| Due to the ever-increasing demand for energy,energy revolution with the characteristics of“clean,environmentally-friendly,safe and efficient”has been the general trend.As a fuel,methanol has the advantages of high energy density,convenient storage and transportation,safety and stability,wide material sources,low cost and easy availability.Direct methanol fuel cells are considered to be one of the most promising clean energy conversion devices.They are simple,portable,and can meet the needs of small electronic devices.Currently,the technical challenge in the development of fuel cells is to develop a membrane electrode assembly with high energy conversion efficiency and sustainable use.The membrane electrode assembly is usually composed of a proton exchange membrane,a catalytic layer,a gas diffusion layer and bipolar plates.The proton exchange membrane,as its core component,has an important influence on the overall performance of fuel cells.Therefore,the design and construction of high-performance proton exchange membrane materials is very crucial.The perfluorosulfonic acid-based polymer electrolyte represented by Nafion?（Du Pont Co.,Ltd.）is the most widely used proton exchange membrane materials.Its unique molecular structure consists of a polytetrafluoroethylene-like hydrophobic skeleton and a side chain terminated with hydrophilic sulfonic acid groups.The perfluorinated backbone provides chemical stability and the alkyl sulfonic acid side chains can aggregate into ion clusters to form an effective proton transport network.However,such perfluorosulfonic acid polymers still have a series of problems such as low reaction kinetic efficiency,material dehydration effect under high temperature and low humidity conditions,and fuel permeation which leads to a sharp drop in cell efficiency.The long-term performance of proton exchange membranes has become one of the difficulties that direct methanol fuel cells cannot be widely promoted.This work aimed at the current problems of low proton conductivity,poor structural stability,and serious fuel penetration at high temperatures in the presence of perfluorosulfonic acid-based polymer electrolytes.Taking advantage of the characteristics of high mechanical strength and good dimensional stability of poly aryl ether ketone materials,poly ether ether ketone with high methanol resistance was reasonably modified.The newly constructed porous matrixes（crystalline type,block type,blending modification,surface modification）were endowed with high proton conductivity while maintaining excellent mechanical properties and dimensional stability.They were combined with polyelectrolyte materials through pore filling technology to prepare proton exchange membranes with high methanol resistance,which greatly improved the power density of materials in high-concentration direct methanol fuel cells,and the relationship between the structure of the composite membrane and the single cell performance had been discussed.Utilizing the excellent comprehensive performance of crystalline poly ether ether ketone,it was prepared into a porous membrane through the reversible chemical modification reaction,and used as the membrane matrix to compound with sulfonated poly ether ether ketone to fabricate the poly ether ether ketone reinforced composite membrane.The porous structure compacted the proton transporting groups and limited the swelling effect of molecular chains,so that the methanol resistance and dimensional stability of the composite membrane had been significantly improved.The methanol permeability was only 3.2×10-7 cm2 s-1,and the swelling rate was only 15%of that of a pristine sulfonated membrane.Meanwhile,the excellent mechanical stability of poly ether ether ketone reduced the influence of the membrane thickness.Finally,the single cell performance of the composite membrane（with a thickness of 15μm）was evaluated in a 10 M methanol solution.The open circuit voltage was 0.53 V and the maximum power density was 34.6 m W cm-2,which was approximately twice of the original sulfonated membrane.Compared with the commercial Nafion,this type of composite membranes is expected to increase the power density of DMFCs.In order to further improve the proton conductivity of composite membranes,a poly aryl ether ketone block copolymer with a low degree of sulfonation was synthesized from the perspective of molecular design,preparing into a porous matrix,and filled with Nafion to obtain the sulfonate poly aryl ether ketone reinforced composite membrane.Block copolymers provided obvious microphase separation structure and larger microscopic sizes,could absorb more water and shorten the distance of proton transition,form continuous and efficient ion channels,help the conduction of protons in the membrane,and avoid the swelling effect caused by high sulfonation degree.The composite membrane showed a proton conductivity of 0.086 S cm-1,similar to Nafion in the through-plane direction.Meanwhile,the reinforcement of the porous matrix improved the mechanical properties and methanol barrier properties of the Nafion-based composite membrane,and the minimum methanol permeability was 4.0×10-7 cm2 s-1.The optimal selectivity of the composite membrane was 1.04×105S s cm-3.The output voltage in a 10 M methanol solution was 0.61 V,and the maximum power density was 46.8 m W cm-2.Compared with the crystalline poly ether ether ketone reinforced composite membrane aforementioned,the proton conductivity of this type of membranes had a certain improvement,which further improved the power density of fuel cells.Although the prepared sulfonated block porous matrix improved the performance degradation caused by the traditional pore-filled composite membrane with the aprotic matrix,in order to maintain the balance between proton conductivity and methanol permeability,its proton conductivity was still lower than the traditional homogeneous membrane.Hence,a poly ether ether ketone modifier with a low degree of sulfonation was prepared,and it was introduced into the porous poly ether ether ketone membrane by means of physical blending.The rearrangement of the hydrophilic groups in the porous membrane was realized by solvent-induced method,and the composite proton exchange membrane enhanced by poly ether ether ketone/sulfonated poly ether ether ketone was obtained by compounding with Nafion.The hydrophilic molecular segments had gathered around the pores through the solvent treatment,forming a highly hydrophilic proton transport channel with a unique phase separation structure,which increased the selectivity of ion channels,greatly improved the performance in a high-concentration methanol solution.Under the test condition of 10 M methanol fuel at 70℃,the open circuit voltage was 0.67V,and the maximum power density was 91.7 m W cm-2.This type of proton exchange membranes exhibited higher proton conductivity than the sulfonated block poly aryl ether ketone reinforced composite membrane,which greatly increased the power density of DMFCs.The composite membrane prepared from the blended sulfonated porous matrix possessed excellent proton conductivity as well as high selectivity,and showed an effective improvement in cell performance in high-concentration methanol fuel.However,the sulfonated degree of the matrix was limited by the preparation process and was difficult to control.Therefore,the pore surface of the poly ether ether ketone porous membrane was pre-modified by the co-deposition reaction of polydopamine/polyethyleneimine to prepare the phosphotungstate-ionic liquid composite functionalized modifier.It was introduced into the pore surface of the modified matrix to form an organic-inorganic composite proton exchange membrane with Nafion.Modification of the nano-modifier directly to the surface of the pores gave the inert matrix the ability to conduct protons.At the same time,due to the electrostatic interaction between the imidazole group in the ionic liquid and the sulfonic acid group of phosphotungstate,the sulfonic acid group in the nano-modifier and the amino group on the surface of the polyetheretherketone porous matrix,acid-base pairs were formed in the composite membrane.The low-resistance proton transport network could reduce the activation energy of proton conduction,increase the proton conductivity(0.11 S cm-1),which reached the level of Nafion.But the methanol resistance of the composite membrane was much higher than that of Nafion.For the single cell performance test,the open circuit voltage was 0.71 V,and the maximum power density was 107 m W cm-2,which was higher than the power density of previous reported pore-filled composite membranes.This work provided a new way to solve the mutual restriction of proton conduction and methanol permeation faced by polyelectrolyte materials,and to design and construct new type of high methanol-resistance proton exchange membranes. |
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