| Since the 21st century,energy and environmental issues have become a hot topic in the international community.How to develop green,environmentally friendly clean energy is an important research subject being carried out by various countries.Fuel cell is a device that can efficiently convert chemical energy stored in fuels and oxidant into electrical energy.Due to its high conversion efficiency,strong modularity,and environmental friendship,it has attracted widespread attention from researchers around the world.As the core component of fuel cells,proton exchange membrane（PEM）has a direct impact on the performance of fuel cells.As the most widely used proton exchange membrane,Nafion membrane has the advantages of high proton conductivity and good dimensional stability.However,it is limited in further development by the high price,the high methanol permeability in direct methanol fuel cells（DMFC）and the chemical degradation by oxidative radical attack in proton exchange membrane fuel cells（PEMFC）.In order to solve above problem,based on the relationship between the distribution state of the modifier in the Nafion membrane and the microstructure of Nafion membrane,the introduction of different functionalized fibrous network backbones or metal organic framework（MOF）induces a directional rearrangement of the hydrophilic phase region in the Nafion membrane,which makes full use of the properties of the modifier in the membrane.The aim is to obtain high-performance,durable and low-thickness Nafion-based modified PEM.As an effective solution to the problem of high methanol permeability of Nafion in DMFC and for the purpose of reducing the amount of Nafion used and lowering costs,Nafion membranes can be modified by an organic-organic blending method.Poly（aryl ether ketone）（PAEK）is widely used in the modification of Nafion membranes with organic-organic components as a polymeric material with high methanol resistance permeation,low price and high modified properties.However,conventional solution blending methods can result in macroscopic phase separation phenomena due to the large surface energy differences between them.Therefore,in Chapter 2,a three-dimensional organic support framework（F-CQPAEK）of crosslinked-quaternized PAEK material was prepared using an electrostatic spinning method.The affinity between PAEK and Nafion was improved by ion pair interface between the quaternary ammonium group（QA）and the sulfonic acid group.Due to the presence of acid-base ion pairs,the sulfonic acid groups are distributed in an orderly pattern along the nanofibers.The electrospinning modification method modulates the microphase self-assembly process of Nafion and optimizes the distribution state of the hydrophilic region,which will construct a long-range ordered high-speed proton transport channel to reduce the activation energy of proton transfer.This method also improves the proton conductivity of the Nafion composite membrane.The proton conductivity of F-CQPAEK-1.0 reached 0.211 S cm-1.The presence of the F-CQPAEK reinforced structure and ion pair resulted in improved dimensional stability and methanol resistance of the Nafion composite membranes.The methanol permeability of the F-CQPAEK-1.5 hybrid membrane was only 31.17%compared to that of Recast Nafion membrane.In the DMFC operation fueled with 10 M methanol solution,the maximum power density（PDmax）of the F-CQPAEK@Nafion-1.0 hybrid membrane reached 65.31m W cm-2,which was 3.54 times higher than that of Recast Nafion membrane.Therefore,the microscopic hydrophilic phase distribution of the Nafion chain segments can be effectively tuned by the method of electrospinning,which improves the proton conductivity and restricts the fuel permeability of the Nafion composite membrane.This modification method provides the possibility of its application in DMFC fueled with a highly concentrated methanol solution.Although the introduction of PAEK nanofiber backbone can effectively solve the problem of high fuel permeability of Nafion,it was found that its anti-oxidative stability was poor due to a large number of ether bonds in the main chain of PAEK,thus inevitably leading to a decrease in the durability of the fuel cell.Furthermore,Nafion can be degraded under the attack of free radicals,which limits its further rapid development.According to the literature,Ce O2,as one of the most commonly used free radical scavenger（FRS）,can effectively improve the antioxidant stability of Nafion.However,as a metal oxide,Ce O2 will migrate under the action of electric field,and even dissolves from PEM to reach the catalyst layer,thereby affecting the activity of the catalyst.In order to improve the anti-oxidative stability of Nafion membrane and solve the problem of Ce O2 loss,the method of organic-inorganic modification of Nafion was used in the third chapter.Ce O2 anchored MOF were prepared by hydrothermal synthesis,and the resulting inorganic particles were