The use of batteries as an energy carrier has increased enormously in the last decade due to the transition to sustainable energy sources. The rechargeable lithium ion battery with its high energy density is the most commonly used solution for both mobility purposes and stationary energy storage systems. The disadvantages of lithium give reason to look for alternative solutions, such as the Na ion and the Mg ion battery, collectively called metal-ion batteries. These developments require advanced research methods and measurement techniques. In this thesis, two measurement techniques are examined and expanded with new possibilities and insights, expanding the range of measurement techniques for research into metal-ion batteries.

In Chapter 2, new methods and techniques are Investigated to perform advanced measurements on the surface of electrode materials with an Atomic Force Microscope (AFM). The use of scanning probe techniques has been introduced in the last decade in battery research. Until now, Scanning probe microscopy techniques are used to determine surface properties where the moving probe makes full or partial contact with the surface to be examined and the probe as a measuring instrument has as little influence as possible on the surface to be examined during these measurements. However, for electrochemical research it may be desirable for the probe to make an active contribution to the process to be investigated. Therefore, it was examined whether the probe can be provided with a small amount of lithium, so that it becomes part of the electro-chemical process. An Atomic Force Microscope (AFM) is combined with a special designed glovebox system and coupled to a Galvanostat/Potentiostat to allow measurements on electrochemical properties. An open cell design with electrical contacts makes it possible to reach the electrode surface with the cantilever so as to perform measurements during battery operation.

A combined AFM-Scanning Electro-Chemical Microscopy (AFM-SECM) approach makes it possible to simultaneously obtain topological information and electrochemical activity. Several methods have been explored to provide the probe tip with a small amount of lithium. The "wet methods" that use liquid electrolyte appear to have significant drawbacks compared to dry methods, in which no electrolyte is used. Two dry methods were found to be best applicable, with one method applying metallic lithium to the tip and the second method forming an alloy with the silicon of the tip. The amount of lithium applied to the tip was measured by determining the shift of the resonance frequency which makes it possible to follow the lithiation process. A Finite Element Method (FEM)-based probe model has been used to simulate this shift due to mass change.

The AFM-Galvanostat/Potentiostat set-up is used to perform electrochemical measurements. Initial measurements with lithiated probes show that we are able to follow ion currents between tip and sample and perform an electrochemical impedance analysis in absence of an interfering Redox-probe, a so called non-faradaic measurement. The active probe method developed in this way can be extended to techniques in which AFM measurements can be combined with mapping electrochemical processes with a spatial resolution of less than 100 nanometer.

In chapter 3 Positron Annihilation Doppler Broadening Spectroscopy (DBPAS) is presented as a powerful method to analyse the origin and development of defect processes in porous silicon structures. Silicon is a promising negative electrode material due to it's high capacity. The main drawback is the extreme expansion when alloying with lithium. The volume changes cause cracks in the electrode material, resulting in accelerated degradation. Several prepared anodes were lithiated (discharged against Li+/Li) and de-lithiated (charged) with different capacities followed by a distinct treatment procedure and an analysis using the Delft Variable Energy Positron Beam. The results presented here show that we can distinguish two different processes attributed to (1) structural changes in silicon as a result of the alloying process, and (2) the formation of defects that initiate degradation of the material. The limit at which the porous material can be used for at least the first two cycles without the occurrence of damage can thus be accurately determined by using the DBPAS technique.

The long-term performance and degradation of porous silicon anodes under repeated lithiation and delithiation (cycling) was investigated in chapter 4. Employing X-ray diffraction (XRD) and positron annihilation spectroscopy as complementary measurement techniques, 20 samples where cycled between 5 and 76 times and lithiated to capacities of 1000, 1200, and 1500 mAh.g⁻¹, representing 28%, 33.5%, and 42% of silicon's theoretical capacity, respectively. . XRD results show an increase in amorphization upon cycling, evidenced by diminishing normalized silicon peak intensities. This is accompanied by a simultaneously decreasing positron S-parameter, which indicates that amorphization results In a decrease in defects. Higher end capacitances show faster amorphization, which may be related to the plastic deformation around the pores that is exacerbated due to mechanical stress. The study highlights the potential of positron annihilation as an indirect measure of amorphization, although additional techniques are essential to elucidate the complex interactions in silicon cycling.

