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High-temperature Electrochemical Impedance Spectroscopy Transmission electron microscopy on energy materials

Periodic Reporting for period 2 - HEIST (High-temperature Electrochemical Impedance Spectroscopy Transmission electron microscopy on energy materials)

Reporting period: 2021-07-01 to 2022-12-31

The great challenge for humankind is to mitigate climate changes by replacing fossil fuels with renewables. We will need to produce synthetic green fuels for transport and we will have to store excess energy produced by solar and wind power for usage in dark and calm weather. High-temperature electrolysis cells as have the potential to convert vast amounts of electrical energy 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 mortem analysis.

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 the HEIST project 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.
The project has three tasks A, B and C.

Task A: Focus on high-temperature EIS-TEM experiments on metal oxide nanofibers. CGO and LSCF nanofibers was produced by electrospinning followed by calcination. For EIS-TEM experiments, a commercial chip-based TEM heating-biasing holder was purchased after testing the most promising systems. A great challenge in task A is mount single fibers on the heating-biasing chip ensuring sufficient contact to the current collectors. A successful procedure was identified. Using theoretical models for conductivity and fiber geometry, expected results for fiber resistances were estimated and used for guidance for sample preparation and EIS measurements. Procedures for conducting EIS-TEM experiments on the fibers were evaluated and developed. From EIS-TEM experiments, temperature dependent EIS data can be directly compared with structural information from TEM images. Fiber structure dynamics is compared to conductivity as a function of temperature in oxygen. The oxidation state of Ce in CGO was also determined in O2 as a function of temperature. Reference EIS experiments with the same type of fibers have been conducted.

Task B: Focus on development of a platforms for high-temperature EIS-TEM experiments on full nano SOEC cells. Cell design was based on results from theoretical modelling EIS spectra for different geometries. Model cells with YSZ, CGO electrodes and Pt and STN current collectors were produced by PLD. Commercial chips and holders have matured since the proposal was written. Therefore, commercial systems were considered for task B as in task A. In addition, a chip was designed and produced at DTU inspired from a previous DTU chip (The “tweezer” chip). The most promising chips for our EIS-TEM experiments was the purchased commercial system. Therefore, the originally planned focus on chip development was replaced by spending more time on the core part of task B which is to develop a robust procedure for mounting and connecting the model cells. Several EIS-TEM experiments were performed. EIS-TEM experiments with single materials (electronic conductors, ionic conductors, and mixed conductors) where performed and the results are a proof-of-concept for the method. EIS-TEM experiments on layered model SOECs/SOFCs are on-going.

Task C has according to plan not started.
The first EIS-TEM experiments have been conducted. The results show for the first time that reliable EIS measurement can be performed in the TEM at elevated temperatures in oxygen and in H2O/H2 mixtured. EIS measurements of samples in the TEM, which nanoscale dimension, are consistent with EIS measurements of bulk materials measured in traditional electrochemical rigs. This means that the EIS-TEM method is now mature for applications in solid state electrochemical experiments.

A variation of procedures for sample preparation of both single materials and full cells have been developed. This includes variations in cell geometries.