| The fuel cells?FCs?system,as a source of clean energy,has received considerable attention,due to its high efficiency and environmental friendliness converting the chemical energy of fuels?such as H2,methanol,ethanol and so on?to electronic energy by electrochemical redox reaction,and without releasing toxic and harmful gas.The rate of its energy conversion is up to 40-60 %.Proton exchange membrane?PEM?fuel cell?PEMFC?,as a kind of FCs system,is composed by anode,cathode and proton exchange membrane.Among them,the PEM has the effect of transporting protons,isolating electons and fuel,which is one of the core components in PEMFC.Currently,sulfonated aromatic polymers,as alternative PEMs to Nafion,possessing the advantages of simple and flexible structure design,low fabrication cost,and high mechanical stability,have attracted wildly attention over the past decades.Among of them,sulfonated poly?arylene ether ketone?s?SPAEKs?have been extensively studied as alternative PEMs due to their high temperature stability,good chemical stability,prominent mechanical property,and low production cost.Main-chain-type SPAEKs PEMs,as a kind of SPAEKs,have the typical distinguished micro-phase-sepration structure from those of PFSA membranes.For PFSA membranes,the water content inside membranes is balanced by the extreme hydrophobicity of the perfluorinated polymer backbones and the extreme hydrophilicity of the terminal sulfonic acid groups.In the presence of water,only the hydrophilic domain of the nanostructure is hydrated to maintain the proton conductivity,while the hydrophobic domains provide mechanical strength.PFSA membranes have more obvious separated,less branched,good connectivity and wide channels.Compared to PFSA membranes,the hydrocarbon backbones of sulfonated hydrocarbon polymers are less hydrophobic and the sulfonic acid functional groups are less acidic and polar.As results,main-chain-type SPAEKs have less separated,highly branched,narrow and dead-end channels.So,nowadays,many reaserch groups simulated the chemical structure of nafion to prepare side-chain-type SPAEKs.Those side-chain-type SPAEKs exhibited similar proton transportion channels to that of PFSA membranes,meanwhile maintaining the prominent properties of PAEK.In chapter two,a series of novel side-chain-type sulfonated naphthalene-based poly?arylene ether ketone?s?SNPAEK-xx?consisting single-naphthalene-based in every repeat unit were successfully prepared by designing the polymer structure and adjusting the ratios between hydrophilic side-chain flexible sulfoalkyl groups with hydrophobic main-chain.The degree of sulfonation?DS?was calculated by 1H NMR method,and the mechanical property,thermal stability,water sorption,proton conductivity and methanol permeability of SNPAEK-xx membranes were further investigated.Though the mechanical properties of SNPAEK-xx membranes were decrease to some extent as the DS increasing,which was because of the plastic effect of side-chain flexible sulfoalkyl structure,all SNPAEK-xx membranes still showed higher mechanical properties than that of Nafion membrane.The DSC measurements showed that no obvious Tg could be observed below decomposition temperature for SNPAEK-xx polymers.And TGA measurements showed that 5 % weight loss temperatures of SNPAEK-xx polymers were all higher than 230 oC,indicating that SNPAEK-xx polymers had sufficient thermal stability for PEM applications.Water sorption experiments showed that SNPAEK-xx membranes had anisotropic membrane swelling in water with lower swelling in thickness than in plane,which was beneficial to improve the dimensional stability.The proton conductivities of SNPAEK-xx membranes increased with DS increasing,the highest proton conductivity value was up to 0.191 S cm-1 at 80 oC.Meanwhile,all SNPAEK-xx membranes had low methanol permeability and higher selectivity than that of Nafion,indicating SNPAEK-xx membranes have great potential for direct methanol fuel cell?DMFC?applications.The plane structure of naphthalene group had strong interaction between each other,which was benifical to increase the aggregation of hydrophilic-phase.The resulting aggregated hydrophilic domains were helpful to form