Investigation On Structure-property Relationship Of Perfluorosulfonic-acid Membrane For Proton Exchange Membrane Fuel Cells

As the critical component of proton exchange membrane fuel cell(PEMFC),perfluorosulfonic-acid(PFSA)membranes' performance and durability,to a large extent,determine the overall performance and durability of PEMFC.Hence,the objective of this dissertation is to investigate the impact of PFSA membrane chemistry structure,cation as well as environmental effect such as ageing on the structure/property relationship of PFSA membrane.In addition,the possible influence it has on membrane durability and stability is also investigated.Understanding factors controlling PFSA membrane structure-property relationship will be helpful for optimizing and designing better materials.The roles of PFSA membrane backbone length and side-chain length play in influencing the structure-property relationship are investigated.Particularly,their effects on water content and proton conductivity are studied,and a universal relationship is found between water content and proton conductivity over a large range of relative humidity.In addition,the relationship between crystallinity and water content as well mechanical property is found,and the structure-property relationship of PFSA membrane is thus established.The impact of cations on PFSA structure-property relationship is studied,in particular the nano-structure changes and corresponding transport property.This study involves nano-structure and conductivity changes over a wide range of water content,ranging from water vapor at different relative humidity(RH)to liquid water.The role of water plays during this process is addressed.The overall properties of Nafion membranes in different cation forms is related to the interaction between fixed sulfonate groups and cations through the amount of water it absorbs.In addition,the water content in the membrane is dominated by storage modulus,revealing the underlying structure-transport-stability relationships.The influence of hygrothermal ageing environment on PFSA structure-property relationship is also investigated.It is found that the EW and storage modulus increase upon hygrothermal ageing while the water uptake,proton conductivity and hydrophilic domain decrease which is attributed to the formation of anhydride through condensation reaction.The extent of hygrothermal ageing effect on PFSA membranes is related to the pretreatment procedure and hygrothermal ageing environment.A universal relationship is found to exist between hydrophilic domain spacing and crystalline domain spacing before and after hygrothermal ageing.The relationship between nano-structure changes and water uptake is also established.The water transport property of PFSA membranes with different chemistry structures is thoroughly studied through water permeation experiments.In particular,the effect of temperature and RH on water transport interfacial resistance and water diffusion coefficient is investigated where interfacial resistance is found to dominate at low temperature and high relative humidity.The relationship between PFSA membrane chemistry structure and water transport property is established.The presence of reinforcement layer on the structure-property relationship of Nafion XL composite membrane is explored.The impact of the introduction of reinforcement on swelling is discussed,and the relationship between water uptake and swelling is established.In addition,the effect of pretreatment on microstructure and performance of Nafion XL composite membrane is also investigated,and the role pretreatment plays in altering the structure and property is elucidated.The thermomechanical property of PFSA membrane is also investigated.The dependence of mechanical property on loading path is studied,and factors influencing PFSA lifetime are discussed.