Preparation And Properties Of Polybenzimidazole Based Novel Polymer Electrolyte Membranes

Polymer electrolyte membrane is the key component of fuel cell,all vanadium flow battery and the other clean energy storage systems.To achieve commercialization and real application,there are several requirements for the polymer membranes such as excellent chemical and electrochemical stability,high ion(proton or hydroxide ions)conductivity,good mechanical strength and low swelling ratio.Polybenzimidazole with excellent chemical and thermail stability,good process capability has been widely evaluated in high temperature proton exchange membrane fuel cells,but the application in low temperature PEMFC,alkaline anion exchange membrane fuel cell and all vanadium redox flow battery is not well reported.In this thesis,a series of polybenzimidazole based membranes are prepared through grafting,blending,crosslinking and related methods,the performance in alkaline anion exchange membrane fuel cell,low temperature proton exchange membrane fuel cell and all vanadium flow battery are evaluated.For anion exchange membrane,two parts of work have been done and are described in chapter 2 and chapter 3 separately.In chapter 2,firstly,polybenzimidazole with pendant amino groups(H₂N-PBI)is synthesized through the reaction of 5-aminoisophthalic acid(APTA)and 3,3'-diaminobenzidine(DAB).Then a series of novel polybenzimidazoles with pendant quaternary ammonium groups(QPBIs)have been synthesized by grafting 4-methyl-4-glycidylmorpholin-4-ium chloride(MGMC)onto the polybenzimidazole with pendant amino groups(H₂N-PBI).The remaining unreacted pendant amino hydrogen atoms of the H₂N-PBI were further used to react with the epoxy groups of bisphenol A diglycidyl ether(BADGE)for cross-linking.The hydroxide conductivity of the cross-linked QPBI membranes increases with the increase in ion exchange capacity(IEC)and temperature.The highest hydroxide conductivity of 0.056 S cm^-1 has been achieved with the QPBI-2/1(here,the‘2/1'refers to the molar ratio of MGMC to H₂N-PBI in feed)membrane at 80?under fully hydrated state.All the QPBI main chains show extremely good chemical stability in strong alkaline solution(6.0 M Na OH)at 60?.However,the quaternary ammonium groups of the QPBI membranes except the QPBI-1/1 are unstable(degradable)in 6.0 M Na OH at 60?leading to large reduction in hydroxide conductivity.Another method to prepare alkaline anion exchange membrane is discussed in chapter 3,that is to replace morpholin quaternary ammonium group with imidazolium.Firstly,poly(1-vinylimidazole)(PVI)is prepared through radical polymerization of 1-vinylimidazole.Then,the PVI is quaternized with 1-bromohexane to yield poly(1-vinyl-3-hexylimidazolium bromide)(QPVI-Hx)with varied degree of quaternization(DQ).A poly(ether benzimidazole)(OPBI)is also synthesized via condensation polymerization of 4,4'-dicarboxyldiphenyl ether(DCDPE)and 3,3'-diaminobenzidine(DAB)in Eaton's reagent at 140?.It is found that the QPVI-Hx exhibited extremely poor film-forming ability.However,blending with the OPBI through covalent cross-linking using 1,6-hexamethylene dibromide as the cross-linker yields free-standing and ductile membranes.The membranes are transparent at different weight fraction of OPBI(20%-50%).The membranes with higher content of OPBI tend to have higher mechanical strength,lower swelling and lower anion conductivity.The highest hydroxide conductivity(16.7 m S cm^-1,80?,in water)is achieved with the membrane OPBI/QPVI-Hx 0.1/0.9 due to its highest IEC.This membrane also exhibits good mechanical properties(tensile strength:33MPa,elongation at break:199%)and moderate swelling ratio(in-plane:13.2%,through-plane:10.4%,60?).With the same concept,another type of imidazolium QPVI is prepared.The PVI is quaternized with benzyl bromide to yield poly(1-vinyl-3-benzylimidazolium bromide)(QPVI-Bz)with varied DQ.Then a series of cross-linked blend membranes are prepared from the QPVI-Bz and the H₂N-PBI with 1,6-dibromohexane and bisphenol A type epoxy resin as the cross-linking agents.By changing the content of the QPVI-Bz,membranes with different IEC values are obtained.The incorporation of H₂N-PBI significantly improves the mechanical properties of membrane due to the reinforce effect as well as the covalent cross-linking.All the prepared membranes are transparent even at high weight fraction of H₂N-PBI(H₂N-PBI/QPVI–Bz 0.5/0.5)indicating good miscibility of the two polymers.The highest hydroxide conductivity(23 m S cm^-1,80?,in water)is achieved with the membrane H₂N-PBI/QPVI-Bz 0.1/0.9 due to its highest