Silver-Modified Ba_1–<i>x</i>Co_0.7Fe_0.2Nb_0.1O_3−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 (Ba_1-<i>x</i>Co_0.7Fe_0.2Nb_0.1O_3-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, <i>Pm</i>3̅<i>m</i>, α = 90°) with a high degree of crystallinity, as demonstrated by X-ray powder diffraction analyses. It was also found that the <i>in situ</i> 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 (Ce_0.9Gd_0.1O_2-δ) electrolyte-supported symmetrical half cell using different Ag-BCFN formulations of Ba_1-<i>x</i>Ag_<i>x</i>Co_0.7Fe_0.2Nb_0.1O_3-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 <i>in situ</i> 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.