The interactions between different components of solid oxide cells (SOCs) are critical issues for achieving the tens of thousands of hour's goal for long-term performance stability and lifetime. The interactions between the ceramic electrolyte, porous ceramic air electrode, and metallic interconnect materials — including solid state interfacial reactions and vaporization/deposition of some volatile elements — have been investigated in the simulated SOC operating environment. The interactions demonstrate the material degradation mechanisms of the cell components and the effects of different factors such as chemical composition and microstructure of the materials, as well as atmosphere and current load on the air electrode side. In the aspect of materials, this work contributes to the degradation mechanism on the air electrode side and provides practical material design criteria for long-term SOC operation. In this research, an yttria-stabilized zirconia electrolyte (YSZ)/strontium-doped lanthanum manganite electrode (LSM)/AISI 441 stainless steel interconnect tri-layer structure has been fabricated in order to simulate the air electrode working environment of a real cell. The tri-layer samples have been treated in dry/moist air atmospheres at 800°C for up to 500 h. The LSM air electrode shows slight grain growth, but the growth is less in moist atmospheres. The amount of Cr deposition on the LSM surface is slightly more for the samples thermally treated in the moist atmospheres. At the YSZ/LSM interface, La enrichment is significant while Mn depletion occurs. The Cr deposition at the YSZ/LSM interface is observed. The stoichiometry of the air electrode is an important factor for the interactions. The air electrode composition has been varied by changing the x value in (La0.8Sr0.2)xMnO₃ from 0.95 to 1.05 (LSM95, LSM100, and LSM105). The enrichment of La at the YSZ/LSM interface inhibits the Cr deposition. The mechanisms of Cr poisoning and LSM elemental surface segregation are discussed. A 200 mA·cm-2 current load have been applied on the simulated cells. Mn is a key element for Cr deposition under polarization. Excessive Mn in the LSM lessens the formation of La-containing phases at the YSZ/LSM interface and accelerates Cr deposition. Deficient Mn in LSM leads to extensive interfacial reaction with YSZ forming more La-containing phase and inhibiting Cr deposition. / Ph. D.

The present doctoral thesis summarises results of investigation focused on the characterisation of materials involved in Solid Oxide Cell technology. The main topic of investigation was the ceramic cell, also known as MEA. Particular attention was given to the role that bi-material interfaces, co-sintering effects and residual stresses play in the resulting mechanical response. The first main goal was to investigate the effects of the manufacturing process (i.e. layer by layer deposition) on the mechanical response; to enable this investigation, electrode layers were screen-printed one by one on the electrolyte support and experimental tests were performed after every layer deposition. The experimental activity started with the measurement of the elastic characteristics. Both elastic and shear moduli were measured via three different techniques at room and high temperature. Then, uniaxial and biaxial flexural strengths were determined via two loading configurations. The analysis of the elastic and fracture behaviours of the MEA revealed that the addition of layers to the electrolyte has a detrimental effect on the final mechanical response. Elastic characteristics and flexural strength of the electrolyte on the MEA level are sensibly reduced. The reasons behind the weakening effect can be ascribed to the presence and redistribution of residual stresses, changes in the crack initiation site, porosity of layers and pre-cracks formation in the electrode layers. Finally, the coefficients of thermal expansion were evaluated via dilatometry on bulk materials serving as inputs for finite elements analyses supporting experiments and results interpretation. The second most important goal was to assess the influence of operating conditions on the integrity of the MEA. Here interactions of ceramic–metal interfaces within the repetition unit operating at high temperatures and as well at both oxidative and reductive atmospheres were investigated. The elastic and fracture responses of MEA extracted from SOC stacks after several hours of service were analysed. Layer delamination and loss of mechanical strength were observed with increasing operational time. Moreover, SEM observations helped to detect significant microstructural changes of the electrodes (e.g. demixing, coarsening, elemental migration and depletion), which might be responsible for decreased electrochemical performances. All the materials presented in this work are part of SOC stacks produced and commercialised by Sunfire GmbH, which is one of the world leading companies in the field.

This report travels through multiple disciplines to seek innovative and sustainable energy solutions for wastewater treatment plants. The first subject is a report about increased global temperatures and an over-exploitation of natural resources that threatens ecosystems worldwide. The situation is urgent where the current trend is a 2°C increase of global temperatures already in 2040. Furthermore, the energy-land nexus becomes increasingly apparent where the world is going from a dependence on easily accessible fossil resources to renewables limited by land allocation. A direction of the required transition is suggested where all actors of the society must contribute to quickly construct a new carbon-neutral resource and energy system. Wastewater treatment is as required today as it is in the future, but it may move towards a more emphasized role where resource management and energy recovery will be increasingly important. This report is a master’s thesis in energy engineering with an ambition to provide some clues, with a focus on energy, to how wastewater treatment plants can be successfully integrated within the future society. A background check is conducted in the cross section between science, society, politics and wastewater treatment. Above this, a layer of technological insights is applied, from where accessible energy pathways can be identified and evaluated. A not so distant step for wastewater treatment plants would be to absorb surplus renewable electricity and store it in chemical storage mediums, since biogas is already commonly produced and many times also refined to vehicle fuel. Such extra steps could be excellent ways of improving the integration of wastewater treatment plants into the society. New and innovative electric grid-connected energy storage technologies are required when large synchronous electric generators are being replaced by ‘smaller’ wind turbines and solar cells which are intermittent (variable) by nature. A transition of the society requires energy storages, balancing of electric grids, waste-resource utilization, energy efficiency measures etcetera… This interdisciplinary approach aims to identify relevant energy technologies for wastewater treatment plants that could represent decisive steps towards sustainability.