This thesis addresses the critical issue of gas crossover in small gap alkaline electrolysers, significantly impacting the safety of hydrogen production systems. The research aims to optimize electrolysis cell design by investigating how bubble coverage, gap size, and electrolyte flow influence hydrogen crossover, particularly for integration with renewable energy sources. Computational simulations using Ansys Fluent revealed that higher bubble coverage and controlled electrolyte flow significantly reduce crossover rates, ensuring safe Hydrogen-to-Oxygen levels. The study highlights the delicate balance required between minimizing gap size and managing crossover risks. Additionally, the research validates numerical results with an analytical model, providing a robust predictive tool for practical applications. The findings offer valuable insights and practical recommendations for the design and operation of alkaline electrolysis systems, enhancing their safety for industrial applications. This work supports global efforts to reduce greenhouse gas emissions, facilitate the transition to renewable energy sources, and advance the hydrogen industry, which is pivotal in the evolving energy landscape.

Hydrogen is used in a variety of industrial applications and can function as a green energy carrier, if produced sus-tainably. Alkaline water electrolysis holds great promise as a production method for green hydrogen, potentially playing an important role in the energy transition. The performance of this technology depends significantly on its electrical efficiency. In some industrial-scale green hydrogen plants, multiple electrolysis cells are coupled together in series or in parallel to form a stack. These electrolyser stacks are being fed with a liquid electrolyte, often a KOH solution, which acts as a good conductor for ions to move between electrodes. The channels through which the electrolyte and gas products are transported in and out of the stack are usually connected to each other via manifolds. Electrolyzer stacks frequently encounter a problem known as shunt current, which is alternatively described as leakage, bypass, or parasitic current in various studies through these channels and manifolds. Math-ematically describing the magnitude and nature of these shunt currents has been the topic of a variety of studies. Being able to adequately measure and quantify shunt currents in an actual stack remains a challenge. This study aims to measure and quantify shunt currents in a novel electrolyser stack design by the employment of hydrogen reference electrodes, copper or silver pseudoreference electrodes, and a magnetic current clamp. Numerous ex-perimental findings have been coupled with mathematical models, imaging and theoretical expectations offering detailed insights in the behaviour of shunt currents with varying external factors. The variable parameters in this shunt current research are the applied current density to the stack and the applied liquid flow rate, by an external pump. The performance of hydrogen reference electrodes, copper and silver wires separately, to measure potential dif-ferences in an electric field generated in the highly alkaline environment of a 6M KOH solution was validated. It was found that the hydrogen reference electrodes functioned accurately and stable, giving conductivity results of the electrolyte 7.5 % above measurements performed with a conductivity probe. The copper wires functioned less predictable and stable, giving values 16 % above the validated value, with a larger spread and less reproducibility. The silver wires showed great potential, providing a value of 3 % above the validated value, but showed less stability in measuring constant potential differences. The plain copper and silver wires only functioned for short term measurements, where a potential difference between a baseline potential was the only predictable outcome. Both lacking a stable redox potential, they were found not suitable for accurately measuring potential differences inside the stack. The hydrogen reference electrodes proved to be useful in quantifying average manifold shunt currents, however leaving uncertainties as to total shunt currents in the experimental set-up used in this research. The magnetic current clamp was used to measure the current running through the external wiring between cells, from which the shunt current could be inferred. These measurements showed much potential in quantifying total shunt currents, but showed a large standard deviation between measurements due to the instantaneous nature of the measure-ments, alongside unpredictable electrode connections interfering with the outcomes...

This study aims to relate the size, number density, and surface coverage of bubbles adhering to a vertical gas-evolving electrode to the current density and shear rate of alkaline electrolyte flow.
A force balance that includes the known forces on the bubbles is formulated. This force balance is made to predict the release radius of the adhered bubbles. Because the electric and Marangoni forces are still an open topic of research, they are approximated using a constant. Also, because the influence of the drag force on bubbles in alkaline water electrolysis is not exactly known, it is approximated using the Stokes drag force with a multiplicative fit factor. An equation for this force balance model that approximates the release radius of the bubbles is formulated. To know the exact value of the shear rate on the electrode, this study uses laminar, fully developed flow. Equations are formulated to relate the fully developed laminar flow to the shear rate.
The bubbles adhered to the electrode are visualized using a high-speed camera. The camera films through a PMMA rod that goes through the first electrode, allowing visualization of the bubble-covered surface of the second electrode. A long stainless steel rectangular duct develops the Laminar flow before it arrives at the electrode.
The experiment is to relate the bubble size, bubble number density, and surface coverage to the shear rate due to forced flow and the current density. Increasing the shear rate has the biggest effect on the largest bubbles adhered to the surface. The maximal radius reduces, while, for the range in shear rate used in this study, the median stays almost constant. The bigger bubbles will be replaced by smaller bubbles. This will reduce the surface coverage and increase the bubble number density. Increasing the current density, for the range in current density used in this study, means an increase in bubble interactions. This means an increase in bubble coalescence, collisions between bubbles, large bubbles being pushed off the surface by smaller ones behind them, and the increased shear rate from natural convection caused by moving bubbles. These increased bubble interactions will result in a reduction in bubble sizes, surface coverage and an increase in bubble number density.
The force balance model is related to the maximal radius of the bubbles. The constant of the electric and Marangoni forces is negative at the anode and positive at the cathode, which is expected because the forces should be attractive at the anode and repulsive at the cathode. The multiplicative fit factor of the drag force due to forced flow indicates a higher force than the Stokes drag force because the flow is not perfectly homogeneous and will flow through other bubbles, which can accelerate the local flow velocities. Also, bubbles will collide at a higher speed with increased forced flow. The multiplicative fit factor of the drag force due to natural convection indicates a higher force than the Stokes drag force because of the increased bubble interactions with increasing current density. Both the higher multiplicative fit factors are observed at the anode and cathode.

