Nanostructuring Strategies to Control Surface Cation Segregation in Double Perovskite Electrodes for Solid Oxide Fuel Cells

Abstract

Development of an intermediate temperature (operating at 973 K or less) solid oxide fuel cell (SOFC) necessitates the design of efficient electrode materials to carry out oxygen reduction reaction (ORR) and transport at the cathode. Towards this, ORR in double perovskites materials of series LnBa1-xSrxCoyFe1-yO5+δ (LnBSCF, Ln = Gd, Pr) was studied. Molecular dynamics (MD) simulations were utilized to calculate the oxygen anion diffusivity (D) in the lattice of the double perovskite structured materials. In general, anisotropicity in oxygen transport was observed. For example, oxygen anion diffusion coefficient for the PrBaCo2O5+δ (PBCO) material was calculated to be 3 x 10-8 cm2 s-1 at 873 K in the a-b (Pr-O and Co-O) direction, which was observed to be higher than in the Ba-O plane (D = 8x10-9 cm2 s-1 at 873 K). On doping, PBCO with Sr and Fe cations, the resultant PrBa1-xSrxCoyFe1-yO5+δ (PBSCF) structure was calculated to show an order of magnitude higher diffusivity (D = 1.1x10--7 cm2 s-1 at 873 K) as compared to PBCO. Trends in calculated diffusion coefficients compared well with the measured electrocatalytic activity of the material reported in other experimental studies. The electrochemical measurements were performed on a geometrically well-defined nanostructured thin-film electrode, fabricated as a symmetric cell using a spray pyrolysis deposition method. Electrochemical experiments on thin-film electrodes provided an insight into the operating mechanism. Following the hypothesis of a characteristic thickness (Lc ) below which the performance was expected to be predominantly controlled by surface reaction, the Lc for layered perovskite PBSCF at 863 K was calculated to be around 3.2 μm. Interestingly, the dense thin-film electrode of PBSCF (1 to 3 μm thick) deposited using the spray pyrolysis method showed a thickness dependent electrochemical performance suggesting bulk diffusion limitation1.In order to understand the origin of this diffusion limited electrochemical performance, density functional theory (DFT) calculations were utilized to calculate the surface energy (γ) and oxygen vacancy (EOV) formation energies in the respective layered structures of PBCO, PBSCF and GBCO. For example, the EOV in the Gd plane (98.4 kJ/mol) of GdBaCo2O5+δ (GBCO) was calculated to be lower than that of Ba plane (EOV = 266.3 kJ/mol). However, the surface energy of the Ba plane was calculated to be minimum (γ =7.2 kJ/mol Å), which makes it the most exposed surface, while it is least diffusive. Due to low surface energy values, Ba cations are known to segregate towards the surface of the double perovskite structure. GBCO nanoparticles of reduced size were expected to provide a desired control on the degree of segregation of the Ba cation. The particle size of GBCO was reduced to 20 nm using a bio-milling approach, wherein the chemically synthesized particles were subjected to fermentation using a microorganism. The measured impedance of the electrode made up of bio-milled nanoparticles was improved by 15% as compared to the chemically synthesized material. MD simulation on the GBCO nanoparticles indicated two distinct regimes of diffusion; one corresponding to the surface and other to the bulk region of the nanoparticle. The thickness (2 nm) of surface shell regime was observed to be particle size independent. The diffusivity of the shell regime (D = 3.9 x 10-10 cm2 s-1 at 873 K) was calculated similar to that of the core (D = 2.51 x 10-10 cm2 s-1 at 873 K)2. Thus, nanoparticles showed improved electrochemical performance as compared to the bulk electrode, likely due to reduced Ba cation segregation in nanostructured electrode. References Anjum, T.S. Khan, M. Agarwal, M. Ali Haider “Identifying the origin of the limiting process in a double perovskite PrBa0.5Sr0.5Co1.5Fe0.5O5+δ Thin-Film Electrode for Solid Oxide Fuel Cells”, ACS Appl. Mater. Interfaces 2019, 11, 28 25243-25253.Anjum, M. Agarwal, T.S. Khan, Prateek, R. K. Gupta, M. Ali Haider “Controlling surface cation segregation in a nanostructured double perovskite GdBaCo2O5+δ electrode for solid oxide fuel cells”, Nanoscale, 2019, 11, 21404-21418.