| As the gradualdrain of fossil fuels and the increasing severity ofenvironmental pollution,developing novel energy and new energy conversation devices have been the research focus in the new energy field in recent years.Direct methanol fuel cell?DMFC?is a kind of device that transforms chemical energy into electric energy directly and possesses the advantages of the easily available fuel,high specific energy and specific power,simple system construction and environmental friendliness,and so on.Proton exchange membrane?PEM?is one key component of DMFCs and has the functions of preventing fuel permeation and providing proton transfer channels.The properties of PEM can immediately affect the properties and working life of fuel cells.Nowadays,the most widely used PEMs are Nafion membranes produced by the DupontCompany,which contain good proton conductivity,high mechanical strength and chemical stability.However,the complex preparation process of perfluoropolymer backbonesof Nafion results in high price and serve environmental pollution.In addition,the disadvantages of Nafion membranes,such as high fuel permeation,proton conductivity decline at high temperature and low humidity condition,et.al.,decrease the energy density and power output of DMFCs seriously.Therefore,developing novel PEMs with high methanol-permeation resistance as alternatives to Nafion has attracted considerable attentions.Sulfonated poly?ether ether ketone??SPEEK?is the sulfonated product of the engineering plasticpoly?ether ether ketone??PEEK?.SPEEK has many advantages,such as low methanol permeability,high mechanical strength,excellent chemical stability and low price,which isconsidered as one of the promising membrane materials to replace Nafion.However,the proton conductivity of SPEEK is closely relying on the degree of sulfonation?DS?.The comparable proton conductivity to Nafion can only be obtained at a high DS value,which simultaneously causes the serious swelling problem.It leads to the restricted application of SPEEK in DMFCs.For the purpose of solving these problems of SPEEK,three modified strategies were applied:introducing inorganic fillers,blending with hydrophobic polymer and preparingsemi-interpenetrating polymer networks.The target is to improve the dimensional stability and methanol-permeability resistance of SPEEK while keeping suitable proton conductivity.As a consequent,the prepared PEMs can be utilized in DMFCs and improving the energy density of the assembled cells by increasing the concentrations of methanol fuel.The research contents of this paper are listed as the following:1.Preparation and Property Investigation of Sulfonated Carbon Nanofibers Doped SPEEK Composite MembranesMicrostructural construction ofpolymer/inorganic filler interface is a key in high performance composite proton exchange membranes.Here,carbon nanofibers?CNFs?were successfully sulfonated to prepare sulfonated CNFs?SCNFs?with sulfuric acid groups on surface.SCNFs weresheared into the average length of several micron metersby ultrasound,which facilitateduniform dispersion in the SPEEK matrixto obtain the SCNF/SPEEK organic/inorganic composite PEMs.The organic/inorganicinterfacial compatibilityalong with dimensional stability was improvedvia the hydrogen bonding interactionbetween sulfonated groups of SCNFs andSPEEK.It was found that the mechanical strength and water uptake of the composite membranes can be improved with decreased methanol permeation upon increasing the amount of SCNF in the polymeric materials.The obtained highest proton conductivity of 0.128 S cm-1 at 60 oC is comparable with the value of Nafion 117,but with substantially lower methanol permeation of 5.02×10-7 cm2 s-1.2.Preparation and Property Investigation of SPEEK/BPPOBlending Proton Exchange MembranesThe aggregation problem of inorganic nanofillers has limited the added amount,leading to the hindered improvement of membrane properties.Therefore,hydrophobic brominated poly?2,6-dimethyl-1,4-phenylene oxide??BPPO?with a high additive level was applied to modify SPEEK.A novel series of SPEEK/BPPO composite PEMs was prepared by blending BPPOwith SPEEK.The Friedel-Craft reaction between bromobenzyl groups and nucleophilic phenyl rings was performed to achieve the crosslinking networks through the heat treating.The fabricated SPEEK/BPPO membranes possess obviously low methanol permeability and swelling behavior,which is attributed to the formed interrupted methanol transport channels arising from the combined properties of the hydrophobic BPPO polymeric backbone,the formed spherical BPPO obstacles and the cross-linking networks.Thus,the SPEEK/BPPO membrane assembled DMFC demonstrates excellentpower output even at 20 M methanol