Proton mixed conductors

8 MIXED PROTON/ELECTRONIC CONDUCTORS 8.8.1 Proton Mixed Conductors 

Acceptor doping in perovskite oxides gives materials with a vacancy population that can act as proton conductors in moist atmospheres (Section 6.9). In addition, the doped materials are generally p-type semiconductors. This means that in moist atmospheres there is the possibility of mixed conductivity involving three charge carriers (H+, O2-, and h ) or four if electrons, e, are included. 

The situation can be illustrated with respect to the acceptor-doped perovskite structure SrZrC 3, with Y3+ substituted for Zr4+ to give compositions SrZrj YVC 3-0.5.V The doping reaction can be written  

The successfully doped materials are generally p-typc semiconductors. This arises from a combination of atmospheric oxygen with the vacancies, thus  

At low oxygen pressure more vacancies will tend to be created  

In reality the conductivity of proton conducting phases (Section 5.3) is more complicated than described. Successfully doped materials are not electronic insulators, as the aforementioned equations imply, but are generally weak p-type semiconductors. This comes about because at high temperatures a small amount of oxygen can react with the defective perovskite to partially fill the vacancies and generate a population of holes  

The situation can be illustrated with respect to the B-site substituted perovskite SrZrj Y 03 (,5, with dopant levels of x=0.05-0.2. The B-site substitution of two lower valence cations is balanced by the formation of one oxygen ion vacancy. 

On exposure to water vapour the oxygen vacancies can also react to form OH  

The resulting phases are good proton conductors. The total conductivity is due to 

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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... 

Our approach is to separate the conduction paths for H+ ions and electrons through the incorporation of a ceramic second phase. This approach essentially eliminates the combined dependence of hydrogen flux on electronic and proton conductivities. The approach is to short-circuit the electron flow-paths so that the overall flux is limited only by the proton conductivity. A similar mixed conducting requirement exists for electrodes in high-temperature proton conducting fuel cells, and some work has been carried out to develop mixed conductors as electrodes [24]. 

Hamakawa S, Hibino T, Iwahara H. Electrochemical hydrogen permeation in a proton-hole mixed conductor and its application to a membrane reactor. J Electrochem Soc. 1994 141(7) 1720. 

Li L, Iglesia E. Modeling and analysis of hydrogen permeation in mixed proton-electronic conductors. Chem Eng Sd. 2003 58 1977-88. 

Figure 1.1a shows schematically the operation of a membrane that is permeable to hydrogen molecules (corresponding to a porous membrane or a dense material in which molecules dissolve and diffuse) or to neutral hydrogen atoms (corresponding to a material in which hydrogen dissolves dissociatively, as in a metal). Figure 1.1b shows schematically how a mixed proton-electron conductor performs the same process by so-caUed ambipolar diffusion of both protons and electrons in the same direction to maintain electroneutrality and zero net current. 

Figure 1.3 Schematic iiiustration ofsequen-tiai use of a mixed oxygen ion eiectron conductor for oxygen separation (upper tube) and mixed proton—eiectron conductor for hydrogen separation. The air flowing inside...

Surface Kinetics of Hydrogen Permeation in Mixed Proton-Electron Conductors 21... 

Tab. 2.2 S ummary of transport numbers of various mixed protonic-electronic conductors in various gas concentration cells. 

Fig. 37.1. Principle of an all solid battery with a protonic conductor as electrolyte and two mixed protonic-electronic conductors as electrodes.

Iwahara, H., Yajima, T., Hibino, T., and Ushida, H. (1993). Performance of solid oxide fuel cell using proton and oxide ion mixed conductors based on BaCei-jcSmjtOs-a Electrochem. Soc. 140 1687-1691. 

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