Efficient energy utilization has been central to human progress, with each new energy source driving societal and technological advancements. While fossil fuels like coal, oil, and natural gas currently dominate, their environmental impact and finite availability underscore the urgent need for sustainable alternatives. Developing environmentally friendly and efficient energy conversion processes is vital to addressing the challenges of resource scarcity and climate change. Magnetocaloric materials (MCMs) are at the forefront of research in advanced cooling technologies due to their ability to exhibit a magnetocaloric effect (MCE). This area of study holds both significant fundamental scientific interest and critical application potential. From a scientific point of view, MCMs are highly intriguing due to their ability to exhibit a giant magnetocaloric effect (GMCE). Uncovering the underlying mechanisms of first-order magnetic transitions (FOMT) plays a pivotal role in advancing research and development in this field [1–6]. From an application perspective, related research primarily focuses on developing 3D-printed bulk materials to achieve gradient composites manufacturing of MCMs with enhanced heat transfer coefficients [7]. This approach is particularly relevant given the narrow operating range of FOMTs, which necessitates multiple material layers to achieve optimal efficiency [8]. However, for the successful commercialization of magnetic cooling and heat pump technologies, the primary future challenge lies in ensuring mechanical stability of GMCMs, which is a critical prerequisite. This thesis explores the unique structure and thermomagnetic properties of Fe2Pbased and potential novel MCM systems using techniques such as magnetization, heat capacity, Mössbauer spectroscopy, Density Functional Theory (DFT), X-ray and neutron diffraction.

In recent years, there has been growing interest in the magnetocaloric effect (MCE) which is a magneto-thermodynamic phenomenon observed in certain magnetic materials. MCE materials undergo temperature changes when subjected to magnetic fields. Researchers are now exploring the possibilities of utilizing this material in biomedical application technologies, as they offer the potential for non-invasive control of their properties through external magnetic fields. However, there is a lack of research on suitable magnetic compositions that exhibit desired magnetic properties for medical purposes, while ensuring no adverse effects on the cells.

This master project explores the potential of a magnetocaloric (Mn,Fe)2(P,Si)-based compound as a biodegradable membrane for multi-cell separation in organ-on-chip platforms. Mn0.65Fe1.30Si0.37P0.65 compound was selected due to its sharp phase transition characteristics and tunable Curie temperature (Tc), which enable self-regulation of temperature within the therapeutic range, thus preventing potential damage to living cells. The synthesized Mn0.65Fe1.30Si0.37P0.65 compound exhibited a transition temperature of 316 K (43°C) with particle sizes ranging from 1-5 µm. A magnetocaloric wax-based composite was synthesized by integrating these particles with a wax component. Characterization under alternating magnetic fields (AMF) showed a significant temperature rise with higher magnetic concentrations and field amplitudes. At an applied AMF of 9 mT and a frequency of 244 kHz, a sample containing 15 vol.% magnetic particles stabilized at 44°C, effectively maintaining controlled temperatures within the therapeutic range (42-47°C) and efficiently melting the wax without harming living cells. Water absorption tests and SEM imaging further demonstrated the composite's low water permeability and minimal development of microcracks over time. These findings validate the potential of magnetocaloric materials with adjustable Curie temperatures for precise self-regulation of temperature in biomedical applications. The observed high heating efficiency and prevention of overheating offer promising opportunities for controlled thermal activation processes across various biomedical fields.

The tunable color-neutral photochromic properties of rare-earth oxyhydride films render them promising for applications in smart windows, memory devices, and photo-catalysts. The investigation of the microstructural and electronic structure changes during the photodarkening-bleaching cycles and their relationship with the optical properties deepens our understanding of the photochromic effect and the memory effect (slower bleaching rate upon cycling) in rare-earth oxyhydride films. This thesis explores the nature and evolution of metallic-like domains and vacancy defects during photodarkening and bleaching by in-situ illumination positron annihilation spectroscopy and spectroscopic ellipsometry…

In this thesis, the formation of ferrihydrite by the iron electrocoagulation (Fe-EC) technology was investigated, focusing on the impact of process conditions such as pH and oxygenation (Chapter 2), as well as the composition of the electrolyte (Chapter 3). The objective was to enhance the removal of contaminants by generating a rust phase (ferrihydrite) with maximum adsorption capacity. Moreover, the influence of local high pH regions on the cathode surface, due to the evolution of H2 and OH- ions, was further examined through recrystallization studies under highly alkaline conditions and in the presence of Fe(II) (Chapter 4).....

This thesis focuses on recycling resources while preserving water quality and availability. This concept is at the basis of a healthy and sustainable society yet works needs to be done. Water scarcity will be a growing challenge that humanity will have to face in the coming years, due to poor resource management and the climate change crisis. Waters cover 70 % of our planet, but only 3 % of it is freshwater, and only 1 % is easily accessible. Already more than 2 billion people live in water-stressed countries.
Moreover, in some ways, this thesis will show how there is a thin line between resources and waste, nutrients and pollutants, impurity and added value. This thin line is both defined by our everyday life choices, the name we give to things, and their related connotation....