more obvious phase separation structure to enhance proton conductivity.To further improve the proton conductivity,a series of novel side-chain-type sulfonated poly?arylene ether ketone?s,consisting of dual-naphtahlene-based in every repeat unit and different contents of flexible sulfoalkyl groups?SDN-PAEK-x?,were successfully prepared in chapter three.The single-naphthalene-based SNPAEK-1.60 in chapter two and recast Nafion membranes were also prepared the same conditions for comparisons.The results showed that SDN-PAEK-x membranes had prominent mechanical properties,and the introduction of dual-naphthalene-based made them exhibit higher mechanical properties than that of SNPAEK-1.60 and recast Nafion membranes.However,the introduction of dual-naphthalene-based structure reduced the elongation at break to some extent,which was because of strong interaction between naphthalene moieties in SDN-PAEK-x prohibited the polymer chain conformational adjustment and further restrained polymer chain stimulation response.The investigation of proton conductivity showed that the highest proton conductivity of SDN-PAEK-x membranes was up to 0.271 S cm-1 at 80 oC.And compared with SNPAEK-1.60 membrane with similar IEC value,SDN-PAEK-1.74 membrane with dual-naphthalene-based exhibited higher proton conductivity,higher proton conductivity normalized against IEC value,and higher effective proton mobility with lower analytical acid concentration.All results indicated that the introduction of dual-naphthalene-based was benifical to improve proton conductivity.Single cell performance investigation showed that SDN-PAEK-1.94 membrane exhibited a good cell performance with maximum power density of 60 m W·cm-2 at 80 oC,which was higher than that of recast Nafion with maximum power density of only 24 m W·cm-2 at 80 oC.All the results indicated that the introduction of dual-naphthalene-based in SDN-PAEK-x membranes improved the comprehensive performances of SDN-PAEK-x membranes,which made them have the great potential to be promising alternative PEMs for DMFC applications.The single-naphthalene-based SNPAEK-xx and dual-naphthalene-based SDN-PAEK-x polymers had been prepared successfully in chapter two and chapter three,respectively.Comparing their comprehensive properties and fabrication cost,single-naphthalene-based SNPAEK-xx was chosen as a Nafion modifier because of its prominent comprehensive properties and lower fabrication cost.In chapter four,SNPAEK-1.35 was prepared because of its similar proton conducicity to Nafion and chosen as Nafion modifier to be blended with raw Nafion to improve comprehensive properties of Nafion membrane.As the exsistence of incompatibility between Nafion and SNPAEK-1.35,a series of Nafion composite membranes?Nafion/SNPAEK-x?were prepared with the mass contents of SNPAEK-1.35 polymer lower than 10.0 %.Incompatibility between Nafion and SNPAEK-1.35 polymer was been insight obvious in TEM viewing.Among them,Nafion/SNPAEK-7.5% membrane exhibited the most well-defined and connective aggregation phase separation structure,which was benifical to improve proton conductivity.Although between Nafion and SNPAEK-1.35 had the incompatibility,Nafion composite membranes still exhibited more prominent proton conductivity properties than that of Nafion.Especially,Nafion/SNPAEK-7.5% membrane exhibited the highest proton conductivities and lowest proton-conductive activation energy.The effect of reducing methanol crossover of Nafion composite membranes made them meet the requirements for PEMFC application.The single cell performances were evaluated using 2 mol L-1 methanol solution and O2 as fuel and oxidant.Nafion composite membranes exhibited higher OCV than that of Nafion.And the maximum power density of Nafion composite membranes were up to 47 m W·cm-2 at 80 oC,which was significantly higher than that of recast Nafion with maximum power density of 24 m W·cm-2 at 80 oC.All results indicated that the introduction of SNPAEK-1.35 polymer is beneficial to enhance the comprehensive properties of Nafion membrane,and makes Nafion composite membranes have the great potential to be applied in DMFCs. |
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