Polarization Induced Changes in LSM Thin Film Electrode Composition Observed by In Operando Raman Spectroscopy and TOF-SIMS

Abstract

For decades strontium doped lanthanum manganite (LSM) electrodes have been the material of choice for cathodes in high temperature solid oxide fuel cells (SOFCs). LSM has relatively high electrical conductivity at high temperatures and has mechanical properties that are well matched to yttria stabilized zirconia (YSZ), a common electrolyte material. Recently, LSM electrodes have been employed in lower temperature (300-500 °C) electrochemical gas purification applications. Several studies have attributed the electrochemical activation of LSM electrodes to changes in the surface stoichiometry under an applied potential.1-3 The presented work explores the polarisation induced changes in LSM electrode composition by utilizing in operando Raman spectroscopy and post mortem ToF-SIMS depth profiling on LSM thin film model electrodes fabricated by pulsed laser deposition on YSZ substrates with a thin (200 nm) CGO barrier layer. Experiments were conducted on cells with 200 nm thick (La0.85Sr0.15)0.9MnO3 ±δ electrodes in 10% O2 at 500°C and 700°C under various electrical polarisations (-0.5V, ±1V and -2.5V). Raman spectra recorded continuously during polarisation showed evidence of shifts in band intensities that were both reversible and dependent on the direction of the applied potential (Figure 1). The spectral changes were assigned to changes in the LSM electronic structure and specifically to changes in the relative oxide concentration in LSM’s near surface region. Ex situ ToF-SIMS depth profiles were recorded through the LSM thin film electrodes and revealed distinct compositional changes throughout the electrodes (Figure 2). The electrode elements and impurities separated into distinct layers that were more pronounced for the stronger applied polarisations. The mechanism behind this separation into “layers” in the LSM electrode poses interesting questions about mass transfer and ion migration in conducting materials subject to electrical polarisation. Figure 1. Representative Raman spectra collected on an LSM electrode at 700 °C with the cell open at circuit voltage (OCV) and polarised at (a) -1 V and (b) +1 V. The peaks at 440 and 600 cm-1 are signatures from the CGO and YSZ electrolyte, respectively. Figure 2. ToF-SIMS depth profile of LSM electrode polarised at -1V at 700 °C for 2 h. Stapled lines mark layers enriched in different species. References 1 M. Backhaus-Ricoult, K. Adib, T. St.Clair, B. Luerssen, L. Gregoratti and A. Barinov, Solid State Ionics, 2008, 179, 891. 2 M. A. Haider and S. McIntosh, J. Electrochem. Soc., 2009, 156, B1369. 3 S. P. Jiang and J. G. Love, Solid State Ionics, 2003, 158, 45. Figure 1