TY - BOOK

T1 - Development of new materials for solid oxide fuel cell cathodes with superior performance and improved long-term stability

AU - Berger, Christian

N1 - no embargo

PY - 2019

Y1 - 2019

N2 - Solid oxide fuel cells (SOFCs) represent a highly efficient and sustainable future technology for stationary energy generation. Nevertheless, several obstacles need to be overcome in order to achieve a broad introduction in the commercial market. In this regard, one of the most critical factors is the limited long-term stability of the cells, especially of the cathode. Degradation of the cathode is frequently caused by contaminants, which are introduced through the air feed, or originate from stack components. These degradation effects can decrease the performance of the stack significantly during operation for several thousand hours. Thus, the development of long-term stable cathode materials with excellent mass and charge transport properties is one of the most important aspects of current research worldwide. The first part of the present thesis focuses on synthesis and characterisation of novel SOFC cathode materials with perovskite structure. The aim is the development of compounds which show fast oxygen exchange kinetics, good ionic and electronic conductivities, as well as high tolerance against critical impurities. Single phase materials are synthesised and characterised regarding mass and charge transport properties and defect chemistry. The second part of the thesis focuses on the optimisation of the oxygen exchange kinetics of alkaline earth-free SOFC cathode materials with K2NiF4-type structure. These generally offer extraordinarily high oxygen diffusivities, but the oxygen exchange kinetics is limited by the surface exchange process. Thus, the aim is the synthesis and characterisation of new K2NiF4-type materials with significantly higher surface exchange coefficients for oxygen. The results of the thesis show impressively that a series of promising SOFC cathode materials with fast oxygen exchange kinetics and high SO2 tolerance could be developed by purposeful variation of the chemical composition. For various compositions of La1-xCaxFeO3 (with x=0.1 for LCF91, x=0.2 for LCF82, and x=0.25 for LCF7525) comprehensive data were acquired on important material parameters such as oxygen exchange kinetics, ionic- and electronic conductivity, thermal expansion coefficient, and oxygen nonstoichiometry as functions of temperature and oxygen partial pressure. Furthermore, the effect of changing the A-site cation from La to Pr at x=0.2 leading to Pr0.8Ca0.2FeO3 (PCF82) is investigated. These studies lead to the conclusion that LCF82 shows the best material properties for application in SOFC-cathodes. However, with respect to increased long-term stability (SO2 tolerance), PCF82 shows the most promise. Within this work, detailed insights into the mechanisms of cathode degradation were obtained down to the nanometre scale. This knowledge was used to develop and validate strategies for the improvement of long-term stability. With PCF82, island-like growth of secondary phases, which are formed by reaction of the cathode material with sulphur dioxide, results in a relatively low degree of coverage of the surface with inactive phases. This characteristic feature of PCF82 leads to an increased long-term stability of the surface oxygen exchange kinetics under critical conditions. With the new K2NiF4-type materials, an increase in the chemical surface exchange coefficient of oxygen could be achieved by partial substitution of Ni with Co.

AB - Solid oxide fuel cells (SOFCs) represent a highly efficient and sustainable future technology for stationary energy generation. Nevertheless, several obstacles need to be overcome in order to achieve a broad introduction in the commercial market. In this regard, one of the most critical factors is the limited long-term stability of the cells, especially of the cathode. Degradation of the cathode is frequently caused by contaminants, which are introduced through the air feed, or originate from stack components. These degradation effects can decrease the performance of the stack significantly during operation for several thousand hours. Thus, the development of long-term stable cathode materials with excellent mass and charge transport properties is one of the most important aspects of current research worldwide. The first part of the present thesis focuses on synthesis and characterisation of novel SOFC cathode materials with perovskite structure. The aim is the development of compounds which show fast oxygen exchange kinetics, good ionic and electronic conductivities, as well as high tolerance against critical impurities. Single phase materials are synthesised and characterised regarding mass and charge transport properties and defect chemistry. The second part of the thesis focuses on the optimisation of the oxygen exchange kinetics of alkaline earth-free SOFC cathode materials with K2NiF4-type structure. These generally offer extraordinarily high oxygen diffusivities, but the oxygen exchange kinetics is limited by the surface exchange process. Thus, the aim is the synthesis and characterisation of new K2NiF4-type materials with significantly higher surface exchange coefficients for oxygen. The results of the thesis show impressively that a series of promising SOFC cathode materials with fast oxygen exchange kinetics and high SO2 tolerance could be developed by purposeful variation of the chemical composition. For various compositions of La1-xCaxFeO3 (with x=0.1 for LCF91, x=0.2 for LCF82, and x=0.25 for LCF7525) comprehensive data were acquired on important material parameters such as oxygen exchange kinetics, ionic- and electronic conductivity, thermal expansion coefficient, and oxygen nonstoichiometry as functions of temperature and oxygen partial pressure. Furthermore, the effect of changing the A-site cation from La to Pr at x=0.2 leading to Pr0.8Ca0.2FeO3 (PCF82) is investigated. These studies lead to the conclusion that LCF82 shows the best material properties for application in SOFC-cathodes. However, with respect to increased long-term stability (SO2 tolerance), PCF82 shows the most promise. Within this work, detailed insights into the mechanisms of cathode degradation were obtained down to the nanometre scale. This knowledge was used to develop and validate strategies for the improvement of long-term stability. With PCF82, island-like growth of secondary phases, which are formed by reaction of the cathode material with sulphur dioxide, results in a relatively low degree of coverage of the surface with inactive phases. This characteristic feature of PCF82 leads to an increased long-term stability of the surface oxygen exchange kinetics under critical conditions. With the new K2NiF4-type materials, an increase in the chemical surface exchange coefficient of oxygen could be achieved by partial substitution of Ni with Co.

KW - perovskite

KW - A2BO4 Ruddlesden-Popper type structure

KW - solid oxide fuel cell cathode

KW - oxygen exchange kinetics

KW - electronic conductivity

KW - ionic conductivity

KW - sulphur dioxide poisoning

KW - Perowskit

KW - A2BO4 Ruddlesden-Popper Phase

KW - Festoxid- Brennstoffzellen Kathode

KW - Sauerstoffaustauschkinetik

KW - Elektronische Leitfähigkeit

KW - Ionische Leitfähigkeit

KW - Schwefeldioxidvergiftung

M3 - Doctoral Thesis

ER -