The electrochemical conversion of captured carbon dioxide (CO2) at low temperatures holds promise as a sustainable method for producing materials and fuel using renewable energy sources. However, technological hurdles such as mass transfer limitations and operational instability hinder its industrial application. This dissertation aims to address these challenges by exploring the use of gas-liquid Taylor flow (series of confined gaseous CO2 bubbles, which are separated from each other by liquid electrolyte and from the channel walls by a thin liquid film) in electrolysis, which can enhance mass transfer without requiring complex electrode designs, potentially improving long-term operational reliability. Additionally, a multi-scale modelling framework is introduced to evaluate electrolyser designs from an economic standpoint, aiding in the identification of bottlenecks and guiding technology development.
In Chapter 2, we propose a tubular electrolyser design operating under gas-liquid Taylor flow to overcome mass transfer limitations. By developing a numerical model, we investigate the relationship between process conditions, mass transfer, and reactor performance. Insights gained from this model allow us to derive an easy-to-use analytical relation to evaluate the impact of changes in inlet flow rates on Faradaic efficiency and current density. We find that long gaseous CO2 bubbles and low velocities enhance the current density towards CO, outperforming traditional H-cells. However, achieving performance comparable to flow-through electrolysers operated with a gas diffusion electrode (GDE) requires means to increase CO2 solubility in the liquid electrolyte, by for example increasing pressure.
Chapter 3 focuses on experimentally testing how Taylor flow influences the electrolyser performance within the established zero-gap water electrolyser concept adapted for CO2 reduction, by employing a silver gauze as the cathode. Our experimental findings reveal that Taylor flow enhances the Faradaic efficiency towards CO compared to single-phase flow, with minimal influence from gas holdup within the studied velocity range. Contrary to the tubular design, high velocities are desirable to increase the Faradaic efficiency towards CO in the rectangular flow channel. We find that further optimisation of
cathode design and fabrication is needed to fully exploit the potential of this electrolyser concept.
In Chapter 4, techno-economic aspects of electrochemical CO2 conversion are addressed, aiming to optimise operational parameters for industrial applications. A multiscale model capturing mass transfer effects over the channel length of a GDE electrolyser is integrated into an economic framework to analyse the interdependencies of key performance variables on the economic outlook. The analysis indicates that optimal current densities may differ significantly from previously reported benchmarks, emphasising the importance of multi–scale modelling for evaluating electrolyser designs under economic considerations.@en

Hydrogen is expected to play a vital role as an energy carrier in the future decarbonized system. Currently, a large portion of the hydrogen produced comes from the steam reforming of methane present in natural gas, which produces significant amounts of carbon dioxide emissions. However, with the growing need to reduce greenhouse gas emissions, a shift from fossil fuels to renewable energy sources is required. As a result, there is a growing emphasis on green hydrogen, i.e., hydrogen generated using renewable power. Green hydrogen has been growing at an exponential rate since 2020 and is expected to account for the majority of hydrogen production by 2050. However, a few studies have recently suggested that hydrogen may indirectly contribute to global warming. It is believed that hydrogen delays the decomposition of methane, a strong greenhouse gas, and thus extends its lifetime in the atmosphere. If green hydrogen is to be the primary fuel in the energy transition, hydrogen emissions from an electrolysis unit should be investigated.

This project focuses on identifying the sources of hydrogen emissions from an electrolysis unit. The goal is to comprehend the depth of this potential issue and investigate possible solutions. This project is carried out in collaboration with Worley, a market leader in the design, construction, and delivery of green H2 facilities. The leakage estimates for the green hydrogen alkaline electrolysis plant are based on Worley’s in-house data. Venting during startup and shutdowns when power is unavailable, as well as hydrogen crossover in the electrolyzer, have been identified as two major contributors to hydrogen emissions. Solutions such as flaring systems to combust the vented hydrogen and battery energy systems to reduce frequent shutdowns and startups are investigated. To reduce emissions from hydrogen crossover, a reactor is modeled to explore the catalytic recombination of hydrogen and oxygen. These solutions are subjected to a techno-economic analysis to determine their viability.