IEC.The membrane H₂N-PBI/QPVI-Bz 0.2/0.8 exhibits the best-balanced performances,i.e.,it has a high tensile strength(33 MPa),a low swelling ratio(8%,60?)and a high hydroxide conductivity(15.1 m S cm^-1,80?,in water).In chapter 4,some research work is done on proton exchange membrane.Firstly,the anhydride-terminated sulfonated polyimide oligomers(SPI-m,m=3,5,7,here m means degree of polymerization)are synthesized via condensation polymerization of excess 1,4,5,8-naphthalenetetracarboxylic dianhydride(NTDA)and 4,4'-diaminodiphenyl ether-2,2'-disulfonic acid(ODADS)in m-cresol in the presence of benzoic acid and isoquinoline at 180?for 20 h.The averaged block length of the SPI oligomers is controlled at 3,5 and 7 by controlling the molar ratios of NTDA to ODADS at 4:3,6:5 and 8:7,respectively.Then a series of sulfonated polyimide/polybenzimidazole cross-linked membranes with the ion exchange capacities(IECs)of 0.70-1.52 meq/g have been prepared via the reaction of the anhydride-terminated sulfonated polyimide oligomers and the polybenzimidazole with pendant amino groups(the same as prepared in chapter 2)in dimethylsulfoxide during the membrane casting process.The proton conductivities of the cross-linked membranes increase with increasing temperature and relative humidity.The cross-linked membrane prepared from the longest oligomer(SPI-7/H₂N-PBI)displays the highest proton conductivity,0.092 S cm^-1at 60?and 100%RH,which is comparable to that of Nafion 117.Water stability test reveals that all the cross-linked membranes could maintain reasonable mechanical strength after being soaked in deionized water at 100?for two months,whereas the noncross-linked NTDA-ODADS/ODA(1/1)(ODA is 4,4'-Diaminodiphenyl ether)membrane which have similar IEC to SPI-7/H₂N-PBI crosslinked membranes,completely lost mechanical strength(could not handle)after being soaked in deionized water at 100?for only 5 h.This indicates that the water stability is greatly improved due to cross-linking.What's more,it's interesting that all the cross-linked membranes show anisotropic swelling behaviors,i.e.the dimensional changes in‘in-plane'direction(?l)are rather small(4-5%),whereas the dimensional changes in‘through-plane'direction were relatively large(12-22%).This is quite different from that of the NTDA-ODADS/ODA(1/1)membrane which showed isotropic dimensional changes.This indicates that the cross-linked membranes might have lamellar structure to some extent,whereas the NTDA-ODADS/ODA(1/1)membrane has homogeneous structure because it is a random copolymer.The small dimensional change in‘in-plane'direction is expected to be helpful for enchancing fuel cell lifetime.The radical oxidative stability of the cross-linked membranes is evaluated by Fenton's test(test conditions:3%H₂O₂+3 ppm Fe SO₄,80?).Fenton's test revealed that all the cross-linked membranes displayed significantly better radical oxidative stability than the pure noncross-linked NTDA-ODADS/ODA(1/1)membrane(30 min).This should be attributed to the excellent radical oxidative stability of the H₂N-PBI.With the development of society,traditional fossil fuels are consumed quickly which lead to environment has been destroyed seriously,so the green and renewable energy sources such as wind and solar energy have become highly attention.However,the renewable energy sources are intermittent in nature,and demand safe and effective large scale electrical energy storage(EES)equipment.Vanadium redox flow battery(VRFB)is one of the most promising EESs because VRFB has long cycle life,low cost and high energy efficiency.A high-performance separator is indispensable for a high performance of VRFB that displayed high energy density,energy efficiency and durability.From chapter 5 to chapter 7,3 different types of membranes are applied in all vanadium flow battery.In chapter 5,a series of H₂SO₄ doped polyetherbenzimidazole(OPBI)membranes are developed for VRFB application.The OPBI is synthesized through the reaction of DCDPE and DAB,the same process as chapter 3.The OPBI membrane with good mechanical strength(tensile strength of 120 MPa)is prepared through casting.The alkaline imidazole ring can be doped with H₂SO₄,and the doped degree is enhanced with the increased H₂SO₄ concentration.When the H₂SO₄ concentration is 10 M,the H₂SO₄ doped degree reaches 120 wt%.The OPBI membranes showed anisotropic swelling behaviors,i.e.the dimensional changes in‘in-plane'direction(?l)are rather small(<5%),which is good.The proton conductivity of OPBI membrane increases with the increasing of H₂SO₄ concentration.The proton conductivity of OPBI membrane doped