Fabrication Of Anion Exchange Membranes With Cross-linked And Ether-modified Structures Via Facile,environment-friendly Methods For Fuel Cell Application

Using a proton exchange membrane,the most prominent low-temperature fuel cells today operate under an acidic condition.Their caustic operating conditions,however,cause serious stability and activity problems for the metal catalysts and finally require the use of platinum-group materials,severely limiting commercial viability.A potential solution is to use an anion exchange membrane(AEM),carrying hydroxide ions instead of protons,to operate the fuel cell in an alkaline environment,which may speed up the oxygen reduction reaction and thus opens the door to cheaper catalysts.Unfortunately,typical AEMs suffer from the severe tradeoff between conductivity and stability(both mechanical and chemical);meanwhile,most of the AEM synthesis processes are complicated and not environment-unfriendly enough.This thesis focuses on the development of cross-linked and ether containing AEMs using facile,low cost and environment-friendly methods(avoiding the use of carcinogenic chloromethyl methyl ether/chloromethyloctyl ether and trimethylamine).The main contents are as follows.To alleviate the conductivity-mechanical robustness tradeoff in a facile way,chapter 2 deals with the fabrication of novel self-cross-linked composite AEMs using benzoxazine(Bz)monomer and polytetrafluoroethylene(PTFE)via thermally induced,in-situ ring-opening polymerization of Bz and subsequent quaternization.The quaternized polybenzoxazine(QPBz)works as a self-cross-linked anion conductive polymer.The synthesized membranes show improved conductivity(26 to 70 mS/cm)and a low swelling ratio;they also show improved alkaline stability for 150 h at 60? in 1 M KOH solution,the decrease in conductivity being only ca.10%.Furthermore,the prepared membrane shows a tensile strength of 23 MPa,a Young's Modulus of 958 MPa and an elongation-at-break of 15%,and also enhanced thermal stability.Another strategy of balancing conductivity and stability is described in chapter 3,where a novel 4-(dimethylamino)butyraldehyde diethyl acetal(DABDA)quaternized polysulfone AEM,or PSf-DABDA was synthesized.The DABDA-derived cation bears a long ether-containing substituent,which imparts the resulting membrane a well-developed microphase separation,and therefore,a high conductivity at relatively low ion exchange capacity(IEC)while low IEC is beneficial for membrane robustness.The PSf-DABDA membrane exhibits a higher room temperature conductivity(21 mS/cm)than trimethylamine(TMA)-and dimethylbutylamine(DMBA)quaternized AEMs(11.8 and 13.2 mS/cm,respectively)with comparable IEC.It experiences a slight conductivity decrease of 5%when treated 150 h in 1 M NaOH at 60?,while the PSf-TMA and PSf-DMBA membranes with a comparable IEC suffer a more serious conductivity decay of 19 and 13.5%,respectively.The better alkali stability of PSf-DABDA is further confirmed by a density functional theoretical study and a structural characterization.The prepared membrane also exhibits enhanced tensile strength ranging from 17.3 to 24.7 MPa under hydrated conditions and improved thermal stability.In chapter 4,DABDA quaternized polysulfone AEMs were cross-linked with an ether-containing cross-linker.The resultant cross-linked membrane structure not only enhanced the dimensional stability of the AEMs but also improved the ion cluster aggregation leading to the creation of better hydrophilic/hydrophobic microphase-separated morphology and ion-conductive channels.As a consequence,it is possible to achieve better-balanced conductivity and stability.The prepared AEMs showed a conductivity of 19.6 mS/cm at 25?,comparable to that of the without ether cross-linked counterpart,but a better mechanical thermal and alkaline stability.Its tensile strength reaches 30 MPa,and when treated in 1 M NaOH solution for 150h,the membrane conductivity did not show appreciable decay.The above ether-enabled cross-linking strategy was further extended for application in another type of AEM.In chapter 5,a novel structured bifunctional cross-linker,namely 1,1'-((ethane-1,2-diylbis(oxy))bis(ethane-2,1-diyl))bis(1,4-diaza-bicyclo[2.2.2]octan-l-ium)(DACEE)chloride,was synthesized and employed for AEM fabrication.DACEE chloride is an ether-containing salt,and can lead to simultaneous quaternization and cross-linking of chloromethylated polysulfone,and create multiple cations in the cross-links.Therefore,wide channels for hydroxide ion transport can be formed in the resulting AEMs.The prepared membranes show relatively high conductivity(31 mS/cm at 25?)at low IEC(1.08 mmol/g)while that of the cross-linked membrane fabricated without using DACEE chloride only exhibits a conductivity of 14.9 mS/cm(IEC=1.12 mmol/g).The prepared membranes also show a rational water absorption and low swelling ratio as well as enhanced alkaline stability over that cross-linked with a non-ether-containing agent.A density functional theory study and physical characterization after the alkaline treatment further confirm the better alkali stability of cross-linked membrane over its counterpart.A H2/O2 fuel cell assembled with the membrane shows an open circuit voltage of 1.01 V and a maximum power density of 217 mW/cm2 at 60?.To conclude,novel AEMs of unique cross-linked and ether-containing structures were designed and prepared using cost-effective,facile and environment-friendly methods.The results obtained demonstrate that the fabricated membranes possess the anticipated qualities such as high conductivity of hydroxide ion at low IEC and moderate water uptake;what's more,in a challenging basic environment,the prepared membranes exhibit better alkaline stability than the traditional main chain type polysulfone AEMs.Therefore,the technical issue of conductivity and robustness tradeoff can be significantly alleviated with these membranes,which are promising for the application in AEMFCs.