solution,superior to the Nafion 117 DMFC.3.Effect of Sulfonated/Non-sulfonated Diamine Cross-linker on Properties of Semi-InterpenetratingMembranesBased on SPEEK/BPPOThere are poor structural uniformity and low proton conductivityproblems existing in the SPEEK/BPPO blending membranes.Here,the semi-IPN membranes were prepared to adjust the micromorphologies and properties of membranes.The semi-interpenetrating polymer network?semi-IPN?PEMs based on SPEEK,BPPO and diamine were prepared by the reaction between bromobenzyl groupsof BPPO and amine groups of diamine linkers.The influence of the non-sulfonated cross-linker and sulfonated cross-linker on the membrane properties was studied.It was noted that the formed crosslinking networks are benefit for reducing the swelling and methanol permeability of the semi-IPN membranes.The sulfonated cross-linker enhances the micro phase separation,leading to improved proton conductivity.However,the PEM fabricated by the non-sulfonated cross-linker has smaller hydrophilic regions,resulting in decreased methanol permeability and the methanol permeability is3.26×10-7 cm2 s-1,which is four times lower than that of the Nafion membrane.The highest relative selectivity of 18.12×104 S s cm-3 was obtained for the PEM based on the non-sulfonated cross-linker,which is 2.7 times higher than that of 6.64×104 S s cm-3for the Nafion 117 membrane.The cell assembled by this membrane displays 3.8 times higher power output than the Nafion 117 DMFC at 5 M methanol solution.4.Effect of Structure and Polarity of Diamine Cross-linker on Properties of Semi-InterpenetratingMembranes Based on SPEEK/BPPOExcept sulfonated/non-sulfonated cross-linkers,the rigidity/flexibility,volume and hydrophilic/hydrophobic peculiarity of cross-linkers can also affect the membrane morphologies and properties.The preparation of semi-IPN membranes is similar to the former work 3.The influence of the structure and polarity of the cross-linkers on the membrane properties was studied.Four kinds of diamine cross-linkers with different structures of flexible,hydrophilic flexible,rigid/bulky and hydrophilic rigid/bulky structures were applied.The results indicate thatthe hydrophilic cross-linker is prone to accelerate micro phase separation and thus,obtain high proton conductivity.Comparing with the hydrophilic flexible cross-linker,the PEM prepared with the hydrophilic rigid cross-linker possesses much higher proton conductivity.In addition,the hydrophobic rigid/bulky fluorine cross-linker is the most cm2 s-1.The assembled DMFC by this membrane gives rise tohighpower output at high concentration methanol solution.In particular,by analyzing the correlation of cell performances,proton conductivity and methanol permeability,we note that methanol permeability is a more important factor than proton conductivity for high concentration DMFCs.5.Effect of Content of Amine-containing Crosslinking Networks on Properties of Semi-Interpenetrating MembranesIt is found that the structure of crosslinking networks has significant effect on the membrane properties,but there is still no paper about the effect of content of amine-containing crosslinking networks on membraneproperties.Here,the hydrophilic flexible cross-linker?2,2'-?Ethylenedioxy?bis?ethylamine??in the work 4 was selected to construct the amine-containing crosslinking networks and the influence of crosslinking network content on membrane properties was systematically investigated.It was observed that the formed acid-base ionic pairs are beneficial for preventing methanol crossover and the methanol permeability decreases as the increasing crosslinking network content.Thelowest methanol permeability of 5×10-8 cm2 s-1 is only 3.8%of the value offered by the Nafion 117membrane.Otherwise,dimensional stability and mechanical strength are also obviously improved.The DMFC assembled by the prepared membrane displays four times higher power output at 5 M methanol solution than the Nafion 117 DMFC.As a result,the PEMs based on the modification of SPEEK were researched.Three methods,namely introducing inorganic fillers,blending with hydrophobic polymer and semi-IPNs,were used to modify SPEEK to improve the dimensional stability and methanol-permeability resistance.By comparing with the two methods of introducing inorganic fillers and blending with hydrophobic polymer,the semi-IPN PEMs possess much better structure uniformity and property controllability.The DMFCs assembled by the prepared PEMs contain excellent power output at high concentration methanol solutions,which improves the energy density of DMFCs.Thus,this paper provides a useful perspective for the research ofnovel PEMs inDMFCs. |
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