In this report, a model is presented to alleviate some of the computational work that goes into the effort of finding the magnetic properties of magnetocaloric materials. The model utilizes an interior point optimization routine to solve for the minimal exchange energy configuration of a system, given the exchange interactions of the material. The model is tested against four materials (Ni, MnO, Fe\textsubscript{2}P and Mn\textsubscript{2}Sb). For Ni and MnO, the exchange interactions are also computed. Three iterations of the model are compared. The base model, which only considers exchange interactions inside a chosen supercell, the base model with the inclusion of boundary conditions, and the base model with boundary conditions and the addition of an algorithm to find optimal solutions. \\ The algorithm analyzes the found results by the optimization routine, and if the result is considered not properly symmetric, runs the optimization routine another time, from a symmetrical starting point obtained from the outcome of the previous run. \\ In all versions of the model, effectiveness (percent of runs that resulted in the optimal configuration) and average run times were recorded. Three initialization methods for the model were used, and also tested for their effectiveness. For the algorithm, a parameter $\gamma$ is introduced that changes the size of some of the moments for the new starting points. Six different values for $\gamma$ were tested for their effectiveness against a test set of suboptimal solutions.
The model with the addition of boundary conditions and the algorithm performed the best out of the three iterations of the model, with an effectiveness of 99.895\%, and an average run time ranging from 0.62 s for 2$\times$2$\times$2 Ni, to 94.64 s for 3$\times$3$\times$3 Fe\textsubscript{2}P, in the case of $\gamma = 0.3$. To conclude, the model with the inclusion of the boundary conditions and the algorithm proves to be a robust method to evaluate the magnetic configuration of a material, especially for smaller systems.

Magnetocaloric heat pumps are those types of heat pumpswhich use a solidMagnetoCaloricMaterial (MCM) in place of a refrigerant to provide the work for the heat pump. MCMs are materials whose temperature changes on the application or removal of a magnetic field. This effect is known as the MagnetoCaloric Effect (MCE). Magnetocaloric heat pumps have the potential in replacing the conventional vapour compression technology for Dutch dwellings, since they have minimum environmental impact due to the absence of the coolant fluids that can harm the environment. A model of the regenerator of the magnetocaloric heat pump is developed in this project having in mind the application of this type of heat pumps to Dutch dwellings. The magnetic field that is used in the development of the model is 0.875 T, with permanent magnets being the source of the magnetic field. A sensitivity analysis is carried out in order to obtain the methods to optimize the regenerator for improving the performance of the heat pump. The performance of the heat pump is measured through its average cooling capacity, average heating capacity, temperature span and Coefficient of Performance (COP) for heating. TheMCM used for the regenerators is Gd with a totalmass of 1.18 kg. The parameter that influences the performance of the heat pump the most is the porosity of the regenerator. The heat losses through the casing have the least impact on the performance of the regenerator. The frequency of the cycle also has a profound impact on the heating capacity and cooling capacity up to temperature spans of around 18 K, but it does not have such an impact on the heating COP. After increasing the frequency, decreasing the particle diameter and decreasing the porosity, the heating capacity and the temperature span are optimized. For a temperature span of 15 K, the heating capacity is 164.2W, and for a temperature span of 20 K, the heating capacity is 99.1W.

Fischer-Tropsch synthesis (FTS) is a catalytic reaction, which involves the production of liquid hydrocarbon fuel from synthesis gas obtained from natural gas, biomass or coal via gasification and steam reforming. From an industrial perspective, both Co and Fe based catalysts have been applied. However, Co-based catalysts are preferred in FTS particularly for gas-to-liquid (GTL) processes as they have high activity, high selectivity to linear hydrocarbons and low activity for the unwanted water-gas shift reaction. However, Co-based catalysts are relatively expensive and deactivate in time. To make the FTS process economically more effective, a stable performance of the catalyst is required. Therefore, studying the catalyst deactivation is an important topic in the development of better industrial catalysts. Oxidation, sintering of active phase and deposition of oxygenated compounds are potential causes for deactivation. The possible role of oxygenates and their effect of catalyst deactivation, however, is less understood. With the aim to investigate the deposition of oxygenated compounds, particularly carboxylates, as hypothetical deactivation mechanism, operando characterisation techniques were adapted to monitor the chemical and physical properties and structure-activity relationship of the catalyst during the reaction. Operando Diffuse Reflectance Infrared Fourier Transform (DRIFT) and Mössbauer emission spectroscopy setups were employed that can be operated at industrially relevant FTS conditions.

