Proton-Electron Conducting Oxides

State-of-the-art proton conductors comprise acceptor-substituted perovskites, such as the barium-based ones (BaCe03, BaZr03, etc.) which exhibit proton conductivities in excess of 0.01 S cm i [51-55] and strontium-based ones (SrCe03) with somewhat lower conductivities. Both BaCe03 and BaZr03 are almost pure ionic conductors, and the electronic conductivity would, as such, rate limit the H2 flux across membranes of these materials [56]. 

BaPr03 and BaTb03 also possess perovskite-related structures and exhibit considerable proton conductivity [57-60]. Because Pr and Tb show mixed valence (III/ 

Acceptor-doped SrCe03 is, without doubt, the mixed electron-proton conducting oxide system most studied, both with respect to actual measurements of 

Because the temperature dependence of the hydrogen flux in SrCe03 exhibits an essentially straight-Une Arrhenius behavior, the electronic conductivity is 

Although acceptor-doped SrCe03 shows the highest reported hydrogen fluxes so far, long-term use of this material in industrial applications may be difficult 

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Defect-structure-wise there are two main routes to a mixed proton-electron conducting oxide. The simplest would be to dissolve protons compensated by electrons originating directly from hydrogen gas ... 

As the mixed proton and electron conductive oxide membrane becomes sufficiently thin, surface kinetics will become important, and difiusion of protons across the membrane will no longer be rate deterrriining for the overall hydrogen flux. Bouwmeester et al. [10] defined a characteristic thickness, L, for membranes where surface kinetics and bulk kinetics are equally important to the flux. Decreasing a membrane s thickness below gives essentially no increase in the flux. 

With the requirement of electronic conductivity, oxides containing cations with mixed valence and, in particular, reducible cations are preferable. Oxides containing transition metals are therefore appropriate alternatives. There are indications based on conductivity measurements that Ti02 could be a possible candidate [83], but no direct measurements of hydrogen permeability have been reported. Tita-nates, in general, however, are interesting because there are a number of materials classes that accommodate oxygen vacancies and may dissolve protons. 

Today, the term solid electrolyte or fast ionic conductor or, sometimes, superionic conductor is used to describe solid materials whose conductivity is wholly due to ionic displacement. Mixed conductors exhibit both ionic and electronic conductivity. Solid electrolytes range from hard, refractory materials, such as 8 mol% Y2C>3-stabilized Zr02(YSZ) or sodium fT-AbCb (NaAluOn), to soft proton-exchange polymeric membranes such as Du Pont s Nafion and include compounds that are stoichiometric (Agl), non-stoichiometric (sodium J3"-A12C>3) or doped (YSZ). The preparation, properties, and some applications of solid electrolytes have been discussed in a number of books2 5 and reviews.6,7 The main commercial application of solid electrolytes is in gas sensors.8,9 Another emerging application is in solid oxide fuel cells.4,5,1, n... 

Currently, the most widely applied electrolyte in PEFCs is Nation, manufactured by DuPont, Dow Chemical, Midland, MI, USA and other chemical companies. The Nation polymer electrolyte is a good proton conductor. Besides, it has very low electron conductivity, and is gas impermeable in order to provide the necessary spatial separation between the anode oxidation and the cathode reduction reactions. 

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