The main goal of this thesis is to better understand the photochromic mechanism in rare-earth (RE) oxyhydride thin films. When illuminated, these materials have shown to darken, and bleach to a transparent state when the illumination source is removed. The RE oxyhydride thin films used in this thesis are deposited using magnetron sputtering and are YOxHy and GdOxHy films with different storage (air and vacuum) and oxidation (air and dry air) conditions. The films were probed using Doppler broadening positron annihilation spectroscopy, spectrophotometric (optical) measurements, X-ray diffraction and spectroscopic ellipsometry, measured before, during and after in or ex situ ultraviolet illumination.

Series of illumination and bleaching cycles have shown to permanently induce vacancy defects, which are most likely divacancies. The photochromic effect is seen as a reversible process, which leads us to believe that the changes seen in the defects are not directly responsible for the photochromic effect. During in-situ photodarkening and bleaching partially reversible changes are seen, whereby systematic shifts in the positron annihilation Doppler parameters are exhibited. This shift correlates with the photochromic effect, where the irreversible formation of vacancies is seen as a result of UV illumination. This leads us to the idea of the formation of metallic domains that may be attributed to H- ions that become mobile under illumination and may cluster to form metallic domains. Furthermore, the formation of the vacancy defects correlates with an increase in bleaching times seen from optical measurements, suggesting that they could play a role in the memory effect. Another optical property seen is a change in the band gap. We have seen that during photodarkening, the band gap tends to widen and decrease during bleaching. This phenomenon can be associated with the Burstein-Moss effect and can be used to estimate the photo-induced electron density in the conduction band. Despite the changes observed from positron annihilation techniques and optical measurements, no significant differences were observed by X-ray diffraction. The lattice constants of the film before and after illumination were shown to be constant.

There are two hypotheses explaining the photochromic mechanism: the segregation of metallic domains as a result of H- ion mobility and an Anderson-Mott insulator-to-metal transition as a result of overlapping electron wave functions. Using spectroscopic ellipsometry, these models were tested for YOxHy. It is shown that the mechanism leading to the darkening of the films is best described by the formation of Y or YHx metallic domains and voids. The percentage of metallic domains and voids grows with illumination time and starts stabilising after 90 minutes of illumination. The fraction of metallic domains (1.9-2.6% after 150 minutes of illumination) is similar in magnitude as seen in previous studies. Furthermore, the voids probed by ellipsometry are much larger than the vacancy-related defects probed by positrons. Such voids could give rise to the formation of positronium in the films, as could be examined in future positron annihilation lifetime spectroscopy (PALS) experiments.

Qualitatively GdOxHy and YOxHy behave similarly when it comes to optical measurements, XRD and DB-PAS data. For both systems, an increase in bleaching time as a function of illumination and bleaching cycles, a widening and shrinking of the band gap and a decrease in contrast are observed. However, there are quantitative differences between these films. The bleaching constant is shorter for GdOxHy and the contrast is higher, which makes it seem more attractive for potential applications. However, GdOxHy could not yet be satisfactorily modelled, in contrast to the case of YOxHy using spectroscopic ellipsometry. The GdOxHy film could not be described by a single homogeneous layer and therefore requires more complex models to describe the behaviour of the film in future work.

As hydrogen can make an important contribution in the energy transition, its detection is necessary for safe application. Optical hydrogen sensors are a promising alternative to conventional hydrogen sensors as they are smaller, less expensive and safer. These sensors show a change in optical transmission upon exposure to hydrogen, allowing the partial hydrogen pressure to be probed. The optical response and the structural properties of Pd capped TaRu thin films are studied. A sensing range of 7 orders of magnitude in partial hydrogen pressure is observed, with a significant change in optical transmission upon hydrogenation. Exposure to hydrogen results in a phase transition of the Pd capping layer, leading to the hysteresis in the optical response of the sensor. This phase transition can be suppressed by alloying the capping layer. A sub-second response time is observed for large hydrogen pressures, which can be improved significantly using an additional PTFE layer. Phase transitions of the TaRu layer are suppressed by the
nanoconfinement of the sensing layer, and a solid solution is formed. The sensing range of TaRu can tuned by alloying Ta with Ru, resulting in a shift of the sensing range to higher hydrogen pressures. Implementing the suggested improvements would result in a hydrogen sensor with a sub-second response time and a tuneable sensing range of 7 orders of magnitude free of any hysteresis, proving TaRu to be an effective material for hydrogen sensing.

