Doctoral defence Anke Hagen

"Performance and Durability of Solid Oxide Fuel Cells"

Professor Anke Hagen
Technical University of Denmark

Opponents

Professor Mari-Ann Einarsrud, NTNU, Norge
Professor John Thomas Sirr Irvine, University of St Andrews, Storbritannien

Both opponents are appointed by DTU and have been part of the assessment committee with the chairman also appointed by DTU, Professor Qingfeng Li, DTU Energi.


Unofficial opponents are to address the moderator:

Provost Rasmus Larsen
Building 101A
Technical University of Denmark
Tel.: 45 25 71 42
e-mail: anlun@adm.dtu.dk

A copy of the dissertation can be obtained by contacting:

Anna-Maria Lund
Office of Research and Relations
Building 101A
Technical University of Denmark
Tel.: 45 25 71 42
e-mail: anlun@adm.dtu.dk

The dissertation:

"Performance and Durability of Solid Oxide Fuel Cells"

English summary
The interplay between materials compositions and structural features determines the performance and durability of solid oxide fuel cells (SOFCs). This exciting technology came at the verge of commercial breakthrough during the time of this research and will certainly contribute to the future energy systems throughout the world based on – among other advantageous properties - the achievable high electrical efficiencies of ~50-70% and the fuel flexibility.

The overall objective of this work was to achieve a better understanding of the relations between materials and their structures in the SOFCs. The approach was to develop and combine a broad range of advanced methods for characterisation – electrochemical, micro structural and spectroscopic - in order to gain profound insight into the complex processes that occur in operating SOFCs. This understanding was achieved largely under conditions as close as possible to real applications for SOFCs.

SOFCs with planar geometry and with the following compositions were investigated: cathodes based on lanthanum strontium manganite (LSM) and lanthanum strontium cobaltite ferrite (LSCF), anodes composed of Ni/yttria stabilised zirconia (YSZ) and Ni/yttria scandia zirconia (ScYSZ), and YSZ or Sc/YSZ electrolytes. A determining impact of structuring on performance and durability of SOFCs was shown on SOFC generations with the same nominal chemical compositions but different structural features in the cathode or anode.

In order to achieve a detailed understanding of the correlation between composition and structure in the electrodes, the electrochemical behaviour of the SOFCs was analysed in detail. It was possible to break down the total SOFC performance in terms of area specific resistance (ASRcell) quantitatively into the contributions from the single SOFC components and interfaces (cathode, anode, electrolyte, etc.). The concept that was developed to enable this break down included testing at a set of well-defined changes of gas conditions, temperatures, and SOFC concepts. In the studied SOFCs with LSM/YSZ cathodes, these electrodes proved to have the largest area specific resistance (ASRcathode) among the cell components. This specific resistance contribution can be decreased through structural improvements with the same chemical cathode composition to a certain extent. Changing the cathode composition towards LSCF - a mixed ionic electronic conductor – the Ni/YSZ or Ni/ScYSZ anode becomes the weakest part, i.e. the electrode with the largest ASR in the SOFC. Also for Ni/YSZ anodes, it was demonstrated that the performance can be tailored through structural modifications such as particle sizes and particle size distributions.

Comprehensive durability studies were carried out with the aim to correlate durability with the externally applied operating conditions, for example temperature, current density, fuel composition, and fuel utilisation for a given SOFC type. The observed degradation mechanisms were identified and related to specific cell components. Based on these findings, possibilities for improvement of the durability were investigated. It was revealed that, in general, the weakest part of a given SOFC generation determines the overall degradation mechanisms. In other words, the component with the highest ASR among the cell components is also usually experiencing the largest degradation.

