Hydrated solid protonic conductors

The more highly conducting solid protonic conductors such as Nafion ,  

HU02P04-4H20(HUP) , heteropoly acid hydrates contain water generally associated with HjO or ions. There- 

Some characteristic values of the proton dynamics in hydrated protonic conductors are reported in Table 21.2. 

The characterization of the vibrational modes involving the hydrated proton is of crucial interest with respect to proton conduction. A huge amount of spectroscopic work has been performed in this field, as nicely summarized in a detailed review extending up to 1988 by Ratcliffe Irish . Since this date, several interesting papers dealing with the INS 

In the more complex HUO2PO4.4H2O compound, the INS spectra were found to be consistent with the presence of both and H502  

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T. A. Wheat, A. Ahmad and A. K. Kuriakose (eds). Progress in Solid Electrolytes (CANMET (Energy, Mines and Resources), Ottawa (1983)) (P. J. Wiseman, Particle Hydrates as Proton Conductors). 

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],... 

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. 

Fig. 3.1. Relationship between the pre-exponential factor

Table 21.2. Specific conductivity a, proton self-diffusion coefficient D, and residence time between reorientations v esfor some hydrated solid state protonic conductors... 

Perovskite Proton Conductor, Fig. 3 Correlation between standard enthalpy of hydration and the electronegativity difference of the A- and B-site elements in the perovskite (Fig. 3). The open points are tabulated by Norby et al. [8], while the solid points are for BaCeo.9-xZr,Yo.i03-6 (BCZY) [9]... 

There are several types of protonic conductors. One of the representative protonic conductors is uranyl phosphoric acid hydrate (HUO2PO4-41120 1111 ). The electrolyte is based on the hydrate. Protonic conductivity occurs through the interexchange of hydrogen by the rotation of H2O or H30 molecules in the hydrates. Therefore, the protonic conductivity of these solid electrolytes decreases with reducing humidity. These electrolytes are restricted to use around room temperature, since they cease to be protonic conductors if they are deltydrated. 

Recently, a protonic conductor based on strontium or barium cerate (SrCeOa, BaCeOs) has been developed. In these cerates, protons do not exist in the solid at all. This feature is apparently different from the two electrolytes mentioned above. One of the specific characteristics of the cerate electrolytes is that they can work as effective conductors at elevated temperatures compared with the hydrate protonic conductor and the ionically exchanged H3O+ or NH4 P- or p"-alumina. 

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