Polymer Architecture Optimization And Property Regulation Of Nano-phase Separated Anion Exchange Membranes

Ion exchange membrane fuel cell(IEMFC),which can directly convert the chemical energy to electricity power in a high-efficient and environmental-friendly manner,is recognized a promising technology to solve the challenge of energy shortage and environment pollution.Traditional IEMFC adopted a proton exchange membrane(PEM,typically Nafion(?))as the gas separator and proton conductor,namely proton exchange membrane fuel cell(PEMFC).However,the dependence on Pt catalyst have been severely hindering its further development and applications.On the other hand,the emerging alkaline anion exchange membrane fuel cell(AAEMFC)which adopts an anion exchange membrane instead,can enable the use of non-precious metal catalyst(Ni,Ag,Fe etc.)due to the faster oxygen reduction kinetics under alkaline condition.Besides,AAEMFC also bears other distinct advantages like more facile water management,wide choice of fuels,lowered chance of catalyst poisoning etc.But there are still remaining problems to be solved in the field of AAEMFC.The power density of AAEMFC is generally much lower compared with PEMFC due to the lack of high performance anion exchange membrane(AEM).Thus,the development of AEM with high conductivity as well as good stability have received increasing attention nowadays.In order to enhance its hydroxide ion conductivity,various strategies have been developed by researchers and one of the most efficient way is to construct high-efficient ion conducting channels within the membrane matrix.By tuning the location of ion conducting groups,particular polymer structure composed of hydrophilic segments as well as hydrophobic segment could be constructed.During the membrane formation process,the phase separation arising from the polar discrimination between the hydrophilic segments and the hydrophobic segment results inter-connected hydrophilic phase.Due to the containment of high density of ion conducting groups,this hydrophilic phase is considered to be the main pathway for hydroxide ion conduction,namely the ion conducting channel.In this dissertation,four types of nano-phase separated AEMs were reported,their merits and importance have been described as follow:1)By analyzing the structural features of the side-chain type AEMs and the densely functionalized AEMs,we can infer that the mobility and the density of ion conducting groups in the ion conducting channels are two key factors that decide the nano-phase separation ability and the hydroxide ion conductivity.To combine the advantages of these two strategies,we have designed 'ionic string' AEMs with two ion conducting groups in one same side chain.The flexibility of side chain ensured the high mobility of ion conducting groups.The increased cation number in the side chains can enhance the hydrophilicity of side chains and facilitate the aggregation of ionic segments.As a result,this type of AEM with densely functionalized side chain is expected to show improved nano-phase separation ability and high hydroxide ion conductivity.2)To enhance the driving force of phase separation process,a crosslinkable double bond was introduced as the termination of AEM side chain.During the membrane forming process,the aggregation of hydrophilic side chains could effectively facilitate the contact between double bonds,thus enhancing the crosslinking degree of AEM.On the other hand,the polymerization of double bonds would in return promote the process of nano-phase separation.As a result,well-connected ion conducting channels as well as improved mechanical properties were achieved at thesame time.3)The enhancement of the AEM performance depends on the construction of high-efficient ion conducting channels.Besides the insufficient mobility and density of ion conducting groups,the entanglement of polymer chain will also hinder the aggregation of ionic segments,thus lower the nano-phase separation ability of AEMs.Therefore,in the fourth chapter of this dissertation,we have designed AEMs with novel ionic clusters.The resulting ionic cluster contains six quaternary ammonium groups thus less number of aggregated ionic clusters were required to form the phase separated morphology.As a result,the synthesized AEM exhibited excellent fuel cell performance with peak power density of 266 mW/cm2.4)In the last chapter of this dissertation,by introducing a long aliphatic tail as the termination of side chain,we have prepared novel AEM with unique polymer architecture with hydrophilic main chain and hydrophobic side chains.Results indicated that this strategy was more effective in constructing the well-connected ion conducting channels.The peak power density of this comb-shaped AEM exceeded 360 mW/cm2.Also attributing to the hydrophobic nature and steric hindrance effect of the aliphatic chain,enhanced alkaline stability was also observed.