The degradation mechanisms did not correlate in a straightforward manner with externally applied SOFC operating conditions such as temperature and current density. For example, some degradation processes such as changes of the Ni particle size were amplified with increasing temperature while LSM/YSZ cathode degradation was larger with decreasing temperature. As a consequence, the probably not expected result of an increase of the SOFC degradation with decreasing temperature was observed. It was revealed that these externally applied operating conditions affect the degradation processes in a more complex way. More relevant are the obtained conditions directly at the active sites and interfaces within the SOFC electrodes, which finally are leading to the specific degradation processes. The challenge was to discover those decisive conditions and how they are related to the externally applied conditions. Fundamental insight was gained through combining different electrochemical and micro structural analysis methods.

In case of SOFCs with LSM/YSZ cathodes being the weakest part, the oxygen partial pressure at the cathode/electrolyte interface was identified to be a key parameter. At critically low oxygen partial pressures an irreversible degradation occurred at this interface. It was revealed how this parameter in turn is affected by the applied temperature, current density, cathode gas, and the specific local interface micro structure. An improved local cathode/electrolyte interface structure in the LSM/YSZ cathode, specifically a higher degree of coverage of the YSZ electrolyte with small LSM particles, led to optimum durability features without changing the fundamental degradation mechanisms.

In SOFC types with mixed ionic electronic conducting cathodes, the Ni/YSZ (or Ni/ScYSZ) anode becomes the part with the highest area specific resistance and determines the overall degradation of the SOFC to large extent. For these SOFC types, the overall steam content and fuel impurities were identified as the most critical factors for durability. The externally applied conditions such as fuel composition, current density and fuel utilisation determine together the overall steam content at the Ni particles close to the anode/electrolyte interface and thus the degradation of the anode. Also in this case, both chemical (YSZ or ScYSZ) and structural modifications significantly tune the specific degradation rates.

A specific feature of SOFCs is the Ni containing anode that acts as heterogeneous catalyst for in situ conversion of hydrocarbon containing fuels to the electrochemical reactants hydrogen and carbon monoxide. This leads to the initially mentioned advantage of SOFCs over other types of low temperature fuel cells: the fuel flexibility. When using hydrocarbon fuels, catalytic and electrochemical processes are running in parallel and again, compositions and structures in the anode affect significantly to what extent the anode degrades and how degradation depends on operating conditions.
Detailed electrochemical characterisation before, during, and after operating the SOFC has been shown to be key methods to identify the relevant performance and degradation effects on SOFC component level and to guide the micro structural analysis. Dedicated effort was spent on developing tailored methodology that enabled identification of the relevant features at the relevant interface. For example, the coverage of the YSZ electrolyte with LSM cathode particles was identified as important structural feature that determined the performance and durability of the whole SOFC of this type. Further, the degree of percolation of Ni particles close to the electrolyte was found to be the determining factor for the durability of SOFCs with mixed ionic electronic conducting cathodes.

The gained detailed knowledge about degradation mechanisms at electrode/electrolyte interfaces contributes to paving the way for innovative ex situ, artificial aging tests and even accelerated test concepts for faster lifetime evaluation. For example, the finding that a specific partial pressure at the cathode/electrolyte interface or anode/electrolyte interface is critical for degradation, can be transferred to dedicated degradation studies outside the SOFC by mimicking the relevant interfaces at component level and expose them to the found conditions or even harsher to accelerate degradation.
X-ray absorption spectroscopy (XAS) proved its great potential to explain conductivity phenomena in electrode materials and to relate those to specific elements in specific locations / coordination environment within the oxidic structure, like in perovskites containing a variety of transition metals.

The XAS method was further developed towards in situ conditions involving the relevant high temperatures and the presence of relevant gasses. Even first steps towards characterisation of the electrochemical behaviour and the oxidation states/geometry of electrode components under SOFC polarisation, i.e. operando conditions, were realised.

Through the combination of results from a variety of methods it was possible to gain better understanding of the interplay between a specific material in the SOFCs and its structure in relation to performance and durability as function of operating parameters.


Time

Fri 19 Jan 18
14:00 - 18:00

Organizer

DTU

Where

DTU

Anker Engelunds Vej 1

2800 Kgs Lyngby

Building 101A, meeting room 1 (1. floor)