Study On In-situ Construction And Optimization Of Self-assembled Hierarchical Proton Conductive Channels In Hyperbranched Highly Selectivite Proton Exchange Membranes

Proton exchange membrane is widely used in electrochemical energy conversion devices,including proton exchange membrane fuel cell(also direct methanol fuel cell,DMFC),the SPE water electrolysis and vanadium redox flow battery(VRB).Nafion membranes domain the proton exchange membrane market due to the excellent proton conductivity,satisfied chemical and oxidative stability.However,the relatively low selectivity of the Nafion membranes for proton transportation limits the large scale application in DMFC and VRB technologies.Low selectivity of Nafion would lead to severe methanol permeation in DMFC application.The high methanol permeation will reduce the power output and energy density of DMFC simultaneously and therefore hinders the large-scale commercialization of DMFC.It is hence urgent to develop novel proton exchange membranes with higher selectivity than the Nafion membrane.In the VRB,the low H+/V selectivity of Nafion membrane means high vanadium permeability through the Nafion membrane during the VRB operation,leading to a decrement on the coulomb efficiency and energy efficiency.Above all,it is important to design and develop proton exchange membrane with high selectivity,excellent chemical stability and low cost for both DMFC and VRB.Well-connected proton conductive channels(PCCs)are required directly affect the proton conductivity and selectivity of proton exchange membrane.Therefore,self-assembled hierarchical PCCs(HPCCs)with high selectivity are in-situ constructed in the hyperbranched proton exchange membranes in this thesis.By tuning the microstructure of polymers,HPCCs are optimized for ultra-high selectivity.1.The concept of HPCCs is verified by synthesis of the hyperbranched polyamide macromolecules,which are able to effectively minimize the methanol permeation and improve the proton conduction.The methanol resistivity of the proton exchange membrane shows 15 times higher than that of Nafion 117.In the HPCCs,the high sulfonic acid groups within the macromolecules were defined as the first-order PCC(FOPCC)and the hydrogen bonding connections among the macromolecules were defined as the secondary-order PCC(SOPCC).Due to the synergistic effect of the FOPCC and SOPCC,the hyperbranched polyamide proton exchange membrane exhibits proton conductivity high to 0.282 S/cm(80 oC).The HPCCs in the polyamide based PEMs was facilely optimized by tuning the monomer for hyperbranched polyamide synthesis.Upon appropriate HPCC optimization,methanol crossover of the PEMs was further reduced with the selectivity improved by 123%.2.Basis on the conception of HPCCs,microstructure of FOPCCs was tuned aiming to improve the methanol resistivity.Two polyamide macromolecules with-COOH end-capping and different –SO3H concentration were synthesized and hybrid with different weight ratio.Due to the different high-SO3 H concentration of the two macromolecules,the FOPCCs consisted of-SO3 H were modified for better methanol-permeation resisitive property and selectivity.3.Similarly,structure of the SOPCCs was optimized by appropriate adjustment of the water retention capacity and the hydrogen bond strength of the SOPCCs to improve the overall performance of the PEMs.Hyperbranched polyamide macromolecules with similar size and different end-capping groups(-COOH and-NH2)were designed and synthesized for the hybrid membranes preparation.The SOPCCs in the membrane was therefore optimized by tuning the mass proportion of the two polyamide macromolecules.The optimization of the SOPCCs would lead to an improvenment on selectivity of 14.1×104 S.s/cm3.4.Based on the previous developed hyper-branched polyamides proton exchange membrane with high selectivity,the application potential of hyper-branched polyamid proton exchange membranes in VRBs was studied.The compact structure of the HPCCs in the hyper-branched polyamide proton exchange membrane effectively blocked the permeation of vanadium ions.The HBP membranes exhibited appropriate chemical and satisfactory properties with high H+/V selectivity to 14.4×104 S.s/cm3,3 times higher than that of the Nafion117 membrane.5.Due to the great chemical stability,hyperbranched polyimide macromolecule was designed and synthesized for high selectivity HPCC construction.With 25% better mechanical strength than that of the polyamide macromolecule proton echange membrane,methanol permeability through the blend polyimide macromolecule proton exchange membrane was low to undetectable.Above all,proton exchange membranes with high selectivity were developed by constructing high efficiency hierarchical self-assembly proton conductive channels via molecular design.With excellent mechanical and oxidative stabilities,the thesis points out a new direction for the development of novel proton exchange membrane for DMFC and VRB application.