Enhanced Stability of BaCoO3-δ Using Doping Process as a Cathode Material for IT-SOFCs
Authors: Yang Xin , Han Xiaochuan , He Tianmin , Du Yanhai 
Published: February 2020

Solid oxide fuel cells (SOFCs) are energy conversion devices that transform the chemical energy stored in fuels to electricity without combustion process involved. The environmentally friendly operation, high energy conversion efficiency, as well as the fuel flexibility are currently attracting a great deal of attention. However, the high operating temperature restricts the commercial application of SOFCs with traditional cathode materials. Then researchers had expended considerable efforts in reducing operating temperature by lowering the polarization resistance of cathodes. In addition, the power density of traditional cathode ceramic oxides shows a severe degradation as the operating temperature drops. The most common ceramic materials used as cathode materials for intermediate temperature solid oxide fuel cell (IT-SOFC) are mixed ionic-electronic conducting (MIECs) perovskites. Therefore, more advanced cathode materials with adequate conductivity and desirable electrochemical performance under intermediate temperature (600-800oC) need to be researched and developed.

 Many investigations suggest that cobalt-based perovskite cathode is one of the most competitive candidates with the most excellent electrocatalytic activity for oxygen reduction and sufficient conductivity around intermediate temperature comparing with other ABO3-type perovskite cathode based on chromium and manganese, etc. The electrocatalytic activity, thermal expansion coefficient (TEC), chemical compatibility with different types of electrolytes as well as structural stability of Cobalt-containing perovskites as cathodes for SOFCs had been widely studied, for example, La(Sr)CoO3-δ and BaCoO3-δ. In A-site ordered perovskites fast oxygen diffusion can be achieved. Base on the study of oxygen permeation rate, A-site sublattice fully occupied by Ba2+ provides the highest oxygen permeability in contrast to Sr2+and Ca2+ in La0.6A0.4Co0.6Fe0.2O3-δ (A=Ca, Sr, Ba). Unfortunately, the sensitivity to CO2 / H2O as well as the structural transition of cubic-phase to hexagonal-phase at RT limited its further application. Cheng, et.al. pointed out that the structural-stabilization can be efficaciously improved by low concentration Nb5+ doping in the B-site. Therefore, researchers made a lot of attempts on improving the structural stability and maintaining the high oxygen permeation rate by B-site doping high oxidation state cation, such as Ta, Ni, Nb, etc.

 In this study, partial substitution of Fe3+ for Co4+ is aimed at reducing the TEC of BaCoO3-δ parent oxide. Ta5+ having similar ionic size with Co4+ as another dopant in B-site will help to improve the oxygen nonstoichiometry corresponding to structural stability by introducing high oxidation states cations into B-stie. BaCo
0.7
Fe
0.2
Ta

0.1
O3-δ (BCFT) oxide as cathode material for proton conducting IT-SOFC had been reported. It shows low cathode polarization resistance in a symmetric cell and a fair cell performance around 255 mWcm-2 fueled with humidified H2. However, the chemical compatibility and the electrocatalytic activity of BFCT as a cathode material for IT-SOFC supported by an oxygen-ion conducting electrolyte are not reported. BFCT powder was synthesized by a solid-state reaction. La
0.9
Sr
0.1
Ga
0.8
Mg
0.2O3-δ (LSGM) was used as supporting electrolyte. LSGM-supported symmetrical cells were fabricated using screen-painting process. X-Ray diffraction and SEM were performed to show the crystallization (see figure. 1). TEC and symmetric cell resistance were investigated to analyze the structural stability of BCFT to be a potential candidate of cathode for IT-SOFCs. A detailed reference list can be found in the manuscript.

Figure 1