Silver-Modified Ba1-xCo0.7Fe0.2Nb0.1O3-δ Perovskite Performing as a Cathodic Catalyst of Intermediate-Temperature Solid Oxide Fuel Cells.

Abstract

A series of silver (Ag)-modified barium cobalt ferrous niobate (Ba1-xCo0.7Fe0.2Nb0.1O3-δ, BCFN) materials were fabricated using a solid-state method by doping silver cations into the A-site of this perovskite matrix (Ag-BCFN). The electrochemical analyses indicated that the Ag-BCFN cathodic catalysts performed superior to the nonmodified catalysts when applied in intermediate-temperature solid oxide fuel cells (IT-SOFCs). These Ag-BCFN cathodic catalysts displayed a cubic perovskite structure (PDF 75-0227, Pm3̅m, α = 90°) with a high degree of crystallinity, as demonstrated by X-ray powder diffraction analyses. It was also found that the in situ exsolution of the silver ion (Ag+) occurred, where 57.9% of doped Ag+ was reduced into metallic Ag particles with size ranging from 5 to 10 nm, as shown by electron microscopic analyses. The cerium gadolinium oxide (Ce0.9Gd0.1O2-δ) electrolyte-supported symmetrical half cell using different Ag-BCFN formulations of Ba1-xAgxCo0.7Fe0.2Nb0.1O3-δ as electrodes showed a polarization resistance as low as 0.233 Ω·cm2 and an exchange current density of 85.336 mA·cm-2 at 650 °C under ambient pressure. The improved electrochemical kinetics is anticipated to be attributed to two reasons: doping of ions (Ag+) in the A-site of perovskite and in situ exsolved silver nanoparticles (Ag NPs) along the edge and on the surface of BCFNs improving the mobile charge and electrical properties of the material. The remaining Ag+ in the A-site induced the electron redistribution, whereas the Ag NPs were found to increase the electrochemically active sites and enable the formation of a triple-phase boundary. These explanations were confirmed by the density functional theory study, indicating that Ag-doping processes lead to a decrease in the formation energy of oxygen vacancies from 1.72 to 1.42 eV upon the partial substitution of Ba2+ by Ag+ cations.