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 systemscenario. In order to become widely commercially available, an improved performance and durability of the cellshas to be achieved [1]. Conventional scanning and transmission SEM and TEM have been often used for ex-situpost mortem characterization of SOFCs and SOECs [2,3].

However, in order to get fundamental insight of themicrostructural development of SOFC/SOEC during operation conditions in-situ studies are necessary [4]. Thedevelopment of advanced TEM chips and holders makes it possible to undertake analysis during exposure to theSOFC/SOEC sample of reactive gas flow, elevated temperatures and electrical biasing in combination.

Thisallows the study of nanostructure development under temperature and electrode polarisation conditions similarto operation conditions.In this work, we have for the first time performed in-situ analysis of a symmetric cell inside a TEM underdifferent configurations. In order to be able to perform in-situ experiments while drawing a current through thesample, we used a homemade TEM chip [5,6] and an 80-300kV Titan ETEM (FEI Company) equipped with animage corrector and a differential pumping system.A symmetric cell was prepared by depositing a cell consisting of three thin films on a strontium titanate (STO)single crystal substrate by pulsed laser deposition (PLD).

Lanthanum strontium cobaltite La0.6Sr0.4CoO3-δ (LSC)was chosen as electrode and yttria stabilized zirconia ZrO2: 8% mol Y2O3 (YSZ) as electrolyte, see figure 1. Highresolution TEM analysis on PLD samples after the deposition, did not reveal any second phase formation at theinterface between YSZ and LSC.

The in-situ experiment was firstly conducted in vacuum at temperature between25 oC and 900 oC. Secondly, it was repeated in presence of oxygen with an oxygen partial pressure of about 2mbar and a maximum temperature of 750 oC. Subsequently, the symmetric cell will be exposed to oxygen at 600 oC and 1 V overpotential within the ETEM.

In order to do that, a symmetric cell has been placed on the chip withthe use of a focus ion beam (FIB) microscope, see figure 2. To do so, a lamella was first extracted by the bulksample and attached to a conventional TEM grid. Afterwards, the grid was tilted by 90 degrees and the lamellawas detached once again and placed on the chip.STEM-EDS investigation was used for ex-situ post mortem analysis.

Finally, a bulk symmetric cell, coming fromthe same batch as the in-situ treated TEM samples, was tested in a furnace with similar environmentalconditions. This comparison is vital for distinguishing possible surface diffusion effects caused by having a thinlamella for in-situ TEM analysis. Electrochemical properties were also investigated by electrochemicalimpedance spectroscopy (EIS).In the figure 3 the cell was heat treated at 400 oC in vacuum, whereas in figure 4, the cell was treated at thesame temperature but in presence of oxygen, with PO2 of 2 mbar.

Comparing the two figures, the cell exposed tooxygen showed structural changes in the LSC thin film in comparison with the sample heated in vacuum. Thesechanges refer to the formation of grains as is confirmed by electron diffraction patterns.