Periodic Reporting for period 1 - HEIST (High-temperature Electrochemical Impedance Spectroscopy Transmission electron microscopy on energy materials)
Reporting period: 2020-01-01 to 2021-06-30
store excess energy produced by solar and wind power for usage in dark and calm weather. Excess energy can be stored
electrochemically by high-temperature electrolysis cells as they have the potential to store vast amounts of electrical energy
by conversion to chemical fuels. Solid oxide electrolysis cell (SOEC) technology is well known and proven, but not price
competitive with storage of fossil fuels.
To drive the SOEC research towards a breakthrough, it is critical to determine relations between electrochemical activity and
structure/composition in the cells. Electrochemical impedance spectroscopy (EIS) is a very powerful method for determining
the contribution from processes in the cell to the overall activity. EIS cannot show structure/composition which is offered by
transmission electron microscopy (TEM). Conventional TEM, however, does not offer insight into active cells, but only post
High-temperature electrochemical TEM is extremely challenging because this requires a) that hard and brittle ceramic cells
are thinned to electron transparency (ca. 100 nm), b) that the cells are carefully designed to allow for characterization of the
layer interfaces, and c) that the cells are characterized during exposure of i) reactive gasses, ii) electrical potentials and iii)
temperatures up to ca. 800 °C.
The aim of HEIST is to cover step a) to c), i.e. to transform TEM into an electrochemical lab for high-temperature
electrochemical experiments including EIS. HEIST will give us “live” images of nanostructures and composition during
operation of the electrochemical cells and thus disclose structure-activity relations. This is important, because the structures
of nanomaterials will transform depending on the electrochemical environment, and post mortem analysis does not offer a
correct representation of the active nanostructures.
Task A focus on high-temperature EIS-TEM experiments on metal oxide nanofibers of electrode and electrolyte materials. Metal oxide nanofibers has been produced by electrospinning followed by calcination. This sample preparation has been optimized. The produced fibers was characterized by TEM. A variations of procedures for handling, and mounting the nanofibers on MEMS chips for conducting EIS-TEM experiments was tried, and a successful procedure was identified. Optimization of this procedures is ongoing. By using mathematical models for conductivity and geometry for the specific nanofibers, expected theoretical results is estimated for measurements using MEMS chips in order to optimize EIS measurements. The first EIS-TEM experiments were conducted on a gadolinia-doped ceria nanofiber. The experiments were successful enough to proof the concept of the EIS-TEM method. The experiments also give new insight into the coupled relation of conductivity and nanostructure of such a fiber.
Task B focus on development of a platforms for high-temperature EIS-TEM experiments on full nano SOEC cells. Different approaches has been followed to determine the optimal MEMS chips design and TEM holders for such experiments. A development has taken place of commercial systems size the proposal was written. Therefore four different commercial systems were considered, tree of which were tested. Two systems were found interesting and one of them were purchased after a tender process. In addition to commercial MEMS chip systems a new chip design was made and the chip was produced. This design was inspired from a previous DTU chip (The “tweezer” chip). The use of the purchased commercial system is promising, and therefore currently has our main focus. But the comparison to the custom made DTU chip is still ongoing. The samples in task B are layered model SOECs/SOFCs prepared by PLD. The specific design of these model cells were based on modelling of theoretical EIS spectra for a number of different cell geometries. After this model cells were produced by PLD, and we now have a selection of model cells to work with. Our main focus is currently on a symmetric cells with yttria-stabilized zirconia electrolyte and gadolinia-doped ceria electrodes. The core part of task B is to develop a robust procedure for mounting and connecting such model SOECs/SOFCs without breaking the sample or shortcutting the cell. This work is ongoing, and so far with enough success to have performed the first EIS-TEM measurements with full cells. We are currently analyzing the results.
Task C has according to plan not started.
First EIS-TEM experiments were conducted on full model nano-SOEC cells.