Perovskite proton conductors

Fig. 4.42 Electrical conductivities of modified perovskite proton conductors (adapted from...

Bhide SV, Virkar AV. Stability of AB l/2B l/203-type mixed Perovskite proton conductors. 

Oxygen Anion Transport in Solid Oxides Perovskite Proton Conductor Solid Electrol34 es Solid State Electrochemistry,... 

Perovskite proton conductors belong to the class of high temperature proton conductors (HTPCs),... 

Perovskite Proton Conductor, Fig. 1 Number of scientific publications per year that include the words proton, conduction, and oxides in their title or keywords. The idea for this graph comes from ref. [3]... 

Generation of Protonic Carriers in Perovskite Proton Conductors... 

Perovskite Proton Conductor, Fig. 2 Structure of SrCeOa, a typical perovskite proton conductra- when doped with a trivalent element on the Ce site. The spheres represent the A-cation, Sr, while the octahedra have the B-cation, Ce at the center, and oxygen at the apices. A strong distortion from the cubic structure is visible the structure is shown in the Pbnm setting... 

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]... 

Perovskite Proton Conductor, Fig. 6 Diagram showing the possible ionic transport numbers for oxide ions, protons, and electron holes in a HTPC, following the relation t x + Iyi + ty, = 1- The preferred regimes for three types of function are shown in the figure... 

The conductivities due to each of these defects are determined by their concentrations multiplied by their mobilities. As a result of the electron-hole equilibrium, these defects cannot both be present in high concentration at the same time. Therefore, perovskite proton conductors normally display protonic, oxide ion and one type of electronic conductivity. For an HTPC in oxidizing conditions the main defects are Vo, OHo, and h/. The situation can be visualized in the diagram of Fig. 6, which indicates the domains favorable for applications in fuel cell electrolytes and hydrogen transport membranes. The domain for oxygen transport membranes (i.e., no proton conduction) is included for the sake of completeness. 

Even though perovskite proton conductors are selected for their protonic conductivity, possessing a minority electronic conductivity component is not necessarily a disadvantage. Figure 7a shows a plot of the total conductivity versus the oxygen partial pressure P02) a logarithmic scale. The curve follows an equation of the type ... 

Perovskite Proton Conductor, Fig. 7 Two transport properties for a typical HTPC, plotted versus the logarithm of oxygen partial pressure (Poi) ( ) Total conductivity (b) Ionic transport number... 

In this equation, cr,- is the ionic conductivity, assumed to be independent of Pq2, while Op° and cr ° are parameters describing the other conductivity components at. Po2 of f hn [15]. In Fig. 7b, the ionic transport number cr,7(7,0, is plotted on the same logarithmic scale. In a fuel cell, for example, an H2/O2 ceU, the emf is dependent on the mean ionic transport number. In Fig. 7b, the mean icMiic transport number is the area ratio of the light colored block to the whole block. As a result, partial electronic conductivity limited to the high and low P02 regions is not a problem. This is also observed in practice, where H2/O2 or H2/air cells based on perovskite proton conductors give open circuit voltages within a few percent of the theoretical values [16]. 

In the first years of work on perovskite proton conductors, many researchers, including the author, assumed that PCFCs would be useful only in coimection with hydrogen fuel cells, while with hydrocarbon fuels, SOFCs would be the only choice. However, work by Coors and coworkers has demonstrated that perovskite proton conductors with minority oxide ion conduction can sustain stable currents for long... 

The range of transport properties and the applications are likely to keep the subject of perovskite proton conductors in the spotlight for many years to come. 

Bhide, S.V., W. Meng, and A.V. Virkar, Stability of Mixed Perovskite Proton Conductors, paper presented at Joint Fuel Cell Technology Review Conference, 1999. 

Although the cathode material used in our fuel cell is popular in SOFCs, the cathode polarization is much smaller than that reported in SOFC literature using bulk electrolyte [14]. Hibino et al. also reported anode and cathode polarization of fuel cells using a perovskite proton conductor as a bulk electrolyte, and the polarization was much larger than our result [15]. These facts suggest that electrolyte thickness may affect the polarization. 

In the case of multinary compounds the composition changes described by the chemical diffusion need not always be a redox effect (as here). Thus, the incorporation of water in the perovskite proton conductors (see Section 5.6) comes about as a result of the coupled diffusion of 2H and 0 , that is an acid-base effect. Mechanistically this corresponds to the coimterdiffusion of oxygen vacancies and proton defects (H-... 

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