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.

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.

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.

Owing to global warming and fast depleting fossil reserves, the world is currently witnessing an energy transition into cleaner and greener energy sources like the wind and solar energy. At present, most of this clean energy is converted into electricity and used up directly or stored in the grids or batteries to be used later. Chemical storage of this energy is an attractive alternative due of its high energy density. Artificial photosynthesis is a hot field wherein researchers try to mimic the photosynthesis process of plants and try to convert solar energy into chemicals that can be stored. Photo-electrochemical (PEC) water splitting is one such pathway that is extensively investigated. However, the ideal semiconductor required to make this conversion efficient and economical is yet to be discovered. A big share of research also focuses on materials or techniques to improve the performance of these PEC devices independent of the semiconductor. Plasmonics, or plasmonic hot electron injection in particular, is such a technique that is being extensively investigated. Plasmonic hot electron injection is a relatively new phenomenon that could have potential application in many fields including photo-electrochemistry and photovoltaics. It is coveted as one of the techniques that could directly improve the performance efficiencies of solar and photo-electrochemical cells. But a fundamental understanding of this phenomenon is still lacking. This is crucial for improving and optimizing this technology before it can have any practical application. In this thesis, photo-electrochemistry is used as a tool to study the plasmonic hot electron injection and shed more light on the fundamental aspects, at the nanoscale, involved in the excitation and injection of hot electrons into the conduction band of a semiconductor. Silver, gold and 50% silver – 50% gold alloy nanoparticles are used in this work to systematically
study the effect of composition, size and concentration of these nanoparticles on the hot electron injection efficiency. Through this work, some fundamental understanding of the factors affecting the energy of the excited hot electron and hot electron injection process is obtained.