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....@en

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.

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.

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.