This study aims to relate the size, number density, and surface coverage of bubbles adhering to a vertical gas-evolving electrode to the current density and shear rate of alkaline electrolyte flow. A force balance that includes the known forces on the bubbles is formulated. This force balance is made to predict the release radius of the adhered bubbles. Because the electric and Marangoni forces are still an open topic of research, they are approximated using a constant. Also, because the influence of the drag force on bubbles in alkaline water electrolysis is not exactly known, it is approximated using the Stokes drag force with a multiplicative fit factor. An equation for this force balance model that approximates the release radius of the bubbles is formulated. To know the exact value of the shear rate on the electrode, this study uses laminar, fully developed flow. Equations are formulated to relate the fully developed laminar flow to the shear rate. The bubbles adhered to the electrode are visualized using a high-speed camera. The camera films through a PMMA rod that goes through the first electrode, allowing visualization of the bubble-covered surface of the second electrode. A long stainless steel rectangular duct develops the Laminar flow before it arrives at the electrode. The experiment is to relate the bubble size, bubble number density, and surface coverage to the shear rate due to forced flow and the current density. Increasing the shear rate has the biggest effect on the largest bubbles adhered to the surface. The maximal radius reduces, while, for the range in shear rate used in this study, the median stays almost constant. The bigger bubbles will be replaced by smaller bubbles. This will reduce the surface coverage and increase the bubble number density. Increasing the current density, for the range in current density used in this study, means an increase in bubble interactions. This means an increase in bubble coalescence, collisions between bubbles, large bubbles being pushed off the surface by smaller ones behind them, and the increased shear rate from natural convection caused by moving bubbles. These increased bubble interactions will result in a reduction in bubble sizes, surface coverage and an increase in bubble number density. The force balance model is related to the maximal radius of the bubbles. The constant of the electric and Marangoni forces is negative at the anode and positive at the cathode, which is expected because the forces should be attractive at the anode and repulsive at the cathode. The multiplicative fit factor of the drag force due to forced flow indicates a higher force than the Stokes drag force because the flow is not perfectly homogeneous and will flow through other bubbles, which can accelerate the local flow velocities. Also, bubbles will collide at a higher speed with increased forced flow. The multiplicative fit factor of the drag force due to natural convection indicates a higher force than the Stokes drag force because of the increased bubble interactions with increasing current density. Both the higher multiplicative fit factors are observed at the anode and cathode.

The global climate crisis and rising greenhouse gas emissions highlight the urgent need to shift from fossil fuels to sustainable energy solutions. Hydrogen is increasingly recognized as a crucial tool for decarbonization, with significant potential to drive the transition to clean energy. This study examines hydrogen production via water electrolysis, focusing on forced-flow alkaline water electrolysers (AWE). It investigates how factors such as current density (j), height above the electrode (z), and superficial liquid velocity (w_l) influence velocity profiles, turbulence, and bubble dynamics within the electrolyser.

The results demonstrate that variations in current density j and height z strongly affect the velocity profile w(x), with higher values promoting buoyancy-driven flows and accelerating the liquid, especially near the electrode where gas flux is highest. These effects are more pronounced on the hydrogen (H2) side, where increased gas production generates larger velocities and broader plume dispersion. In contrast, higher superficial liquid velocity w_l shifts the flow regime toward forced convection, attenuating bubble plume effects and reducing the contrast between plume and bulk flow.

Turbulence intensity I(x) also depends on j, z, and w_l, increasing with higher j or z due to intensified bubble interactions and buoyancy effects, particularly near the hydrogen electrode. However, increased w_l reduces turbulence in buoyancy-dominated regions by structuring the plume. The model aligns well with experimental data, especially when fitting velocity profiles to a step-function regime for natural convection, achieving scaling exponents consistent with theoretical predictions.

This study identifies the Richardson number (Ri) as a valuable dimensionless measure of convection type, with a transition from natural to mixed convection observed around Ri = 1. Ri unifies the effects of j, z, and w_l on velocity distribution, offering enhanced design flexibility for electrolysers. Bubble-induced turbulence and wakes caused by large bubbles are the primary drivers of dispersion within the electrolyser, accounting for over 98% of total observed dispersion. This finding underscores the dominant role of turbulent dispersion over hydrodynamic or shear-induced effects in this forced-flow system.

