Extended polymer electrolyte membrane

The concept of a promoter can also be extended to the case of substances which enhance the performance of an electrocatalyst by accelerating the rate of an electrocatalytic reaction. This can be quite important for the performance, e.g., of low temperature (polymer electrolyte membrane, PEM) fuel cells where poisoning of the anodic Pt electrocatalyst (reaction 1.7) by trace amounts of strongly adsorbed CO poses a serious problem. Such a promoter which when added to the Pt electrocatalyst would accelerate the desired reaction (1.5 or 1.7) could be termed an electrocatalytic promoter, or electropromoter, but this concept will not be dealt with in the present book, where the term promoter will always be used for substances which enhance the performance of a catalyst. 

A second commercially available electrolyzer technology is the solid polymer electrolyte membrane (PEM). PEM electrolysis (PEME) is also referred to as solid polymer electrolyte (SPE) or polymer electrolyte membrane (also, PEM), but all represent a system that incorporates a solid proton-conducting membrane which is not electrically conductive. The membrane serves a dual purpose, as the gas separation device and ion (proton) conductor. High-purity deionized (DI) water is required in PEM-based electrolysis, and PEM electrolyzer manufacturer regularly recommend a minimum of 1 MQ-cm resistive water to extend stack life. 

From all that has been said above, it can be concluded that polymer electrolyte membrane fuel cells, working at elevated temperatures, are highly promising. Many difficulties must still be overcome in order to develop models, which will function in a stable and reliable manner, and for extended periods of time. At present, about 90% of all publications on fuel cells are concerned precisely with the attempts to overcome these difficulties. Most of the publications deal with research into new varieties of membrane materials. Some results of these works are described in the reviews on elevated-temperature-polymer electrolyte fuel cells (Zhang et al., 2006 Shao et al., 2007). 

Figure 6.2 Various configurations for extended-area polymer batteries. From left to right concertino, Swiss-roll and flat-plate versions. Electrode 1 (anode), electrolytic membrane, electrode 2 (cathode), current collector.

It is well known today that perhaps the most dramatic application of the fuel cell—an electrochemical device that may be based in the future upon the oxidation of aliphatic hydrocarbons— was in the Gemini Space Mission. In this application, the cell was based upon the use of a solid polymer electrolyte —a cation-exchange membrane in its acid form—but with hydrogen and oxygen as the fuels rather than an aliphatic hydrocarbon. Considerable research and development preceded and supported these successful missions and the units demonstrated that indeed the H2/O2 fuel cell was capable of extended performance at relatively high current densities—2l capability of fundamental importance in commercial applications. 

The low cost and excellent oxidation and thermal stability of phosphoric acid doped polybenzimidazole (PBI) prompted researchers at Case Western University (Samms et a/., 1996 Wang et /., 1996a,b,c) to develop this membrane as a polymer electrolyte for DMFCs. After investigation of the thermal stability of PBI doped with phosphoric acid up to 600°C, it was concluded that this membrane is adequate for use as PEM in a high temperature fuel cell. These studies may also be extended to other polymers like the polybenzimidazobenzophenanthrolines (PBIPAs) which exhibit excellent thermal and mechanical properties (Zhou and Lu, 1994). 

Concentration of Electrolyte Myer and Sievers"" applied the Donnan equilibrium to charged membranes and developed a quantitative theory of membrane selectivity. They expressed this selectivity in terms of a selectivity constant, which they defined as the concentration of fixed ions attached to the polymer network. They determined the selectivity constant of a number of membranes by the measurement of diffusion potentials. Nasini etal and Kumins"" extended the measurements to paint and varnish films. 

Modeling an electrochemical interface by the equivalent circuit (EC) representation approach has been exceptionally popular in studies of electrodes modified with polymer membranes, although an analytical approach based on transport equations derived from irreversible thermodynamics was also attempted [6,7]. ECs are typically composed of numerous ideal electrical components, which attempt to represent the redox electrochemistry of the polymer itself, its highly developed morphology, the interpenetration of the electrolyte solution and the polymer matrix, and the extended electrochemical double layer established between the solution and the polymer with variable localized properties (degree of oxidation, porosity, conductivity, etc.). 

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