Electrochemical TEM experiments on solid oxide cells

Research output: Book/ReportPh.D. thesis

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Abstract

In response to climate change and its potential negative impacts, there is a growing trend of using sustainable energy to replace fossil fuels. Developing related fuel production and storage technologies is becoming increasingly important. Solid oxide electrolysis/fuel cell (SOEC/SOFC) has become a research hotspot due to its high energy conversion efficiency, relatively lower cost, low noise, and scalability. However, its degradation mechanisms at high working temperatures are still unclear. In order to enable larger-scale commercial applications, further optimization is required in terms of cost and operational stability. The performance of SOC-related components, such as electrodes and electrolytes, needs to be improved, and their degradation mechanisms need to be further studied. Advanced characterization methods, especially in situ techniques, are necessary to introduce.

Electrochemical impedance spectroscopy is a powerful technique in electrochemistry, and one of its advantages is allowing operando characterization on a running system without causing damage to the research object. It works by applying an oscillating voltage/current (the frequency usually ranges from 0.01 to 1000000 Hz) and collecting response current/voltage. By fitting the data with an equivalent electric circuit model (ECM) of the electrochemical system, one can get the contributions of different processes in the reaction.

Environmental TEM can inlet different gases into the column and, with a DENS solutions lighting holder, heating and biasing can be also applied to the sample. This unique combination allows us to mimic the practical environments of running a solid oxide electrolysis/fuel cell/materials. In addition, by combining simultaneous EIS or biasing signal, one can investigate the structural/elemental evolutions and their effect on the electrochemical performance on a running system inside the ETEM.

In this PhD project, I focus on the application of electrochemical ETEM to investigate the properties of solid oxide materials/structures used for SOFC/SOEC, which can potentially also be applied to batteries and thermoelectric devices. Firstly, I identify the key considerations for conducting solid-state electrochemical TEM experiments, including sample preparation, electrochemical measurement, failure factors, and optimization methods. I developed an entire sample preparation process using FIB-SEM and provided examples of typical EIS spectra for pure electronic, pure ionic, and mixed ionic and electronic conductors, as well as cell structures. Next, I demonstrate the feasibility of combining EIS with environmental TEM to investigate the electrochemical properties of a micro CGO sample in reactive gases at elevated temperatures, the derived transport and surface reaction data are consistent with those from literatures. Finally, I conducted the high temperature EIS-TEM measurement on a nano CGO-YSZ-CGO model cell and applied a high temperature biasing measurement on an nano LSC-YSZ-LSC cell in different atmospheres and at high temperatures. The results from EIS-TEM and biasing-TEM are consistent with we expected with two arcs contributed by transport and surface properties respectively. For high temperature biasing-TEM experiments, the (S)TEM structure analysis, the STEM-EDS, and STEM-EELS results revealed the decomposition of LSC layers under the cathodic biasing. Those results demonstrated the feasibility of apply high temperature EIS-TEM or biasing-TEM experiments on the cell structures in the ETEM.

Generally speaking, this work demonstrates the feasibility of performing high temperature electrochemical measurements on solid oxide samples inside the ETEM, which can serve as an important methods for future
studies on energy materials or cell studies.
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherTechnical University of Denmark
Number of pages184
Publication statusPublished - 2023

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