Detached bubbles significantly enhance liquid and gas flow through buoyancy-driven effects and turbulence. Optimizing parameters such as j, z, w_l, and Ri can improve electrolyser performance, paving the way for more efficient hydrogen production technologies. These advancements are critical for accelerating the transition to a low-carbon future.

Alkaline water electrolysis emerges as a promising technology for green hydrogen production, playing a significant role in global decarbonization. Nickel-based electrodes are widely used in alkaline water electrolysis due to their excellent catalytic properties for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). However, nickel electrodes often experience a decrease in activity over time. Attempts of existing literature to investigate nickel deactivation employ conditions that differ from industry standards. Therefore, a more profound understanding of these phenomena under industry-relevant conditions is crucial for averting specific degradation pathways in future electrode design. This thesis investigates the deactivation phenomena and the timescales of untreated nickel electrodes during the OER and HER at 323 K and 30 wt% KOH, employing a current density of 1 A cm-2. The electrolyte concentration and current density match industry standards, and the temperature aligns more with industrial practices than literature. The OER overpotential increases under these conditions in 2.5 hours by 0.19 V and still increases after this period. The HER overpotential increases by 0.65 V and stabilizes after 20 minutes. Electrochemical and surface analytical tests suggest that both reactions primarily deactivate due to a redox reaction.

In this research project, multiphase behaviour in zero gap alkaline water electrolyzers is investigated, focusing on the influence of various parameters including current density, electrode-wall distance and electrode height. This involves a cell containing a vertical electrode pair separated by a membrane submerged in an electrolyte that has a liquid-air interface near the top of the electrodes. The study aims to identify and understand turbulent phenomena through different analysis methods. Additionally, the outcomes derived from the conducted experiments serve as validation data for concurrent modeling research.
The analysis begins by examining vortex formation on the cathodic side, where hydrogen bubbles originate, using Particle Image Velocimetry (PIV). The results indicate the presence of bubble vortices, especially near the top of the channel. At electrode-wall distances of 1.0 cm or more, the bubbles that fail to reach the liquid-air interface tend to circulate extensively within the channel without exhibiting any noticeable discrepancies, such as small vortices. It should be noted that some low-velocity vortices, typically on the order of millimeters per second, can be observed within the bulk of the channel. As a general rule, shorter electrode-wall distances lead to increased vortex formation throughout the entire channel. Additionally, higher current densities induce a greater amount of these vortices or chaotic and irregular motion of the bubbles in the bulk region. In modern configurations, elastic elements, such as materials of thin wires with high porosity and low solid volume fraction, are strategically incorporated into the electrolyzer design to enhance mechanical resilience and accommodate variations in pressure and temperature. These elements serve to ensure the stability and reliable performance of the electrolyzer system under diverse operational conditions. Much of the chaotic behaviour of the bubbles seems to be counteracted when the electrode-wall distance is large (1.5 cm) while an elastic element is placed in between the cathode and opposing wall. When the whole channel is filled by the elastic element (0.6 cm electrode-wall distance), this results in more small-velocity vortices than without the element.
Another examined phenomenon is the presence of a bubble mist. This mist arises when the bubbles that originate at the electrode do not escape the channel at the liquid-air interface, but recirculate towards the opposing wall and downwards from there. Light intensity coming from the bubble clouds is used to gain and calibrate results. Increasing the current density and decreasing the electrode-wall distance lead to deeper bubble mist plumes. Surprisingly, elevated electrolyte height directly influences mist depth, with larger electrolyte heights showing the highest mist depths, even when dividing these by the exposed electrode height. For these measurements, diamond shaped apertures on the electrodes are used. For another series of experiments where higher current densities are examined, slotted apertures are used and the effect of lowering the electrode-wall distance on the bubble mist depth is diminished. Whether this is due to the type of electrodes or the different measurement circumstances (for example lighting) can not be concluded from the results. This emphasizes the need for consistent experimental conditions.
Thirdly, the continuous release and upward movement of gas bubbles originating at the cathode, i.e. the bubble plumes, are investigated. The effect of changing the current density is investigated along different heights of the electrodes. The analysis performed employs three different strategies to assess turbulent behaviour. This includes the PIV method used initially to depict vortices, the light intensity coming from the bubble clouds and visual analysis to find discrepancies like bubble bursts and big bubble trajectories. The PIV method shows capabilities in tracking the plume width and depicting turbulent behaviour. The light intensity analysis however proves challenging due to disparities in lighting. Improved lighting and higher-resolution cameras could yield more reliable results, enabling further investigation into correlations between parameters and turbulence, such as bubble bursts.

The increasing awareness and urgency of climate change have led to an increase in investments and research into power sources not reliant on fossil fuels. Proton exchange membrane (PEM) fuel cells are a promising technology for automotive, maritime, and auxiliary power applications converting chemical energy into electricity. In order for this upcoming technology to compete with well-established alternatives such as diesel generators and combustion engines, it is of vital importance to improve the power density and durability.

PEM fuel cells produce water and heat during operation. The presence of superfluous liquid water in the fuel cell stack gives rise to flooding of the electrodes, which hampers operation and induces degradation processes. On the other hand, it is of great importance to maintain a high membrane humidification to reduce Ohmic losses over the membranes and avoid the formation of cracks. Therefore, water management plays a vital role both in maintaining the power density and guaranteeing durability.

