Yttria doped ceria electrolytes

Figure 46. Performance characteristics of a cathode-supported thin film Ni—YSZ/YSZ/LSM fuel cell at 600 °C in humidified H2 and air with and without a dense protective yttria-doped ceria (YDC) protection layer introduced between the porous LSM cathode and the thin-film electrolyte. (Reprinted with permission from ref 296. Copyright 1997 Elsevier.)...

SOFCs employ a ceramic oxide (ceria- or yttria-doped zirconia, electrolyte... 

The anodes consisting of a nickel catalyst and of cermet mixed with yttria-doped zirconia electrolyte that are used in conventional solid oxide fuel cells also lose their ability to work at lower temperatures because of a loss of conductivity by the ceramic. This suggests that, for the ceramic in the anode, a material having a higher conductivity at intermediate temperatures should be used. It was in fact shown that an anode made with a nickel/samaria-doped ceria cermet has a much lower polarization than the conventional variant. 

Barnett, Perry, and Kaufmann (75) found that fuel cells using 8 im thick yttria-stabiUzed zirconia (YSZ) electrolytes provide low ohmic loss. Furthermore, adding thin porous yttria-doped ceria (YDC) layers on either side of the YSZ yielded much-reduced interfacial resistance at both LSM cathodes and Ni-YSZ anodes. The cells provided higher power densities than previously reported below 700 °C, e g., 300 and 480 mW/cm at 600 and 650 °C, respectively (measured in 97 percent H2 and 3 percent H2O and air), and also provided high power densities at higher temperatures, e g., 760 mW/cm at 750 °C. Other data (Figure 7-25) from the University of Utah (73) show power densities of 1.75 W/cm with H2/air and 2.9 W/cm with H2/O2 at 800 °C for an anode-supported cell. However, no data is presented with regard to electrodes or electrolyte thickness or composition. 

One approach to overcoming the limitations of nickel anodes, which has met with some success, is to augment the oxidation activity of Ni/YSZ cermets through the addition of an oxide-based oxidation catalyst. For example, stable operation on dry methane has been reported at 650°C in an SOFC using an yttria-doped ceria interlayer between the YSZ electrolyte and the Ni/YSZ cermet anode [61]. Ceria is a well-known oxidation catalyst, and might be expected to increase the activity of the anode for the electrochemical oxidation of methane. This approach still requires, however, that the operating temperature be maintained below 700°C to suppress carbon deposition reactions that take place rai nickel. 

The experiments were performed on an anode supported planar Solid Oxide Fuel Cell which consists of a 525—610 pm thick anode with two layers (both made of MO/8YSZ cermet functional layer 5—10 pm thick support layer 520—600 p thick) a 4—6 pm thick dense electrolyte Yo,i6Zro.g402 (8YSZ) a 2—4 pm thick barrier layer made of yttria doped ceria (YDC) the cathode consists of a 20—30 pm thick layer made of porous lanthanum strontium cobalt ferrite oxide... 

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]. 

Yttria-doped zirconia and gadolinia-doped ceria oxygen ion conductors and strontium yttrium zirconium oxide proton conductors are being investigated as the solid electrolyte. 

Trunec has described the thermoplastic extrusion of thin-wall tubes made of yttria-stabilized zirconia and gadolinia-doped ceria [Tru 04], These ceramics are used for solid oxide electrolyte applications, e.g. solid oxide fuel cells. The thermoplastic binder system used consists of ethylene-vinyl acetate copolymer, parafHn wax and stearic acid. With this system tubes with an outer diameter of 10.5 mm and wall thicknesses of 290 and 280 pm could be fabricated. 

