Zirconia ceria-based electrolytes

K. Eguchi, T. Hatagishi, and H. Arai, Power generation and steam electrolysis characteristics of an electrochemical cell with a zirconia- or ceria-based electrolyte, Solid State Ionics, 86-8 1245-1249 (1996). 

The maximum value of conductivity for ceria-based electrolytes is attained at a certain concentration of the dopant oxides, in a matmer similar to the zirconia electrolytes. As shown in fig. 7 (Takahashi and Iwahara 1966), the conductivity of Ce02 doped with... 

The electrolytic domain for a ceria-based electrolyte, at any given temperature, is narrower than that of a zirconia-based electrolyte, such that electronic conductivity is acquired in reducing atmospheres. As shown in fig. 8 (Steele et al. 1994), the electrolytic domain of Ce02-0d0i.5 (10 mol%) at 1000 K is such that the electrolyte is not suitable for use at oxygen pressures below 10 atm. The high ionic conductivity, however, makes the electrolyte of considerable interest at low temperatures and/or in environments of moderate oxygen potential. 

As already discussed, the electrolyte is the most important component of a SOFC, the properties of which determine critical parameters such as cell performance and temperature of operation. Because of this, much time has been devoted to developing and understanding the materials and their properties and Raman spectroscopy has been a key tool. This section summarises the key results of studies that have used Raman spectroscopy to investigate electrolytes for SOFCs and is split into three parts. The first focuses on Zirconia based materials the conventional electrolyte of choice. The second will summarise the results of investigations on Ceria based electrolytes the frontrunner electrolyte for intermediate temperature SOFCs. Other novel electrolytes which have some potential for reduced temperature operation will be summarised in the final section. 

For the particular cases of zirconia- and ceria-based electrolytes, the ionic conductivity is mostly related to dopant nature, composition, microstructure, local structure, impurity and processing, and so on [19,20]. In many cases, the... 

Much has been published on ceria-based fuel cells however, to date, only Ceres Power Ltd., Crawly, UK, is developing SOFCs with ceria-based electrolytes, whereas numerous companies are developing SOFCs with zirconia-based electrolytes. Ceres Power Ltd. is developing a ceria-based electrolyte on a metal support, where the target operating temperature is 500-600 Ceres Power Ltd. is focus-... 

Doped ceria has been suggested as an alternative electrolyte for low temperature SOFCs [6, 31, 32]. Reviews on the electrical conductivity and conduction mechanism in ceria-based electrolytes have been presented by Mogensen et al. [33] and Steele [34], Ceria possesses the same fluorite structure as the stabilised zirconia. Mobile oxygen vacancies are introduced by substituting Ce " with trivalent rare earth ions as shown in Eq. (1). The conductivity of doped ceria systems depends on the kind of dopant and its concentration. A typical dopant concentration dependence of the electrical conductivity in the (Ce02)i -x(Sm203)x system as reported by Yahiro etal. [3 5] is shown in Figure 4.9. 

T. Hashida, K. Sato, Y. Takeyama, T. Kawada, J. Mizusaki, Deformation and fracture characteristics of zirconia and ceria-based electrolytes for SOFCs under reducing atmospheres. ECS Trans. 25(2), 1565-1572 (2009)... 

The doped ceria (Ce02) based electrolytes, most typically Gadolioiiim doped ceria (GDC), but also Sm doped (SDC), are probably the next most commonly considered electrolyte. Ceria has the same fluorite structure as doped zirconia, with a similar conduction mechanism. However, the ceria fluorite phase is stable at aU temperatures and so the dopants are used purely to increase the vacancy concentration, and hence conductivity. Ceria-based electrolytes exhibit much better ionic conductivity at lower temperatures flran YSZ, able to operate down to about 500 °C. 

The practical maximum ternperamre for a ceria-based electrolyte is at most 600 °C, at which temperature the reduction causes too much electronic conductivity and the cell eflhciency drops too far. Protective layers of zirconia are sometimes applied to prevent such short circuits... 

A key factor in the possible applications of oxide ion conductors is that, for use as an electrolyte, their electronic transport number should be as low as possible. While the stabilised zirconias have an oxide ion transport number of unity in a wide range of atmospheres and oxygen partial pressures, the BijOj-based materials are easily reduced at low oxygen partial pressures. This leads to the generation of electrons, from the reaction 20 Oj + 4e, and hence to a significant electronic transport number. Thus, although BijOj-based materials are the best oxide ion conductors, they cannot be used as the solid electrolyte in, for example, fuel cell or sensor applications. Similar, but less marked, effects occur with ceria-based materials, due to the tendency of Ce ions to become reduced to Ce +. 

Another way to decrease the anodic overpotential is to intercalate a mixed conductor between the yttria stabilized zirconia electrolyte and the metallic anode. Such a combination enlarges the reaction area which theoretically lowers the anodic overpotential. Tedmon et al. [93] pointed out a significant decrease of polarization when ceria-based solid solutions like (Ce02)o.6 (LaO, 5)04 are used as anode materials for SOFCs. This effect is generally attributed to the mixed conductivity resulting from the partial reduction of Ce4+ to Ce3+ in the reducing fuel atmosphere. A similar behaviour was observed in water vapor electrolysis at high temperature when the surface zirconia electrolyte is doped with ceria [94, 95]. 

Oxides exhibiting only high ion conductivity are mainly fluorite-related structures based on zirconia or ceria. Zirconia-based electrolytes are currently used in solid oxide fuel cells (SOFCs). The MIEC oxides are more attractive for separative membrane applications, and these oxides mainly belong to the following types fluorite-related oxides doped to improve their electron conduction, - ... 

It is considered that the bulk area specific resistance i o must be lower than l o = k/<7 = 0.15 Qcm, where L is the electrolyte thickness and a is its total conductivity, predominantly ionic [39]. At present, fabrication technology allows the preparation of reliable supported structures with film thicknesses in the range 10-15 pm consequently, the electrolyte ionic conductivity must be higher than 10 Scm. As shown in Figure 12.9, a few electrolytes (ceria-based oxides, stabihzed zirconias, and doped gallates) exceed this minimum ionic conductivity above 500 °C. 

Figure 13.20 Outputs of mixed potential C3H5 sensors with gold electrodes and various ceria- and zirconia-based electrolytes [228, 229, 231, 233, 236-238],...

Watanabe M, Uchida H, Yoshida M (1997) Effect of ionic conductivity of zirconia electrolytes on the polarization behavior of ceria-based anodes in solid oxide fuel cells. J Electrochem Soc 144(5) 1739-11743... 

In a review by Hui et al. (2007), various approaches to enhancing the ionic conductivity of zirconia- and ceria-based solid electrolytes in the light of composition, microstructure, and processing are described. 

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