Flare systems and battery energy systems are both deemed feasible. In the long
run, however, installing a battery energy system would be preferable to combusting the hydrogen product. In comparison to other battery technologies such as Li-ion and lead-acid batteries, vanadium redox flow battery systems have been found to provide the maximum incentives and highest optimal capacities at the lowest overall costs. To avoid emissions from hydrogen crossover in a low-pressure alkaline electrolysis unit, the most cost-effective design involves a single-stage compression followed by a scrubber, heater, and reactor. However, this design is still costly because the annualized costs are four times greater than the costs offset by emissions reductions per year. Governments can encourage the adoption of such solutions by providing financial incentives to businesses.

As industrialization advances rapidly in pursuit of refining the quality of human life, there has been a release of organic micropollutants (OMPs) such as pharmaceuticals into various water reserves. This has endangered the ecosystem and the possibilities of water recovery. Commonly used tertiary wastewater treatment technologies have proven to, not under all conditions, be less effective in OMP removal. Hence, Advanced Oxidation Processes (AOPs) have gained attention in the recent past due to their production of reactive oxidative species (ROS) that unselectively degrade OMPs. Photocatalysis (PC), an AOP that uses solar radiation to produce ROS, has been investigated earlier, but shows low efficiency due to fast electron-hole recombination. Photoelectrocatalysis (PEC) is a modified version that additionally uses electrical energy, thereby reducing the recombination and improving the OMP removal efficiency. Bismuth vanadate (BiVO4) photocatalyst has gained importance due to its narrow band gap of 2.4 eV and hence, efficient absorption in visible light spectrum. Thus, this research focused on investigating PEC using BiVO4 electrodes for the removal of OMPs in aqueous solutions. The BiVO4 electrodes were fabricated using dip-coating from 1 to 5 layers, and electrodeposition at -0.2 V and -0.4 V, each at durations of 2, 5, 7, 10 and 15 minutes. They were then characterized using analytical techniques to investigate their structural, optical and optoelectronic properties. Subsequently, all the BiVO4 electrodes were used for the photoelectrocatalytic degradation of Acetaminophen (ACT). The electrodes fabricated by dip-coating were shown to achieve superior degradation efficiencies of ACT of over 99% in 5 hours, due to optimum surface morphology and band gap. It was seen that their varied surface structures played an important role in improving OMP degradation, and compensated for their low average quantum efficiency at 7%, as compared to that of electrodeposition at 14%. Next, the photoelectrocatalytic degradation of multiple OMPs in a solution was studied, and it was determined that although the ROS unselectively targeted all the OMPs, some were degraded quicker than others due to their chemical structure. Scavenging experiments were also carried out that affirmed the role played by ROS in the photoelectrocatalytic degradation of the OMPs. Eventually, the degradation of OMPs in secondary wastewater treatment effluent was conducted to test its usage in purification applications. Overall, PEC using BiVO4 electrodes was found to be feasible and successful in OMP degradation, and has the potential to be developed for usage in environmental remediation strategies like water treatment and recovery.

