The shift towards a carbon-neutral economy represents a considerable challenge in the search of sustainable carbon-sources. The electrolysis of CO2 presents an attractive, direct pathway to produce sustainable fuels using only CO2, water, and electricity. The rapid shift towards high reaction rates and industrially relevant process conditions in just a few short years has opened new research directions towards understanding the intricacy of the complex interfaces involved in this reaction. The two key enabling technologies: gas-diffusion electrodes and membrane electrode assemblies, have greatly increased efficiency metrics of CO2 electrolyzers, but many questions on the homogeneity and activity of these electrodes remain unanswered....

As a response to climate change, substantial efforts are being made to achieve global net-zero greenhouse gas emissions by the year of 2050, as established by the Paris agreement. To decrease reliance on fossil fuels, we are transitioning to renewable energies and electrifying various sectors. However, certain segments of the global supply chain will still require carbon-based chemicals and energy carriers. CO2 electrolysis allows the use renewable electricity to electrochemically reduce air-captured CO2, producing chemical building blocks such as CO, ethylene and formate. These chemicals can then be converted into larger hydrocarbons, e.g. into synthetic diesel using the Fischer-Tropsch process. In this way, CO2 electrolysis can aid in closing the carbon cycle by converting CO2 emissions into valuable chemicals and fuels. Despite its promise, a few hurdles still hamper the industrialization of CO2 electrolysis. These are the relatively low energy efficiency, salt deposition, the inefficient use of CO2 due to carbonate cross-over and the necessity of scarce iridium-based anode catalysts. Most of the mentioned challenges can potentially be solved by novel, or optimized ionexchange membranes (IEMs) - these have a pivotal role in the process since they provide a conductive medium to selectively transport ions between the electrodes. Increasing the IEM’s ionic conductivity and permselectivity can increase the energy efficiency of the process and decrease cation cross-over and therefore salt deposition. Furthermore, an OH– selective membrane which rejects other anions such as carbonate, can potentially solve the carbonate-cross over issue. In this way, the goal of this work is to develop novel IEMs to address each of the challenges tied with CO2 electrolysis...

The main objective of the research as described in this thesis was to enhance the piezo- and electrical properties of bismuth ferrite-based (BiFeO3 or BFO) piezoceramics at room and elevated temperatures, using both chemical and physical approaches.

Fossil resources are being depleted, while the concentration of CO2 in the atmosphere rises. There clearly is a strong and immediate need for a radical energy transition. Renewable energy is growing strongly as supplier of fossil-free electricity, but it needs large-scale and long-term storage options in order to be able to make a real impact on our total energy market. Petroleum is not only a feedstock for energy resources such as kerosene and gasoline, but also for many materials and chemicals that are being used in everyday life. A technology called electrochemical CO2 reduction (CO2R) can provide a solution to both these issues: renewably generated electricity is converted in any carbon-based energy containing molecule. The purpose (large-scale energy storage, mobile energy carrier or chemical) determines the molecule that should be produced. Electrochemical CO2 reduction is demonstrated at the lab scale. In order for it to become economically feasible and scalable to use in industry, additional research is needed to find better catalysts, understand the reactions and to improve cell design. The need for research includes a need for operando studies to investigate the exact physical and structural state of the catalyst under operating conditions. Also, relevant reaction intermediates cannot be studied without operando experiments. This thesis therefore focusses on the question how we can use existing operando characterisation techniques to study electrochemical systems. Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy is used to study reaction mechanisms on silver electrodes. A front-irradiation, surface-sensitive cell design for an electrochemical cell enabling operando X-ray absorption spectroscopy (XAS) studies on modified silver electrodes during CO2 reduction is presented. A guide on how to conduct operando XAS experiments on gas diffusion electrode-based CO2 reduction catalysts and its results are demonstrated. Oxygen and vanadium are studied with X-ray Raman scattering (XRS) in order to elucidate the electronic effect of photocharging on bismuth vanadate photoelectrodes for solar water splitting.