This research is aimed at designing and optimising a fully renewable energy system for the GCC countries of the Arabian Peninsula for the year 2050 . This includes deriving the energy potential of open-field PV and wind technologies in the region. Apart from the electricity demand of the power sector, the hydrogen demand of the steel, ammonia and road&rail transport sectors is also considered, as well as the export of hydrogen in the liquid form. Different scenarios are considered for a better understanding of the operation of the system. Li-ion batteries and gaseous hydrogen vessels are employed to balance production and demand, while reconversion technologies are also part of the system together with powerlines and hydrogen pipelines. The study shows that the potential of onshore wind and solar energy is way greater than total energy demand, while offshore wind potential is of a comparable size to total energy demand. On average, solar energy is the cheapest source of electricity at 2.9 €c/kWh, followed by onshore wind at 4.9 €c/kWh and offshore wind at 9.9 €c/kWh. However, the minimum cost registered by solar and onshore wind technologies is almost the same at around 2.2 €c/kWh, while offshore turbines do not fall below 5.0 €c/kWh. In a cooperation scenario considering local electricity and hydrogen demand, total annual costs are equal to 130 €bln/year, 21% less than a similar scenario where countries are isolated from each other. Twice as much electricity is produced through wind turbines than PV farms under the cooperation scheme, whereas solar energy slightly dominates the electricity mix in the isolation scenario. Besides, the system relies more on curtailment and battery storage than reconversion. The coupling of the power and hydrogen sectors leads to a total costs reduction of 7%. The LCOE found for the system is of 6.1 €c/kWh, which is around 20% more than the current LCOE of modern gas turbines in the region. The LCOH of gaseous and liquid hydrogen at the port is of 2.5 and 3.0 €/kg respectively, which is 2-3 times more expensive than crude oil and several times more expensive than LNG based on current breakeven costs in the region.

The transition from stable fossil fuel energy production to intermittent renewable sources poses a huge challenge to our generation. The mismatch between electricity supply and demand will become an increasingly
pressing problem, due to the growing share of renewable energy production and the electrification of the automotive industry. The upcoming field of Solar Redox Flow Batteries (SRFB) proposes a possible solution in the search for feasible energy storage. Integrating a photovoltaic module with a redox flow battery results in a highly reversible storage system, which has both the advantage of flexibility in capacity and charge/discharge power, as well as the direct and efficient coupling to solar energy generation. However, due to its early stage in development, the SRFB has several challenges to overcome, like low charging efficiencies and energy densities. To improve SRFB efficiency, this work provides a performance estimation model, to identify general sensitivities of SRFB efficiency with respect to the loss mechanisms associated with the device, such as (but not limited to) kinetic overpotentials and resistances, to establish device design principles. This model is also extended to simulate realistic environments, using real (daily/seasonally dependent) solar radiation and ambient temperature data. Moreover, a fit tool is realized to identify losses in a specific system, by varying parameters and mimicking experimental linear scan voltammetry data. Experimentally, several conducting layers are tested for their kinetics and conductivity and impact on device performance. A thin film of platinum is found to result in the best kinetics and lowest resistance. However, carbon-coated electrodes also show promising results and could be a feasible and cheaper alternative to platinum, when applying layer-optimization. Based on the design principles derived from the modeled and experimental results, a single-junction siliconbased SRFB is developed, based on a ferri-/ferrocyanide electrolyte, coupled with Cu2+=+, yielding high solar-to-chemical charging efficiency of 9.4%. This efficiency is close to the simulated efficiency, while multiple other SRFB photo-charging experiments follow modeled trends, validating the model.

Since the industrial revolution the use of fossil fuels has increased exponentially, this increase caused a higher concentration of carbon dioxide in the atmosphere, which is responsible for climate change. To prevent a large-scale change of the earth’s climate humanity needs to switch to renewable energy sources. A large drawback of current renewable energy sources is that they almost all exclusively produce electricity which is hard to store for long periods of time. A possible solution to this energy storage issue is the electrochemical reduction of CO2 with electricity from renewable sources. Catalysts used for this process are transition metals but there are several problems that prevent large-scale implementation. Two fundamental problems with these catalysts are: Low activity causing low reaction rates, and low selectivity causing the formation of undesired products. A possible solution was proposed by Lim et al. (2014)1 where they suggested doping a silver electrode with sulphur to enhance the selectivity and activity for carbon monoxide production. This proposal was put the test in this thesis where silver was doped with sulphur by two different methods and were characterized by different techniques and tested at four different potentials. The method where a sample was immersed in a sulphur containing solution and subsequently annealed proofed to be the most effective way to dope a silver electrode. With the help of gas chromatography it was found out that at lower overpotentials the addition of sulphur is beneficial to its performance to produce CO but at higher potentials this advantage is lost. However if the results of this thesis are compared to literature it is clear that doping with sulphur is not effective. Furthermore it was seen that the sulphur doped Ag electrodes were not very stable and therefore not suited for large-scale implementation.

