Proton-permeable polymeric membrane

The proton-exchange membrane (PEM) fuel cell uses a thin, permeable polymeric membrane as the electrolyte. The membrane is very small and hght and in order to catalyse the reaction, platinum electrodes are used on either side of the membrane. Within the PEM fuel cell unit, hydrogen molecules are supplied at the anode and split into hydrogen... 

Apart from water uptake and proton conductivity, another important physicochemical property of a polymeric membrane for PEM fuel cells is its gas permeation, which can be considered a measure of membrane impermeability toward reactant species. Permeability is dehned as the product of diffusivity and solubility ... 

In the search for PEMs with lower alcohol permeability than Nafion and other perfuorinated membranes, without degradation of the proton conductivity, a number of new polymeric membranes were synthetized and characterized, such as sulfonated polyimides, poly(arylene ether)s, polysulfones, poly(vinyl alcohol), polystyrenes, and acid-doped polybenzimidazoles. A comprehensive discussion of the properties of these alternative membranes is given in Chap. 6, along with those of Nafion and Nafion composites. 

Gas sensors usually incorporate a conventional ion-selective electrode surrounded by a thin film of an intermediate electrolyte solution and enclosed by a gas-permeable membrane. An internal reference electrode is usually included, so that the sensor represents a complete electrochemical cell. The gas (of interest) in the sample solution diffuses through the membrane and comes to equilibrium with the internal electrolyte solution. In the internal compartment, between the membrane and the ion-selective electrode, the gas undergoes a chemical reaction, consuming or forming an ion to be detected by the ion-selective electrode. (Protonation equilibria in conjunction with a pH electrode are most common.) Since the local activity of this ion is proportional to the amount of gas dissolved in the sample, the electrode response is directly related to the concentration of the gas in the sample. The response is usually linear over a range of typically four orders of magnitude the upper limit is determined by the concentration of the inner electrolyte solution. The permeable membrane is the key to the electrode s gas selectivity. Two types of polymeric material, microporous and homogeneous, are used to form the... 

Among the many applications of NMR techniques, the most essential application of NMR is the characterization of molecular structures of polymeric proton-conducting materials, in terms of one-dimensional (ID) and two-dimensional (2D) NMR techniques. The second type of application is to characterize the dynamic properties of protons, water, and alcohol molecules. These dynamic properties are directly related to the mobility of protons, the macroscopic transport of water in terms of difilision and the electroosmosis, and the alcohol crossover or permeability, respectively. The third type of application is related to the identification of the water types soaked in the proton exchange membranes and the visuafization of water distribution in the proton exchange membranes. 

Poly(phenylene oxide) (PPO) is a thermoplastic, linear, noncrystalline polyether commercially produced by the oxidative polymerization of 2,6-dimethylphenol in the presence of a copper-amine catalyst. PPO has become one of the most important engineering plastics widely used for a broad range of applications due to its unique combination of mechanical properties, low moisture absorption, excellent electrical insulation property, dimension stability and inherent flame resistance. This chapter describes the recent development of this polymer, particularly on the production, application, compounding, properties of its alloys and their general process conditions. The polymerization mechanism and thermal degradation pathways are reviewed and new potential applications driven by the increasing environmental concerns in battery industry, gas permeability and proton-conducting membranes are discussed. 

In summary, this section demonstrates that there are many kinds of membranes with functionalized non-fluorinated backbones that have been investigated the recent years in order to substitute the expensive Nafion PEM with properties (especially proton conductivity, methanol permeability, and mechanical stability) superior to those of Nafion. The last category of polymeric PEMs is the acid-base polymer systems, which are presented in the following section. 

Polyaniline (PANI) nanocomposite membranes are also prepared by a sol-gel process, embedding silica in the hydrophilic clusters (Nafion) followed by its deposition by redox polymerization [51]. PANI modified the membrane structure and reduced the methanol crossover, while silica Incorporation improved the conductivity and stability. Zeolite has been incorporated as potential filler for PEMs, either by blending or by infiltration in swelled membrane, to reduce the methanol permeability and enhance the thermal stabihty [52,53]. Although the fuel cell performance of these membranes was Inferior compared with pristine Nafion membrane, incorporation of semipermeable particles is an effective method to engineer the transport properties of composite membranes. Chen et al [54] reported nanocomposite membranes by in situ hydrothermal crystallization method, with similar proton conductivity, but low methanol permeability (40% less) in comparison with Nafion membrane. These membranes showed higher OCV (3%) and power density (21%) than Nafion. 

Some hybrid membranes prepared by mixing various polymeric ionomers with AI2O3 or with Si02 ceramic powders were described in Ref. [194]. Some improvements due to the mixing of PVDF with AI2O3 were noted. However, neither proton conductivity nor permeability to methanol changed dramatically as a result of such treatment [194]. The proton conductivity of hybrids prepared by mixing sPEEK with a zeolite did not exceed 8mS/cm, even at 140°C [195]. 

---