Structure Design Of Anion Exchange Membrane For Alkaline Fuel Cell And PH 7 Redox Flow Battery

Fuel cell can efficiently convert hydrogen energy into electrical energy without emission of polluting chemicals,thereby is appropriate for new energy vehicle.Among various fuel cell types,proton exchange membrane fuel cell is most developed while its application is impeded by the cost due to the use of noble metal catalyst and expensive Nafion membrane.In contrast,alkaline fuel cell,using non-precious metal catalyst and inexpensive anion exchange membrane(AEM),is more promising for practical application.However,exploring an AEM with high hydroxide conductivity and long-term stability remains a challenge.Flow batteries can stabilize the power grid of solar and wind energies with their storage and scalability features.The most widely studied flow battery is the all-vanadium redox system.However,some limitations,such as the scarcity of vanadium,the strong acid ambient and the high-cost Nafion membrane,delay or prevent the widespread commercialization of this technology.Therefore,researchers start to focus on aqueous organic redox flow battery because of safety operation,affordable material cost and high power density.Considering the corrosivity and stability of device,pH 7 redox flow battery,using an AEM to fast conduct Cl-and steadily separating cathode from anode,is the best choice.To address the challenge of alkaline fuel cell and pH 7 aqueous organic redox flow battery,designing and preparing AEMs with novel structure and superior property are vital.Most commercial AEMs are main-chain type,resulting in the alkaline degradation of polymer main chain induced by cation charges and the limited mobility of functional groups.Therefore,they are unstable in alkaline environment and exhibit low hydroxide conductivity.Inspired by the structure of Nafion membrane,researchers designed a pendant type AEM and demonstrated that this membrane is very stable in alkali and can fast transport hydroxide ions.However,to achieve the structure,noble metal catalysts and extreme conditions are always needed,according to previous researches.Besides,to improve the mechanical stability and mitigate the water swelling of AEM,cross-linking,as an effective strategy,is commonly utilized and can be readily carried out.However,AEMs cross-linked via covalent bonds cannot be dissolved or melted anymore and that makes them difficult to reprocess,recycle or heal the "injured" or"retired" AEMs.Therefore,to make AEMs more stable,sustainable and cost-effective,exploring alkaline stable,recyclable and self-healing AEMs that can be readily synthesized is critical.The detailed contents are summarized as follows:(1)A pendant type AEM with flexible quaternary ammonium groups attached to poly(2,6-dimethyl-1,4-phenylene oxide)backbone via length-tunable spacers were successfully prepared by mild and one-pot Wittig reaction.The membrane with 6C spacers exhibits highest hydroxide conductivity(99.5 mS/cm at 80 °C),restricted water swelling ratio(2.91%at 30 ?)and good chemical stability in alkali at 80 ?.(2)To fabricate recyclable AEMs,we developed an AEM cross-linked with disulfide bonds.When cross-linked,the membrane shows enhanced mechanical strength and suppressed water swelling.Exploiting the formation of disulfide bonds from-SH groups,the membrane can be readily cross-linked in alkali and recycled by reversibly breaking disulfide bonds with dithiothreitol.The recycled membrane solution can be directly utilized to cast a new AEM.During the recycling process,the membrane shows decreased hydroxide conductivity while acceptable fuel cell performance.(3)We incorporate self-healing to an AEM through reversible and temperature-controlled Diels-Alder(DA)reaction.We first synthesized a block copolymer from vinylbenzyl chloride and 2-((4-vinylbenzyloxy)methyl)furan by RAFT polymerization.The benzylic chloride moiety was quaternized with trimethylamine to conduct anions while bismaleimide was added to dynamically connect the furan moieties via DA reaction.By solution casting and reacting at 80 ?,an AEM with dynamic network was prepared.When evaluated for practical application,we found the AEM has a Cl'conductivity of 32.7 mS cm-1 at 80 ? and man-made cracks on the membrane can be self-healed.We then tested its performance in a pH 7 aqueous organic redox flow battery,where it delivered stable cycling performances over 100 consecutive charge/discharge cycles,with a coulombic efficiency of>97%and an energy efficiency of>79%.