Design,synthesis And Properties Of Highly Branched Side-chain-type Polybenzimidazole Anion Exchange Membranes

Anion exchange membrane fuel cells（AEMFCs）have attracted wide attentions because of their advantages such as high electrode reaction rate and available for non-precious metal catalysts.Anion exchange membranes（AEMs）are as one of the critical constituents of AEMFCs,which directly determines the output power and service life of fuel cells.Unfortunately,the current AEMs are facing two major challenges:（1）low hydroxide conductivity;（2）poor alkaline stability,which makes AEMs hard to meet the practical requirements of AEMFCs,immensely impeding the further development and large-scale applications of AEMFCs.Therefore,simultaneously developing and preparing AEMs with high hydroxide conductivity,robust alkaline stability and excellent mechanical properties is of great significance to promote the development of AEMFCs.Polybenzimidazole（PBI）possesses excellent physical chemistry properties,which has been widely used in high temperature proton exchange membranes and redox flow battery separators fields.However,there are relatively few reports about PBI-based AEMs,especially for side-chain-type PBI-based AEMs.Piperidinium cationic groups exhibit excellent alkaline stability owing to the existence of strain-free six-membered ring andβ—H,which has recently become a research hotspot.In addition,our groups have found that the expanded free volume of branched structure in polymer chains is conductive to improving the performance of membranes.Based on above,in terms of material selection and membrane structure design,a series of high performance side-chain-type PBI-based AEMs have been prepared by design of polymer architecture under taking PBI as the backbones and piperidinium cationic group as the functional groups of AEMs.This study looks forward to providing an effective strategy to address the existing problems of the current AEMs.The main contents are as follows:（1）By molecular design,four kinds of side-chain-type PBI-based AEMs with different molecular topologies structure were successfully synthesized and prepared.The results demonstrated that the branched structure containing hydrophobic side chain remarkablely enhanced the hydroxide conductivity and alkaline stability of AEMs,comparing with the branched non-fluorine,linear fluorine-containing and linear non-fluorine structures under the same IEC.AFM and SAXS proved that the synergistic effect of the branched structure and fluorinated hydrophobic side chain induced the formation of a well-defined microphase separation structure and constructed an efficient ion transport channels.Therefore,FB-OPBI-6-g-MPRD-2.5 membrane exhibited the highest hydroxide conductivity,up to 80.8m S cm^-1 at 80℃.This membrane still maintained about 96.55%of its initial conductivity after being immersed in a 1 mol L^-1 aqueous NaOH solution at 60℃ for 593 h.（2）To further improve hydroxide conductivity and alkaline stability of AEMs.Based on the previous chapter,piperidinium-functionalized highly branched polybenzimidazole containing hydrophobic side chain with differert IEC were prepared via manipulating the grafting degree of Br-6-MPRD onto B-OPBI-6.The results revealed that the water uptake,swelling ratio and hydroxide conductivity of AEMs increased with the increasion of IEC,while the mechanical properties and alkaline stability decreased with the increasion of IEC.FB-OPBI-6-g-MPRD-3.0 membrane had the highest IEC,showing the distinctest microphase separation morphology,whose hydroxide conductivity reached 92.40 m S cm^-1(>80.8 m S cm^-1)at 80℃.FB-OPBI-6-g-MPRD-1.0 membrane had the lowest IEC,which could retain over 2078 h in a 1 mol L^-1 aqueous NaOH solution at 80℃ without obvious degredation.（3）To reduce the grafting degree and achieve high hydroxide conductivity at relatively low swelling and IEC,branched ionic clustered side-chain functionalized highly branched polybenzimidazole AEMs were synthesized and prepared.AFM,SAXS and SEM indicated that the as-prepared FB-OPBI-6-HQA-x membranes formed a clear hydrophilic/hydrophoobic phase-separated structure after introducing rigid branch structure and flexible branch ionic cluster side chains into polymer main chain at the same time,expanding free volume within membranes and constructing high efficiency ion transport channels.FB-OPBI-6-HQA-100%membrane achieved the highest hydroxide conductivity of 70 m S cm^-1 at 80℃.When this membrane was soaked in a 1 mol L^-1 aqueous NaOH solution at 60℃ for 87 h,its hydroxide conductivity was only lost by 5.99%（<10%）,showing excellent alkaline stability.