrane to CL and hindering the O2 mass transport.In order to investigate the quantitative effects of PA leaching on O2 mass transport in CL,an ultra microelectrode electrochemical testing technique is independently designed and developed.The parameters such as diffusion coefficients,solubility and permeability of O2 in CL are successfully obtained,and the effects of PA leaching and binder composition in CL on O2 mass transport are quantitatively studied.Experimental results confirm that PA leaching causes the dynamic change of binder PDL and a certain degree of PA leaching is beneficial to O2 mass transport in CL.The application of this technique is of great significance for deeply understanding the factors affecting the stability of HT-PEMFCs and formulating corresponding improvement strategies.In order to gain a more realistic and profound understanding of the O2 mass transport process in CL,a high-temperature ultra-microelectrode technique is designed and developed by combining the electromagnetic shielding box and micro-current amplifiers.The quantitative effects of temperature on the O2 mass transport in CL under real working conditions of HT-PEMFCs are investigated.According to Cohen and Turnbull’s free volume theory and hard sphere model,a "ball-basket" model describing the O2 diffusion process and a dual-mode model describing the O2 dissolution process are established.All-atom force field and molecular dynamics simulations reveal the O2 mass transport mechanism in CL,which further and theoretically confirms the point that a certain degree of PA leaching is beneficial to the O2 mass transport.Exploring PA leaching regulation methods and seeking other preparation strategies are important directions for the subsequent development of HT-PEM.In this thesis,a strategy for the preparation of HT-PEM using water as proton transportation carriers is attempted.SiO2 nanoparticles are introduced into the ionic clusters of commercial Nafion membranes by a confined acid-catalysis method.The water retention capacity and conductivity of the Nafion/SiO2 composite membranes under high temperature and low humidity are improved by using highly hydrophilic SiO2 nanoparticles.Combined with in situ synchrotron radiation SAXS technique,the effects of temperature on the structure of ionic clusters in composite membranes are explored,and the variation of proton transport network structure and connectivity with temperature is found.In general,in the process of researching and solving the PA leaching problem of HT-PEM,two kinds of novel membranes with high resistance to PA leaching are designed and prepared,and the normal/high temperature ultra-microelectrode techniques are developed in this thesis.The quantitative effect of PA leaching on oxygen mass transport behavior in CL is innovatively revealed,and the preparation strategy s of HT-PEM using water as proton transport carriers are preliminarily explored.These works have important significance in the development of novel high-efficiency HT-PEM materials and the further development of HT-PEMFCs. |
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