| Proton exchange membrane fuel cells(PEMFCs)have attracted much attention due to their high efficiency and cleanliness.As a key part of PEMFCs,proton exchange membrane(PEM)have been used to insulate fuel,support catalysts and conduct protons,which make it the focus and hotspot of the research.In the field of high-temperature proton exchange membrane(HTPEM),a polybenzimidazole(PBI)membrane doped with phosphoric acid(PA)is representative.Compared with low-temperature proton exchange membrane fuel cells(LTPEMFCs),the assembled HTPEMFCs using PA-doped PBI membrane own high cathode reaction kinetics.Moreover,the water management system has been simplified,and the tolerance of the catalyst to CO has been improved too.However,PA-doped PBI membranes have demerits such as poor proton conductivity and mechanical performance,poor processability,PA leakage,and poor long-term stability.In recent years,various methods for improving the performance of PBI structures have been reported,such as changing the main chain structure,introducing basic groups,grafting PA,synthesizing high molecular weight polymers,preparing composite materials,and covalently crosslinking using small molecules.However,these methods are more or less problematic and further research and improvement is needed.More recently,in the field of LTPEM,our group found that the modification of the polymer backbone by the three-dimensional dendritic branch structure can effectively increase the free volume inside the polymer film.At the same time,the intricate connections between these branches can effectively improve the oxidative stability of the PEM.Under the guidance of these research results,series of branched PBI derivative membranes were synthesized and prepared for the drawbacks of PA-doped PBI-type HTPEMs,and the properties applied in the field of HTPEM were studied and compared.The content is as follows:(1)Three kinds of OPBIs with different branching structures and various degrees of branching were prepared,and the properties of these prepared membranes were characterized and compared.The results show that the choice of a bulky rigid branched structure R2 and a degree of branching of 6% allows the branched membrane to achieve optimum overall performance.Among them,the highest proton conductivity can reach 0.053 S cm-1,and the peak power density of the selected OPBI-R2-6 membrane is 222 mW cm-2(160?,H2/Air,under anhydrous conditions).(2)In order to further improve the proton conductivity of the branched PBI membranes,based on the first part,a novel branched star block PBI membranes was designed and prepared.On the one hand,the existence of the branching structure effectively improves the free volume of the PBI membrane,so that the membrane can absorb more PA;on the other hand,the existence of the block structure facilitates the formation of the microphase-separated proton transport channel,leading to high-efficiency proton transfer.The results show that the proton conductivity of PA-doped branched star-shaped block PBI membrane reaches 0.15 S cm-1 at 160 ? under anhydrous condition,and the corresponding activation energy is 5.6 KJ mol-1.These results indicate a significant increase in proton conductivity.(3)The branched PBI with fluorine-containing aliphatic chain was prepared by utilizing a bulky rigid branched structure R2 and a branching degree of 6%.Meantime,the macromolecular covalently cross-linked branched PBI membrane was designed and prepared with the principle of Mannich base bridge formed by thermal ring-opening polymerization of benzoxazine.The results show that the mechanical properties,proton conductivity and oxidation stability of the membranes are effectively improved.At the same time,the single cells assembled with PA-doped cross-linked branched PBI membrane(H2/O2)exhibited high peak power density(690 m W cm-2)under anhydrous condition at 160 ?.The membrane also showed no significant degradation during the 200 h life test.The results basically meet the requirements of commercialization. |
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