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 La_0.3Sr_0.7TiO_3+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 (E_a) 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 (R_p) 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 R_p 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 R_p 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.

References:

[1]. Prakash, B. Shri, S. Senthil Kumar, and S. T. Aruna. "Properties and development of Ni/YSZ as an anode material in solid oxide fuel cell: a review." Renewable and Sustainable Energy Reviews36 (2014): 149-179.

[2]. Tiwari, Pankaj Kr, and Suddhasatwa Basu. "CeO₂ and Nb₂O₅ modified Ni-YSZ anode for solid oxide fuel cell." Ionics(2017): 1-7. DOI: 10.1007/s11581-016-1945-1.

[3]. Colomer, M. T., and J. A. Kilner. "Ni-doped lanthanum gallate perovskites: Synthesis and structural, microstructural, and electrical characterization." Solid State Ionics 182.1 (2011): 76-81.