This thesis is written under the auspices of both TU Delft and PowerCell Group, a manufacturer of PEM fuel cell systems located in Gothenburg, Sweden. Currently, a substantial amount of water condenses in the anode subsystem of PowerCell’s PEM fuel cells in certain operating ranges which subsequently enters the fuel cell stack. This study identifies the influence of certain system operating parameters on responses as the water crossover through the membranes, the relative humidity at the stack inlet, the temperature at the inlet of the stack, the condensation rate in the mixing chamber of the recirculation loop, and the mass flow rates of liquid water and water vapor in and out of the stack. Multiphysics simulation software provided by Gamma technologies is used to simulate 5600 operating points in which the system operating parameters are varied according to the Latin Hypercube sampling method. These simulations give a clear overview of the influence of the operating parameters on the aforementioned responses over the entire operating range of PowerCell’s PS-100 system.

The simulated experiments are subsequently used as a basis to construct metamodels. These metamodels predict the behavior of the system based on the operating parameters varied in the 5600 simulations. The metamodels are constructed both as Krigings and as multilayer perceptrons (MLPs). Kriging is a statistical method which produces an output for a certain response based on known input data where input points which resemble the unknown point are given a greater weight. MLPs are neural networks which recognise patterns in input data and use those to predict an output for a certain response. Finally, the operating parameters are optimized using the metamodels to minimize the liquid water mass flow rate into the stack, to prevent condensation in the mixing chamber of the anode subsystem, and to target a certain inlet relative humidity of the hydrogen feed to the stack. Concluding from these optimizations it would be beneficial to preheat the hydrogen before it reaches the mixing chamber. A number of alternative designs for the anode loop are proposed which require further investigation.

In an effort to make water electrolysis more efficient and simple, flow-through electrolyzers eliminate the need for a gas-separating membrane between the electrodes. However, replacing the membrane with a forced flow field brings new challenges and considerations. In this research, the design of a flow-through electrolyzer is optimized for minimum voltage losses due to bubble formation, pressure drop, and electrolytic resistance. These parameters are all influenced by the potassium hydroxide concentration, the inter-electrode gap and the feed flow rate. The effect on the power dissipation of a flow-through electrolyzer is analytically modelled and validated using experiments, where the design variables are varied.

Additionally, the effect on the resistance in the electrolyte of keeping the gasses dissolved in the electrolyte is studied using IV curves of degassed electrolyte. The gasses in the electrolyte were purged with a vacuum chamber. This degassed electrolyte showed a reduction in overpotential of 150 mV at max. However, this reduction in overpotential was outweighed by the energy requirements to degas the electrolyte. Besides degassing, the effect of suppressing bubble formation by different flow rates was investigated. It was found that there was no noticeable reduction in overpotential between the state where bubbles are suppressed and bubbles were formed.

In addition to the effect of keeping the gasses dissolved, an analytical model was constructed to describe the necessary flow rates to mitigate the electrical resistance due to bubbles in the electrolyte. In the analytical model, solubility plays a big role in determining the necessary flow rate, as solubility is related to the emergence of bubbles. Contrarily, experiments showed the effect of solubility was found to be rather low. By varying the gap width, it was shown that the shear rate at the wall is a better indicator for bubble removal and therefore reduction in resistance due to bubbles.

Furthermore, the shape of the discharge channel was changed to promote uniform flow across the electrodes. The uniform flow should make the product removal at every part of the electrolyzer equal, such that there are no stagnant zones where gas accumulation could build up. This new discharge channel shape was analysed using COMSOL Multiphysics by comparing it with a conventional straight discharge channel. The variable discharge channel outperformed the straight discharge channel in creating a uniform flow across the electrodes for Euler numbers bigger than 10, meaning that the inertial forces are negligible compared to the pressure drop. During experiments, the electrolyzer with a variable discharge channel was tested. This electrolyzer configuration did not perform as well as expected from the theory and simulations, having a higher pressure drop and electric resistance than the conventional electrolyzer. The reason why the variable discharge channel performed poorly was inconclusive.

Lastly, the performance of the various variables was evaluated using the total power dissipation as a function of the current density. This took both the pressure drop across the system and the electrical power consumption of the electrolyzer into account. It was found that using a high electrolyte of 6M potassium hydroxide (KOH) together with a small inter-electrode gap gave the lowest energy dissipation per kg produced hydrogen gas. However, increasing the KOH concentration increases the viscosity and thereby the pressure drop. The theoretical optimum for KOH concentration was calculated to be 5M for this flow-through electrolyzer.