With green energy well on its way to becoming a necessity rather than a commodity it is vital to not only focus on improving the source of energy, but also the efficiency of the eventual energy usage. Vapor-compression based household refrigerators are one of the most used electrical appliances in the common household and consume electricity throughout the day, so finding a more efficient alternative would be beneficial. One such alternative uses the magnetocaloric effect as cooling mechanism and could theoretically reach a much higher efficiency. However, the search for the right material with the right characteristics for the commercial realization of this technology is still ongoing and this thesis aims to assist in making progress in said search. The focus of this thesis is on the substitution of manganese with nickel in Mn-Fe-P-Si compounds and how the magnetocaloric properties of these compounds hold up against similar compounds that had manganese substituted with cobalt as this lowered the Curie temperature and decreased the thermal hysteresis, which are favourable results. Unfortunately, there are a few factors such as price and criticality that motivated replacing cobalt with a close relative such as nickel in hope of achieving similar results. Measurements done with a superconducting quantum interference device (SQUID) indicated that the synthesized Mn-Fe-Ni-P-Si compounds have very similar magnetic properties to the Mn-Fe-Co-P-Si compounds. Differential scanning calorimetry (DSC) measurements showed that both compounds reduce thermal hysteresis similarly, but the nickel compounds are capable of lowering the curie temperature much more with increased doping compared to the cobalt compounds. X-ray powder diffraction (XRD) analysis proved that nickel is substituted in the proper place and that the decreasing Curie temperature is the result of a change in the ratio of the unit cell parameters a and c. Nickel thus showed to be an excellent replacement of cobalt to be used to substitute manganese with to lower the Curie temperature of the initial Mn-Fe-P-Si compound and reduce thermal hysteresis. More research could be done on further tuning the composition by for example increasing the silicon doping as this has shown to increase the Curie temperature rather than decrease it whilst also reducing the thermal hysteresis. Combined with the results from this thesis it could lead to a composition with more favourable magnetocaloric characteristics.

High efficiency, low cost, and long stability are three key factors for the wide application of photovoltaics (PV) which are currently intensively studied in order to meet the increasing global renewable energy demand. Currently, the PV market is mainly based on silicon. However, solar cells based on silicon may not be capable to meet the long-term global energy demand due to their relatively high costs and high energy required for the synthesis of silicon wafers, opening the door to conventional thin films (such as Cu(In1-xGax)Se2 (CIGS)) and innovative thin films (e.g. semiconductor quantum dots (QDs), based on PbS, PbSe, CdSe QDs). The advantages of QDs as solar cell materials are the low-temperature synthesis process, the tunable band gap via control of the composition and size, and the promise of physical mechanisms that may increase efficiency above the Shockley-Queisser limit, such as multiple exciton generation (MEG), in which more than one exciton is created from a single photon. However, the low efficiency, with a current laboratory record just above 10%, and the durability are still limitations on their widespread application in the PV market. It is very important to understand the surface structure and surface-ligand interactions in order to improve the efficiency and stability of QD solar cells. For CIGS solar cells, research-cell efficiencies have reached 22.6%, which is just below the efficiencies of Si-based solar cells. Besides, various deposition approaches have been developed that can supply high-efficiency, low-cost and large-area solar cell devices. However, it is still a challenge to guarantee long-term stability of CIGS modules. CIGS solar cells can be well protected by sealing into glass plates, but this in turn increases the manufacturing cost. Therefore, understanding of the degradation mechanism is necessary.
Positron techniques are powerful tools to study the surface composition of QDs and to determine the types of open space deficiencies in thin film materials. For QDs, previous studies provided indications that positrons can trap and annihilate at the surfaces of semiconductor QDs and can effectively probe the surface composition and electronic structure of colloidal semiconductor QDs. For CIGS, previous depth-sensitive positron experiments indicated the sensitivity of positrons to probe the types of vacancy-related defects in CIGS.

