| Polymer electrolyte membrane fuel cell is a new type of energy conversion device,which has the merits of high energy conversion efficiency,quick starting speed,and environmentally friendly etc.Polymer electrolyte membrane is the core component of polymer electrolyte membrane fuel cell,which directly determines the performance and lifetime of fuel cell.Polymer electrolyte membrane can not only isolated the fuel and oxidant,but also hinder the transmission of electron.The most important thing is that it can be used as the ion carrier and promote the ion transport.Polymer electrolyte membrane can be divided into two categories: proton exchange membrane and anion exchange membrane.Currently,the commercialized proton exchange membrane is Nafion membrane.The commercialization process of Nafion membrane has been seriously hindered due to its complex preparation process,high cost and poor performance at medium-high temperature.Therefore,the development of novel polymer electrolyte membrane with low cost and excellent performance has become the focus of research work.At present,the research focuses on the development of polymer electrolyte membrane is mainly concentrated in the following two aspects: one is to develop the proton exchange membrane with low cost,simple preparation process,high proton conductivity and excellent thermal stability;the other one is to develop anion exchange membrane with good alkaline stability and high ion conductivity,which can reduce or avoid the use of noble metals platinum catalyst.Aromatic polymers,such as poly(arylene ether ketone)and poly(arylene ether sulfone)have excellent thermodynamic stability,good chemical stability and relatively low cost.The modified aromatic polymers have been widely used in the field of polymer electrolyte membrane,because they can inherit the original advantages of aromatic polymers.However,as proton exchange membranes,sulfonated aromatic polymers exhibit poor dimensional stability and high methanol permeability at high operating temperature and high degree of sulfonation,while as the anion exchange membranes,quaternary ammonium aromatic polymers usually suffer from low ion conductivity and the decomposition of quaternary ammonium groups under strong alkaline environment.In order to solve the above problems and develop high performance polymer electrolyte membrane materials,this paper designs a series of targeted solutions.First of all,for sulfonated aromatic proton exchange membranes,we adopted the method of crosslinking modification and side chain sulfonation to solve the problems of high methanol permeability,poor dimensional stability and low proton conductivity.Sulfonated poly(arylene ether ketone sulfone)s containing pendant amino groups(Am-SPAEKS)were synthesized by condensation polymerization.Sulfonated poly(vinyl alcohol)(SPVA)was prepared by post sulfonation.The SPVA was used as macromolecular crosslinker due to its high molecular weight and large amount of functional hydroxyl groups.The crosslinked membranes were prepared by heat treatment.The water uptake and swelling ratio of the crosslinked membrnaes were all controlled below 10 % at 80 oC.The methanol permeability coefficients of the crosslinked membranes were 0.32 × 10-7cm2 s-1 at 20 °C and 0.96 × 10-7 cm2 s-1 at 60 °C,respectively,when the SPVA content reach at 80 %.The crosslinking modification can effectively improve the dimensional stability of the membranes.However,sulfonic acid groups are consumed during the reaction,which affects the proton conductivity of the membranes.It is found that it is more conducive to improve the performance of membrane when the sulfonic acid group is located on the side chain of polymer.This is because that the sulfonic acid groups attached to the flexible side chains,which is far away from the polymer main chain.In this way,the mobility of the sulfonic acid groups could be affected by the polymer backbone at relatively low degree.Thus the formation of ionic transportation channel could be promoted and the proton conductivities could be improved.In addition,the hydrophilic/hydrophobic phase structure can effectively inhibit the water swelling behavior of the polymer membrane,thus improving the dimensional stability of the membrane.Therefore,in the fourth chapter,we have prepared the side-chain type sulfonated poly(arylene ether ketone sulfone)s crosslinked membrane with different degree of crosslinking.S-SPAEKS-2 showed the highest water uptake(20.62 %,at 80 oC)and very low swelling ratio(7.09 %,at 80 oC).S-SPAEKS-2 exhibited the highest proton conductivities(0.046 S cm-1at 20 oC,0.095 S cm-1at 80oC).The results showed that the method of preparing side-chain type sulfonated crosslinked membrane can effectively solve the problem of poor dimensional stability and low proton conductivity.However,the functional groups can be consumed during crosslinking reaction,which can reduce the amount of sulfonic acid groups and affect the proton conductivity.In order to further improve the proton conductivity,sulfonated poly(arylene ether ketone sulfone)s copolymers(S-Am-SPAEKS)containing sulfonic acid groups both in the main chain and side chain were prepared.Novel side-chain type crosslinked composite membranes were prepared by the combination of S-Am-SPAEKS and sulfonated poly(arylene ether ketone sulfone)s containing carboxylic acid groups(C-SPAEKS).The water uptake of S-Am-SPAEKS-2.0/C-SPAEKS was 12.6 % at 80 oC.And the swelling ratio of S-Am-SPAEKS-2.0/C-SPAEKS was only 3.59 % at the same temperature.In addition,the proton conductivity of S-Am-SPAEKS-2.0/C-SPAEKS was 0.135 S cm-1(at80 oC),which was higher than that of Nafion(0.100 S cm-1).This is because that the hydrophilic/hydrophobic phase structure and the strong acid-base interaction between the amino and carboxyl groups are conducive to the construction of proton transport channels and the proton transport.For the anion exchange membrane,the stability of imidazole group is higher than that of quaternary ammonium group in alkaline environment.A novel series of imidazolium-functionalized anion exchange membranes(Im-Am-PAEKS),based on poly(arylene ether ketone sulfone)s containing pendant amino groups were synthesized for alkaline fuel cells.The 1-allyl-3-methylimidazolium chloride was introduced by grafting reaction.The Im-Am-PAEKS membranes exhibited excellent alkaline stability.The structure of Im-Am-PAEKS membrane was not changed after alkaline treatment(2M NaOH aqueous solution)for 300 h.The conductivity of Im-Am-PAEKS-3 membrane after alkaline treatment only reduced to 87.2 % of its initial ionic conductivity.The ionic conductivities of the Im-Am-PAEKS membrnaes increased with the increasing imidazole group content.The Im-Am-PAEKS-3 had the highest conductivity(0.139?10-2 S cm-1)at80 oC.In summary,the results indicate that the imidazole functionalized poly(aryl ether ketone sulfone)s anion exchange membranes have a great potential and application prospect in alkaline fuel cells. |
PDF Full Text Request