Typical electrolyte materials for SOFCs are oxides with low valence element substitutions, sometimes named acceptor dopants [13, 95] which create oxygen vacancies through charge compensation. For SOFC applications, there are various materials that have been explored as electrolyte, yttria-doped zirconia (YSZ) and gadolinium-doped ceria (GDC) are the most common materials used for the oxideconducting electrolyte. Above 800 °C, YSZ becomes a conductor of oxygen ions (02-) zirconia-based SOFC operates between 800 and 1100 °C. The ionic conductivity of YSZ is 0.02 S m at 800 °C and 0.1 S cm at 1000 °C. A thin electrolyte (25-50 (im) ensures that the contribution of electrolyte to the ohmic loss in the SOFC is kept to a minimum. 

Liu, Q.L., Khor, KA., Chan, SXI., and Chen, X.J. (2006) Anode-supported solid oxide fuel cell with yttria-stabilized zirconia/gadolinia-doped ceria bilayer electrolyte prepared by wet ceramic cosintering process. J. Power Sources,... 

Doshi et al. (1999) used an electrolyte of ceria-doped gallate (CeGO) 30 p,m thick. In the cell, a two-phase cathode 250 p,m thick was used that consisted of a cermet of 45 to 55% silver and electrolyte material in the form of yttria-doped bismuth oxide (YDB). An Ni-CeGO cermet served as the anode. At a working temperature of 500°C and a ceU voltage of 0.6 V, the current density was 100 mA/cm at a cell voltage of 0.4 V, the current density was 3(K) mA/cm. It is interesting to note that in this cell, the contributions of the ohmic resistance of the electrolyte (0.7 cm ) and of cathode polarization (0.6 cm ) to the... 

In most SOFCs, the main contribution to rjohm is from the electrolyte, since its (e.g. yttria-stabilised zirconia, YSZ) ionic resistivity is much greater than electronic resistivities of the cathode (e.g. Sr-doped LaMnOs, LSM), and the anode (e.g. Ni + YSZ cermet). For example, the ionic resistivity of YSZ at 800°C is 50 J2cra. By contrast, electronic resistivity of LSM is 10 Qcm and that of the Ni + YSZ cermet is on the order of 10 S2cm. Thus, the electrolyte contribution to ohmic polarisation can be large, especially in thick electrolyte-supported cells. The recent move towards electrode-supported cells, in which electrolyte is a thin film of 5 to 30 microns, reduces the ohmic polarisation. Also, the use of higher conductivity electrolyte materials such as doped ceria and lanthanum gallate lowers the ohmic polarisation. 

Doped and undoped ceria exhibit ionic and electronic conduction at low oxygen partial pressures. Ce" " gets reduced to Ce ", which is the main cause for mechanical failure due to lattice expansion (for low oxygen partial pressure). The electrolyte/electrode interface may be delaminated due to the formation of cracks. When 40-50% of Ce" " ions are substituted with Gd " ions (Ceo.6Gdo.40i.8-CGO), then a balance between lattice parameters and conductivity with the YSZ electrolyte and an anchoring YSZ layer provides the ability to resist the thermal expansion mismatch. Uchida and co-workers obtained the electrocatalytic activities for the SDC and Yttria doped Ce02 (YDC) for the electrodes sintered at 1150 and 1250°C. The current density of 0.2-0.25 PJcvc is observed for SDC and YDC anodes,... 

The most common electrolyte material is YSZ. Another central one is yttria-(YDC) or gadolinia-doped ceria (GDC). Despite its higher ionic conductivity than 8 YSZ, its drawback are non-negligible electronic conduction at low oxygen partial pressure and isothermal expansion. It can also serve as a compatibility layer, to prevent undesirable reactions between the YSZ electrolyte and an LSCF cathode. The CTEs of both materials exhibit temperature dependence. SRU typically have to withstand spatial temperature differences of 100 K, e.g. 973-1073 K. The corresponding variation of the CTE between RT and 1073 K in one data set is of approximately 9.8—11.0 x 10 for 8YSZ and 12.1 —12.9 x 10 K for GDC, which can lead to imprecision in the calculation of the stress field. 

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