Microfluidic extraction is a promising alternative for existing extraction methods of radioisotopes due to the high surface-to-volume ratio. When assessing the compatibility of microfluidic material with the extraction system, there are three critical aspects—compatibility with organic solvents, compatibility with the radiochemistry, and, most importantly, the resistance to radiation-induced damage. Resistance to radiation-induced damage includes the primary reactions of the radiation with the matter, such as bond cleavage and the indirect damage caused by radiation due to free radicals. On the other hand, polymers are an interesting alternative as they are cheap and suitable for producing microfluidic chips. Therefore, in this experimental study, multiple interactions with polymers and quartz caused by radiation are assessed. In general, most radiation-induced modifications can be traced back to changes taking place in the structure of the material. Some of the changes have been attributed to the scission of the polymer chains, promotion of cross-linkages, breaking of covalent bonds, formation of carbon clusters, and liberation of volatile products. However, these materials must not lose their mechanical and chemical properties to maintain a well-functioning microfluidic extraction system. Atomic layer deposition (ALD) is an interesting surface modification method due to its self-limiting nature and atomic precision control. With ALD, it is possible to deposit a nano-scale layer of metal oxide that reduces surface-induced damage without changing much of its bulk properties. Furthermore, with ALD, it is possible to change the wettability of the material to make them suited for microfluidic liquid-liquid extraction. This study showed that a 40nm T iO2 thin-film was able to stabilize the surface modifications during high flux electron irradiation. A similar pattern is found between the samples coated with VALD as radiation-induced surface activation is achieved. Briding oxygen that usually is present on the surface gets replaced by carboxyl groups and increases the surface energy to increase the hydrophilicity of the surface. Furthermore, polycarbonate, quartz, and high-density polyethylene showed impressive radiation resistance up to 5MGy as Young’s modulus showed no significant difference. With this study, it is possible to use nano-coating to stabilize the radiation-induced surface activation and can be helpful for surface modifications in multiple fields.

Electrochemical reduction of CO2 using renewable energy sources is one of the promising avenues to pursue towards mitigating the emissions of the notorious CO2. However, the CO2 electrolysis in aqueous systems, due to the low solubility of CO2, are severely limited by mass transfer. State of the art review shows that a significant amount of the research is done to improve mass transfer, where a variety of electrolyser designs were studied. Despite the effort, the challenge to enhance mass transfer remains and is the focus of the present work. To improve mass transfer, an innovative concept is proposed - Taylor flow in an electrochemical flow cell. Taylor flow has been extensively studied in the literature, especially in micro channels and monolith reactors. Therefore the characteristics of the Taylor flow are known to a certain extent. But they were never tested in an electrochemical system. For that reason, a numerical investigation is carried out to assess the performance of Taylor flow on the flow cell. A simplified 2D model was formulated using a unit cell strategy and is verified based on experimental data and theoretical concepts. The effect of dissolving bubbles is also modelled using a quasi-steady-state analysis The results of the 2D model show a significant improvement in the performance, i.e. in the current densities of the electrochemical cell compared to a typical flow cell. The maximum calculated current densities increase by an order of magnitude under certain flow conditions. The dissolution studies showed that the current densities deteriorate with time. Nevertheless, the overall performance is still higher than the typical flow cell. Finally, based on the insights from the 2D model, a 1D model is suggested to estimate the current densities and dissolution rates. The present study showed promising results for using Taylor flow in an electrochemical cell. The proposed 2D model can help in aiding future modelling studies while the 1D model can give simple estimates for the experimental work.

Air pollution is a worldwide prevailing issue affecting the health of human beings in urban areas requiring urgent measures to adopted. This graduation project targets on adoption of passive air remediation techniques that can be widely applied into urban areas. Within the passive technical measures available, integrating greeneries on the infrastructure and photocatalytic materials prove promising to be incorporated in a broader perspective. Photocatalytic materials one-time installation, easy to maintain and self-cleaning properties has led to the selection of this material to quantify its air purifying capability.
TiO2 coating has been used in building materials for its self-cleaning property. However, its capability to break down NOx compounds has broadened its scope for pollution abatement. Though it can be applied as a coating, the performance of the photocatalyst in urban scenarios is affected by contact wind speeds and incident UV irradiation. Also, increased surface area provides more area of photo catalytically active sites. These factors are governed by the geometrical form of the panel. Hence, the main objective of this research is to design a façade panel with enlarged surface area and surface roughness to manipulate the environmental factors to favor photocatalysis. Also, its air purification effect is quantified to understand the effectiveness of this passive strategy and the design.
The façade concept has been designed following a series of strategies to respond to wind and irradiation. The concept with optimum surface enlargement and irradiation is developed into modular panels which are materialized by choosing the material with least environmental impact. The final part of this graduation project is about applying these panels in the street canyons, a controlled urban environment and evaluating the pollution abatement. The results indicate a pollution abatement of 3.5% - 8.9% in winter and 18% - 37.5% in percentage in summer for London.