On the development of proton conducting materials for technological applications
Introduction
Although high proton conductivity has been reported for a large number of solid compounds and materials [1]and many of them have been suggested for applications as electrolytes in electrochemical cells, their use in technological devices is still quite limited. Apart from the fact that proton conductivity in the solid state remains significantly below the upper limit for proton conductivity in liquids, major problems arise from the numerous additional material requirements, other than proton conductivity, which have to be met for a particular application.
Therefore, the simultaneous optimisation of all relevant material properties appears to be an interesting route in the development of new proton conducting materials for specific applications. Such a strategy is promising especially when the mutual dependences of the relevant properties can be expressed in terms of the same structural, thermodynamical and dynamical parameters.
Along this line, this paper presents a few issues in the selection or development of:
(i) high temperature proton conducting perovskite type oxides,
(ii) low temperature proton conducting hydrated polymer membranes,
(iii) intermediate temperature proton conducting materials relying on proton transfer between nitrogen acting as proton donor as well as proton acceptor.
The first two have attracted increasing interest in recent years especially because of their potential use as separator materials in fuel cells, electrochemical reactors and sensors. Whereas the existence of proton conductivity in these families of compounds has been known for a long time, the presented approach also leads to materials based on distinctly different chemistries. This may even open up new fields of application as demonstrated for the third case.
Section snippets
Perovskite type oxides
The systematic investigation of proton conducting perovskite type oxides (ABO3) started with the work of Takahashi and Iwahara in 1980 [2]. It was already this paper which reported proton conductivity in indium doped zirconates (In:SrZrO3), the only class of oxides, which has later been developed for a technological application, i.e. as a separator material for high temperature potentiometric hydrogen sensors [3].
In contrast to this application, the use of oxides in fuel cells, electrochemical
Low temperature proton conducting polymers
Stimulated by the legislative pollution control in most industrialised nations, the development of polymer electrolyte membrane fuel cells (PEM-FC) has attracted an increasing interest. Generally perfluorinated ion exchange membranes in their protonic form such as Nafion are being used as membrane materials. These combine the required chemical, electrochemical and mechanical stability with high proton conductivity. In fact, such membranes have been used in hydrogen PEM fuel cells for a long
Intermediate temperature proton conductors
Whereas some proton conducting oxides and hydrated polymers are close to being technologically applied at high (>500°C) or low temperatures (<90°C) respectively, only few materials have been reported to show high proton conductivity at intermediate temperatures (100–400°C). This gap arises from the fact that all compounds, for which proton conductivity has been reported so far, belong to a limited number of families of compounds with respect to the species `solvating' the proton [1].
In the case
Acknowledgements
The author thanks J. Clauß and G. Frank (Hoechst AG, Frankfurt) for providing the polymers, A. Fuchs for technical assistance and W. Münch, H.J. Schlüter and B. Gibson for reading the proofs. Part of the work is being supported by the BMBF under the contract number 0329567
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