Water proton conductors

Polymer Electrolyte Fuel Cell. The electrolyte in a PEFC is an ion-exchange (qv) membrane, a fluorinated sulfonic acid polymer, which is a proton conductor (see Membrane technology). The only Hquid present in this fuel cell is the product water thus corrosion problems are minimal. Water management in the membrane is critical for efficient performance. The fuel cell must operate under conditions where the by-product water does not evaporate faster than it is produced because the membrane must be hydrated to maintain acceptable proton conductivity. Because of the limitation on the operating temperature, usually less than 120°C, H2-rich gas having Htde or no (

A number of factors must be taken into account when the diagrammatic representation of mixed proton conductivity is attempted. The behavior of the solid depends upon the temperature, the dopant concentration, the partial pressure of oxygen, and the partial pressure of hydrogen or water vapor. Schematic representation of defect concentrations in mixed proton conductors on a Brouwer diagram therefore requires a four-dimensional depiction. A three-dimensional plot can be constructed if two variables, often temperature and dopant concentration, are fixed (Fig. 8.18a). It is often clearer to use two-dimensional sections of such a plot, constructed with three variables fixed (Fig. 8.18h-8.18<7). 

Typically, Nation ionomer is the predominant additive in the catalyst layer. However, other types of CLs with various hygroscopic or proton conductor additives have also been developed for fuel cells operafed xmder low relative humidity (RH) and/or at elevated temperatures. Many studies have reported the use of hygroscopic y-Al203 [52] and silica [53,54] in the CE to improve the water retention capacity and make such CEs viable for operation af lower relative humidity and/or elevated temperature. Alternatively, proton conducting materials such as ZrP [55] or heteropoly acid HEA [56] have also been added... 

All acidic proton conductors discussed so far in this review have relied on the presence of large amounts of water (A = 10—30) as a mobile phase for the conduction of protons. Current targets for automotive use of hydrogen/air fuel cells are 120 °C and 50% or lower relative humidity. Under these conditions, the conductivity of the membrane decreases due to low water uptake at 50% relative humidity and thus creates large resistive losses in the cell. To meet the needs of advanced fuel cell systems, membranes will have to function without large amounts of absorbed water. Organic—inorganic composites are one preferred approach. ... 

Imidazole proton conductors have been explored as water replacement solvents and have also been attached to polymer backbones to replace the acid/ water complex in current PEMs (Figure 45). 

Not surprisingly water in the form of hydroxyl groups can become incorporated into the titanate structure and the evolution of this during subsequent processing stages towards the sintered ceramic has to be planned for. In its fundamentals the situation is identical to that described in Section 4.6.1 concerning the development of proton conductors. 

Fig. 4.26 The elements of the hydrogen fuel cell. Note-, (i) The student is reminded that a chemical species which loses electrons is oxidized one that gains electrons, reduced, (ii) The proton H is transported through the polymer electrolyte attached to water molecules, (H20)nH which causes a water management problem. Current research is aimed at developing proton conductors able to operate in the region of 200 °C when the proton migrates unattached to water molecules.

In the case of the high temperature SOFC discussed below the principles outlined above equally apply. The technical differences are that the cell runs typically on hydrocarbon fuels (e.g. natural or coal-gas) and that the electrolyte is an oxygen ion conductor rather than a proton conductor. The complex fuel molecules, in the presence of the water molecule and at the high operating... 

More recently, H. Iwahara et al. [20] reported that some compounds having the perovskite structure (see Section 2.7.3) become proton conductors if hydrogen is introduced into the crystal, and the solubility of water and proton mobility in perovskites are now actively researched topics [21]. The perovskites which can be tailored to exhibit high protonic conductivity have compositions of the type... 

The PEFC was first developed for the Gemini space vehicle by General Electric, USA. In this fuel cell type, the electrolyte is an ion-exchange membrane, specifically, a fluorinated sulfonic acid polymer or other similar solid polymer. In general, the polymer consists of a polytetrafluoroethylene (Teflon) backbone with a perfluorinated side chain that is terminated with a sulfonic acid group, which is an outstanding proton conductor. Hydration of the membrane yields dissociation and solvation of the proton of the acid group, since the solvated protons are mobile within the polymer. Subsequently, the only liquid necessary for the operation of this fuel cell type is water [7,8],... 

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. 

In the presence of water this polyelectrolyte is an excellent protonic conductor, although it suffers some degradation in a working fuel cell. Probably the most successful polyelectrolytes developed so far are based on perfluorinated polymers, the first of which was Nafion , with the structure ... 

Most proton conducting solids until fairly recently were inorganic hydrates and stable conductivities much above 150 °C in such materials are seldom found (i.e. after the water is lost, conductivity disappears) moreover, the conductivity is highly dependent on the water content, including the surface water of the material. Thus there is a need to develop proton conductors for various applications in the higher temperature range, which are not dependent on water content. 

Bhide, S.V. and Virkar, A.V., Stability of BaCeOj-based proton conductors in water-containing atmospheres, / Electrochem. Soc., 146, 2038-2044 (1999). 

PEMFCs are characterized by the use of proton exchange membranes as electrolytes, which are good electronic insulators and very good proton conductors in the presence of water. 

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