Preparation And Characterization Of Imidazole Functionallized Polymers For High Temperature Proton Exchange Membranes

Fuel cell is a kind of energy conversion device,which can directly convert the chemical energy from the fuel and oxidants into electric energy via electrochemical reations.Among the various types of fuel cells,the proton exchange membrane fuel cell(PEMFC)is considered as the preferred power source for using in various vehicle power supply and portable sources due to its advantages such as rapid start,high power density and simple construction.However,the limitation on proton exchange membrane(PEM)includes complex preparation,poor mechanical properties,excessive swelling after acid doping,high cost and so on.All of these impede the development and application of the PEMs.A series of membrane electrolyte materials were designed and preprared by various methods,such as crosslinking,organic/inorganic or organic/organic composites.The performance of those membranes as high temperature proton exchange membranes(HT-PEMs)was comprehensively studied.Evaluation of the prepared membranes as electrolytes for PEMFCs was made by providing necessary experimental data.Polyepichlorolydrin(PECH)has the characteristics of high temperature resistance,corrosion resistance and low price.To my knowlege,PECH is mainly used for preparation of anion exchange membranes,seldom used for PEM.In addition,the flexibility and mechanical strength of the PECH membrane need to be improved.Imidazole grafted PECH membranes with good flexibility and good mechanical properties are introduced in chapter three.Both methylimidazole(Melm)and triethoxysilypyldihydroimidazole(SiIm)were grafted onto the PECH to obtain the proton conductors after doping phosphoric acids.The results indicate that the composite membranes have good thermal stability and high conductivity.A conductivity as high as 0.068 S/cm was reached by the composite membranes at 180℃.The PECH-50%SiIm-50%MeIm/PTFE based fuel cell exhibited an open circuit voltage of 0.92 V.At 0.5 V,the current density of PECH-50%SiIm-50%Melm/PTFE based cell increased from 128 mA/cm2 to 225 mA/cm2 when the temperature changed from 120℃ to 180℃.Polyphenylene oxide(PPO)is widely applied to prepare PEMs,while its rigid phenyl groups lead to stress cracking and poor tensile stress.In chapter four,the composite membrane of imidazole functionalized PPO(BPPO-SiIm-MeIm)with micro-porous PTFE is described.The skeleton structure of PTFE has great toughness and a large number of pores in the structure benefits to composite the ionic conductor(BPPO-SiIm-MeIm)and PTFE.The results indicate that BPPO-SiIm-Melm/PTFE had good thermal stability and it was stable at temperature less than 220℃.The conductivities of BPPO-60%SiIm-40%Melm/PTFE/2.37PA and BPPO-50%SiIm-50%MeIm/PTFE/2.42PA at 180℃ were 0.079 S/cm and 0.081 S/cm,respectively.Their values of tensile stress at room temperature were 4.17 MPa and 3.63 MPa,respectively.Polyvinyl chloride(PVC)has the advantages of forming film easily,low price and great compatibility.In chapter five,the prepared composite membranes based on PVC are presented.Different proportions of MeIm and Silm were grafted onto the PVC first.The hydrolysis reaction of SiIm was then occurred in sulfuric acid solution to form a crosslinked Si-O-Si network structure for reducing swelling and enhancing mechanical properties of the membranes.The results show that at 180℃，the conductivity of PVC-100%SiIm/2.80PA and PVC-90%SiIm-10%MeIm/1.52PA were 0.112 S/cm and 0.076 S/cm.Their values of tensile stress were 6.01 MPa and 11.82 MPa at room temperature,respectively.