Nanoparticle Synthesis and Oxygen Anion Diffusion in Double Perovskite GdBaCo2-xFexO5+δ Electrodes for SOFC

Field: Materials Science (Physical Sciences)

Nanostructured perovskite electrode materials have shown promising applications in improving the electrochemical performance of a solid oxide fuel cell (SOFC). An integrated bio and chemo-synthesis method is developed to obtain reduced particle size of layered perovskite materials. Double perovskite materials of composition GdBaCo 2 O 5+δ was synthesized chemically and subjected to particle size reduction by using a bio-milling process[1], in which a fungal treatment was applied. Sacchromyces cerevisiae , was cultured in 250 mL YPD medium for 96 hours at 301 K. Reduced GBCO particles of sizes lesser than 20 nm were observed using HR-TEM (Figure 1a). XRD pattern confirms the presence of pure double perovskite phase in the synthesised nanoparticle of GBCO. In order to test the electro-catalytic activity of the GBCO nanoparticle, symmetric cell on gadolinium doped ceria (Gd=20%) electrolyte were fabricated using both chemically and bio-milled GBCO electrodes. The area specific resistance (ASR) for bio-milled electrode was measured by electrochemical impedance spectroscopy and was calculated to be 11.47 Ω cm 2 at 873 K. The ASR was observed to decrease on increasing temperature following the Arrhenius law with an activation energy of 83 kJ/mol in the temperature range of 823-1023 K (Figure 1b). Molecular dynamics (MD) simulations were performed to study the transport of oxygen anion in GBCO (Figure 1c). The diffusion coefficient of oxygen anion in bulk GBCO was calculated to be 5 x 10 -8 cm 2 s -1 at 873 K. The diffusivity was found to be thermally activated with an activation energy of 50.8 kJ/mol. The diffusion coefficient in a-b plan (Figure 1d) was observed to be one order of magnitude higher as compared to the c-axis (D= 3.33 x 10 -9 cm 2 s -1 , 873 K) indicating towards the anisotropic diffusion. The effect of iron doping at the B-site on oxygen anion diffusivity was calculated in co-doped LnBa 0.5 Sr 0.5 Co 2-x Fe x O 5+δ structures. Levels of iron doping was varied from x = 0.0, x = 0.5 to x = 1.0. The maximum diffusion was calculated for x=1.0 in GdBa 0.5 Sr 0.5 CoFeO 5.5 structure. On the contrary, in PrBa 0.5 Sr 0.5 Co 1.5 Fe 0.5 O 5.5 (PBSCF) and NdBa 0.5 Sr 0.5 Co 1.5 Fe 0.5 O 5.5 the maximum diffusivity was calculated at x = 0.5 (Table 1). The trends in diffusivity on varying Fe doping level in LnBa 0.5 Sr 0.5 Co 2-x Fe x O 5+δ electrodes follow the same trend in their measured electrochemical performance [2]. The results thus obtained indicates that the bulk diffusion could be limiting. Experimental studies led by Choi et al. on co-doped double perovskite structures have reported a high peak power density (~2.2 W/cm 2 ) of SOFC fabricated with a PBSCF electrode. In order to understand the effect of A- and B-site dopant on oxygen anion transport the diffusion coefficient of PBSCF was calculated by MD simulations. The diffusion coefficient of PBSCF was calculated to be 1.1 x 10 -7 cm 2 s -1 at 873 K (Table 1). The diffusion coefficient of PBSCF was observed to be one order of magnitude higher in comparison to the A-site doped PrBa 0.5 Sr 0.5 Co 2 O 5.5 (8.33 x 10 -8 cm 2 s -1 ) and B-site doped PrBaCo 1.5 Fe 0.5 O 5.5 (8 x 10 -7 cm 2 s -1 ) structure, calculated at 873 K [3]. References: [1] I. Uddin and P. Poddar, Int. J. Innov. Biol. Res. , vol. 2, pp. 1–5, 2013. [2] S. Choi, S. Yoo, J. Kim, S. Park, A. Jun, S. Sengodan, et al. Sci. Rep. , vol. 3, no. 2013, pp. 2426, 2013. [3] U. Anjum, S. Vashishtha, N. Sinha, and M. A. Haider, Solid State Ionics , vol. 280, pp. 24–29, 2015. Figure 1