Polymer Structure Design And Performance Of Anion Exchange Membrane For Alkaline Membrane Fuel Cell

Due to the excessive development and use of coal,oil,natural gas and other fossil energy sources,excessive CO₂ gas emissions have led to a series of problems such as global warming and melting of glaciers.Therefore,the development and utilization of new clean energy is the subject of competition and cooperation among countries in the world.Among them,China's goal is to achieve a carbon peak by 2030 and achieve carbon neutrality by 2060.Therefore,it is necessary to vigorously develop green energy.Among them,as a clean secondary energy source,hydrogen energy is one of the important energy sources to achieve the goal of carbon neutrality and has attracted much attention.The fuel cell is an effective device which can convert hydrogen energy into electric energy directly and has a wide application prospect in the field of automobile.The traditional proton exchange membrane fuel cell(PEMFC)relies on the precious platinum as catalysts because it works under acidic conditions.At the same time,commercial perfluorosulfonic acid membrane(Nafion)is also very expensive.Therefore,the high cost of PEMFC greatly limits its commercialization and wide application.Compared with PEMFC,the alkaline membrane fuel cell(AMFC)works under alkaline conditions,and has faster oxygen reduction reaction kinetics.Therefore,the choice of catalyst is greatly expanded,the use of precious metal platinum is reduced.Non-precious metal catalysts such as Ni and Ag can be used to reduce fuel cell cost.However,the development of AMFC is mainly limited by its key component of anion exchange membrane(AEM).The bottlenecks and challenges facing AEM are mainly in the following two aspects:(1)Increasing the ionic conductivity is often accompanied by an increase in the swelling of the membrane due to water absorption,resulting in a decrease in the mechanical properties of the membrane.Therefore,it is necessary to solve the"trade-off"effect between ionic conductivity and dimensional stability.(2)Under alkaline conditions,cationic groups and polymer backbone in AEM are easily attacked by nucleophilic OH^-to cause complex degradation reactions,which reduce the service life of AEM.Therefore,in order to solve the bottleneck of the current AEM development and provide references for the preparation of AEM materials with better comprehensive performance,the research contents of my dissertation are as follows.(1)Through the molecular structure design,the hydrophilic/hydrophobic microphase separation morphology is constructed,which effectively solves the"trade-off"problem of the ion conductivity and dimensional stability of the membrane.First,the polyaryl ether ketone copolymer was prepared by polycondensation of methyl hydroquinone and4,4'-difluorodiphenyl ketone.The introduced hydrophilic cationic group was also used as a crosslinking agent to promote the formation of the microphase separation structure in the membrane,and improve the mechanical properties and dimensional stability of the membrane.Play the synergistic effect of side chain and cross-linked structure to prepare cross-linked side chain type poly(ether ether ketone)membranes,and explore the influence of its structure on AEM performance.(2)Furthermore,ion cluster type AEM was constructed by adjusting the polymer topological structure.Several hydrophilic cationic groups are introduced into the same structural unit of the hydrophobic main chain,and obvious hydrophilic and hydrophobic differences are produced between the dense hydrophilic cationic groups and the hydrophobic main chain,thus constructing the hydrophilic/hydrophobic microphase separation structure.By introducing hydrophilic cross-linking agent as an additional driving force,the formation of hydrophilic/hydrophobic microphase separation structure can be promoted,and the size and morphology of ion clusters can be regulated by regulating the proportion of hydrophilic and hydrophobic groups,so as to construct an ideal ion transport channel and improve the transport efficiency of OH^-.At 80?,the ionic conductivity of Im PEK-0.4 membrane is 0.083 S/cm,and the swelling ratio of Im PEK-0.4membrane is less than 8%.(3)Through molecular design,an obvious hydrophilic/hydrophobic phase separation structure based on ion cluster type AEM is further constructed,and the size and morphology of ion transport channels are controlled by controlling the ratio of hydrophilic groups and hydrophobic groups.The improvement of ionic conductivity is achieved under the condition of low water absorption and low expansion,which effectively solves the problem of"trade-off"between ionic conductivity and membrane dimensional stability.The relationships among the molecular structure,microstructure and properties of AEM are established.At the same time,the introduction of substituents increases the steric hindrance of cationic groups,inhibits the attack of hydroxide ions,and improves the chemical stability of AEM.The conductivity of the Im-PEK-0.8 membrane increases to 0.148 S/cm at 80°C.At the same time,the swelling ratio of the membrane does not exceed 20%,exhibiting excellent dimensional stability.After immersing in 2 mol/L Na OH for 240 h,the ionic conductivity of the membrane does not drop by more than 20%.(4)In order to further extend the overall service life of AEM,the alkaline stability of the main chain and cationic groups were optimized simultaneously.Therefore,the AEM based on poly(biphenyl-piperidine)was prepared.The cross-linking structure and hydrophilic/hydrophobic microphase separation structure were constructed to improve the comprehensive performance of AEM.After soaking in 5 mol/L Na OH solution at 80°C for30 days,the ionic conductivity of PAP-OH-8%decreased by 10.8%,showing excellent alkaline stability.In addition,the maximum power density of PAP-OH-8%is 290 m W/cm².