In SITU Transmission Electron Microscopy on Operating Electrochemical CELLS

Solid oxide cells (SOC) have the potential of playing a significant role in the future efficient energy system scenario. In order to become widely commercially available, an improved performance and durability of the cells has to be achieved [1]. Conventional scanning and transmission SEM and TEM have been often used for ex-situ post mortem characterization of SOFCs and SOECs [2,3]. However, in order to get fundamental insight of the microstructural development of SOFC/SOEC during operation conditions in situ studies are necessary [4]. In this work, we use advanced chip-based TEM holders to analyse SOFC/SOEC samples during exposure of reactive gas flows, elevated temperatures and electrical biasing in combination. This allows monitoring the nanostructure development under temperature and electrode polarisation conditions similar to operation conditions. Specifically, an in situ analysis of a symmetric SOFC/SOEC cell is analysed inside the TEM under different reaction conditions. In order to perform in situ experiments while drawing a current through the sample, we used a homemade TEM chip [5,6] and an 80-300kV Titan ETEM (FEI Company) equipped with an image corrector and a differential pumping system. A symmetric cell was prepared by depositing three thin films on a strontium titanate (STO) single crystal substrate by pulsed laser deposition (PLD). Lanthanum strontium cobaltite La 0.6 Sr 0.4 CoO 3- δ (LSC) was chosen as electrode and yttria stabilized zirconia ZrO 2 : 8% mol Y 2 O 3 (YSZ) as electrolyte. High resolution TEM analysis on PLD samples after the deposition did not reveal any second phase formation at the interface between YSZ and LSC. An in situ experiment was firstly conducted in vacuum at temperature between 25 o C and 900 o C. Secondly, it was repeated in presence of oxygen with an oxygen partial pressure of about 2 mbar and a maximum temperature of 750 o C. Subsequently, the symmetric cell will be exposed to oxygen at 600 o C and 1 V overpotential within the ETEM. In order to do that, a symmetric cell has been placed on a chip with the use of a focus ion beam (FIB) microscope, see figure 1. STEM-EDS with a JEOL 3000F TEM, STEM-EELS and Energy Filter (EF) TEM with a 1MV JEOL ETEM [7] were used for ex situ post mortem analysis. Finally, a bulk symmetric cell, coming from the same batch as the in situ treated TEM samples, was tested in a furnace with similar environmental conditions. This comparison is vital for distinguishing possible surface diffusion effects caused by having a thin lamella for in situ TEM analysis. Electrochemical properties were also investigated by electrochemical impedance spectroscopy (EIS). Figure 2 compared results from two different in situ TEM experiments where for the images (a) and (b) the sample was kept in vacuum and exposed to different temperatures, and for the images (c) and (d) the sample was exposed to different temperatures and an oxygen partial pressure of 2 mbar. The comparison of the two experiments revealed a faster grain growth for the sample exposed to both oxygen and high temperature. For the sample analysed in vacuum, ulterior analysis showed an agglomeration of cobalt at the interface between STN and LSC, figure 3. [1] A. Atkinson et al., Nature Materials 2004, 3, 17-27. [2] R. Knibbe et al., Journal of The Electrochemical Society 2010, 157, B1209-B1217. [3] N. Imanishi et al., Solid State Ionics 2006, 177, 2165-2173. [4] M. L. Traulsen et al., ECS Transactions 2015, 66, 3-20. [5] C. Kallesøe et al., Nano Letters 2012, 12, 2965-2970. [6] S. B. Alam et al., Nano Letters 2015, 15, 6535-6541. [7] N. Tanaka et al., Microscopy 2013, 62, 205-215. Figure 1