| In the 21st century when the global energy crisis has been intensified,how to promote the environmental pollution control better and faster has become an increasingly vital issue for various countries.A fuel cell,as an energy conversion device that directly converts fuel chemical energy into electric energy facilitated by electrochemical processes,has demonstrated diverse merits including high conversion efficiency,high-degree environmental friendliness,high modularity,adequate sources,low heat loss,etc.The excellent performance consistently gains extensive attention from all walks of life.In particular,the relevant research and application progress of fuel cells has stepped into a critical development stage since the Chinese government officially announced the carbon peaking and carbon neutrality goals in 2021.A proton exchange membrane（PEM）is the core of fuel cells,and enhancing its performance is essentially crucial to the performance enhancement of the entire battery system.Therefore,the main research to promote the further commercialization of fuel cells lies in improving PEM’s properties.In a fuel cell,PEM mainly plays the role of proton transport and fuel block,which means transporting the protons generated from the oxidation reactions of fuels at the anode to the cathode to react with the oxidant.In the meanwhile,it also isolates the two electrodes so that the redox reactions in the energy conversion process take place independently at their respective electrode side.Therefore,raising the transport efficiency and fuel blocking rate of PEM are the keys to enhancing the membrane performance.Based on the research objectives above,this thesis has pivoted upon the structure-activity relationship between the protons during the migration and diffusion process in membranes,the molecular chemical structure,and the nano-microphase arrangement of the polymer membrane matrix.The degree of hydrophilic-hydrophobic bi-phase separation in the membrane was improved by adjusting the chain structures of polymer molecules and introducing guest particles to dope modifications.The proton transport channel in the membrane,as well as its phase structure,were also improved by these methods,which optimized the membrane performance.In Chapter 3,it was proposed that the modification of PEM composed by a single polymer material could selectively avoid the direct regulation of hydrophilic segments of macromolecules.Instead,the conformation adjustment of hydrophobic domains conducted promoted the optimization of the micro-phase structure in hydrophilic domains by boosting the orderly changes in structures of the hydrophobic domain.It not only helped to raise the separation degree of hydrophilic and hydrophobic microphases but also enhanced the stability of the hydrophobic domains in the membrane’s main body.In the concrete work,a series of poly aryl ether ketone macromolecules with rigid structures of benzene rings and naphthalene rings in main chains were synthesized.The hydrophilic side chains with sulfonate in the end were connected to the rigid naphthalene ring to conduct post-sulfonation upon the polymers.Subsequently,by adjusting the molecular polarity matching mode that controlled the microphase self-assembly process of the driving hydrophobic domains,our indirect regulation methods proved to facilitate the improvement of the connection degree of hydrophilic ion clusters,reduce the mass transfer resistance in PEM,and enabled the optimization adjustment of proton transport channels.The experimental outcome showed that the maximum power density of the corresponding membrane was 94.71m W·cm-2 at 80℃,was with 2 M methanol solution as the anode and oxygen as the cathode.In the case where H2 was set as the anode and air as the cathode,the maximum power density reached 485.09 m W·cm-2.In addition,combined with the calculation of density functional theory（DFT）,the process of free radicals attacking polymer molecules was analyzed in terms of thermodynamics and kinetics.It has been proved that adjusting the molecular polarity can interfere with the attack process of free radicals by affecting the distribution of the electron cloud in the molecule,hence improving the antioxidant stability of membranes.Chapter 4 was mainly subsequent research of the problems encountered in the previous chapter.The performance of membranes was improved by introducing functional macromolecules into the main membrane matrix through organic-organic composition in order to solve the inevitable bottleneck restrictions on the property enhancement of single-polymer membranes.The introduction of guest doping realized the merits of materials and improved the performance.In the specific work,recast Nafion,a typical case of commercialized PEM,was employed as the main membrane matrix for regulation.According to the approaches with higher efficiency of polarity matching,the polymer SDF-PEAK was prepared and introduced to recast Nafion as a guest subject.Inside the membrane,SDF-PEAK played the role of a membrane skeleton molecule,supporting and optimizing the mechanical strength and dimensional stability of the composite membrane.In the meantime,the Nafion molecule’s self-assembly was guided and promoted to heighten the separation degree of the hydrophilic-hydrophobic nanophase,which made the hydrophilic ion clusters in the main membrane matrix aggregate to form a proton transport channel with certain connectivity.In addition,the adaptation in the solution formula and the optimization in membrane preparation methods actualized the increasing compatibility that reached the 20%compound amount between the two polar molecules in maximum.At 80℃,the composite membrane CHPEM-15 displayed the highest conductivity(0.239 S·cm-1)and selective permeability(1.32×105 S·s·cm-3).By testing its battery performance,the maximum power density of CHPEM-15 reached 127.88 m W·cm-2,which was nearly 50%higher than that of Nafion..Chapter 5 further investigated the higher synergistic interaction between the membrane matrix and functional fillers.Specially speaking,in the relevant research,a type of carbonized polymer quantum dots（CPDs）with a size of approximately 2 nm and a surface functionalized by amino modification was prepared and introduced into Nafion as guest particles so as to compose CHPEM composite membranes.Due to the tiny size and selective acid-base interactions,CPDs could also directly regulate the hydrophilic microphase structure that contained sulfonate in the membrane at a molecular level.While promoting the‘de novo construction’of proton transport channels,CPDs also assembled themselves in the channel to actualize the role of nano-screening.This not only improved the proton conductivity of composite membranes but also reduced the methanol penetration,improving the membrane selectivity and realized a higher extent of synergistic interactions between the membrane matrix and functional fillers.At 80℃,the conductivity of CHPEM-15 was 0.239 S·cm-1;and when methanol solution acted as the anode,the maximum power density of a single cell could reach 127.88 m W·cm-2.Finally,Chapter 6 explored the synergistic effect of multi-component doped particles to improve the comprehensive performance of membrane matrix so as to solve the problem encountered in the previous chapter,which was regarding the insufficient enhancement of matrix performance by single fillers.Specially speaking,this chapter was founded on and extended the conclusion obtained in Chapter 5,and different polymer quantum dots were prepared including s-CPDs with rich sulfonic acid groups on the surface,A-CPDs with iminogroup and sulfonic groups on the surface（1:1 in ratio）,and functional mesoporous silica V-MSN.After s-CPDs were assembled in V-MSN,the surface of the particles was functionalized to obtain CMP grafted with the shell containing tertiary amine group.Then,CMP and A-CPDs were introduced into Nafion to modify the membrane matrix based on the synergistic action of multi-component composite fillers.The doping particles with better water-retaining capacity could not only attract and facilitate the aggregation of sulfonic groups through acid-base interactions,improving the penetration degree of the proton transport channel,but also raise the hygroscopic and water-retaining properties of the channel,and improve the internal hydration proton network,in order to reduce the proton transport energy barrier in the composite membrane.The conductivity of CMP&A-10 composite membrane was the highest,reaching 0.288 S·cm-1and 0.091 S·cm-1 at 80℃under100%RH and 80%RH,respectively.These figures were over 86%and 81%higher than those of RN-PEM,respectively.When the anode was methanol solution at 80℃,the maximum power density of CMP5&A-10 reaches 143.66 m W·cm-2;When the anode was hydrogen and the cathode was air,the maximum power density of CMP5&A-10became 572.47 m W·cm-2,which was 70%and 82%higher than that of the original membrane,respectively. |
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