Alumina proton conduction

In the previous section, /1-aluminas were discussed. If one replaces Na+ by H30+ or NH4 in these oxide compounds by, for example, electrolysis, then proton conducting materials can be obtained. Their ionic conductivity, however, is relatively low (10—5 Q-1 cm-1) because of the strong interactions between the small H+ ions and O2- ions in the conduction plane (OH ). The electrical conductivity is markedly higher in crystals of NH4 -/ -Ga203. 

In proton exchange membrane fuel cells, perhaps the most divulgate type of fuel cells, a proton-conducting polymer membrane acts as the electrolyte separating the anode and cathode sides. Porous anaodic alumina (Bocchetta et al., 2007) and mesoporous anastase ceramic membranes have been recently introduced in this field (Mioc et al., 1997 Colomer and Anderson, 2001 Colomer, 2006). 

Zirconium phosphate, besides the traditional intercalation processes, has been used to produce pillared compounds, such as those involving mixed Fe-Cr oxides [3]. Furthermore, it has been shown [4] that the protonic conductivity of a-zirconium phosphate can be enhanced by the formation of a composite with alumina or silica, an example of synergic cooperation between the oxides and phosphates which are the main classes of compounds highhghted in the book. 

Acidity-basicity effects in proton-conducting materials depend not only on the atomic and bonding properties of the central anionic species, but also on the properties of the aggregate surface and on the state of aggregation as can be observed in a discussion of the true compositions of weak and medium strength acidic materials such as tin dioxide, P"-alumina and antimonic acid hydrates (Table 1.1). 

N. Baffler, J. C. Badot and Ph. Colomban, Protonic conductivity of p" and ion-rich p-alumina. II ammonium compounds. Solid State Ionics 13 (1984) 233-6. 

H. Ikawa, K. Shima, T. Tsurumi and O. Fukunaga, Protonic conductivities in single crystal and polycrystalline of p"-alumina type NH -gallate, in Solid State Ionic Devices, B. V. R. Chowdari and S. Radhakrishna (eds) (World Scientific, Singapore (1988)) 497-502. 

Glipa, X., Leloup, J. M., Jones, D. J., and Roziere, J., 1997b, Enhancement of the protonic conductivity of a-zirconium phosphate by composite formation with alumina or silica. Solid state Ionics 97 227-232. 

Since the discovery of the MAg4I5 compounds, very many more solid electrolytes have been reported. Sodium, lithium, copper, proton, oxide and fluoride solid electrolytes are now well known. An important example is sodium /3-alumina which was discussed in Chapter 8. The conductance... 

Direct studies of the effect of pressure on surface charging are rare. The conductance of an anatase dispersion in HCI increased with pressure [179], This suggests a release of pre-adsorbed HCI from the surface at elevated pressure. On the other hand, the pressure effect was negligible in anatase dispersions in water or in NaCl. The experimental setup was designed to study desorption kinetics, and only the sign of the pressure effect could be determined. A similar method was used to study the pressure effect on proton adsorption on alumina dispersions in water [2928,3059], and in NaCl [2928], HNO,. and NaNO, [927] solutions, and the effect was negligible for a pressure of about 10 Pa. On the other hand, the same pressure had a substantial effect on uptake of heavy-metal cations [927] and of anions [2928,3059] on alumina. 

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