Solid-state phase transformations in steels cover a broad range of aspects. The underlying physics behind these phase transformations usually include nucleation, diffusion, lattice reconstruction and interactions between solutes and grain boundaries and interfaces. These features and the fact that the events take place at high temperatures, in the bulk, at time scales ranging from milliseconds to hours and at length scales ranging from atomic dimensions to millimeters make even the most widely studied phase transformation in steel, the austenite-ferrite phase transformation, only approximately understood and therefore an attractive topic for investigation. Quantitative data obtained by sophisticated physical characterization techniques in combination with supporting physical microstructural models addressing the relevant length and time scale are required to bring the field of ferrous physical metallurgy further.
This thesis focusses on two new approaches to orchestrate phase transformations in steels such that more physical insight is obtained or that new properties can be reached: (1) cyclic partial austenite-ferrite phase transformations that are designed to unravel the grain growth, and more specifically the interface mobility by avoiding concurrent nucleation of new phases. This topic is studied by computational studies and 3D neutron depolarization studies that are capable to in-situ monitor the ferrite grain size and fraction. (2) Self healing of creep damage by site selective precipitation of supersaturated iron-based alloys. A strong preference for precipitation at free creep cavity surfaces compared to that in the bulk can result in a filling of creep cavities and a significant extension of the creep life time. To make this self-healing mechanism applicable for creep-resistant steels, a search for an alternative healing agent for Au in Fe is to be executed and new design recipes need to be extracted on the basis of the experimental input from advanced characterization techniques such as electron microscopy and X-ray nanotomography.

The sustainable energy transition relies on energy storage technology. Crucial to meet the ever increasing energy storage demand is profound understanding of the governing processes. Yet, due to the inherent difficulty to study light ions with conventional techniques, limited methodology is available for operando monitoring of lithium ion batteries. A non-invasive and versatile alternative is Neutron Depth Profiling(NDP). This technique provides information on the spatial and temporal lithium concentration during (dis)charge. In this work NDP is used to shed light on key challenges for lithium ion batteries. The results provide detailed understanding of electrode parameters such as tortuosity and Li-ion transport. This allows to reduce the battery internal resistance or increase charging current, thereby reducing charge times. Post lithium ion battery technology relies on a reversible lithium-metal anode, this would enable batteries based on the conversion reaction of lithium with oxygen or sulfur. Furthermore, a lithium metal anode can double pack level energy density when employing current cathodes. Using NDP, we can monitor the lithium concentration profile as the material is plated. This allows to study the dependency with respect to current density, electrolyte composition and cycling history. Moreover NDP allows to follow lithium independent of oxidation state. Hence enabling to monitor battery failure originating from lithium polysulfide dissolution in the liquid electrolyte. The findings as presented rationalize electrode design towards high energy-dense, safe and low-cost Li-S batteries. The thesis concludes with a revolutionary concept based on a gas filled gridpix time projection chamber. A gridpix detector allows a 3D particle trace reconstruction. Hence a 3D spatial isotope specific, lithium-6, distribution is obtained. This technique would cater for a whole new range of topics to study.

Computational modeling is shaping the fundamental understanding of key thermodynamic and kinetic properties in batteries, the importance of which is undeniable for the implementation of next-generation batteries, mobile and large-scale applications (chapter 1). In the present thesis, we employ density functional theory (DFT) at the nanoscale and phase field modeling at the mesoscale (chapter 2) to study both state-of-the-art and novel battery chemistries...

In this thesis, the boundary conditions for the development of giant magnetocaloric materials are investigated. The magnetocaloric effect is found in magnetic materials, when they are subjected to an external magnetic field. This leads to abrupt magnetization changes that cause a temperature change in the material. Materials based on Fe2P show giant temperature changes around room temperature and are especially suited for cooling applications. This is due to the large magnetization change that can be realized with the application of a relatively small magnetic field around the ferro- to paramagnetic phase transition. This transition is of a first order, giving rise to latent heat and rise to
‘‘giant’’ effects. After earlier studies investigated the relation between microscopic and macroscopic properties of these materials, the focus of this thesis is on the electronic factors that play a role in the stability and phase transitions of these compounds. After all, when the mechanism behind these phase transitions is clear, is it easier to search for new materials that show similar phase transitions. Two strategies are possible: elucidating the mechanism of Fe2P-based materials or investigating materials that show similar phase transitions. The latter is described in the next paragraph...

Mn-Fe-P-Si compounds are, to date, the most promising materials for refrigeration and energy conversion applications due to the combination of giant magnetocaloric effect (GMCE) and low material cost. The nature of magneto-elastic transition was studied on both length- and time-scales using neutron diffraction, muSR and Mӧssbauer spectroscopy techniques for Mn-Fe-P-Si compounds. The revealed strong spin-lattice coupling is responsible for the different magnetic orders, the tunability of the phase transition, and the excellent magnetocaloric properties in these compounds. This thesis contributes to a better understanding of the spin-lattice coupling and phase-transition behavior for Mn-Fe-P-Si compounds.