| Fuel cell is a kind of clean electrochemical technique converting chemical energy to electric energy directly.As compared to acidic fuel cell,alkaline fuel cell has the advantages of using non-precious metal catalyst,high activity of oxygen reduction.As one of the main parts,alkaline polymer electrolyte membrane acts as the barrier of fuel and medium of OH-conduction.The main challenge of alkaline polymer electrolyte membrane is obtaining high OH-conductivity and excellent chemical stability,as a result of poor OH-diffusivity(as compared with H+)and low dissociation of functional group(as compared with SO3-)as well as a large amount of free active OH-and O2-.Physical electrospinning and chemical grafting are adopted to design micro-structure and regulate ion channels of polymer electrolyte membrane leading to significantly enhanced of hydroxide conductivity and alkaline stability.Furtherly,oxidation stability test method of functional group is established to study oxidation stability and mechanism of oxidation degradation.On the one hand,physical electrospinning is adopted to induce ionic clusters to move thus regulating ion channel connectivity.And imidazolium functionalized polysulfone(IMPSf)electrolyte membrane with excellent interfacial compatibility between fiber mats and pore filler is fabricated by using same material as electro spinning solution and pore filler component based on the differences in solubility of IMPSf,solving the problem that easy blocking of ion channels caused by interface incompatibility.Ionic conduction channel formation mechanism that the strong electric field induced ionic groups within the single fiber tend to fiber surface,forming along the fiber orientation of long range order ionic conduction channel is revealed by electrospinning IMPSf.OH-conductivity and alkaline stability are enhanced significantly.OH-conductivity reaches 1.7 folds of IMPSf cast membrane with same IEC at 20 ℃ and 70.2 mS·cm-1 at 60℃(IEC=1.78 mmol·g-1),residual ionic conductivity is 40%(1M 60℃ KOH、24h),2 folds of cast membrane.Power density reaches 87 mW cm-1.And then,functionalized MWCNT(FMWCNT)/IMPSf composite electrospun electrolyte membrane is fabricated,solving the tradeoff between hydroxide conductivity and mechanical strength.FMWCNT is induced to overcome van der Waals force by high voltage electrostatic field and fixed in the single fiber along fiber orientation in the process of the fiber jet,increasing dispersion and interion aggregation.And then FMWCNT/IMPSf composite electrospun composite electrolyte membrane with enhanced fiber content is obtained by hot pressing.Imidazolium groups on both the MWCNT and PSf interact with each other,reducing ion transfer barrier to 10.8-11.8 kJ mol-1,much lower than that of IMPSf electrospun membrane(13.3 kJ·mol-1)and cast membrane(16.5 kJ-mol-1).OH-conductivity,tensile strength and alkaline stability increase remarkably,in which OH-conductivity of FMWCNT/IMPSf electrospun membrane with 0.4 wt.%FMWCNT(E 0.4)reaches 67.5 mS·cm-1 at 30℃,2.1 and 3.3 folds of IMPSf electrospun and cast membrane without FMWCNT,respectively.The maximum tensile strength reaches 24.4 MPa,1.5 folds of IMPSf electrospun membrane without FMWCNT.Residual conductivity of E 0.4 membrane is 61.7%(1M 60℃ KOH 24h),higher than cast membrane with same doping amount(54.9%).The maximum single fuel cell power density reaches 102.5 mW·cm-2.On the other hand,chemical grafting method is adopted to induce self-assembly of molecules in the membrane to form a high efficiency ion conduction channel.A novel approach is proposed to design anion exchange membranes(AEMs)containing pendent imidazolium side chains with flexible ether-containing spacer by the Williamson etherification between chloromethylated polysulfone and as-synthesized hydroxyl-bearing imidazolium,solving the problem that special function group on monomer is needed for grating ether-containing side chain.Enhanced side chain mobility and ionic static electricity induce hydrophilic/hydrophobic micro-separation and regulate ion channels.Ion transport barrier of membrane is reduced to 18.54 kJ·mol-1,lower than that of IMPSf electrolyte membrane with-CH2-as spacer(24.82 kJ·mol-1).High OH-conductivity,low swelling ratio and high alkaline stability are achieved at low IEC simultaneously,in which OH-conductivity and swelling ratio reaches 72 mS·cm-1 at and 7.3%at 60℃,respectively and residual conductivity is 93.4%(1M 60℃ KOH 168h).In the end,oxidation stability of polymer electrolyte is studied furtherly.Oxidation stability test reagent(KO2/18-crown-6/DMSO)is designed and established to generate O2-radical,solving the problem that the study of oxidation stability of cation can’t be studied as lack of oxidation stability test reagent.And then using the test system to investigate the oxidation stability degradation mechanism of cation functionalized group.Changes of structure of imidazolium、quaternary ammonium and phosphonium was characterized by 1H NMR、13C NMR、31P NMR and MS.The oxidation stability order is:quaternary ammonium>phosphonium>imidazolium.It is likely to occur deprotonation reaction for benzyl proton.Imidazolium is prone to occur open-loop degradation.β-H is the degradation position for quaternary ammonium while central P atom is the reaction position for phosphonium cation,giving P=O as degradation product.It provides theoretical guidance for the preparation of alkaline polymer electrolyte membranes with high chemical stability. |
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