A Study On The Polymer Electrolyte For Direct Methanol Fuel Cell Applications

Direct methanol fuel cells (DMFC) have attracted considerable attention as an alternative to the present power sources, since they offer numerous benefits, including high efficiency, high power density, low or zero emissions and easy fuel carriage. DMFC are expected to find wide application as a portable or mobile power, exemplified by battery for cellular phones or engine for electric vehicles. However, at present, two major technology problems in DMFC still restrict their performances and applications. One problem is the crossover of methanol through the electrolyte membrane. The other is slow methanol oxidation kinetics on the anode catalyst. In this study, development of methanol barrier conducting membranes was attempted. Heteropoly acids (HPAs) are known to be the most conductive solids among the inorganic solid electrolytes at room temperature, and they are considered as a new kind of electrolyte for fuel cell applications. Worrying about the nature of the HPAs water-soluble, previous researchers confined their consideration of HPA application within the scope of H2 feed proton exchange membrane fuel cells (PEMFC). In this work, we went further by studying the feasibility of adopting HPAS for liquid DMFC applications. Three kinds of inorganic-organic composite membranes including HPAs doped polyvinyl alcohol (HPAs/PVA), phosphotungstic acid (PWA) doped sulfonated poly(ether ether ketone) (PWA/SPEEK) and PWA doped sulfonated phenolphthalein poly(ether sulfone) (PWA/SPES-C) were prepared. Because of the hydrogen bonds between these hydrophilic polymers and heteropoly acids, these composite membranes are stable in methanol water solution. The effect of the kind of heteropoly acids, the content of PWA and operated temperature on the proton conductivity and methanol permeability of these composite membranes was investigated. Proton conductivity of these composite membranes is about 10-1~10-2S/cm at 100℃, which is better than that of Nafion? 115 membrane under the same condition. Methanol permeability of these membranes is about an order of magnitude lower than that of Nafion? 115 membrane.SPEEK and SPES-C membranes with various degree of sulfonation (DS) were prepared and their feasibility for DMFC applications was first studied. Proton conductivity of SPEEK membrane (DS=46.71%) and SPES-C membrane (DS=70.21%) is about 10-2S/cm at 100℃, which is closed to that of Nafion? 115 membrane under the same condition. Methanol permeability of these two membranes is about 10-7cm2/s, which is considerably smaller than that of Nafion? 115 membrane. In addition, the DMFC performance of the SPEEK membrane was better than that of Nafion? 115 membrane at 80℃.We first studied the influence of curing of SPEEK membranes on their proton conductivity and methanol permeability. It was discovered that the residual DMF solvent would affect the proton conductivity and methanol permeability of SPEEK membranes. Methanol fuel cell with alkaline electrolyte might be attractive, for it will allow much more choice of catalyses. But new type of electrolyte membrane is needed as a first step to realize this idea. So we also had a preliminary study on novel anion conducting membrane for alkaline direct methanol fuel cell applications. Through chloromethylated, aminate and hydrolytic reaction, OH-form quaternized PES-C membranes were synthesized. Conductivity in NaOH solution of these membranes is about 10-4S/cm at room temperature. Methanol permeability of these membranes is about 10-8cm2/s.