Increasing tensions over global warming, talks about a sustainable future and a huge imbalance in closure of the carbon cycle indicate a response for developing efficient conversion of CO2 and syngas obtained from renewable sources. Thermochemical conversion of carbon oxides (CO and CO2) in combination with hydrogen to produce methanol in the presence of catalyst provides a pathway to close this carbon cycle. Steady state activity tests were carried out in a small integral reactor for methanol synthesis from a mixture of either CO/H2 or CO2/H2. The temperature was varied from 200 to 300°C, while the total pressure was held constant for CO/H2 at 85 bar and CO2/H2 at 60 bar keeping stoichiometric flow of hydrogen at GHSV of 24,000 hr¡1. Four different metal oxides namely ZnO, ZrO2,MgO and CeO2 were investigated for support effects on active Cu along with different combinations among them while keeping commercial catalyst as the benchmark. Catalysts were prepared using urea hydrolysis method. It was found that ZrO2 and MgO show higher selectivity however the latter does not exhibit comparable conversion as the commercial catalyst for CO2 hydrogenation. Detailed GHSV study for Cu-ZrO2 paint a completely different picture showing higher methanol selectivity (64%) with increasing space velocity (at GHSV of 32,000 hr¡1). In case of COhydrogenation, commercial catalyst performs the best, albeit displaying signs of carbon deposition at higher temperature (280°, 300°C). This situation is circumvented by employing ZnO/MgO combination as a support. Cu-CeO2 exhibited characteristics of an excellent water gas shift catalyst. This led to a novel configuration of mixed bed consisting of Cu-CeO2 with commercial catalyst. Results indicate that this combination improves themethanol yield by atleast 30% as compared to commercial catalyst at a high GHSV of 24,000 hr¡1.

The Oxygen Evolution Reaction (OER) is usually the go-to reaction for water-based electrolysis processes. However, this reaction can impede the performance and large-scale implementation of such processes due to its sluggish kinetics and the lack of value of its product stream. In order to boost the feasibility of electrochemical processes, research is being conducted on alternative anodic reactions. Even though thermodynamically less favorable, the peroxide evolution reaction yields an added-value product. In this study, the industrial relevance of the anodic peroxide evolution reaction was investigated. The aim of this thesis was to assess the potential for industrial application of this reaction. To do so, the electrochemical parameters of influence and the mechanisms behind the latter were investigated.
The evaluation of both economic and technical feasibility of the process was achieved by following three main threads. Firstly, a gross margin model was introduced. This model allowed to define performance targets of the electrolytic process based on viability requirements. Secondly, electrochemical experiments were carried out. Materials fit for large scale implementation were to be identified. Tin oxide- based materials (SnO2, Sn3O4, ITO and FTO) were investigated due to the stability of tin oxide in a large pH window. Carbon-based materials (CFP, PTFE-coated CFP and GDE) were investigated for their high current density responses and high peroxide yield. Once these materials were identified, systematic studies on electrolyte effects were carried out. Finally, product characterization methods were investigated in order to understand the role of ions such as HCO3 – and CO32– in the enhanced electrochemical production of peroxide.

The experiments presented in this thesis have provided new insights regarding the photochromic properties of rare earth oxyhydrides (REOxHy). We discovered that thin films of rare-earth (Y, Dy, Er, Gd, Nd) oxyhydrides show unique photochromic properties under ambient conditions. We showed that by direct current reactive magnetron sputtering of rare earth metal targets in an Ar and H2 atmosphere a metallic dihydride thin films can be made. However, above a certain critical deposition pressure, a semiconductor REOxHy thin film is formed upon exposing the films to air. These films show photochromic properties. Compared to YOxHy, the optical bandgaps of the lanthanide-based oxyhydrides are smaller, while photochromic contrast and kinetics show large variation among different cations. The photon energy required to obtain a photochromic effect is given by the optical band gap of the material, as shown in the energy threshold measurements. Photon energies larger than the band gap are required to photo-darken the rare-earth oxyhydrides. The photochromic process is reversible and bleaching occurs through thermal bleaching and possibly through interaction with the light of longer wavelengths (optical bleaching). However, the latter needs to be confirmed with further experiments. We have shown for the first time also that semiconducting YH3 does not exhibit photochromic properties, revealing the importance of the presence of both oxide and hydride ions for the photochromic effect in rare-earth oxyhydrides.

A model is developed to simulate NaCl concentrations across a cation exchange membrane in the transient regime. The convectionless Nersnt-Planck equation is combined with the continuity equation to obtain a partial dierential equation in time and space. The boundary conditions at the edges of the cell for this equationare a constant bulk concentration. Furthermore, in presence of an applied potential, the potential gradient is assumed to be linear with dierent values for the gradient inside and outside the membrane. This dierent value inside the membrane is due to Donnan potentials that arise at the membrane/electrolyteinterfaces. On top of this, because of the Donnan exclusion, the concentrationsare not continuous on the interface. To account for this, the cell is splitinto three regions: the membrane and two electrolyte regions on either side of the membrane. The boundary conditions to tie these regions together, is that the ux should be continuous across the interface. The problem has the form of an initial value problem. The partial differential equation is discretized using a finite difference method and is integrated using the forward Euler method.The results show an accumulation of ions at the membrane interface closest to the positive electrode, while ions are depleted at the other interface. This accumulation of ions is too big however, with the Na+ concentration at one point reaching 12M after only 0.10 seconds, while the concentration was 1M atthe start. This is clearly not what one would expect to happen physically. Therewere, however, no errors found in the numerical method as it was possible tocorrectly deduce and predict concentration changes, based on the concentrations and ux in a certain time step. Therefore it appears that there is an error in the assumptions that were made when developing the model. It is believed that this error lies in the assumption that the potential gradients only has two different values. This assumption ignores the diffusion boundary layers on the outside of the membrane. In this diffusion boundary layer, the potential gradient has another, different value. Therefore, to obtain more accurate results, the cell should actually be split into five different regions: the membrane, two diffusion boundary layers and two electrolyte solutions.

