Ceria electrolytes fuel cell performance

Lanthanum gallate (LaGaOs) is doped with divalent metal ions [e.g., strontium and/or magnesium Lai t Sr cMg Gai-3,03 (LSGM) usually, x andy have values between 0 and 1]. Unlike ceria-based electrolytes, this electrolyte can also be used at low oxygen partial pressures without the menace of developing electronic conduction. When in contact with cathodes of the LSM type, some diffusion of manganese and/or cobalt from the cathode into the electrolyte is possible, but this has little effect on fuel cell performance. One defect of this electrolyte material is its complicated preparation. 

Figure 17. Fuel cell performance obtained from the SSC (anode) GDC (electrolyte) I copper-ceria (anode) cell, shown in Fig. 15.

It is well established that sulfur compounds even in low parts per million concentrations in fuel gas are detrimental to MCFCs. The principal sulfur compound that has an adverse effect on cell performance is H2S. A nickel anode at anodic potentials reacts with H2S to form nickel sulfide. Chemisorption on Ni surfaces occurs, which can block active electrochemical sites. The tolerance of MCFCs to sulfur compounds is strongly dependent on temperature, pressure, gas composition, cell components, and system operation (i.e., recycle, venting, and gas cleanup). Nickel anode at anodic potentials reacts with H2S to form nickel sulfide. Moreover, oxidation of H2S in a combustion reaction, when recycling system is used, causes subsequent reaction with carbonate ions in the electrolyte [1]. Some researchers have tried to overcome this problem with additional device such as sulfur removal reactor. If the anode itself has a high tolerance to sulfur, the additional device is not required, hence, cutting the capital cost for MCFC plant. To enhance the anode performance on sulfur tolerance, ceria coating on anode is proposed. The main reason is that ceria can react with H2S [2,3] to protect Ni anode. 

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.)...

Ceria affords a number of important applications, such as catalysts in redox reactions (Kaspar et al., 1999, 2000 Trovarelli, 2002), electrode and electrolyte materials in fuel cells, optical films, polishing materials, and gas sensors. In order to improve the performance and/or stability of ceria materials, the doped materials, solid solutions and composites based on ceria are fabricated. For example, the ceria-zirconia solid solution is used in the three way catalyst, rare earth (such as Sm, Gd, or Y) doped ceria is used in solid state fuel cells, and ceria-noble metal or ceria-metal oxide composite catalysts are used for water-gas-shift (WGS) reaction and selective CO oxidation. 

Uchida, H., Arisaka, S.,and Watanabe, M. (1999). High performance electrode for medium-temperature solid oxide fuel cells—La(Sr)CoC>3 cathode with ceria interlayer on zirconia electrolyte. Electrochem. Solid State Lett. 2 428-430. 

Liu Y, He T, Wang J, Shu W (2005) The effect of Pr co-dopant on the performance of sohd oxide fuel cells with Sm-doped ceria electrolyte. J AUoy Compd 389 317-322... 

Table 12.3 Performance of fuel cells with a ceria-based electrolyte. If not specified the anode gas is humidified ( 3%) hydrogen and the cathode gas is air. Further details of the design of the cells are in Table 12.4. 

Ce02 host is substituted with either Sm or Gd (Cei-jcSm Oz Sj CSO, and Cei Gd 02 5, CGO), creating significant vacancy concentrations. Use of these ceria-based materials is limited by the redox characteristics of the Ce3+/4+ couple, with reduction occurring at temperatures above about 650 °C leading to a reduction of the ionic transport number. This in turn can lead to short circuits within the cell and hence a loss of performance. However, as conductivity in ceria-based compounds is sufficient at temperatures below 650 °C for fuel cell electrolytes, the issue is then one of suitably active cathodes, addressed in Section 2.1.3. 

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... 

---