Pure and CoO1–x‐doped Ce0.8Gd0.2O1.9 (CGO) have been analyzed with constant heating rate dilatometry. Doping changes the grain boundary structure of CGO resulting in enhanced mass transport and in increased densification rates. Significant changes in activation energies have been detected. While pure CGO exhibits a significant amount of surface diffusion, cobalt oxide doping enhances grain boundary diffusion. The obtained activation energies are 4.70±0.32 eV for CGO and 6.24±0.32 eV for CoO1−x‐doped CGO. The higher activation energy of CoO1−x‐doped CGO is a result of the formation of intergranular films. It is suggested that sintering of CoO1−x‐doped CGO occurs by rearrangement and grain boundary diffusion whereas grain boundary and volume diffusion govern the densification of pure CGO.

A solid oxide fuel cell (SOFC) is a solid-state energy conversion system that converts chemical energy into electrical energy and heat at elevated temperatures. Its bipolar cells are electrochemical devices with an anode, electrolyte, and cathode that can be arranged in a planar or tubular design with separated gas chambers for fuel and oxidant. Single chamber setups have bipolar cells with reaction selective electrodes and no separation between anode and cathode compartments. A nickel/yttria-stabilized-zirconia (YSZ) cermet is the most investigated and currently most widespread anode material for the use with hydrogen as fuel. In recent years, however, doped ceria cermet anodes with nickel or copper and ceria as the ceramic phase have been introduced together with ceria as electrolyte material for the use with hydrocarbon fuels. The state-of-the-art electrolyte material is YSZ of high ionic and nearly no electronic conductivity at temperatures between 800-1000 °C. In order to reduce SOFC system costs, a reduction of operation temperatures to 600-800 °C is desirable and electrolytes with higher ionic conductivities than YSZ are aimed for such as bismuth oxide, lanthanum gallate or mixed conducting ceria and the use of thin electrolytes. Proton conducting perovskites are researched as alternatives to conventional oxygen conducting electrolyte materials. At the cathode, the reduction of molecular oxygen takes place predominantly on the surface. Today's state-of-the-art cathodes are La x Sr 1-x MnO 3-d for SOFC operating at high temperature i.e. 800-1000 °C, or mixed conducting La x Sr 1-x Co y Fe 1-y O 3-d for intermediate temperature operation, i.e. 600-800 °C. Among the variety of alternative materials, Sm x Sr 1-x CoO 3-d and Ba x Sr 1-x Co x Fe 1-x O 3-d are perovskites that show very good oxygen reduction properties. This paper reviews the materials that are used in solid oxide fuel cells and their properties as well as novel materials that are potentially applied in the near future. The possible designs of single bipolar cells are also reviewed.

The sintering of ceria solid solutions, such as Ce 0.9 Gd 0.1 O 1.95 (CGO10), is strongly promoted by the addition of 1 cat% of cobalt oxide, lowering the maximum sintering temperature by 200 • C and triplicating the maximum densification rate. This change in sintering behavior results from cobalt ion segregated at the grain boundaries. An average cobalt ion boundary coverage is at maximum 3.0 ± 1.9 at/nm 2 and is shown to depend on the cooling rate. Coverage by segregated gadolinium is also found and amounts to 13.2 ± 11.4 at/nm 2 for a slowly cooled sample. From cobalt excess measured at the boundary, an estimated concentration of only 0.06 cat% of cobalt oxide is necessary to promote the sintering effect. The remaining amount of cobalt oxide is found in triple points and as particles in clusters. It is expected that the amount of cobalt oxide necessary for fast densification can be reduced with a doping process that distributes the additives more homogeneously.

The conductivity of cobalt oxide doped Ce 0.9 Gd 0.1 O 1.95 (CGO10) of various doping concentrations, sintering temperatures, dwell times, and cooling rates was investigated by 4-point DC conductivity measurements. In cobalt oxide doped CGO10, an enhanced total conductivity occuring with a low activation energy of 0.54 eV was detected below 250 • C in quenched samples. If the same samples were cooled down slowly, only the ionic conductivity of undoped CGO with an activation energy of 0.8 eV was found. The increased conductivity is attributed to a percolating network of an electronically conducting grain boundary phase rich in CoO, which can be retained by quenching from temperatures between 900 and 1000 • C.