Influence of Ca-doping in layered perovskite PrBaCo2O5+d on the phase transition and cathodic performance of a solid oxide fuel cell

Abstract

Solid oxide fuel cells (SOFCs) are prospective energy devices that convert chemical energy to electricity with low carbon emissions, high efficiency, and long term-stability in a wide range of fuels. Conventional SOFCs at high operating temperatures (∼1273 K), however, suffer from considerable problems, such as thermal stress from thermal expansion coefficient (TEC) mismatch and chemical instability from undesirable reactions between the electrode and electrolyte. Due to these reasons, many researchers have attempted to alter the operating temperature in an intermediate range (773–973 K) to guarantee long-term stability and reduce the costs of components. Reducing the operating temperature, however, leads to serious problems associated with a decrease of the electro-catalytic activity for the oxygen reduction reaction (ORR) over the cathode.1 In this respect, intensive efforts have been devoted to developing alternative cathode materials that can achieve superior and stable electrochemical performance at lower temperatures.

Mixed ionic and electronic conductors (MIECs) have been introduced as cathode materials because extended reaction sites can be achieved at not only the triple phase boundary (gas, ionic conductor, and electronic conductor) but also at the two phase boundary (electrode and gas phase).2,3 Among them, cobalt and/or iron based simple and layered perovskite MIECs have been proposed as cathode materials for SOFCs due to their excellent electrochemical performance at intermediate temperatures.4–10 In particular, layered perovskite MIEC oxides have received attention because of their rapid oxygen kinetics such as high oxygen diffusion (D) and surface-exchange coefficients (k), as compared with those of ABO3-type simple perovskite oxides.11–13 Generally, the structure of the cation ordered layered perovskite family has a chemical formula of AA′B2O5+δ, where A is a trivalent lanthanide (Ln) ion (Ln = Pr, Nd, Sm, Ho and Gd), A′ is a relatively large size Ba, and B is a first row transition metal ion or a mixture of these ions.3,12,14–18 The ionic radius difference between large size Ba and relatively smaller size lanthanide generates a two layer alternating stacking sequence of [BaO][CoO2][LnOδ][CoO2] along the c-axis with localized excess oxygen ions at the Ln-O plane, and localized excess oxygen ions thereupon provide a disorder-free channel for ion motion that enhances the oxygen ion diffusivity.16,19 Among the various kinds of layered perovskites, cobalt containing PrBaCo2O5+δ exhibits a low area specific resistance (ASR) and excellent single cell performance.12,16 Furthermore, it is confirmed that strontium doping into the Ba site of PrBaCo2O5+δ enhances the electrical conductivity and the electrochemical properties.20 However, there are some drawbacks such as redox and chemical instability originating from the formation of a secondary phase, e.g., BaCO3 or SrCO3, from the reaction with CO2 in the atmosphere and Sr segregation on the surface.12,21,22 Lee et al. reported the effect of a smaller size mismatch between the host and dopant in order to reduce Sr segregation, consequently leading to a more stable cathode surface.23 Recently, Yoo et al. observed enhanced stability and electro-chemical performance with the substitution of smaller Ca instead of Sr into the Ba site of NdBaCo2O5+δ and first-principles density functional theory (DFT) calculations confirmed these favorable properties according to the thermodynamic behavior of mobile oxygen species.13

In this regard, we systematically studied the effect of calcium substitution into the Ba site of PrBa1−xCaxCo2O5+δ (x = 0, 0.1, 0.2, 0.3 and 0.4) with increasing Ca amounts on the structural, physicochemical, electrical, and electro-chemical properties. Interestingly, we observed a phase transition from a layered perovskite to a simple perovskite structure with increasing Ca content. In addition, we studied the correlation of the phase transition between electrical, electrochemical, and thermal expansion behaviors, respectively.