This master thesis introduces a recently surfaced method to produce hydrogen and oxygen using alkaline water electrolysis. A hydrophilic porous diaphragm is used that can feed electrolyte laterally to two porous electrodes that are placed against it. Consequently, hydrogen and oxygen are produced directly at the electrodes without any bubble formation. More specifically, this master thesis aims to investigate the thermal behaviour of such a capillary-fed electrolyzer and the limits to which capillary-fed electrolysis can be sustained.
A small scale capillary-fed cell and a larger segmented cell are manufactured during the project. Both cells are used to perform experiments at constant current. Capillary rise in diaphragms with and without compression deviate from the Lucas-Washburn model for capillary rise. Particularly the polyethersulfone material outperforms the diaphragms that were studied. It follows the Lucas-Washburn equation relatively well and exhibits two times deeper penetration under compression. Furthermore, experiments have been performed studying the relation between electrolytic concentration and capillary wicking. Although more highly concentrated KOH solutions for electrolytes increase conductivity over time, it has shown that for the utilised diaphragm a KOH solution surpassing 3 moles per litre shows problematic capillarity. Contact angle experiments with 6M KOH have shown a hydrophobic contact angle of ̄Θ=78.14° compared to complete imbibition, i.e Θ = 0°, for 1,2 and 3M KOH solutions. The origin of these extremely large contact angles on the polyethersulfone substrate is not fully understood. 
The study has culminated in performing experiments at constant current and low concentration KOH utilising flushing and dilution techniques to prevent precipitation at the electrodes. The small cell reached an overall heat transfer coefficient of h = 121 W/m²K, showing very large heat production for a relatively small surface area. For the segmented cell, electrolyte supply with a bottom-up feed resulted in h = 19.13 W/m²K in which precipitation occurred. The top-down feed showed that the top segment has the largest areal heat dissipation. The heat transfer coefficient equates to h = 25.73 W/m²K. A final enhancement has been made by introducing Nickel Felt Fiber conducting layers into the cell to improve electronic conductivity. This resulted in an increased range of current densities that could be reached at lower overpotentials compared to absence of these conductive layers. This resulted in the bottom most segment to exhibit the largest areal heat dissipation. The heat transfer coefficient here equates to h = 43.86 W/m²K.
This study takes another important step into the regime of capillary-fed electrolysis and alternative electrolysis methods in general. Introducing an alternative design, assessing this design from experimentation and reporting on subsequent steps that can be taken.

A water electrolyzer is an electrochemical device that produces hydrogen and oxygen from water via electricity. A sustainable way of producing hydrogen with renewable electricity, also known as green hydrogen, could enable its use in decarbonisation of sectors that depend on fossil fuels. However, to scale up the production of green hydrogen, larger water electrolyzers are required, which presents challenges. Water electrolyzers produce hydrogen in the form of bubbles, which are transported away through outlet channels. The geometry of the outlet channel is important for performance and safety. A channel that is too wide can result in unwanted ionic current leakage, reducing efficiency and increasing the risk of explosion. To reduce the leakage current, the electrical resistance in the channel can be increased to make it difficult for ions to flow through. However, a narrow channel can increase the hydraulic resistance, leading to clogging due to bubble build-up. This study focuses on the characterization of the slug flow regimes that eventually lead to clogging of the outlet channel in a novel electrolyzer design. To characterize the flow when blockage develops, electrochemical tests with industrially relevant 6M KOH were carried out at room temperature and ambient pressure in conjunction with high-speed video recording. The examined channel was 2 mm high, 3 mm wide, and 40 mm long. The experiments showed that clogging occurs for certain flow regimes. The hydraulic configuration caused clogging at the hydrogen side, which caused the liquid flow prefer to travel via the anode rather than the cathode, which was reflected in the temperature plots. Clogging did not occur when the superficial gas velocity was greater than 0.3 cm/s, which corresponds to a current density of 140 mA/cm2. The clogging was also not visible at a superficial liquid velocity greater than 0.25 cm/s. Below these values, clogging is expected to take place for the investigated channel dimensions. The bubble diameters in the bubble layer in front of the channel were measured with a microscopic camera. The mean bubble diameter was 0.4 mm, which is 80% smaller than the height of the channel through which they flow. However, exceptional bubble sizes that were 3 to 4 times the channel height, which ultimately led to clogging of the channel, were also present. The bubble layer thickness and velocity at increasing liquid flow rates and current densities were measured using a high-speed camera. At low current densities, the velocity and thickness of the bubble layer were not affected by the liquid flow rate. At higher current densities, a higher liquid flow rate appeared to reduce the thickness of the bubble layer, but not the velocity of the bubble layer. The bubble layer thickness as well as the velocity strongly depend on the applied current density (and thus the gas flow velocity). This knowledge might be, when the flow regimes are scaled to larger electrolyzer dimensions accordingly, essential for future electrolyzer manufacturers and operators for the operation and design of larger-scale electrolyzers. It will give insight into which flow regimes should be avoided in order to avoid clogging and thus explosion risks during operation.

Alkaline water electrolysis holds significant promise for the production of green hydrogen, playing a crucial role in decarbonization across various industrial sectors. The operational effectiveness of this technology hinges on its electrical efficiency. This is intrinsically tied to the formation and behaviour of gas bubbles at the electrode-electrolyte interface, influencing its overall performance. In the Zero-Gap configuration, the electrodes and the membrane are directly pressed against each other to reduce the ohmic resistance between the electrodes. However, this possibly introduces large bubble overpotentials, reduces active surface area, or facilitates gas crossover. Hypotheses for avoiding these shortcomings include introducing a small space between the electrode and membrane to allow bubbles to escape. This study aims to visualise and quantify the behaviour of bubbles in the near-electrode space, presenting numerous experimental findings and images from high-speed photography. The analysis encompasses multiple physical phenomena influencing bubble behaviour, offering comprehensive insights into their dynamics.

