Oxygen Transport Kinetics of Mixed Ionic-Electronic Conductors by Coupling Focused Ion Beam Tomography and Electrochemical Impedance Spectroscopy

Abstract

The oxygen reduction reaction of mixed ionic-electronic conducting (MIEC) cathodes Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ (BSCF) and La 0.58 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) is investigated by detailed electrochemical impedance spectroscopy and focused ion beam tomography. The oxygen transport parameters of these materials are usually determined for model systems, such as dense bulks or thin films. However in the present study, probable differences regarding the time and thermal history of the samples (e.g. ambient poisoning gases, grain coarsening, secondary phase or surface segregation, etc.) were avoided by the in situ sintering of electrodes with nominal stoichiometry under synthetic air. The microstructural parameters of the electrodes are obtained by 3D FIB-SEM reconstruction tomography and subsequently used in combination with the Adler-Lane-Steele analytical model to calculate a simulated cathode resistance. Large discrepancies are observed compared to the electrochemical impedance measurements. In particular, the electrochemical impedance measurements do not show a Gerischer behavior, as expected for MIEC materials controlled by a coupled surface-exchange and bulk diffusion. Analysis by distribution function of relaxation times (DRT) reveals four individual processes taking place, indicating a surface-exchange controlled behavior with a reaction zone similar to the particle size. Bulk diffusion (D) and surface-exchange (k) coefficients from literature are critically discussed and tentative surface-exchange coefficients (k) for both MIECs are given. High-temperature devices such as solid oxide fuel cells (SOFCs) and oxygen transport membranes (OTMs) are promising systems for working towards zero-emissions power generation. The selected materials require high chemical and microstructural stability, as well as high catalytic activity during the oxygen reduction reaction (ORR), under normal operational conditions (i.e. operational temperature and oxygen partial pressure). Particularly interesting are the mixed ionic-electronic conducting (MIEC) materials, based on the ABO 3 perovskite structure. Their material properties can be flexibly customized, giving solid solutions with high ionic-electronic conductivity and hence good transport properties. More specifically, La 0.58 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) has become established as the stateof-the-art SOFC cathode, while Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ (BSCF) has presented outstanding oxygen permeation flux in its cubic phase. 1-3The surface-exchange coefficient (k) and the oxygen diffusion coefficient (D) provide key information about the oxygen transport mechanism that dominates the cathodic ORR. Many publications have assessed the D and k values of MIEC materials, most commonly by; electrical conductivity relaxation (ECR) measurements for dense bulks [4][5][6][7][8] and temperature for LSCF cathodes. 17 Here it should be pointed out that the well-known ALS model can only be applied if the measured impedance shows a Gerischer impedance, which indicates tha...

Gd0.2Ce0.8O2 Diffusion Barrier Layer between (La0.58Sr0.4)(Co0.2Fe0.8)O3−δ Cathode and Y0.16Zr0.84O2 Electrolyte for Solid Oxide Fuel Cells: Effect of Barrier Layer Sintering Temperature on Microstructure

“…For reasons for clarity, we used ZView to subtract the gas diffusion process in the impedance which is caused by the static ambient air in the gas channels of the test setup . The impedance then only comprises losses associated with the LSCF/GDC/YDZ interface, namely, the oxygen reduction reaction, interfacial resistances between contact mesh/LSCF and LSCF/GDC, as well as additional losses that can be associated with the occurrence of secondary phases …”

La0.6Sr0.4Co0.2Fe0.8O3-δ nanofiber cathode for intermediate-temperature solid oxide fuel cells by water-based sol-gel electrospinning: Synthesis and electrochemical behaviour

“…3 reports that Lou et al[51] have obtained an Rp = 0.42 Ω cm 2 with 100 nm granular La0.6Sr0.4Co0.2Fe0.8O3-δ electrode, which demonstrates that improvement can be obtained through further miniaturization of the internal electrode morphology. An additional confirmation is provided by the studies conducted by Ivers-Tiffée and co-workers[52,53,54,55], who have analyzed thin-film La0.6Sr0.4CoO3-δ cathodes obtained through metal organic deposition (MOD), with average grain and pore size in the sub-100nm regime (17 to 90 nm), obtaining an Rp as low as 9 10 -3 Ω cm 2 at 600°C. The EIS spectra reported in[52], different from the spectra reported in the present work, display five distinct processes, suggesting a somehow different physical mechanism.In addition to size reduction, other techniques of electrode improvement have been proposed, including infiltration[15,16,17,18,19,20,21,22,23].…”

Understanding of Oxygen Reduction Reaction on Perovskite-Type Ba0.5Sr0.5Fe0.91Al0.09O3-δ and Ba0.5Sr0.5Fe0.8Cu0.2O3-δ Using AC Impedance Spectroscopy Genetic Programming

“…55 Therefore, peak I in our model and GE are both intertwined with the same process in the ORR mechanism, of oxygen surface exchange and solid state diffusion. 56 The four systems are quite different in their impedance at a given temperature and therefore it is hard to find a single temperature in which they can be compared. Thus, the three better (more conductive) systems are brought in Table 2 at a single temperature and the values of the worst one, the BSCZGY-BSFAl system, are shown at a higher temperature.…”