Fabrication And Intensive Studies Of Microstructures And Macro-Properties Of Nafion Composite Proton Exchange Membranes

Nowadays,energy dilemma,environmental pollution and water scarcity etc.are growing problems.Thus,green power sources such as solar energy,hydrogen energy,wind energy etc.attract much attention as the promising alternative power sources in many fields including military science,environmental protection transportation and communication technology and so on.Recently,proton exchange membrane fuel cells?PEMFCs?show extensive application prospects as the alternative energy sources,because of their high energy conversion efficiency,high power density,low pollution,and fast start-up.Proton exchange membrane?PEM?,which provides proton conduction between fuel and oxidant like hydrogen and oxygen,is one of the key components of PEMFCs.Nafion membranes?the perfluorinated sulfonic-acid?PFSA?based membranes?have been widely used as PEMs in PEMFCs due to their remarkable proton conductivity and chemical stability under various humidity and temperature conditions.However,the widespread application of PEMFCs is greatly limited by the sharp degradation in electrochemical properties of the proton exchange membranes under high temperature and low humidity conditions.In this study,the microstructure evolutions in pristine Nafion and kinds of hybrid Nafion membranes were investigated by the positron annihilation technology,which is correlated to the macroscopic properties,such as mechanical strength,water uptake and proton conductivity,of the membranes under various environmental conditions.1.Effects of thermal annealing on free volumes,backbone mobility,crystallinity and proton conductivity of Nafion membranes were investigated and discussed.During water uptake,results suggested that the variations in free volumes in Nafion membranes were closely associated with the mobility of Nafion backbones.A significant decrement in mobility of Nafion backbones was found in Nafion membranes annealed at higher temperatures,which was attributed to the gradual recrystallization in Nafion membranes induced by thermal annealing.Interestingly,a dramatic decrement in the mobility of Nafion backbones caused by a significant increment in crystallinity of Nafion membranes was found with increasing annealing temperature,which is likely attributed to the melting and re-crystallization of a large number of small,imperfect crystallites in Nafion membranes.From point of view of the high water uptake,mechanical property,and proton conductivity,annealing at 140? of Nafion membranes is helpful for their application in the industrial PEMFCs.2.Pristine and metal oxide nanoparticles/Nafion hybrid membranes were prepared.Due to the incorporation of those hydrophilic metal oxides in membranes,both the Nafion-Ti0₂ and Nafion-Si0₂ membranes exhibited obviously enhanced water retention abilities.As a result,proton transportation in them is more efficient at lower relative humidities than in the pristine one.Particularly,well-connected water channel network for high efficiency proton transportation in Nafion-Ti0₂ membranes were found can be formed under a considerably low relative humidity of?40%RH.At high humidities,the formation of extra ionic-water cluster phases around TiO₂ or SiO₂ nanoparticles in Nafion matrix provided additional proton transport pathways and reduced the overall tortuosity of proton transport network in the Nafion-Ti0₂ or Nafion-Si0₂ hybrid membranes,which is responsible for their obviously enhanced proton conductivity.Particularly,with the water content reaches?4.5 wt%,the percolation of ionic-water clusters in Nafion membranes results in a percolation behavior of proton conductivity.Meanwhile,results showed that numerous water channels could be formed with the water content of?6 wt%.3.With increasing environmental humidity,PALS results showed that the variations in o-Ps lifetime??0-Ps?in both pristine and metal oxide nanoparticles/Nafion hybrid membranes were closely associated with the development of hydrophilic ion clusters.With increasing humidity,the maximum value of ?0-Ps corresponds to the formation of well-connected water channel network in membranes,regardless of metal oxide additives.The phase morphology in Nafion,which basically governs its unique proton transport capability,is very sensitive to the ambient environment.Since the free volumes in ionic-water cluster phase and Nafion matrix phase are significantly different in size,two average o-Ps lifetime distributions with good fittings of positron annihilation lifetime spectra?for Nafion membranes with relatively high water content?enable one to find out two o-Ps lifetime components?bimodal lifetime distributions?,which respectively represent the free volumes in the ionic-water cluster phase and the Nafion matrix phase.The variations in the intensities of these two o-Ps lifetime components vividly demonstrated the enlargement of ionic-water cluster phase in Nafion membranes during water uptake.4.The high-performance sulfonated carbon nanotubes/Nafion hybrid membranes?Su-CNTs/Nafion?for the PEMFCs were prepared,and the mechanism of the microstructures on the macroscopic properties of membranes was intensively studied.PALS results revealed that the Su-CNTs or CNTs in hybrid membranes show significant reinforcement on Nafion molecular chains,which retarded the development of ionic-water clusters in membranes as a function of relative humidity.Proton conductivities in Su-CNTs/Nafion composite membranes were remarkably enhanced due to the formation of numerous proton conducting pathways?water channels?along the Su-CNTs.In particular,these proton conducting pathways interconnected isolated ionic-water clusters at low humidity,and resulted in less tortuosity of the water channel network at high humidity.At a relatively high temperature of 135 ?,proton conductivity of Su-CNTs/Nafion composite membranes still maintained a high level because the reinforcement of Su-CNTs on Nafion molecular chains reduced the evaporation of water molecules from membranes as well as the hydrophilic Su-CNTs were helpful for binding water molecules.5.A certain amount of?1 wt%,3 wt%,5 wt%?sulfonic acid groups modified multi walled carbon nano tubes?Su-CNTs?were suspended in Nafion solutions,and the suspension were casted in magnetic fields of?1.0 T,at room temperature.In this way,the Su-CNTs in fabricated membranes tend to orient with the tube axis parallel to the fields because of the susceptibility anisotropy,therefore the new type of Nafion/oriented Su-CNTs composite membranes has been prepared.In the membranes,numerous water channels formed along the Su-CNTs tend to orient with the tube axis parallel to the fields,which results in strong anisotropic proton conductivities of Nafion/oriented Su-CNTs composite membranes membranes casted under magnetic fields.Therefore,the magnetic orientation of carbon nanotubes provides another method to further increase the proton conductivity in Nafion/Su-CNTs membranes.