The investigation begins by examining the effect of electric fields on electrolytic bubbles. First, an electrowetting experiment was conducted on a polished nickel electrode. Surprisingly, it was discovered that this effect had no observable influence on the contact angle of bubbles with the electrode when subjected to voltages up to 1.7 V. Furthermore, an electrophoresis experiment revealed that the application of an electric field was found to have a minimal impact on the trajectory of both oxygen and hydrogen bubbles. The electric field had a maximum strength of 15 kV/m. This finding implies that electric fields do not significantly alter the behaviour of electrolytic bubbles in this context.

Two distinct cell configurations were employed further to assess bubble behaviour and bubble influence on AWE overpotentials. The classical Zero-Gap cell design was evaluated, equipped with a spacer to introduce a small gap between the electrode and membrane. Interestingly, linear sweep voltammetry experiments conducted on the Zero-Gap cell configuration contradicted previous literature, suggesting that imperfect gap control might explain this discrepancy.

In parallel, a novel cell design allowed for imaging an electrode's membrane side, which was analyzed using chronopotentiometry. Surprisingly, bubble resistances were found to be independent of current density. Additionally, the study revealed that electrode geometry is essential in influencing the potential increases due to double-layer charging and bubble evolution. The analysis of chronopotentiometry data provided insights into the periodic flow of large bubble slugs on the membrane side of the electrode, with the characteristic time of the flow features being inversely proportional to the current density. The utilization of this configuration led to the acquisition of unforeseen images from the electrode's membrane side.

Finally, a Particle Image Velocimetry (PIV) analysis of the recorded data yielded insights into the velocity fields surrounding an expanded metal electrode in a Zero-Gap configuration. The investigation revealed that bubble highways were established owing to the specific electrode geometry, exhibiting a vertical velocity approximately 1.4 times greater than the average vertical velocity.

In summary, this study delivers comprehensive visualizations of bubble dynamics near the electrode and quantifies their influence on the performance of alkaline water electrolysis. Moreover, the utilization of High-Speed - High-Resolution images enables unprecedented insights into the visualization of bubble behaviour on the electrode. These findings significantly contribute to our understanding of bubble behaviour in the vicinity of water-splitting electrodes and provide essential information for validating more comprehensive models of this technology, ultimately aiding in advancing green hydrogen production and the decarbonization of industrial sectors.

This study focuses on water electrolysis and aims to provide suggestions for the company XINTC Global, for the design of the electrolyser and the accompanying flow network. The most important parameters for this project are the dimensions of the cell, the geometry of the piping network, the electrode gap, the volumetric flow rate and circulation velocity. These parameters influence the performance of the cell. As a result, the production of hydrogen is optimized by fine-tuning these parameters. In addition, the performance in different electrolyser geometries is maximized assuming free circulation of the electrolyte. The geometries differ in the number of cells and the stacking arrangement and they are the single cell, single module and horizontally stacked geometries. The performance indicators are the flow velocity through the electrode gap and the volumetric velocity through the electrode gap, which is a product of the velocity, electrode gap and electrode depth. Furthermore, suggestions on the other dimensions are given, with the aim to maximize the flow rate or velocity. To study the behaviour of the electrolysis system, I developed a Python code that simulates the flow through the electrolysis cell and channel network. This numerical model is compared to an analytical model and experimental data from the literature. Finally, the simulations predicted that the optimum electrode gap width is dependent on the total number of cells. The results with the turbulent flow model, showed that the optimum electrode gaps were 1.85 mm 0.250 mm and 0.250 mm for a single module configuration with 1, 60, and 240 electrolysis cells, respectively. Moreover, the horizontal stacking arrangement of the modules, resulted in larger optimum electrode gaps, for the same number of cells.

Reducing the concentration of hydrogen and oxygen dissolved in the electrolyte helps to increase the efficiency of alkaline water electrolysis. This thesis provides an account of the evolution of dissolved concentration in the vicinity of the electrode surface for alkaline water electrolysis for the bubble size, the electrode height, the dissolved gas uptake by bubbles, and the bubble generation at the electrode surface. Included are an analytical and numerical model that not often include local electrokinetic effects coupled with a gas-liquid flow model. As is commonly found for experimental data on dissolved hydrogen and oxygen, the dissolved species is either an average concentration measured relatively far from the electrode surface or an average concentration at the electrode surface without any local effect of the electrode kinetics. In this thesis an agreement is found for the analytical derived natural convection up to an height of 0.01 [m] with the numerical model. The fraction of dissolved hydrogen and oxygen taken up by the gas bubbles is enhanced for smaller bubbles, a high frequency of bubbles generated, and an increased mass transfer of dissolved gas at the electrode. Moreover, a clear difference is found for the dissolved hydrogen and oxygen evolution near the electrode for horizontal and vertical electrodes. Horizontal electrodes have more dissolved gas at the electrodes, likely due to the dissolved gas not being able to transfer to the gas bubbles as easily for vertical electrodes. Also, for both vertical and horizontal electrodes the dissolved gas concentration flattens for increased current density, likely due to homogeneous nucleation. For an increasing electrode height a lowering of the dissolved gas was observed, associated with an increased dissolved gas uptake by the bubbles. Most of these local effects are able to be modelled using an analytical and numerically combined model. These simulations greatly improve the understanding of dissolved hydrogen and oxygen evolution in the vicinity of electrodes.

