Solid oxide fuel cells are used as stationary power plants for electricity production. Despite having a very high efficiency of 90% they haven’t gained a world-wide commercial usage, due to their very high operating temperatures, and high production cost. However, there is a lot of ongoing research with the aim of developing intermediate-temperature solid oxide fuel cells (IT-SOFCs) that could operate at temperatures below 800°C. Cathodes are the most studied components of IT-SOFCs, since decreasing operating temperature results in slow oxygen reduction reaction(ORR) kinetics and large polarization losses. Perovskite related metal oxides have become very popular materials that could make suitable cathodes for IT-SOFCs. In this work an evaluation of several materials belonging to three different material groups have been studied: single layer perovskites, with a general formula of ABO3, double layer perovskites, with a general formula of AA’B2O6 and Ruddlesden-Popper phase, with a general formula of An+1BnO3n+1. Power generating capabilities of those materials have been studied on an electrolyte supported cell, cathode/LSGM9182/Ni-Fe. IR drop and overpotential of the cathode was measured and activation energy of the ORR for each material has been calculated. The double layer perovskite cobaltites offer a significant drop in overpotential, increase in conductivity compared to their single layer counterpart, while being able to generate significant amount of power. Ruddlesden-Popper phase materials offer the lowest activation energy values amongst the researched materials, but offer limited power generation values in the setup they were tested. Both of double layer perovskites and Ruddlesden-Popper based materials have opportunities for their performance to be improved. / Fastoxidbränsleceller används som stationära kraftverk för elproduktion. Trots att de har en mycket hög effektivitet på 90% har de inte fått en världsomspännande kommersiell användning på grund av deras mycket höga driftstemperaturer och hög produktionskostnad. Det är emellertid mycket pågående forskning med sikte på att utveckla intermediär temperatur fastoxidbränsleceller (IT-SOFC) som kan fungera vid temperaturer under 800 ° C. Katod är de mest studerade komponenterna i IT-SOFC, eftersom minskad driftstemperatur resulterar i kinetik med långsam syrereduktion (ORR) och stora polarisationsförluster. Perovskite-relaterade metalloxider har blivit mycket populära material som kan göra lämpliga katoder för IT-SOFC. I detta arbete har en utvärdering av flera material som hör till tre olika materialgrupper studerats: singelskikt perovskiter, med en generell formel för ABO3, dubbelskikt perovskiter, med en generell formel av AA'B2O6 och Ruddlesden-Popper-fasen med en allmän formel för An + 1BnO3n + 1. Effektgenereringskapaciteten hos dessa material har studerats på en elektrolytbärbar cell, katod / LSGM9182 / Ni-Fe. IR-droppe och överpotential hos katoden mättes och aktiveringsenergin för ORR för varje material har beräknats. Dubbelskiktet perovskit koboltiter ger en signifikant minskning av överpotentialen, ökad ledningsförmåga jämfört med deras enkelskikt motpart, samtidigt som man kan generera betydande mängden kraft. Ruddlesden-Popper-fasmaterial erbjuder de lägsta aktiveringsenergivärdena bland de undersökta materialen, men erbjuder begränsade kraftproduktionsvärden i den inställning de testades. Både av dubbelskiktet perovskiter och Ruddlesden-Popper-baserade material har möjligheter att förbättra deras prestanda.

The application of the entropy concept to stabilize oxide systems opens the possibility of discovering new materials with unique structural and functional properties. High-entropy alloys and oxides, which are based on the entropy stabilization concept and composed of multi-principal elements, have the potential to tailor structural and functional properties to meet specific needs. The study of lanthanum-based perovskite materials that benefit from the entropy stabilization approach is a promising area of research.However, the inherent randomness of multi-principal elements presents new challenges, making it difficult to predict their behavior. To understand these difficulties, we have initiated a methodical investigation of La-based medium- and high-entropy perovskite oxides. This study focuses on the synthesis, characterization, sintering mechanism, and electrical conductivity properties of nine La1-xCax(A1/3, B1/3, C1/3)O3 medium-entropy perovskite oxide systems (A, B, and C = three combination of Cr or Co or Fe or Ni or Mn) and one La1-xCax(Cr0.2Co0.2Fe0.2Ni0.2Mn0.2)O3 high-entropy perovskite oxide system (for x = 0.1 to 0.3). This research aims to provide better understanding of: (1) synthesis process, (2) temperature of single-phase formation, (3) the impact of various combinations of multiple B-site transitional elements and Ca doping on crystal structure, and microstructure (4) sintering mechanism and (5) electrical conductivity properties.