In connection with attempts to increase the Solar Redox Flow Battery (SRFB) viability, an investigation was done into the charge carrier transport mechanisms entailed at the photoelectrode interlayer interfaces. On the basis of a preliminary study, the investigation focused on the development of a suitable conducting layer at the electrolyte/electrode interface as well as the development and control of a hole extraction layer, based on the device electronic structure and energy levels matching. Research was conducted into the properties of various prospective materials that could improve charge carrier transport at the electrolyte/photoelectrode interface. Once the optimal configuration is determined, the photoelectrode and redox couples are prepared and characterized and the photocharging performance of the device is optimized. Furthermore, the control of the formation conditions of a hole extraction layer took place, in order to yield improved internal charge carrier separation and transport. Various hole transport layers were assessed in SRFB system in relation to the photo-charging performance. The described process resulted in the development of a conducting layer through the growth of well distributed islands, which led to a high solar-to-chemical conversion of 9.4%. Through the control of the formation conditions of the hole extraction layer, better spatial separation of the photogenerated charge carriers was achieved, leading to improved device performance. Despite the enhanced viability of each device in terms of efficiency, further research into the long-term stability is essential for its practical implementation. This mainly relates to electrolyte degradation to form solid precipitates and photocorrosion of the absorber semiconductor materials in acidic conditions.

Our civilization is facing one of the biggest challenges in its entire history: the energy transition. The growing demand for energy, the scarcity of resources and climate change are the three main factors driving the change. Firstly, the world population is expected to keep on growing, and many poor countries are projected to experience rapid economic development in the near feature, which calls for the continued growth of global energy supply. Secondly, fossil fuels, the back-bone of the current energy-infrastructure, while still sufficient in quantity for the upcoming decades, are finite. In addition, the combustion of these fossil fuels is leading to global climate change, which is a great threat to humanity as we know it, in environmental, economical, political and sociological terms. Thus, a move away from relying on fossil fuels and towards renewable, clean and sustainable energy resources like the Sun, is both necessary and inevitable…

The global transition from fossil-based resources to renewable energy is critically important to address the sharply increasing threat of global climate change and to ensure long-term energy security. One attractive candidate to substitute for conventional fossil fuels is hydrogen. Hydrogen is an excellent energy carrier that can be directly converted into electricity via fuel cells, or be combined with carbon dioxide (CO2) or carbon monoxide (CO) to form high energy density synthetic fuels. Most of the current industrial methods for hydrogen production, however, is by steam reforming of natural gas, which releases CO2 as a by-product, making it environmentally unsustainable. Photoelectrochemical (PEC) water splitting, on the other hand, is a carbon-neutral approach that enables the conversion and storage of the abundant solar energy into hydrogen using only renewable and clean resources. This process uses semiconductors to capture and convert sunlight into photogenerated charge carriers (i.e., electrons and holes), and electrocatalysts to facilitate the multi-charge transfer process for the oxidation and reduction of water to oxygen and hydrogen, respectively.

Photochromic smart windows form a promising class of energy-saving technologies for the built environment. In recent years, sputter-deposited yttrium oxyhydride (YO_xH_y) coatings have been gaining attention due to their desirable photochromic properties such as colour-neutral darkening, high contrast, and apparent durability. For commercial application in smart windows several issues remain to be addressed, e.g their yellowish colour and bleaching time scales of the order of hours. Tailoring these photochromic properties is difficult because the mechanisms involved are largely unknown.

In this thesis, the influence of deposition pressure on photochromic contrast and film stability is linked to film microstructure to optimise the photochromic properties for smart window applications. Both contrast and stability are improved by obtaining a densest-possible microstructure. As a first step towards achieving colourless transparency, proof of concept is provided for the ability to tune the band gap by co-sputtering with dysprosium.

A metallic yttrium dihydride (YH_y) domain growth darkening model is introduced to describe the photochromic behaviour of YO_xH_y. The model appears to be consistent with observations on film porosity, contrast, kinetics, and irreversible darkening of YH_y. This might introduce possibilities to limiting bleaching times, as well as giving better insights into the photochromic mechanisms for YO_xH_y.