Zero Emission Fuels B.V (ZEF) is currently in the process of designing and prototyping a sustainable methanol micro-plant, which captures water vapor and carbon dioxide from the atmosphere and uses those components to eventually produce methanol. In order to produce methanol, hydrogen is required which is produced from the captured water by electrolysis in a zero gap design, high pressure alkaline water electrolyser. The pressure in the electrolysis setup is regulated at a desired operating pressure of 50 bar by a pressure control system, designed by ZEF.

Insight is required in how the flow of electrolyte and bubbles behaves in the electrolysis setup and the electrolyser cells, in order to implement design or operation condition changes to allow for an efficient operation in the eventual micro-plant.

This study focuses on the experimental characterization of the flow and bubble phenomena in the ZEF electrolyser design, during operation at a desired pressure of 50 bar, with the use of visualization by video footage inside the electrolyser cells. Camera equipment used for the visualization consists of a microscope camera and a GoPro camera. An ultrasound Doppler velocimetry device (UDV) was used to measure the electrolyte flow velocity and direction in different parts of the electrolysis setup, in order to substantiate findings made by the camera equipment.

Experiments were conducted with an electrolysis setup based on the current electrolyser design made by ZEF, with the addition of a transparent add-on to allow for visualization of the flow and bubbles inside an electrolyser cell on the oxygen side. First the operation of the electrolyser was characterized by means of IV curves and temperatures, followed by visualization experiments for varying operating pressures and current densities at different parts of the electrolyser cell.

One of the main observations made during the visualization experiments is the periodic occurrence of stagnation of electrolyte and gas flow out of the electrolyser. Stagnation of the flow occurs inside the electrolyser cells on the oxygen side, at the top channels leading the flow out of the electrolyser cells, to eventually a flash tank. Different multiphase flow regimes were observed during active flow through the top channels, which vary based on the operating current density, pressure and temperature.
Bubble behavior was observed at different positions on the electrode in the electrolyser cell and possible enhancement of gas crossover was detected in the electrolysis setup and investigated.

The effect of the pressure control system on the operation of the electrolyser was also investigated and showed to affect the electrolyte flow direction in the electrolysis system.
Models were made to validate and explain the findings from the visualization video footage and characterization of the electrolyser. A previous made model that characterizes the flow in the electrolysis system is used for comparison.

Due to the lack of a temperature regulation system, a finned tube heat exchanger was modelled to be implemented in the experimental setup as for the eventual electrolysis setup.

Based on the findings from the experiments, recommendations are proposed regarding the experimental and eventual design, and for further development of the high pressure alkaline water electrolyser and the electrolysis system.

During water electrolysis for the production of hydrogen and oxygen, many bubbles are formed at the electrodes. The presence of these bubbles causes a number of side effects such as a significant increase in the total cell resistance (e.g., electrical circuit resistances, transport related resistances, and electrochemical reaction resistances). The efficiency of water electrolysis is therefore closely related to the presence of bubbles in the electrolyte.
In this work, the focus is on the numerical modeling of the evolution of the bubble layer in a vertical channel with gasevolving electrodes on either side of the channel. One of the problems is the growth of rising bubbles along the electrode, which affects the shape and thickness of the bubble layer. This bubble growth depends on four main phenomena, e.g., bubble coalescence, change in hydrostatic pressure, presence of water vapor, and mass transfer of dissolved gas. Of these, the mass transfer of
dissolved gas contributes the most to bubble growth. Accounting for this growth can be crucial to the
computational effort to accurately simulate the bubble plume shape and thickness.
By coupling a multiphase flow model to the transport of dilute species, the bubble layer could be simulated. The simulation results were compared with experimental data, which showed that including mass transfer in the simulations could accurately simulate the bubble layer (in shape and thickness) over a wide range of different operating conditions.