then grafted with long alkyl flexible sulfonic acid side chains.The multifunctional inorganic nanoparticles Ce O2-MNCS were then introduced into the Nafion matrix with different proportions to obtain the Ce O2-MNCS@Nafion-x series of Nafion composite membranes.This modification method not only improved the anti-oxidative stability of Nafion composite membranes,but also used the electrostatic interaction between-NH2 and-SO3-groups to rearrange the microscopic phase-separated structure of Nafion,which improved the proton conductivity of Nafion composite membranes and fuel cell performance.The proton conductivity of the Ce O2-MNCS@Nafion-1.5 hybrid membrane reached 0.239 S cm-1,and the power density in the hydrogen/air fuel cell reached 591.47 m W cm-2,which were higher than Recast Nafion membrane tested under the same conditions.The introduction of Ce O2-MNCS effectively improved the durability of the Nafion composite membrane,and its OCV loss rate was 0.54 m V h-1 in the hydrogen/air fuel cell in-situ stability test,accounting for only 24.77%of Recast Nafion membrane.This indicates that Ce O2-MNCS with radical trapping function can effectively improve the durability of Nafion composite membranes.Therefore,in the fourth chapter,a layer of polydopamine was first deposited on the surface of PTFE to reduce the surface energy of PTFE and improve the affinity between PTFE and Nafion matrix.Utilizing the excellent metal chelating ability of dopamine,a layer of amino MOF material UIO-66-NH2 was grown in situ on the surface of PTFE fibers.Then,the Nafion solution was mixed with different proportions of caffeic acid（CA）as combined with the modified PTFE by the casting method.Through thermal cross-linking treatment,CA and UIO-66-NH2 were successfully grated together due to the carboxyl group with the amino group.The thin and dense PTFE-reinforced Nafion composite membranes PPUC-Nafion-x obtained using the above method possessed higher mechanical strength than Recast Nafion membrane.Under the promotion of electrostatic force,the sulfonic acid groups at the end of the side chain of Nafion were rearranged along the PFTE fiber network,constructing a continuous proton transport channel and a dynamic hydrated hydrogen bond network,which improved the proton conductivity and fuel cell performance of the Nafion composite membranes.The PPUC-Nafion-3 composite membrane exhibited a higher proton conductivity of 0.212S cm-1 and the PDmax is 812.64 m W cm-2.The anti-oxidative stability of the Nafion composite membrane was also improved due to the introduction of CA.The OCV drop rate of PEMFC assembled with PPUC-Nafion-3 composite membrane decreased from1.91 m V h-1 of Recast Nafion membrane to 1.11 m V h-1.Meanwhile,the presence of UIO-66-NH2 also limited the loss of CA and regulated the distribution of CA to some extent.From the work in the previous two chapters,it was observed that the use of free radical scavengers to modify perfluorosulfonic acid-based proton exchange membranes could effectively improve the durability of PEMs,and the use of acid-base pair interactions optimized the microscopic phase separation structure of Nafion and improved the proton conductivity of the composite membranes.However,CA was randomly distributed in the Nafion matrix due to the lack of significant interaction between CA and Nafion chain segments.There was still a risk of loss of free CA in the Nafion matrix.Moreover,the excessive incorporation of CA can also reduce the performance of Nafion membrane.Therefore,in Chapter 5,CA was grafted directly onto the surface of UIO-66-NH2 via an amidation reaction between the-COOH group of CA and the-NH2 group of UIO-66-NH2,and then PTFE-reinforced membranes CA-PPU with free radical scavenging functionality were prepared.The pores of CA-PPU were then filled with Nafion solution to obtain thin CA-PPU@Nafion membranes with a thickness of around 25μm.Compared with the work in the previous chapter,the ionic exchange capacity and proton conductivity of the CA-PPU@Nafion composite membrane were improved.At 80°C,the proton conductivity of the CA-PPU@Nafion composite membrane reached 0.242 S cm-1.The fuel cell performance reached 876.78m W cm-2,which was higher than that of the PPUC-Nafion-3 composite film prepared in the previous chapter and 1.8 times higher than that of Recast Nafion(486.30 m W cm-2).In the durability test of the PEMFC assembled with the CA-PPU@Nafion composite membrane,the OCV drop rate was only 0.86 m V h-1,which was only accounted for 77.5%of the OCV decay for the PPUC-Nafion-3 membrane in the previous chapter and 45.0%of Recast Nafion membrane.This suggests that modulating the distribution of caffeic acid and reducing its distance from active sites susceptible to free radical attack can effectively perform the free radical scavenging function of CA and further improve the durability of PEM in fuel cells. |
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