| In the current trend of increasing exhaustion of traditional energy sources,proton exchange membrane fuel cells?PEMFCs?have gradually entered the areas of people's daily life after years of technology accumulation.The transfer characteristics of proton exchange membranes?PEMs?,which work as a heart component of cell systems,have a direct impact on the final output performances.Currently,the commercialized PEMs,for example,Nafion?membranes based on the structure of perfluorosulfonic acids,show more superior performance at low temperature.However,the extremely high temperature may lead to the volatilization of water molecules,thus weakening the performances of cells based on Nafion?membranes.On the contrary,while maintaining a better level of proton conduction,the costs of high-temperature proton exchange membranes?HT-PEMs?could be reduced due to the improvement of catalytic efficiency,the acceleration of electrode reaction,as well as the simplification of water-heat circulating system.Still,they cannot meet the actual demand of starting at low temperature for vehicles in the cold regions.Therefore,developing PEMs that can work efficiently in a wide temperature range has become an urgent issue at this stage.In our paper,the aromatic poly?2,6-dimethyl-1,4-phenyleneoxide??PPO?was modified by imidazole-functionalization or sulfonation to act as the membrane matrix,and then the phosphonic acid-based siloxane with considerable performance at high temperature was incorporated into the matrix to prepare the composite membranes.It is expected that dual proton transfer channels of acid-base sites and hydrogen bond networks could be constructed within the membranes,thus enhancing proton conduction abilities of PEMs and alleviating the dependence of them on temperatures.The details are listed as follows:?1?The PPO were modified with alkaline 1-Methylimidazole groups to obtain a membrane matrix of QPPO-MIm by nucleophilic substitution reaction.Then a series of high-temperature proton exchange membranes were prepared by incorporating phosphonic acid-based siloxane from Amino Trimethylene Phosphonic Acid?ATMP?and 3-Aminopropyltriethoxysilane?APTES?into cross-linked 1-Methylimidazole-functionalized poly?2,6-dimethyl-1,4-phenyleneoxide?.Induced by Coulomb effect,acid-base pairs were formed among phosphonyl groups of ATMP and imidazole rings,imino or amino groups of APTES.The existence of hydrogen bond networks was also confirmed by variable temperature infrared spectroscopy?VT-FTIR?.In addition,the backbones of PPO were immobilized into cross-linking networks of Si-O-Si formed by hydrolysis-polycondensation of APTES.Therefore,the resultant membranes named as HTM-X exhibited superior mechanical properties and stability.The result showed that HTM-3 membrane,whose molar ratio of P to Si was 3:4,possessed the highest proton conductivity of 8.48×10-2 S cm-1 at low relative humidity of 5%and 160?C with contribution of dual proton transfer channels of acid-base pairs and hydrogen bond networks constructed alongside the inorganic-organic interfaces.?2?A series of QPPO/SPPO-X membranes were prepared by incorporating the sulfonated poly?2,6-dimethyl-1,4-phenyleneoxide??SPPO?into the system of imidazole-functionalized poly?2,6-dimethyl-1,4-phenyleneoxide?/phosphonic acid-based siloxane?QPPO-MIm/ATMP-APTES?.After successfully embedded into the membranes,the SPPO was well dispersed and closely related to the original system of QPPO-MIm/ATMP-APTES,which is beneficial to enriching the acid-base pairs and hydrogen bond networks.The SPPO also participated in the improvement of cross-linking structures of APTES.As a result,the mechanical strength and stability of the membranes were further strengthened.With the increasing content of SPPO,the densities of proton dissociation and transfer sites increased correspondingly.The acid-base pairs were enriched and arranged in an orderly manner.Moreover,the involvement of two kinds of proton transfer units,sulfonic acid groups and phosphonyl groups,widened the operating temperature range of membranes.Therefore,the QPPO/SPPO-30 membrane,whose addition of SPPO was 30 wt.%,possessed the highest proton conductivities of 0.1131 S cm-1 at 100%RH,80?C and 0.1049 S cm-1at 5%RH,160?C,respectively. |
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