Correlation of Time-Dependent Oxygen Surface Exchange Kinetics with Surface Chemistry of La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ Catalysts

Abstract

The Sr segregation at the surface of a perovskite La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) oxygen electrode is detrimental to the electrochemical performance and durability of energy conversion devices such as solid oxide fuel cells. However, a quantitative correlation of degradation of the oxygen surface exchange kinetics with Sr precipitation formation at the LSCF surface is not clearly understood yet. Herein, the correlation of the time-dependent degradation mechanisms of the LSCF catalysts with respect to Sr segregation phenomenon at the surface were investigated at 800 °C for a prolonged annealing time (∼800 h) by combining in situ electrochemical measurements, and ex situ chemical and structural analyses at the multiscale. The in situ monitored surface exchange coefficient (k chem) was found to drastically drop by ∼86% over the 800 h, and it was accompanied by the formation of Sr-containing secondary phases on the bulk LSCF surface, as expected. However, the estimated coverage of Sr segregation on the LSCF surface was only ∼15%, even after 800 h of aging time, showing significant deviation from the k chem degradation rate (∼86%). The surface chemistry evolution at the clean surface area, which is believed to be electrochemically active, was further analyzed on the nanoscale. The quantified results showed that the Sr elemental fraction of the A-site at the outermost surface of the LSCF samples was becoming deficient from ∼4.0 at 0 h to ∼0.27 at 800 h annealing. Interestingly, the time-dependent behavioral tendencies between k chem degradation and surface Sr fractional changes were highly analogous. Thus, our results suggest that this Sr deficiency at the clean surface region more dominantly impacts the degradation process rather than an electrochemical activity passivation by the SrO x precipitates, which has been shown to be a major degradation mechanism of LSCF performance.

The effect of sub-surface strontium depletion on oxygen diffusion in La0.6Sr0.4Co0.2Fe0.8O3−δ

“…Strontium segregation has predominantly been reported as a steady increase from bulk stoichiometry to near 100% coverage at the surface, 16,29 although some reports observe subsurface strontium depletion. 25,32 This work conrms the presence of this sub-surface strontium depletion region, however, it also opens the question as to which pathway the strontium (and the lanthanum) is able to diffuse through the material. Kubicek et al showed that, in Pulse Laser Deposited (PLD) La 0.6 Sr 0.4 -CoO 3Àd thin lms, at 720 C the strontium self-diffusion coef-cient (D g ) is on the order of 10 À18 cm 2 s À1 and strontium selfdiffusion coefficient in a grain boundary (D gb ) was on the order of 10 À15 cm 2 s À1 .…”

“…Many reports show enhance strontium concentrations over approximately the rst 6-20 nm of the material, 16,[28][29][30][31] however, strontium deciency has also been observed over the same depth scale. 25,32 Additionally, no correlation has been made between the changes in immediate sub-surface composition and oxygen exchange/transport properties in MIECs.…”

An Elementary Kinetic Model for the LSCF and LSCF-CGO Electrodes of Solid Oxide Cells: Impact of Operating Conditions and Degradation on the Electrode Response

“…As the SrO is an electrical insulating compound blocking the reactions of oxygen exchange, the process leads to the LSCF surface passivation [21,22,23,24]. For example, Wang et al [23] have shown that the global exchange kinetic constant after ageing at 700°C-800°C is decreased by an order of magnitude and they attributed this evolution to the LSCF surface passivation.…”

“…However, Kim et al [22] have recently suggested that the decrease of 'cannot be accounted for only the surface coverage of inactive Sr segregation' and they proposed that the kinetic constant could also be affected by the stoichiometry change with Sr deficiency on the 'clean' part of the LSCF surface. Moreover, the oxygen chemical diffusivity (or ionic conductivity) of LSCF is also decreased by the loss of Sr in the bulk of electrode due to the LSCF decomposition [22,25]. All these phenomena are expected to cause the degradation of the electrode performances even if their impact on the electrode response remains nowadays unclear.…”

“…It has been more recently highlighted that a change of reaction mechanism arises at low anodic over-potential, from the bulk to the surface path (defined by the direct charge transfer at the Triple Phase Boundary lines (TPBls)) [30,31]. Regarding the LSCF-CGO composite, some studies have suggested that the reaction mechanism is entirely controlled by the surface path whatever the electrode polarization [22,32].…”