Preparation And Properties Of Alkaline Anion Exchange Membrane Based On SEBS For Fuel Cell Applications

Recently,the energy shortage and environmental pollution problems are becoming more and more serious,which can not only affect the global politics and economy,but also has a very important significance for the sustainable development of human society.Thus,development and utilization of the new energy has become the key to solving these problems.And the hydrogen energy stands out among many new energy sources due to its clean,efficient and convenient characteristics.Among many kinds of fuel cell types,polymer membrane fuel cells have shown the fastest development and application in the fields of household fixed power supplies and mobile power supplies such as automobiles and aerospace.Polymer membrane fuel cells can be divided into proton exchange membrane fuel cells(PEMFCs)and anion exchange membrane fuel cells(AEMFCs)according to the different ion forms conducted by the electrolyte membranes.Although the PEMFCs under the acidic working environment are more widely used,the highly expensive cost of the core materials restricts its further development,such as precious metal catalysts and proton exchange membranes.Correspondingly,the AEMFCs with alkaline working environment is believed as a potential alternative to PEMFCs,because of the use of inexpensive non-platinum group metal catalysts and the faster oxygen reduction electrode kinetics.Anion exchange membranes(AEMs),as one of the most essential materials in AEMFCs,can directly determin the energy output of the fuel cells,which reqires AEMs must possess high ion conductivity,low swelling ratio,great chemical and thermal stability,excellent mechanical properties and low cost simultaneously.Recent studies have shown that constructing a hydrophilic/hydrophobic microphase separation morphology can promote the formation of ion channels,which can accelerate ion migration and reduce the internal resistance of fuel cell.Also,the high-speed ion transportation channels can help to achieve high ion conductivity at lower ion exchange capacity(IEC).While,lower IEC value means lower swelling ratio and better dimensional stability.Poly(styrene-b-(ethylene-co-butylene)-b-styrene)(SEBS)has the characteristics of alternating soft and hard blocks,which can accelerate the construction of microscopic phase separation morphology.In addition,this all carbon-based polymer backbone without any heteroatoms ensures excellent alkaline stability.In this study,SEBS was chosen as the polymer backbone to prepare AEMs with excellent comprehensive properties via organic polymer compound strategy,multi-cation crosslinked strategy and comb-shaped structure strategy.The relationship between conductivity,microstructure,dimensional and chemical stability and molecular structure was explored,and the scientific principles in the preparation of SEBS-based alkaline membranes were revaled,aiming at tapping the potential of SEBS polymers in the AEM applications.The primary coverage as follows:(1)In order to improve the ion conductivity and film-forming properties of the AEMs,the crosslinking network between SEBS and PPO was conducted to prepare the T3PPO-c-SEBS crosslinked membranes with multi-cation side chain tethering on the backbones.SEBS helps to improve the chemical stability and optimize the microphase morphology,while PPO helps to improve the film formation properties and ionic conductivity.According to this theory,the prepared T3PPO-c-SEBS membranes combine the advantages of these two kinds of polymer and show enhanced comprehensive properties for AEM applications.There SEBS-c-30%SEBS membrane exhibited high comductivity of 69.6 mS/cm at 80?,due to the distinct ion conduct channels with an interdomain spacing of 18.48 nm.Besides,thanks to the crosslinked architecture,the dimensional and chemical stability and the mechanical properties of the membrane are significantly improved.(2)On the basis of the above reaserch study,poly(biphenyl piperidine)(PBP)with better alkaline resistance was used for the preparation of PBP-c-SEBS composite membranes,where the piperidine is used as the cation conducting group to further improve the chemical stability.PBP-c-SEBS crosslinked membranes could maintain a hydroxide ion conductivity of more than 95%after immersing in the alkaline solution for 800 h at 80?,which proves that these membranes have good alkaline stability.In addition,the optimized microphase separation morphology can promote ion transport(? was 81.68 mS/cm,80?),and the crosslinked structure can restrain the swelling(Sr was 9.1%,20?).Besides,the PBP-c-SEBS membranes also becomes flexible and tough after crosslinking,which has a positive significance for fuel cell assembly and the practical work of fuel cells.(3)The comb-shaped structure was introduced into the SEBS copolymer framework for the first time,and the effects of hydrophobic side chains with different length on the swelling behavior,water uptake,microphase separation and ionic conductivity were systematically studied.The results shown that the prepared AEMs with hydrophobic side chain of 16 carbon atoms(SEBS-C16)exhibite the most excellent performance.In addition,the comb structure can improve the solubility of the polymer and it can solve the gelation problem in the process of functional modification of SEBS.Meanwhile,using molecular simulation,the principle of comb-shaped SEBS in improving solubility was explored,which laid a theoretical foundation for the modification of SEBS copolymer.(4)On the basis of comb-shaped SEBS-C16 with enhanced solubility,multi-cation crosslinker was grafted on the polymer backbone to prepare multi-cation crosslinked membranes(SEBS-C16-xC4).The addition of multi-cation cross-linker can not only improve the dimensional stability and mechanical properties,but also optimize the microphase morphology.The ordered microphase separated morphology with an interdomain spacing of 18.87 nm provides a hydroxide ion conductivity of 77.78 mS/cm(80?).Moreover,after 1700 hours of alkaline stability test(2 M NaOH,80?),the molecular structure of the multi-cation cross-linked membrane did not show significant degradation,and the ion conductivity remained 94.13%.The maximum single cell power of 182 mW/cm2 also confirmed the excellent practical value of this type of membrane.