Nonlinear Impedance Analysis of La_0.4Sr_0.6Co_0.2Fe_0.8O_3-δThin Film Oxygen Electrodes

Abstract

Linear and nonlinear electrochemical impedance spectroscopy (EIS, NLEIS) were used to study 20 nm thin film La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF-6428) electrodes at 600 • C in oxygen environments. LSCF films were epitaxially deposited on single crystal yttria-stabilized zirconia (YSZ) with a 5 nm gadolinium-doped ceria (GDC) protective interlayer. Impedance measurements reveal an oxygen storage capacity similar to independent thermogravimetry measurements on semi-porous pellets. However, the impedance data fail to obey a homogeneous semiconductor point-defect model. Two consistent scenarios were considered: a homogeneous film with non-ideal thermodynamics (constrained by thermogravimetry measurements), or an inhomogeneous film (constrained by a semiconductor point-defect model with a Sr maldistribution). The latter interpretation suggests that gradients in Sr composition would have to extend beyond the space-charge region of the gas-electrode interface. While there is growing evidence supporting an equilibrium Sr segregation at the LSCF surface monolayer, a long-range, non-equilibrium Sr stratification caused by electrode processing conditions offers a possible explanation for the large volume of highly reducible LSCF. Additionally, all thin films exhibited fluctuations in both linear and nonlinear impedance over the hundred-hour measurement period. This behavior is inconsistent with changes solely in the surface rate coefficient and possibly caused by variations in the surface thermodynamics over exposure time. The rate of oxygen reduction/evolution on oxide surfaces continues to limit the performance of intermediate temperature (500-800• C) solid oxide fuel cells (SOFCs) and electrolysis cells (SOECs).1 La 1-x Sr x Co 1-y Fe y O 3-δ (LSCF), and particularly the composition La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF-6428), exhibits a good balance of kinetic activity, ionic and electronic transport properties, and chemical stability, resulting in its prevalence as an oxygen electrode. 2-4Electrochemical measurements on LSCF-6428, including conductivity relaxation, 3,5 coulometry, 4,6 and impedance techniques 2,7,8 have underscored the importance of bulk oxygen vacancies in governing rates of O 2 reduction and evolution. However, a clear understanding of how these vacancies contribute to the mechanism and rate-limiting step(s) of O 2 exchange remains elusive.The thermodynamic understanding of La 1-x Sr x Co 1-y Fe y O 3-δ derives largely from measurements of equilibrium oxygen nonstoichiometry (δ 0 ) upon exposure to different oxygen environments and temperatures. 4,[9][10][11] In the limit of high B-site iron content (y > 0.6), workers have interpreted this δ 0 − p O2 − T relationship using a ptype point-defect model originally developed for LSF (LSCF, with y = 1), 12 where electrons are localized on iron centers of discrete charge and defects are considered dilute.4,9 For Co-rich compositions (y < 0.4), a metallic model used to describe LSC (LSCF, with y = 0) yields better agreement.4 While La 0.6 Sr 0.4 Co 0.2...

Controlling Oxygen Mobility in Ruddlesden–Popper Oxides

“…So far, substantial efforts have been focused on developing ABO 3 perovskite oxides, which exhibit fascinating physicochemical properties—i.e., high electronic and ionic conductivities and high catalytic activities [1,2,3]—for use in energy applications. Mixed ionic and electronic conductors (MIECs), such as La 1−x Sr x CoO 3−δ (LSC 113 ) [4,5,6,7,8,9,10,11,12] and La 1−x Sr x Co 1−y Fe y O 3−δ (LSCF 113 ) [13,14,15,16,17,18,19,20,21,22,23,24,25,26], that show high degrees of oxygen deficiency are commonly used to promote oxygen diffusivity and surface exchange kinetics at intermediate temperatures. However, these materials suffer from some serious inherent limitations, including poor thermal and chemical stability [27,28,29,30] and long-term instability [22,31,32,33,34].…”

Controlling the Oxygen Electrocatalysis on Perovskite and Layered Oxide Thin Films for Solid Oxide Fuel Cell Cathodes

“…Therefore, high electrochemical performance at the cathode could be achieved relative to undoped ABO 3 perovskite oxides. In this sense, LSC 113 [17,21,27,28,46,64,73,75,76] and La 1−x Sr x Co 1−y F y O 3−δ (LSCF 113 ) [22,45,47,66,[77][78][79][80][81][82][83][84][85][86] perovskite oxides are commonly used as these oxides are known to show oxygen deficiency and the oxygen nonstoichiometry (δ) affects the electrochemical properties, conductivity and lattice expansion of the materials.…”

Achieving Fast Oxygen Reduction on Oxide Electrodes by Creating 3D Multiscale Micro-Nano Structures for Low-Temperature Solid Oxide Fuel Cells

“…Considerable effort has been devoted to the enhancement of the ORR activity at the cathode by mainly two different approaches, which are the development of new cathode materials and modulating structural architectures. In the case of the development of novel mixed ionic and electronic conducting (MIEC) materials, layered Ruddlesden–Popper phases and double perovskites were extensively studied. − While such materials exhibited comparable ORR kinetics to state-of-the-art cathode materials such as La 1– x Sr x Co 1– y Fe y O 3−δ (LSCF), − further improvement in the ORR kinetics is required for low-temperature (LT)-SOFCs. Alternatively, structural modifications such as increasing the reactive surface area or reducing the thickness of electrolytes can also be used to enhance the ORR.…”