Mixed protonic conductors

A number of factors must be taken into account when the diagrammatic representation of mixed proton conductivity is attempted. The behavior of the solid depends upon the temperature, the dopant concentration, the partial pressure of oxygen, and the partial pressure of hydrogen or water vapor. Schematic representation of defect concentrations in mixed proton conductors on a Brouwer diagram therefore requires a four-dimensional depiction. A three-dimensional plot can be constructed if two variables, often temperature and dopant concentration, are fixed (Fig. 8.18a). It is often clearer to use two-dimensional sections of such a plot, constructed with three variables fixed (Fig. 8.18h-8.18<7). 

Figure 8.18 Schematic representation of defect concentrations in mixed proton conductors on a Brouwer diagram (a) three-dimensional plot with two variables fixed (b)-(d) two-dimensional plots with three variables fixed.

Figure 5.7 The variation in transport number for proton, oxygen ion and hole conductivity for the mixed proton conductor SrZr YO (Data from Huang and Petrie (1995))...

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. 

Jacobsen et al. have demonstrated a Schottky diode with a mixed ion conductor, Pt/CeO/SiC, which showed very high sensitivity to hydrogen at 500°C after annealing at 700°C [89]. It was proposed that the annealing introduced oxygen vacancies in the CeO layer, which were suitable for interaction with protons, thus causing... 

Figure 2 shows the structure of this sensor which is similar to that of the potentiometric sensor reported earlier (10). The only difference is that in this sensor a short circuit current between the sensing electrode and the counter electrode is measured with an ammeter. The proton conductor, antimonic acid (Sb205 2H20), was prepared from antimony trioxide and hydrogen peroxide according to a method described elsewhere (7,14). The sample powder was mixed with... 

Modification of the sensor structure. The above amperometric sensor has a rather complicated construction, because the sample gas (H2 + air) is separated from the reference air. So, we tried to simplify the sensor structure as shown in Figure 9. As proton conductor we used a thin antimonic acid membrane (mixed with Teflon powder) of 0.2 mm thickness. This membrane is thin and porous enough to allow a part of the sample gas to permeate. On the other hand, the counter Pt electrode was covered with Teflon and Epoxy resin in order to avoid a direct contact with the sample gas. 

The use of a mixed oxygen ion-electronic conductor membrane for oxygen separation with direct reforming of methane, followed by the use of a mixed protonic-electronic membrane conductor for hydrogen extraction has also been proposed in the literature [34]. The products are thus pure hydrogen and synthesis gas with reduced hydrogen content, the latter suitable, for example, in the Fish-er-Tropsch synthesis of methanol [34]. 

Proton-conducting materials [38-47], analogous to oxygen conductors but with stationary oxygen anions, can show mixed protonic-electronic conductivity, without considerable oxygen transport in hydrogen or water atmospheres [40,41], These materials have not been widely studied in comparison... 

Preparation of BC Y powders for ANL-1 and -2 membranes is described in detail elsewhere. BCY and metal powders were mixed together to prepare powders for ANL-la and -2a membranes. Powders for ANL-3 membranes were prepared by mixing one of two hydrogen transport metals with ceramic powders that are reported to be poor proton conductors. All membranes contained 40 vol% metal, except where otherwise noted. The powder mixtures were pressed uniaxially to prepare disks ( 22 mm in diameter and 2 mm thick) for sintering. Cermet membranes were sintered in either air or 4% H2/balance He, N2, or Ar in the temperature range of 1,350 to 1,420°C. 

There is a class of nonporous materials called proton conductors which are made from mixed oxides and do not involve transport of molecular or ionic species (other than proton) through the membrane. Conduction of protons can enhance the reaction rate and selectivity of the reaction involved. Unlike oxygen conductors, proton conductors used in a fuel cell configuration have the advantage of avoiding dilution of the fuel with the reaction products [Iwahara ct al., 1986]. Furthermore, by eliminating direct contact of fuel with oxygen, safety concern is reduced and selectivity of the chemical products can be improved. The subject, however, will not be covered in this book. 

Ceramic electrochemical reactors are currently undergoing intense investigation, the aim being not only to generate electricity but also to produce chemicals. Typically, ceramic dense membranes are either pure ionic (solid electrolyte SE) conductors or mixed ionic-electronic conductors (MIECs). In this chapter we review the developments of cells that involve a dense solid electrolyte (oxide-ion or proton conductor), where the electrical transfer of matter requires an external circuitry. When a dense ceramic membrane exhibits a mixed ionic-electronic conduction, the driving force for mass transport is a differential partial pressure applied across the membrane (this point is not considered in this chapter, although relevant information is available in specific reviews). 

Virkar, A.V. (2001) Transport of H2, O2 and H2O through single-phase, two-phase and multi-phase mixed proton, oxygen and electron hole conductors. Solid State Ionics, 140, 275-83. 

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

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...

Fluxes in a Mixed Proton, Oxygen Ion, and Electron Conductor... 

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