Multi-block Sulfonated Proton Exchange Membrane Bearing Base-Modified Side Chains:Syntheses And Fuel Cell Applications

To address the increase in population and associated resource shortages around the world during the past half-century,there has been an urgent need for regenerable and sustainable energy sources,which has gained unprecedented attention regarding the development of fuel cell technologies.In this study,we will synthesize a novel sulfonated poly（arylene ether nitrile）（SPAEN）multi-block structure by introducing a key component,alkyl basic group side chains,into the hydrophobic segments.It is expected that the modified membranes simultaneously have excellent proton conductivity at low RHs and antistress property at various water contents.The effects of the type and content of basic groups on the membrane properties were studied by introducing basic groups with different p K_avalues and controlling the control of basic block length.The basic groups involved in this study are benzoxazole（Bo,p K_a=0.5）,benzotriazole（Bt,p K_a=1.6）,and propylamine group（Bc,p K_a=10.5）.The study will be divided into two parts.Firstly,a series polymers bearing alkyl Bo（M-BO）,Bt（M-BT）,and Bc（M-BC）side chain were synthesized via classic polycondensation in the presence of K₂CO₃between F-terminated and OH-terminated oligomers.The modified structure could facilitate polymer phase-separation,and it effectively suppresses the excessive swelling of all membranes owing to strong electrostatic interactions between the basic group and sulfonic acid groups.The proton conductivity of the M-BC was significantly lowered owing to the strong basicity of propylamino groups,which may neutralize the membrane acidity.It is worth noting that M-BT exhibited the most superior performance among all three membranes,which was probably due to the“protonation-deprotonation”loop formed between the hydrophilic and hydrophobic moieties,strengthening the conductivity by lowering the ion transfer barrier.In order to further study the effect of basic group content on the performance of proton exchange membranes,we prepared three SPAEN-Bt series proton exchange membranes with different Bt block ratios.The modified structure could facilitate polymer phase-separation and generate self-standing films with excellent mechanical properties,and it effectively suppresses the excessive swelling of the membrane owing to strong electrostatic interactions between the Bt chains and sulfonic acid groups.Moreover,the Bt unit could act as both a proton acceptor and proton donor,causing a dramatic increase in the ion conductivity of the membrane.The most optimal membrane possesses an ionexchange capacity of 2.15 meq g^-1and exhibits a weaker relative humidity（RH）dependence and higher proton conductivity than the commercial Nafion 212 over the entire RH range.Remarkably,the maximum power output of the fuel cell based on the most optimal membrane reaches 1090,856,and 451 m W cm^-2at 95%,70%,and 30%RH,respectively,which are more than 2 times higher than those of the non-Bt analogue.Further,the current densities（I0.6）ranging up to 1500 and 1000 m A cm^-2（0.6 V）at 95%and 70%RH are both much higher than those of the Nafion.Our study provides a novel methodology for the design of aromatic ionomer structures with excellent performances for practical fuel cell application.Overall,based on the practical operating conditions of a PEM fuel cell,our results comprehensively reveal the influences on the physicochemical properties brought about by the different basic groups,providing basic data and theoretical support for the future design of high-performance PEMs.