Mechanical pressure offers unique control over the energy landscape of chemical reactions, opening up pathways that are inaccessible through conventional thermochemistry. We hypothesize that the reduced dimensionality defines the conformational space of the high-pressure reaction, giving rise to new selectivity that is unavailable in 3D systems. Here, we demonstrate this concept through the pressure-controlled topochemical polymerization of the diacetylene molecule deca‐3,5‐diyn‐1‐amine (DDA) incorporated in the two-dimensional (2D) perovskite [DDA]2PbBr4. Compression at 3 GPa drives the first topochemical polymerization through 1,2 addition, forming a polyene product at room temperature. The reaction is initiated by the mechanical bending of the linear DDA molecule, a mechanism fundamentally different from the 1,4-addition in 3D solids. Importantly, pressure hinders the second 1,2-addition by disfavoring the gauche conformation between the remaining acetylene groups, allowing for the selective formation of polyene versus polyacene products. We characterize the reaction mechanisms and products using spectroscopies (Raman, X-ray photoelectron, ultraviolet-visible), X-ray diffraction and density-functional theory simulations. These results highlight the important role of dimensionality in high-pressure chemistry, and offers a new paradigm for creating low-dimensional functional materials.

At the high-temperature conditions of the Earth's interior, lattice anharmonic effects in crystalline mineral phases can become pronounced. Anharmonicity, i.e., deviations of vibrations from harmonic oscillations, is caused by phonon-phonon interactions. Knowledge of lattice anharmonicity is essential to elucidate distinctive thermal properties in solids. Yet, accurate investigations of anharmonicity encounter difficulties owing to cumbersome computations. Here we present anharmonic property calculations with the phonon quasiparticle approach for various solids. The phonon quasiparticle approach efficiently and reliably addresses lattice anharmonicity by combining molecular dynamics and lattice dynamics calculations. It characterizes anharmonic phonons by extracting renormalized frequency and phonon lifetime from the mode-projected velocity autocorrelation function without explicitly computing higher-order interatomic force constants. In principle, it accounts for full anharmonic effects and overcomes finite-size effects typical of molecular dynamics. The validity and effectiveness of the current approach are demonstrated in computations of temperature-induced frequency shifts, anharmonic thermodynamics, phase boundaries, and lattice thermal conductivities of both weakly and strongly anharmonic, both insulating and metallic, and both simple and complex systems. These materials include a simple model crystal, Si with diamond structure, minerals of geophysical significance, MgSiO₃ perovskite and postperovskite, cubic CaSiO₃ perovskite, and B8 and B2 phases of FeO. Accurate anharmonic thermodynamic properties, phase boundaries, and lattice thermal conductivities presented in this thesis are important for geodynamic modeling. The theoretical framework validated in this thesis also enables predictive studies of various anharmonic materials which could not be previously addressed by conventional approaches, such as quasiharmonic approximation for thermodynamics calculations and finite displacement method for anharmonic lattice dynamics calculations.

Humanity has great energy demands, and must simultaneously combat climate change by curbing anthropogenic greenhouse gas emissions. Perovskite solar cells (PSC) provide a low-carbon energy source, at lower production costs than traditional silicon-based solar cells. PSC's suffer some issues with long-term stability. This report presents a measurement aimed at better understanding interfacial dynamics of PSC's, using X-ray Photo-electron Spectroscopy (XPS). By collecting data at the synchrotron BESSY II, material compositions at different depths in the cell were successfully measured. An unexplained shift in binding energy was observed for configurations with an external light source on and off.

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