A comparative study of three different membrane electrode assemblies for electrochemical CO2 reduction is performed in a flowcell setup utilizing gaseous CO2 as a reagent. The membrane electrode assembly consists of a bipolar or exchange membrane, an Ag-mesh catalyst and a gas diffusion layer. The counterreaction occurring on the opposite of the electrode assembly is oxygen evolution performed in a 1 M KOH anolyte. While using a fixed cell potential of 2.5 V, it was found that the bipolar membrane configuration produces 4 to 6 times more current versus an anion exchange membrane assembly due to the low local pH enabled at the cathode due to the bipolar membrane. This difference comparatively benefitted both CO2 reduction and the competing H2 evolution. It was further found that the surrounding infrastructure, namely the titanium sample holder, was almost exclusively performing H2 evolution, both inhibiting reactions on the Ag-mesh and preventing the formed products from being transported to the cathodic gas phase. Instead, all formed products were detected in the anolyte compartment. To circumvent these issues the sample holder was removed, and ionomer solution was applied to the Ag-mesh increase conductivity between the bipolar membrane and the Ag catalyst. During operation the membrane ultimately delaminated due to internal gas accumulation. The main conclusions from this work are that the examined membrane electrode assemblies using Ag-mesh as a cathode are unable to perform CO2 reduction while the Ag-mesh is located too far from the gas diffusion layer. Even with layer compression between all components, only trace amounts of CO were produced, implying that contact between the CO2 reduction catalyst and the CO2 source was not sufficiently present. Membrane electrode assemblies might then be improved by utilising a layer of conductive liquid between the membrane and Ag-mesh. In an alternative approach a gas diffusion electrode was produced through the sputtering of Ag directly onto the gas diffusion layer. Even without optimisation this system outperformed all produced membrane electrode assemblies in terms of CO2 reduction.

The worlds energy demand is increasing however, the majority of the energy supply is still based on fossil fuels. The consumption of fossil fuels results in the emission of carbon dioxide which is one of the main greenhouse gases responsible for the ongoing climate change. In order to reduce the carbon dioxide emissions, more and more energy is produced in a re-newable way. Mainly from solar and wind energy. This energy is however intermittent in char-acter and comes in the form of electricity. To overcome the intermittency, the excess pro-duced electricity has to be stored in an efficient reversible way, so that the energy can re-leased again when it is needed. One possible solution that tackles both problems is the elec-trochemical reduction of carbon dioxide in aqueous electrolytes into highly valuable chemicals, or so called solar fuels. In this study, a new infrared spectroelectrochemical cell with an easy exchangeable electrocatalyst is developed to study the reaction mechanism on a silver cata-lyst in 0.1 M KCl and 0.1 M KHCO3 electrolytes at applied potentials of -1.4, -1.6 and -1.8 V vs a Ag/AgCl reference electrode. The performance of the new developed cell is compared to the already proven ATR cell configuration. Investigation of the infrared signal strength showed high infrared signals at angles of incidence of the infrared beam between 30° and 45°. The measured infrared absorption spectra in the thin layer flow cell show three absorption bands which are also present in the obtained infrared absorption spectra of the ATR cell. In the ATR spectra however, three further infrared absorption bands can be observed. Due to the lower number of absorption bands present in the FTIR spectra measured in the new designed thin layer flow cell, it can be concluded that despite the high IR signal no new insights on the reac-tion mechanism of the carbon dioxide reduction reaction can be acquired.

An exponential growth in CO2 concentration over the past few decades has led to an accelerated impact of climate change on planet earth. In a bid to curb these emissions, people across the globe are slowly transitioning towards renewable energy sources with battery technology aiding this growth. Given that battery technology is still in its nascent stage, the “Electrochemical reduction of CO2” could be a viable solution supporting it without decelerating the momentum gained towards renewable development. Although plausible, the direct reduction of CO2 to liquid fuels entails huge energy expenditure thus requiring the implementation of catalysts. Unique in its ability, palladium reversibly reduces CO2 to formic acid making it an interesting candidate for the reduction reaction. In addition to the production of formic acid, palladium is also know to produce carbon monoxide (CO) which completely deactivates the surface preventing further reactions from occurring.

Thus the aim of the current study is focused on analysing the electrochemical reduction of CO2 on palladium thin films using surface enhanced infrared absorption spectroscopy to better understand the deactivation mechanisms of CO on the palladium thin film. The smoothness of the as- sputtered 15 nm palladium thin film with a RMS roughness of 0.511 nm and partially coalesced islands were ascertained, thus requiring surface activation to introduce the enhancement mechanism. Experimental analysis of CO2 reduction on the palladium thin film was performed to unearth significant insights through the combination of electrochemical analysis techniques with surface enhanced infrared absorption spectroscopy. Results obtained through implementation of these methodologies provided substantial information not only on the influence of the palladium-hydrogen system on the electrochemical reduction of CO2 but also on the impact of alkali metal cations on the palladium-hydrogen system and the CO2 reduction reaction over the sputtered palladium thin film. CO formation, accumulation and desorption coupled with hydrogen evolution and desorption were some of the few avenues that were enumerated upon during the experimental investigation. The identity of CO chemisorbed on the palladium thin film along with bicarbonate direct/ indirect reduction to form CO was confirmed through the utilization of N2 saturated C13 NaHCO3 solution. In addition to the analysis of the reduction reaction, emphasis on the oxidation of CO was also provided suggesting the formation of dense CO structures with the existence of strong CO dipole – dipole coupling on the palladium surface.