(Invited) Investigating Transport Pathways on La0.6Sr0.4CoO3-
δ Thin Films with Frequency-Resolved X-Ray Imaging
Authors: Adler Stuart B. , Gerwe Brian S , Mizuno Keita , Sekizawa Oki , Nitta Kiyofumi , Amezawa Koji 
Published: December 2020

Enhancing oxygen electrode performance in SOFCs is generally hindered by insufficient understanding regarding the rates of surface exchange and ionic diffusion processes. Spatial averaging of rates taking place at different locations within the electrode structure prevent AC impedance and DC polarization techniques from unambiguously ascertaining mechanistic details. To probe local electrode rates, workers leveraged operando micro-X-ray absorption spectroscopy (µ-XAS) to capture one-dimensional images of oxygen chemical potential distributions in patterned thin film La0.6Sr0.4CoO3-δ electrodes under steady polarization.1–3 They directly measured the active region length for behavior co-limited by diffusion and kinetics, with surprising results regarding the role of three-phase boundaries (interface of gas, electrode, and electrolyte). Although measuring the active region length indicates relative contributions of kinetics and diffusion, measurements under co-limited conditions cannot determine their absolute rates.

We recently extended those measurements into the frequency domain by collecting one-dimensional µ-XAS images under sinusoidal voltage perturbations. Therefore, we maintain the spatial resolution of µ-XAS and gain the benefit of probing physics by timescale (a feature of AC impedance spectroscopy). Our results from this technique – called frequency-resolved X-ray absorption spectroscopy (fr-XAS) – agree quantitatively with a one-dimensional Gerischer model, yielding transport and kinetic rate parameters from the images. Extracted parameters agree remarkably well with literature values from porous electrodes and show similar oxygen partial pressure dependence. AC impedance measurements of the patterned electrodes are not Gerischer-shaped, likely from parallel processes unrelated to oxygen exchange and transport, preventing meaningful interpretation.4,5 Contrasting interpretability showcases the power of local, frequency-resolved measurements for isolating pertinent phenomena amid extraneous factors.

Higher harmonic distributions from the nonlinear response to moderate amplitude perturbations allow us to examine relative contributions of bulk and surface diffusion. Fr-XAS images collected over a range of perturbation amplitudes are analyzed analogously to nonlinear electrochemical spectroscopy data to extract amplitude independent coefficients.6 The sign and magnitude for the real and imaginary components of these coefficients indicate relative rates of bulk and surface diffusion. Ongoing work focuses on determining relative rates of these parallel transport pathways in patterned electrodes featuring triple-phase boundaries and double-phase boundaries (interface of electrode and electrolyte).

(1) Fujimaki, Y.; Watanabe, H.; Terada, Y.; Nakamura, T.; Yashiro, K.; Hashimoto, S.; Kawada, T.; Amezawa, K. Direct Evaluation of Oxygen Chemical Potential Distribution in an SOFC Cathode by In Situ X-Ray Absorption Spectroscopy. ECS Trans.
2013, 57 (1), 1925–1932. https://doi.org/10.1149/05701.1925ecst.

(2) Amezawa, K.; Fujimaki, Y.; Nakamura, T.; Bagarinao, K. D.-; Yamaji, K.; Nitta, K.; Terada, Y.; Iguchi, F.; Yashiro, K.; Yugami, H.; Kawada, T. (Invited) Determination of Effective Reaction Area in a Mixed-Conducting SOFC Cathode. ECS Trans.
2015, 66 (2), 129–135. https://doi.org/10.1149/06602.0129ecst.

(3) Amezawa, K.; Fujimaki, Y.; Mizuno, K.; Kimura, Y.; Nakamura, T.; Nitta, K.; Terada, Y.; Iguchi, F.; Yugami, H.; Yashiro, K.; Kawada, T. (Invited) Triple Phase Boundary Reaction in a Mixed-Conducting SOFC Cathode. ECS Trans.
2017, 77 (10), 41–47. https://doi.org/10.1149/07710.0041ecst.

(4) Lu, Y.; Kreller, C.; Adler, S. B. Measurement and Modeling of the Impedance Characteristics of Porous La1 − x Sr x CoO3 − δ Electrodes. J. Electrochem. Soc.
2009, 156 (4), B513–B525. https://doi.org/10.1149/1.3079337.

(5) Søgaard, M.; Hendriksen, P. V.; Mogensen, M.; Poulsen, F. W.; Skou, E. Oxygen Nonstoichiometry and Transport Properties of Strontium Substituted Lanthanum Cobaltite. Solid State Ionics
2006, 177 (37), 3285–3296. https://doi.org/10.1016/j.ssi.2006.09.005.

(6) Wilson, J. R.; Schwartz, D. T.; Adler, S. B. Nonlinear Electrochemical Impedance Spectroscopy for Solid Oxide Fuel Cell Cathode Materials. Electrochimica Acta
2006, 51 (8–9), 1389–1402. https://doi.org/10.1016/j.electacta.2005.02.109.