Acid-doped membranes

The methanol uptake of a grafted Nafion/poly(glycidyl methacrylate) membrane was studied by Mohy Eldin et al. [162]. The membrane, its amine derivate, and the phosphoric acid -doped membrane exhibit an important enhance of the methanol and water uptake with increasing grafting percentages. 

Yu S, Zhang H, Xiao L, Choe EW, Benicewicz BC. Synthesis of poly (2,2 -(L4-phenylene) 5,5 -bibenzimidazole) (para-pbi) and phosphoric acid doped membrane for fuel cells. Fuel Cells 2009 9(4) 318-24. 

As explained in a previous section, a membrane electrode assembly (MEA) consists of the polymer membrane that is sandwiched between an anode and a cathode electrode, respectively. The electrodes are composed of a conductive carbon network that supports a catalyst on a gas diffusion layer. An additive, such as polytetra-fluoroethylene (PTEE), helps bind the Pt/C catalyst to the gas diffusion layer. At the anode, the catalyst facihtates the oxidation of hydrogen into its constituent electrons and protons. As the protons are passed through the acid-doped membrane to the cathode, the electrons are passed through an external circuit, thereby creating electricity. Einally, the electrons and protons react with oxygen at the cathode electrode to form water as the final reaction product. 

For phosphoric acid-doped polybenzimidazole membranes with an intermediate Ap Ta (3.5), hydrogen bonds are likely formed as shown by infrared spectroscopy [61, 62] and molecular dynamic simulation [63]. The overall proton conductivity of the acid-doped membranes is at least an order of magnitude lower than that of pure phosphoric acid due to the presence of the solid polymer phase. The polymer does not seem to interrupt the extensive hydrogen bond network of the phosphoric acid, though it does decrease the percolation within the liqmd-like part of the phosphoric acid domain. As a result, the proton conducting mechanism remains the same, i.e., primarily via the Grotthuss... 

Polybenzimidazoles are normally highly soluble in polyphosphoric acid (PPA) which is commonly used as polycondensation solvent. From this solution, phosphoric acid-doped membranes can be obtained directly by casting the crude mixture as a thin film. A sol-gel transition is subsequently induced by hydrolyzing the PPA (good solvent) to orthophosphoric acid (poor solvent) under an atmosphere with carefully controlled temperature and humidity to control the acid content of the membrane [1]. This approach is further reviewed in Chap. 10. 

At room temperature the conductivity of phosphoric acid-doped mPBl was found to decrease with increasing pressure, which is also observed for pure phosphoric acid due to the increase in viscosity [32]. Based on the calculations of the activation volumes it was however concluded that the phosphoric acid-doped membrane behaves strikingly different than aqueous phosphoric acid but similar to... 

Aili D, Hansen MK, Pan C et al (2011) Phosphoric acid doped membranes based on Nafion , PBI and their blends—membrane preparation, characterization and steam electrolysis testing. Int J Hydrogen Energy 36 6985-6993... 

Differences in acid loading, conductivity, mechanical properties, inherent viscosity and fuel cell performance have been shown to vary depending on the method through which the PBI was synthesized and processed. Two methods of processing have been shown to produce greater conductivity than the conventional imbibing method. Recent reviews by Savinell, Bjerrum, Li, Benicewicz, and Schmidt discuss PBI membranes for fuel cells prepared by many different methods [5-7], In this review, we will focus on the preparation, properties and fuel cell performance of acid-doped membranes made by a unique sol-gel method termed the PPA process. 

Key words electrolyte, membrane, proton conductor, perfluorosulfonic acid, ionomer, acid-doped membranes, sulfonic acid, phosphonic acid, heterocycle,... 

Keywords Acid-doped membranes High-temperature PEMFC Polybenzimidazole Polyphosphoric Acid Process... 

Asensio et al. [71] also developed a method for producing acid-doped membranes by direct casting from an AB-PBI/phosphoric acid (PA)/methane-sulfonic acid (MSA) solution. The methanesulfonic acid was evaporated to produce a very homogenous, nearly transparent film with controlled composition and up to 3 moles PA/BI. This method of preparation was much more convenient than the typical multi-step, organic solvent based process. 

Most of these acid-polymer combinations from the early work exhibit proton conductivity less than 10 S/cm at room temperature. If the acid content is higher, the plastifying effect of the excessive acid leads to the formation of a soft paste, unable to be processed into membranes. A breakthrough was achieved when polybenzimidazole (PBl) was hrst proposed for preparing acid-doped membranes [21]. Since then phosphoric acid-doped PBl membranes have been successfully developed and... 

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