Preparation And Properties Of High Temperature Proton Exchange Membranes

Fuel Cell is a kind of electrochemical device which can convert the chemical energy of fuel and oxidizer directly into electric energy without going through the combustion process.As one of the most important green technologies of the 21 st century,fuel cell have many outstanding advantages,such as: high energy conversion rate,low pollutant emissions,wide range of sources of fuel and higher safety factor.Facing the world energy shortage and environmental pollution problems,fuel cell have unparalleled advantages,it can help people solve the environmental pollution problems and dependence on oil and other non renewable resources.Fuel cell can be divided into many types,one of the most widely studied and applied is the proton exchange membrane fuel cell(PEMFC).Proton exchange membrane fuel cell has the characteristics of high energy density,long life,etc.it can be used in aerospace,military and electric vehicles and other fields.Among them,as the core component of fuel cell,proton exchange membrane plays the role of conducting proton,isolating fuel and electrons.Currently,the world's most successful commercialization of proton exchange membrane is perfluorinated sulfonic acid membrane Nafion,developed by the United States Du Pont Co.It has many outstanding advantages,such as high proton conductivity,good chemical stability,etc.However,there still exist many shortcomings and deficiencies:(1)High production cost.The coefficient of perfluorinated compounds synthesis and sulfonated technology are very high;(2)High methanol permeability;(3)The best working temperature of Nafion membrane is 80 ?,it will lead a sharp drop in water content when the temperature is above 80 ?,and the conductivity will decrease drastically.In order to overcome these shortcomings,countries around the world are using non-fluorine materials to prepare proton exchange membrane to improve the heat resistance of the polymer and reduce the dependence of the membrane on water molecules.Increasing the operating temperature of the fuel cell can greatly reduce the poisoning of the catalyst.At present,one of the most current research and application of high temperature proton exchange membrane is the phosphate doped polybenzimidazole(PBI)membrane.PBI has excellent thermal stability and phosphoric acid adsorption capacity,with a high operating temperature,however there are also some shortcomings and deficiencies that limit the use of materials,such as:(1)It is easy to cause experiment operator physical injury because of the strong toxicity of monomer.(2)In the synthesis process of PBI,the conversion rate of monomer is not high,so it is difficult to achieve complete polymerization,and the product is prone to its own degradation reaction;(3)It is difficult for PBI to dissolve in common solvents(NMP,DMSO,etc.).Researchers attempt to synthesize low molecular weight PBI polymers,however,low molecular weight will reduce the mechanical properties of membrane materials.Based on the above ideas,researchers try to prepare new high-temperature proton exchange membrane materials with higher molecular weight,better thermal stability and side chain containing basic heterocyclic groups.We assumed that,the phosphonic acid molecules can be adsorbed by the basic groups with strong side chain motility and the phosphorous acid doping content of the membrane material is increased to obtain the proton exchange membrane material with excellent comprehensive performance.In this paper,first of all,bromomethylated poly(aryl ether ketone)was prepared by bromomethylation reaction of N-bromosuccinimide(NBS)reagent at the benzyl position of poly(aryl ether ketone).The quaternized poly(aryl ether ketone)containing 1-butylimidazolium onium salt on side chain was synthesized by reacting bromomethylated poly(aryl ether ketone)with 1-butylimidazole.Compared with PBI,the cost of the polymer is lower,and the content of the imidazolium group in the structural unit can be precisely controlled,thereby controlling the doping level of the phosphoric acid in the membrane.However,the phosphoric acid doped high temperature proton exchange membrane usually has a significant swelling phenomenon of the volume at the higher phosphoric acid doping level,resulting in a decrease in the mechanical strength of the film,deterioration of the dimensional stability,and even loss of the basic mechanical strength of the material.In order to improve the mechanical strength of the phosphonic acid doped film without losing the proton conductivity,in this paper,the inorganic phase precursors containing quaternary ammonium groups were prepared by ring opening reaction of aminopropyltrimethoxysilane(KH550)and 2,3-epoxypropyltrimethylammonium chloride(EPTAC).Then,the precursor solution and the membrane solution were mixed uniformly,and the Si-O-Si network structure was generated by hydrolysis reaction,which improved thethermal and mechanical stability of the composite film.At the same time,due to the presence of Si-OH and quaternary ammonium groups in the inorganic phase,it has a certain adsorption capacity of phosphoric acid,and can improve the proton conductivity of the composite membrane.