Comparative Study of Metal Impregnated Lanthanum Doped Strontium Titanate Anodes and Conventional Nickel Oxide Anode for Solid Oxide Fuel Cell Application

Abstract

In conventional SOFC, composite anode made up of nickel oxide (NiO) and yttria stabilized zirconia (YSZ) are used [1]. NiO is an excellent anode for SOFC application because of its high electrochemical activity towards H2 oxidation. But the amount of NiO used is around 30- 40 wt% to reach a percolation threshold [2]. Agglomeration of nickel particles due to sintering is a serious concern during SOFC operation at elevated temperatures (800 ˚C). In order to address the above mentioned issue, we have developed doped strontium titanate (SrTiO3) based perovskite anodes. Perovskite anodes are mixed ionic and electronic conductors (MIEC) providing enhanced triple phase boundary (TPB) which lowers the over-potential losses compared to pure conductors [3]. To increase the electrical conductivity, lanthanum (La) has been doped at A-site of SrTiO3 using solid state route (SSR), to form La0.3Sr0.7TiO3+δ (LST). Physical characterizations like field emission scanning electron microscope (FESEM), x-ray diffraction (XRD) and image analysis (ImageJ) were carried out before and after reduction of anodes in H2 environment to analyze their microstructure, phase stability and particle sizes as shown in Figure 1 and 2. Conductivity measurements were carried out using two probe conductivity measurement system to determine the activation energy (Ea) required in reducing the anodes in H2 environment. Along with physical characterizations, electro-chemical characterizations were carried out as described. The halfcell (LST/ YSZ) gave a high polarization resistance (Rp) of 62 Ω cm2, low current density of 20 mA cm-2 and power density of 5 mW cm-2 at 800 ˚C. We impregnate the porous anode with varying NiO content using wet impregnation technique and observed that LST impregnated with 4 wt% NiO forms a percolated network and provided enhanced performance. A decrease in Rp to 4 Ω cm2 and increase in current density to 156 mA cm-2 in 4 wt% NiO impregnated LST halfcells is seen at 800 ˚C. The performance increased sharply upto 900 mA cm-2 in case of halfcell with LST anode when impregnated with both nickel(4%) and ceria(6%) with low Rp of 0.5 Ω cm2, giving a maximum power density of 200 mW cm-2 at 800 ˚C as shown in Figure 3(a, b). The above obtained power density is better in performance compared to the one given by half cells of NiO-YSZ anodes [2]. Thus, metal impregnated A-site doped perovskite anodes can be potential substitute for NiO-YSZ anodes for SOFC application. Key words:A-site doped perovskites, Mixed ionic and electronic conductor, Activation energy and half-cell study.