Zero Emission Fuels B.V. (ZEF) is a start-up company developing a fully autonomous methanol synthesis micro-plant that will be energy driven by solar panels. The process implements an alkaline water electrolyzer, which supplies the methanol synthesis reactor with hydrogen. The electrolysis system includes a small-scale stack of cells and is designed to operate at 90 ^oC and the high pressure of 50 bar, with a 30% potassium hydroxide electrolyte. The system is also self-pressurized through the continuous accumulation of the electrolysis gases in the closed flash separation vessels. To control the process, the company has designed a novel system that aims to maintain the gas pressure at 50 bar and the liquid electrolyte level inside the flash separators at a fixed point. 
The present work has two main objectives. The first is to characterize the transient dynamic response of the company's current experimental electrolysis setup under the effect of the operating conditions and control. The second objective is to predict the level of gas crossover that is induced during the system's operation and evaluate the risk of explosive mixtures formation. Two different models were developed to fulfill the research targets.
The first model is based on a 1-d transient hydraulic network analysis. Simulations were conducted using Simulink for a current density range of 500-5000 A/m². The model predicts the electrolyte flow and pressure response in the various elements and locations of the network respectively, indicating also the oscillatory behavior induced by the operation of the valves. A key finding is the high differential pressure between the stack anodes-cathodes that is caused when the valves open, posing a danger for the integrity of the separators between the half-cells.
The second model was developed in MATLAB and uses the predicted flow response of the first model to estimate the crossovers by solving numerically the unsteady 1-d advection-diffusion equation in the corresponding elements. The effect of the supersaturated electrode boundary layer was also considered as an enhancing parameter of the mass transport through the separators of the cells and a sensitivity analysis was conducted. The main finding is that this parameter has an important effect, especially on the hydrogen crossover inducing 1% higher hydrogen impurity when it is increased by a factor of 10. The crossovers of both hydrogen and oxygen are found to gradually increase and reach an almost stable value under the effect of the control system. A comparison of the model with preliminary experimental data indicates a supersaturation intensity between 5 and 15 times higher than the solubility of the components. In this range, the system will safely operate above 2000 A/m² without the formation of explosive mixtures. 

The increasing penetration of renewable energy in the energy grid causes intermittency, resulting in fluctuating prices. This in turn forges windows of opportunity for the electrochemical production of chemicals that would otherwise be too expensive. This research aims to optimize a continuous-flow-through reactor hosting the production of chemicals by mediated electrochemical oxidation, using organic aminoxyls (i.e., electrocatalysts), specifically 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) and 4-acetamido-TEMPO (ACT).
The first chapter addresses the main motivation behind- and the aims of this research. In the second chapter, an extensive literature review is provided on the physico-chemical and electro-catalytic properties of the TEMPO mediators, that are important for the modelling of the electrochemical reactor. The third chapter describes the analytical- and computational models that were created to investigate the influence of mediator properties, current density, specific surface area, velocity, and electrode thickness on the energy efficiency of the reactor. More specifically, a flow-through electrochemical reactor with metal foam electrodes and divided anolyte and catholyte compartments.
The solutions obtained from the analytical model are in good agreement with the solutions of the computational model. The analytical model was extended with the Hatta analysis to include mass transfer in the porous electrode. It was found that ohmic losses, due to an expanding reaction zone from the membrane inward in the anode, caused by mass-transfer and/or kinetic limitations of the mediator, pose the biggest obstacle in reaching high conversion with reasonable efficiency.
The main result of this study is a mathematical formula, that allows for quick prediction of the performance of mediated electrochemical oxidation in a flow-through reactor with a porous foam electrode.

Electrochemical reduction of the \ce{CO2} into the chemically valuable \ce{CO} at an industrial scale is a promising way to reverse the worrying rise of \ce{CO2} levels in the atmosphere. Profitable operation at an industrial scale requires high CO partial current densities, producing a CO-rich output, while keeping the energy consumption and the price of the electrolyser low. Conventional \ce{CO2} reduction (\ce{CO2}R) to CO electrolysers suffer from the competing hydrogen evolution (HEV) reaction and mass transport limitations. Therefore, they can produce a maximum CO partial current densities of 20 mA cm$^{-2}$. The current densities can be increased by working at higher pressures or using gas diffusion electrodes, but to date, both require high energy inputs. Therefore, we investigated the potential of using a flow-through electrolyser (FTE) to produce \ce{CO} partial current densities exceeding the 20 mA cm$^{-2}$, while keeping the potential attractive. Porous flow-through electrodes can overcome mass transport limitations by their large reactive surface area and their ability to decrease the diffusion boundary layer thickness. To obtain suitable porous electrodes, we electrodeposited Ag on microporous Ti substrates. We used an aqueous \ce{CO2}-saturated \ce{KHCO_3} solution, as a well-proven electrolyte for \ce{CO2}R to \ce{CO} on a Ag catalyst. Our FTE was capable of overcoming the mass transport limitations. In a one-off test, it produced a \ce{CO} partial current density of 100 mA cm$^{-2}$, at a Faradaic efficiency (FE) of 55 \%. We could reproduce \ce{CO} partial current densities of 60 mA cm$^{-2}$ at a FE of 25 \% in a 0.05 M \ce{KHCO_3} solution. We experienced a decrease in activity towards CO when using more concentrated electrolytes. However, high cell potentials of more than 5 V, were required to exceed \ce{CO} partial current densities of 20 mA cm$^{-2}$. The required high potentials can be ascribed to ohmic losses and low selectivities towards CO. We believe that the insufficient deposition of the Ag on the microporous Ti electrode was the main reason for the low selectivity. Implementing the improvements, that are mentioned in the report, could contribute to make the FTE a promising candidate to reduce \ce{CO2} to CO at an industrial scale effectively.