Preparation And Property Studies Of Glass-based Proton Exchange Membrane

Proton exchange membrane (PEM) is the core material of proton exchange membranefuel cells (PEMFC). Commercialized organic perfluorinated sulfonic acid proton exchangemembrane (Nafion membrane) has such advantages as high proton conductivity andchemical stability but there also exist limitations like high costs, complex synthesis process.And because it is the fluoride polymers, its recycling and disposal is really difficult, whichbrings environmental burden. Moreover, in high-humidity conditions where fuel cells operate,a perfluorosulfonic acid proton exchange membrane swells and deforms easily, making thefuel (particularly methanol) easy to permeate through the proton exchange membrane, as toform a chemical short-circuit, resulting in the output performance of the fuel cell reduced.Inorganic proton conductive materials, characterized by the high electrical conductivity, goodthermal stability and chemical stability and low cost, become a new hot spot in thedevelopment of proton exchange membrane. In this paper, a new glass-substrate protonexchange membrane is developed by sol-gel method, including glass-substrate protonexchange membrane, the flexible composite proton exchange membrane that is prepared bycombined glass with hydrocarbon polymer composites, and we also characterize theseobtained proton exchange membranes.By sol-gel method combined with low temperature hydrothermal treatment, thefluorinated phosphorous silicon glass exchange membrane with uniform structure (SiO2-P2O5(Nafion)) is successfully prepared. The skeleton of glass membrane is made up of thedeposited nano-SiO2particles in a diameter of5-10nm, and has a worm-like connectedstructure. Studies have shown that the phosphoric acid not only enables the glass membrane tohave high proton conductivity but also functions as a pore-forming agent in the gel process.By changing the content of phosphoric acid and Nafion, the pore size and porosity can bebalanced, further optimizing the performance of the glass film. The proton conductivity of glass membrane at90℃,70%relative humidity excesses1×10-1S·cm-1, and has good thermalstability. However, the innate high brittleness of inorganic proton exchange membrane makesthe pressing process of MEA unable to be implemented. And the impedance loss, includingthe interface resistance, makes the peak power density under normal test conditions very low(SiO2-P2O5(Nafion) membrane only42.6mW·cm-2). In this paper, the MEA preparationmethods applied for glass membrane is thoroughly studied and battery assembly and testconditions are optimized. As a result, the peak power density of the fuel cell based on theglass membrane reaches207mW·cm-2, open-circuit voltage (OCV)0.94V.In order to overcome the brittleness of inorganic membranes, composite proton exchangemembrane (SiO2-P2O5(Nafion)/SPEEK) was successfully prepared by mechanical millingprocess, based on fluorinated phosphorous silicon glass powder and hydrocarbon polymerswith low cost and high strength. Mechanical milling makes the composite membrane maintaingood flexibility even the proportion of glass powder is70wt.%. SEM analysis shows thatSiO2-P2O5(Nafion) glass powder embeds in the SPEEK matrix uniformly and the compositemembrane has no the appearance of significant phase separation. The addition of thehigh-conductivity glass powder makes the conductivity of composite membrane reach1.5×10-2S·cm-1at80℃and90%RH conditions, much higher than that of the pure SPEEKpolymer membrane. The flexible composite membrane makes it possible to prepare MEA byhot pressing, and the peak power density of the H2/O2single cell reaches322mW·cm-2that isbased on6NPS/4SPEEK composite membrane. In addition, the tests of direct methanol fuelcell, which is based on the SiO2-P2O5(Nafion)/SPEEK composite membrane, are conducted.The factors affecting the peak output power density of a single cell are studied, includingmethanol permeability, operating temperature, flow rate, and oxygen back pressure ofmethanol and other test conditions. The study shows that the methanol permeability of thecomposite membrane is lower than that of Nafion (7.5×10-7cm2·s-1) and the peak powerdensity of a single cell output reached73.1mW·cm-2at85℃and under the conditions of2mL·min-1methanol flow rate as well as0.2MPa oxygen back pressure.In order to reduce costs, ceasing to use the expensive perfluorinated sulfonic acidpolymer, we further synthesized fluoride-free phosphorous silicon glass/hydrocarbon polymercomposite proton exchange membrane (SiO2-P2O5/SPEEK). The addition of high heat-resistance inorganic SiO2-P2O5glass powder and the high heat-resistance of SPEEKitself enable the thermal stability of the composite membrane to significantly increase, whichhelps to prevent the local overheating of membrane that results in the failure of the compositemembrane during the operation of fuel cell and, meanwhile, endows the fuel cell with thepotential of operating at80-100℃. Meanwhile phosphosilicate glass itself has high protonconductivity, so the conductivity of SiO2-P2O5/SPEEK composite membrane can reaches10-2S·cm-1under90%RH conditions. The peak power density of the H2/O2single cell based onthe6Si7P3/4SPEEK composite membrane reaches355.6mW·cm-2and open-circuit voltage(OCV)0.94V.Compared with low-temperature PEMFCs, intermediate temperature proton exchangemembrane fuel cells (operating temperature>100℃) have a higher resistance to CO, fasterelectrode kinetics of catalytic, no cathode flooding and relatively simple system design and soon. In order to develop a new proton exchange membrane used for intermediate temperaturefuel cell, in the present work, N, N-diethyl-methylamine bistrifloromethylsulfonyl imide([dema][TfOH]) proton ionic liquid is used as the intermediate temperature proton conductor;and we prepare the glass proton exchange membrane containing ionic liquid (SiO2/[dema][TfOH]) by sol-gel method where the porous glass of SiO2serves as the substrate. At atemperature range of120℃-220℃and non-humidified condition, the glass membrane has ahigh ionic conductivity reaching the magnitude of10-2S·cm-1. The test results of H2/O2singlecell under anhydrous conditions at60℃,100℃and140℃respectively indicate thatSiO2/[dema][TfOH] glass membrane shows good performance in conductivity and impedingfuel permeability (OCV reaches0.98V at60℃and H2/O2condition). But because of thebrittleness and large internal resistance caused by MEA, the single cell output power densityis only2.8mW·cm-2at140℃in the anhydrous environment. In order to obtain high outputpower density, further research is needed, including catalytic electrode preparation, MEAprocess and the study of high temperature test parameters concerning to the entire system ofsingle cell.