Controlling surface cation segregation in a nanostructured double perovskite GdBaCo2O5+δ electrode for solid oxide fuel cells†
Mechanistic studies, utilizing molecular dynamics (MD) and density functional theory (DFT) calculations, were undertaken to provide a molecular level explanation of Ba cation segregation in double perovskite GdBaCo2O5+δ (GBCO) electrodes. The energy (γ) of the terminal surface having only Ba cations, indicated the surface to be the most stable (γ = 6.7 kJ mol−1 Å−2) as compared to the other surfaces. MD simulations elaborated on the cation disorder in the near surface region where Ba cations in the subsurface region were observed to migrate towards the surface. This led to a disruption in cation ordering with a propensity to form multiphases in the near surface region. In the near surface zone, oxygen anion diffusivity was observed to be reduced by an order of magnitude (D = 1.6 × 10−11 cm2 s−1 at 873 K) as compared to the bulk oxygen anion diffusivity value (D = 1.96 × 10−10 cm2 s−1 at 873 K). A novel idea was then proposed to control the degree of surface segregation of Ba cations by applying nanostructuring of the GBCO material in the form of nanoparticles. MD simulations elucidated that the near surface region having a high degree of cation disorder in the nanostructured GBCO may regain back the oxygen anion diffusivity value (D = 3.98 × 10−10 cm2 s−1, at 873 K) comparable to the bulk core region (D = 2.51 × 10−10 cm2 s−1, at 873 K). A proof of concept experiment was setup to test this hypothesis. The electrochemical performance of the electrode, fabricated using GBCO nanoparticles, was measured to improve by 15% as compared to the electrode synthesized with a bulk size GBCO material. This was attributed to the control in Ba-cation segregation, obtained on nanostructuring which resulted in higher oxygen anion transport in the near-surface region of the electrode material. XPS characterization of the surface of the nanostructured GBCO materials supported this assertion.