Preparation And Properties Of Anion Exchange Membranes Based On Polybenzimidazole And Poly (Aryl Ether Ketone)

Ion exchange membrane is a kind of solid electrolyte membrane with ion-transporting functional groups,which plays an important role in traditional chemical industry such as chlor-alkali industry and sewage treatment,due to its selection and transmission characteristics of specific ions.In recent years,global warming has promoted researches on non-fossil energy.The development of clean alternative energy power generation has become a worldwide challenge.Anion exchange membrane fuel cell is a new clean power generation technology which converts chemical energy into electric energy directly and efficiently.It consists of two electrodes?positive and negative?and an electrolyte membrane transmitting ions between electrodes.This electrolyte membrane in the fuel cell serves as anion transfer and fuel blocker simultaneously.Under high pH value conditions,anion exchange membranes are vulnerable and tend to be attacked by hydroxide ions,thus shortening the service life of the devices.Therefore,how to improve the conductivity and long-term stability of anion exchange membrane has become one of the hot research spots for researchers.Generally,in order to improve the ionic conductivity,the most direct approach is to increase the number of ion transport functional groups?or titled as ion-exchange capacity IEC?in the membrane.However,the hydrophilicity of the membrane increases with the increase of IEC,thus causing excessive water absorption and excessive swelling ratio,severely impacting the stability of the membrane or even dissolving the membrane.Therefore,this paper focuses on imidazole and ferrocene cations to explore their effect on hydroxide conductivity and membrane integrity.The effect of microphase separation on hydroxide conductivity is also discussed.Among various ion-transporting functional group structures,imidazole ring conjugated structure can enhance the interaction with hydroxide ions.In addition,the imidazole group has good chemical stability.So it can be an ideal ion-transporting functional group.Despite this,the species of ion-transporting functional groups are supposed to be improved.For example,ferrocene can be converted into ferrocene cations and used in anion exchange membranes.Unlike traditional quaternary amine cations,the metallocene structure can avoid Huffman rearrangement,from which the quaternary amine structure might suffer.However,according to the reported literature,the reports about ferrocene cations having been used in anion exchange membrane are rare.Therefore,it is necessary to explore the potential application of this metal cation.On the other hand,the microstructure of the anion exchange membrane will also affect the comprehensive performance.The ionic functional groups gathering in the membrane to form ion clusters and separate from the main hydrophobic polymer chain is called hydrophilic/hydrophobic microphase separation.The formation of ion clusters provides facilities for the rapid transportation of ions in the hydrophilic phase.The polymer mainchains can be packed tightly to construct a dense membrane in the process of membrane formation.In this paper,the as-prepared membranes are comprehensively characterized and analyzed such as oxidation stability,thermal stability,micromorphology,ion conductivity,etc.?1?Polybenzimidazole is a polymer with high thermal stability and mechanical strength.At the same time,the C2 position on imidazole being replaced with benzol will increase the steric hindrance effect,thus reducing the risk of imidazole ring decomposition caused by basic or hydroxyl radical attack.However,the anion conductivity of pristine polybenzimidazole membranes is not ideal.In order to further improve the anion transportation performance,an?phosphotungstic acid-ionic liquid?composite filler was synthesized and introduced to polybenzimidazole membranes.The results reveal that imidazole-based fillers can effectively improve the ion-exchange capacity and facilitate hydroxide transporting.The hydroxide conductivity improved from 35.5 mS cm^-1 to 75.8mS cm^-1.?2?According to the results of the above studies,direct composition may cause the filler unstable in the membrane matrix for long-term operation.In order to overcome this shortcoming,this section is committed to strengthening the fillers stability within the polymer matrix.It's worthy to be noted that this part will explore a new metal cation as ion-transporting functional group.We will explore the potential usability of ferrocene cations.Menshutkin reaction was used to prepare methoxysilane ferrocene precursor.Hydroxyl-containing polybenzimidazole was synthesized according to traditional polybenzimidazole synthesis process.By hydrolyzing methoxylsilane with hydroxyl groups,ferrocene precursor is anchored on the polymer main chains.Then ferrocene is further oxidized to cationic form to promote hydroxide conductivity.At the same time,the ring-metal cation structure of ferrocene cations is stable,avoiding Huffman degradation reactions that may occur on traditional quaternary amine cations.The results show that the ferrocene cations anchored in the membrane can promote the hydroxide conductivity.The highest conductivity is 105 mS cm^-1 at 80 ^oC.?3?In addition to searching the effect of ion-transporting functional groups on ion conductivity,the effect of microstructure of the membrane has also been investigated.In order to prepare anion exchange membrane with a microstructure of hydrophilic/hydrophobic phase separation,in this part,tetramethyl poly?aryl ether ketone?is used as the main chain and brominated into benzyl bromide form.Then imidazole polyionic liquid is synthesized.A certain proportion of vinylimidazole is introduced into the polyionic liquid chains.The reaction of benzyl bromide with the nucleophilic N3 position on the imidazole ring can integrate the rigid hydrophobic poly?aryl ether ketone?mainchain and the flexible hydrophilic polyionic liquid mainchain,thus establishing a cross-linking network.Due to the different properties of the two molecular chains,an obvious micro-hydrophobic phase separation structure can be obtained.It is interesting to find that the prepared membrane samples exhibit specific swelling behavior:swelling ratio in the thickness direction can be ten times of that in the length direction.This means that sufficient water absorption is ensured without losing dimensional stability,which promotes the conduction of hydroxide ions.