with 10 M H₂SO₄ is 4.3?10^-3 S cm^-1at room temperature and 75%RH,which is half of Nafion 117(9.1?10^-3 S cm^-1).When doped with 5M of 10M H₂SO₄,even the thickness of OPBI membrane is only 28um,the elimination of VO₂⁺ion crossover performance is excellent.The OCV of VRFB implemented with OPBI(10 M)membrane starts to decrease slightly after 216 h,from 1.50 to 1.31 V;wheras the the OCV of VRFB with Nafion 117 membrane starts to decrease after 46 h,from 1.50 to 0.70 V.The charge/discharge cycle test reveals that single cell with OPBI(10 M)keeps stable under 60 m A cm^-2 after 200 cycles,the energy efficiency is comparable to Nafion 117 integrated single cell.In chapter 6,the crosslinked OPBI/QPVI-Hx membrane prepared in chapter 3 is also applied in VRFB.This series of membranes show excellent VO₂⁺diffusion inhibition property,3-4 orders in magnitude lower VO₂⁺permeability than Nafion117 because of the Donnan exclusion effect resulting from the positively charged imidazolium groups and vanadium cations.The VRFBs assembled with the cross-linked OPBI/QPVI-Hx blend membranes exhibited much higher CE and slower self-discharge rate than that assembled with Nafion117 because of the extremely lower vanadium crossover of the former.The VRFB assembled with the OPBI/QPVI-75%(3:7)membrane shows little performance decay after 300 charge-discharge cycles indicating good durability of the membranes.In chapter 7,a new type of sulfonated poly(ether benzimidazole)with pendant quaternary ammonium groups based membranes are prepared for the application in all vanadium flow battery.Firstly,a series of polybenzimidazole copolymers with varied content of pendant amino groups have been synthesized by condensation polymerization of 4,4'-dicarboxydiphenyl ether(DCDPE),5-aminoisophthalic acid(APTA)and3,3'-diaminobenzidine(DAB)in polyphosphoric acid at 190?for 20 h.The resulting copolymers undergo post-sulfonatation in fuming sulfuric acid at100?yield the highly sulfonated polybenzimidazoles(SOPBI-NH₂(x/y),‘x/y'refers to the monomer molar ratio of DCDPE to APTA.A series of covalently cross-linked membranes(CSOPBI-NH₂(x/y))with good mechanical properties are fabricated by solution casting technique using bisphenol A epoxy resin as a cross-linker.The CSOPBI membranes show 3–4 orders of magnitude lower VO²⁺permeability and 6-30 times higher ion diffusion selectivity(H⁺/VO²⁺)than Nafion117.The charge-discharge behaviors of the vanadium redox flow batteries(VRFBs)assembled with the CSOPBI-NH₂(x/y)membranes and Nafion 117 are investigated and compared.The VRFBs assembled with the CSOPBI membranes exhibit significantly higher columbic efficiency and lower self-discharge rate than that assembled with Nafion 117 owing to the extremely lower vanadium cations crossover of the former.The VRFB assembled with the CSOPBI-NH₂(9/1)membrane exhibits high energy efficiency(85%at 60 m A cm^-2)and little decay in performance is observed after 300 charge-discharge cycles.All these three types of membranes mentioned from chapter 5 to chapter 7,including H₂SO₄ doped polyetherbenzimidazole(OPBI)membranes,crosslinked OPBI/QPVI-Hx anion exchange membranes and covalently cross-linked cation exchange membranes CSOPBI-NH₂(x/y),all are promising in VRFB application.Besides the polybenzimidazole based membranes elaborated above,this thesis also discussed the research work on e PTFE based sulfonated polystyrene crosslinked composite membrane(SPS/e PTFE)in appendix.The SPS with IEC of 3.98 meq g^-1is synthesized through post-sulfonation of polystyrene resins.Then the e PTFE membrane after hydrophilic treatement is coated with SPS solution and at last cross-linked in P₂O₅/CH₃SO₃H solution(80?,5-15 min),a series of SPS/e PTFE composite membranes with different IEC and crosslinking density is prepared.Compared with noncross-linked membrane,the crosslinked SPS/e PTFE composite membranes have better tensile strength,distinguished decreased swelling ratio and water uptake,and the decrease rate of proton conductivity is not obvious.For example,for the composite membrane with 15 mins crosslinking treatment,the proton conductivity is 0.16 S cm^-1 at 80?,comparable to Nafion 112;the dimensional change is only 12%in‘in-plane'direction(?l)and 3%in‘through-plane'direction(?t)wheras for the accordingly noncross-linked membrane the SPS is total dissolved in DI water at 80?and lose the proton conducting property.This type of crosslinked composite membrane is easy to prepare and costs low,the performance is good and could be a promising candidate for VRFB or PEMFC application.The related work is summarized in chapter 6.