Structural Design And Synthesis Of Highly Stable Alkaline Polymer Electrolytes

Alkaline polymer electrolytes(APEs)are a new promising class of materials that enable get rid of noble metal to be used as catalysts in electrochemical energy conversion devices.The latest research in 2012 found that,upon being grafted with cations in proximity,some polar groups in the backbone of such APEs can be attacked by OH-,leading to the backbone degradation in alkaline environment obviously,which effect the life of fuel cell directly,this is the new challenge faced by APEs study.A systematic study about how to improve the backbone stability of aromatic ether based APEs will be demonstrated in this thesis,the main results are summarized as follows:1.Four functional modified aromatic ether based backbones(polysulfone,polyphenylsulfone,polyetheretherketone and polyphenylether)were synthesized successfully,the amount of quaternary ammoniums on APEs were controlled by the grafting degree of functional modification groups.The degradation of bakebones based on quaternary ammonium polysulfone(QAPS),quaternary ammonium polyphenylsulfone(QAPPSU),quaternary ammonium polyetheretherketone(QAPEEK)and quaternary ammonium polyphenylether(QAPPO)with similar ion exchange capacity(IEC)in a hot alkaline solution(1 mol/L KOH solution at 80℃)for 30 days were compared,the result shows that weight loss of QAPS(IEC=1.03mmol/g)was close to 50%,weight loss of QAPPSU(IEC = 1.02 mmol/g)was 33%,weight loss of QAPPO(IEC = 1.35 mmol/g)was 28%and weight loss of QAPEEK(IEC = 0.91 mmol/g)was 8%,weight loss of bare backbones were not higher than 5%.These observations unambiguously indicate that backbone of APEs will be attacked by OH-easily when the backbone with polar groups,more polar groups embedded in backbone,degradation of backbone in alkaline solution more easier.The QAPEEK here with unusual high backbone stable,that is because there are crosslinked structure in QAPEEK.2.Three types of APEs(Cx-aQAPS、Cx-aQAPPSU and Cx-aQAPPO)based on polysulfone backbone,polyphenylsulfone backbone and polyphenylether backbone were designed and synthesized successfully.These three types of self-aggregated APEs with hydrophobic side chain exhibited a significantly improved chemical stability of backbone when compared to each APEs without side chains under similar IEC,the weight loss after the 30-day test were not more than 10 wt%,close to the level of the pristine polymers,which means the polymer backbone protected from OH-by the additional hydrophobic structure is an effective way to improve the backbone stability of APEs.3.Series of APEs with a pendant type QA(pQA)which was linked to the backbone through a long side chain based on polysulfone backbone,polyphenylsulfone backbone and polyphenylether backbone were designed and synthesized successfully,these pQA-type APEs denoted as pQAPS,pQAPPSU,and pQAPPO.After a stability test in 1 mol/L KOH solution at 80℃ for 30 days,all pQA-type APEs(pQAPS,pQAPPSU and pQAPPO)exhibited a weight loss as low as 8 wt%and remarkably lower than that of the QA-type APEs(QAPS,QAPPSU,and QAPPO).4.APEs which cation and polysulfone backbone separated by 1,3,5 and 7 atoms were designed and synthesized successfully,and named QAPS,QAdalPS,pQAPS and pQAdalPS.The backbone weight loss of QAPS(IEC = 1.00 mmol/g),QAdalPS(IEC = 0.99 mmol/g),pQAPS(IEC-1.95 mmol/g)and pQAdalPS(IEC-1.91 mmol/g)changed by test days were compared,after 30 days,weight loss of QAPS is about 45%,weight loss of QAdalPS is not more than 25%,weight loss of pQAPS is not exceed 10%and weight loss of pQAdalPS is about 15%,QAdalPS with cation far away from backbone with 5 atoms is the most stable.The results indicate that the cation-induced degradation can be ignored when the distance between cation and backbone over 5 atoms,and meanwhile the effect of microphase segregation in membrane to backbone stability will become dominant.5.Most above designed structures can improve backbone chemical stability,however,microphase segregation in these membranes morphology will lead to bad membrane mechanical properties,which hindered the application for real.Under this circumstance,a long-chain multi-cations crosslinked APE membrane based on polyphenylether backbone was developed,and denoted as x(QH)3QPPO.Compared to TMHDA crosslinked PPO(x(QH)QPPO),x(QH)3QPPO membrane showed higher ionic conductivity,more chemical and mechanical stability.With the IEC of 3.59 mmol/g,x(QH)3QPPO-40 showed the conductivity of 110.2 mS/cm at 80℃ and swelling degree of 25.0%.After 30 days stability test,the IEC loss ratio and ionic conductivity loss ratio of x(QH)3QPPO-40 were just 22%and 25%.The tensile stress and elongation at break of x(QH)3QPPO-40 were 18.5 MPa and 34.4%under room temperature and 100%relative humidity.The peak power density of the anion exchange membrane fuel cell based on x(QH)3QPPO-40 membrane at 60℃ was 0.302 W/cm2,which was 3 times greater than that of the one based on the QPPO-40.