Structural Design Of Anion Exchange Membranes With Enhanced Alkaline Resistence

Fuel cells are supposed to be the most promising energy conversion equipment as an alternative to traditional fuels in the 21st century due to the high conversion efficiency,low environmental pollution,safety,portability and easy operation.Among the various types of FCs,proton exchange membrane FCs(PEMFCs)have been widely developed because of their compact device architectures,fast startup,high power density.However,the wide use of PEMFCs is hindered by their severe dependence on platinum catalysts,complex water management,and the materials of PEMs(such as Nafion,which cannot be used at high temperature because of the evaporation of water and depression of conductivity).Currently,anion exchange membrane FCs(AEMFCs)have been regarded as a most promising alternative to PEMFCs because of the use of non-precious metal catalysts and the fast electrode reaction kinetics in alkaline environments.One of the major challenges in AEMFCs is the construction of anion exchange membrane(AEM)with mechanical toughness,robust chemical stability,and limited swelling while providing high ionic conductivity.Thus,the design of polymer backbone structures and cationic groups to enhance the chemical stability and improve the ionic conductivity of AEMs is the focus of researches.In this study,AEMs with excellent comprehensive performance were prepared by designing AEMs with different structures and exploring the influence of structures on membrane properties to meet the practical application requirements.To improve the overall performance of AEM for fuel cells,a series of composite AEM was prepared via a facile approach using ionic liquid-functionalized graphene oxide(IL-GO)and imidazolium-functionalized poly(2,6-dimethy1-1,4-phenylene oxide)(ImPPO).The prepared ImPPO/IL-GO-0.5%(0.5%refers to the weight percentage of IL-GO)membranes with an ionic exchange capacity(IEC)of 1.9 meq g1 achieved a hydroxide conductivity of 78.5 mS cm-1.The high conductivity of ImPPO/IL-GO-0.5%is due to the ionic cluster size of composite membranes(3.43-3.58 nm)is larger than that of the pristine membrane(3.30 nm).The ionic cluster morphology is confirmed by atomic force microscopy(AFM)and small angle X-ray scattering(SAXS).Moreover,the AEMs retained approximately 70%of their initial hydroxide conductivity after the alkaline stability test in a 2 M aqueous NaOH at 80?for 480 h,indicating a good alkaline stability.A single cell using ImPPO/IL-GO-0.5%exhibits a peak power density of 136 mW cm-2 at 60? under 100%related humidity,thereby having a great potential for fuel cells.A series of highly alkaline durable crosslinked AEM is prepared via elimination reaction using a sodium salt of 4,4'(5')-di(hydroxybenzo)-18-crown-6 as the crosslinker,and quaternary ammonium(QA)as the cation group.The ionic conductivity,ionic exchange capacity(IEC)and mechanical property of the AEMs are investigated.The highest ionic conductivity(about 117 mS cm-1)of the PPO-CE0.10QA0.90 membranes is obtained at 80? and 100%relative humidity.The alkaline stability of the AEMs is determined by inspecting the decline in ionic conductivity,mechanical property and the change in chemical structure after the alkaline stability test.The ionic conductivity has only a 6.0%decrease,the tensile strength has an 11.8 MPa reduction and the elongation at break has a 2.7%depression after 480 h test in 2 M KOH at 80?.13C NMR spectra confirm a negligible change in chemical structure of the AEMs after the alkaline resistance test.The peak power density(about 195 mW cm-2)is achieved under a current density of 350 mA cm-2 at 80?.Additionally,the AEMs with a novel crosslinked structure decompose above 230? and exhibit decent thermal stability.Herein,a novel strategy to improve the performance of AEM is adopted to build a crosslinked structure by tethering the rigid poly(2,6-dimethyl-1,4-phenylene oxide)(PPO)backbone and the flexible poly(4-vinylphenol)(PVP)backbone using an oscillational chain.The incompatibility between the PPO and PVP backbones makes them self-aggregated resulting in ocurrence of distinct microphase separation and development of ion transport channels in the anion exchange membranes to facilitate the transport of hydroxide ion.The microphase separation of the as-prepared AEMs has been revealed by transmission electron microscopy(TEM)and AFM.The existence of the hydrophilic PVP chain also ensures enough water uptake to enhance the conductivity of the AEMs.A maximum ionic conductivity of 134 mS cm-1 is achieved at 80? by the PPO-c-PVP-40%(40%is the molar content of PVP chains)membrane.A maximum power density of 173 mW cm-2 at 80? is obtained by a H2/O2 single cell using the PPO-c-PVP-40%membrane electrode assembly.The PPO-c-PVP-40%membrane also exhibits robust alkaline stability(retains about 90%ionic conductivity after 480 h alkaline stability test)and excellent mechanical property.Those render the AEMs a potential membrane electrode material for fuel cell applications.A series of hyperbranched poly(arylene ether ketone)(HBPAEK)AEM was synthesized by polycondensation and quaternization using 1-(N',N'-dimethylamino)6,12-(N,N,N-trimethylammonium)dodecane bromide.The highest ionic conductivity of the HBPAEK-28-18 AEM with ionic exchange capacity of 2.21 meq g-1 is 122.5 mS cm-1 at 80?.The chemical stability of the AEMs was examined upon exposure to a 2M KOH solution at 80? by monitoring the change in ionic conductivity,which decreased from 122.5 to 104.2 mS cm-1 after 500 h.1H NMR spectra confirmed the chemical stability of the anion exchange membrane during this period.The hyperbranched structure can not only improve the conductivity,but also increase the chemical stability of the AEMs.The H2/O2 single cell using HBPAEK-28-18 exhibits a maximum power density of 188.6 mW cm-2 under a current density of 450 mA cm-2 at 80?.Furthermore,the hyperbranched structure AEMs also exhibited good thermal stability and mechanical property.A series of AEM based on Troger's base polymers has been prepared.The AEMs made from Tb-poly(crown ether)s(Tb-PCEs)show good comprehensive performance.The influence of crown ether on the conductivity and alkaline stability of AEMs has been investigated in detail.The formation of hydronium ion-crown ether complexes and an obvious microphase separated structure formed by the existence of crown ether can enhance the conductivity of the AEMs.The maximum ionic conductivity of 141.5 mS cm-1 is achieved from the Tb-PCEs based a AEM(Tb-PCEs-1)at 80? under 100%humidity.The ion-dipole interaction of the Na+with crown ether can protect the quaternary ammonium from the attack of OH-to improve the alkaline stability of AEMs.After 480 h alkaline treatment,the ionic conductivity of TbPCEs-1 only decreases by 6%.The single cell fabricated by the Tb-PCEs-1 shows a peak power density of 0.202 W cm-2 at 80?.The prominent physicochemical properties are attributed to the well-developed microstructure of the Tb-PCEs,as revealed by TEM,AFM and SAXS observations.