As the demand for sustainable energy grows, large-scale energy storage solutions are becoming in- creasingly essential to balance power supply and demand, ensuring grid stability and security. Acid- base flow batteries (ABFBs) have emerged as a promising energy storage solution due to their cost- effective electrolytes, high energy density, and environmental advantages. However, co-ion crossover in ABFBs remains a critical challenge, leading to pH imbalances, capacity fade, and reduced efficiency over cycling. In this study, the mechanistic aspects of co-ion transport in ABFBs are investigated, and their impact on battery performance is evaluated. Experiments are conducted under varying electrolyte concentrations, state of charge, current densities, and temperatures to quantify crossover ion flux and correlate it with key performance metrics, including round-trip efficiency, power density, and capacity fade rate. The results provide insight into the co-ion crossover mechanisms and their influence on the cycling stability of the battery. These findings contribute to the optimization of ABFBs by identifying operational strategies to mitigate co-ion crossover and enhance overall performance, advancing the development of high-efficiency ABFB systems. Keywords: Acid-base flow batteries, co-ion crossover, cycling stability, ion transport phenomena, ca- pacity fade, bipolar membranes, ion exchange membranes

The negative environmental effects of large-scale use of fossil fuels, chemicals, and energy-intensive processes are forcing us to develop green alternatives to mitigate the climate crisis. Electrochemistry, powered by renewable energy, provides us with a direct way to produce precursors or materials from green electricity and benign reagents such as water, carbon dioxide (CO2), or oxygen. A widely known example is water electrolysis to produce hydrogen as a green fuel and chemical building blocks for other chemicals. Alternatively, CO2 can be fed to an electrolyzer to produce sustainable carbon-based materials - such as pharmaceuticals, paints, and synthetic fuels - that are otherwise obtained from fossil fuels. Electrochemistry is therefore a versatile and promising technology that could provide a strong foundation for sustainable alternatives to various environmentally unfriendly processes.
Because electrochemical systems are often studied as replacements for well-established and optimized industrial processes, the benchmarks to achieve an economically viable and competitive status are high. Most importantly, the processes must be efficient with materials and energy,
resulting in requirements such as high current density, energy efficiency and product selectivity. These are hampered in many systems by poor solubility of reagents in water, such as the aforementioned CO2 and oxygen. The low reagent concentration results in reagent depletion, intensified competition with parasitic side reactions, and low selectivity at industrially relevant current densities. Such systems are severely limited by the slow mass transport and availability of reagents towards and at the electrode surface.

In 2023, renewable energy generation reached an all-time high, with 29% of all electricity coming from renewable sources. However, electrical energy will not be able to fully replace fossil fuels, as the intermittency of renewable sources requires additional solutions to match the energy demand. Additionally, conversion to chemical bonds is required to supply chemicals for plastics, fertilizers, steel, etc. Electrolysis – particularly water and CO2 electrolysis – offer a promising solution to these problems by converting renewable electricity into fuels and chemical building blocks. Although electrolysis processes are very promising for the energy transition, their costs are currently still too high.
This thesis focusses on the role that gas bubbles have on the performance of electrolysers: the formation of gas bubbles is inevitable in most electrolysers, since the common electrolysis products (e.g. H2, O2 or CO) have a poor solubility in water. Controlling the behaviour of gas bubbles offers a pathway to lower the cell voltage or improve the mass transport, which allows operation at higher operating current densities. This could help with decreasing the costs of electrolysers, bringing them closer to competing with fossil fuel-based processes...

As part of the global energy transition, large amounts of hydrogen will be needed to provide fuel for heavy-duty transport, heat for industrial processes, and as a feedstock in chemical processes. To be environmentally sustainable, this hydrogen has to be generated using electricity via water electrolysis. With an increase in hydrogen production, however, the additional demand for freshwater in water electrolysis will start to compete with other freshwater consumers, especially humans. One approach to lessen the freshwater impact of water electrolysis is to use seawater as the process feed and perform in-situ water purification. Among others, this purification can be done by using forward osmosis membranes. However, previous research on this topic has only been performed at low current densities, while simultaneously demonstrating high cell overpotentials. Therefore, the overall aim of this thesis is to quantify the attainable current density of the forward osmosis-based purification method, and to optimise the cell performance through use of a zero-gap design. In addition, an evaluation of performance and material degradation is pursued. To this end, tests were conducted to quantify the water flux over the forward osmosis membrane within the given system. Next, a variety of long-term electrochemical tests were performed, both with and without water purification, and at different current densities. Furthermore, the cell voltages before and after testing were broken down into anode, cathode, and membrane contributions to gauge performance degradation and locate its origins. Finally, a variety of testing methods such as SEM, EDX, and XPS were used to evaluate the observed material and performance degradation and its causes. The maximum current density attainable within the designed system is found to to amount to 62.39 mA/cm2 . Furthermore, an initial performance improvement is found for the zero-gap cell design. This performance improvement is attributed to lower concentration and bubble overpotentials. However, the cell design exhibits strong material and performance degradation under all investigated testing parameters. The performance degradation is caused by increasing anode and membrane overpotentials. The anode overpotential increase is hypothesised to be due to the dissolution of a catalytically-active iron and nickel-rich anode top layer. The membrane overpotential is hypothesised to be due to membrane blockage by material deposits, and sodium-proton substitution in the membrane. In contrast, stable, or even improving, cathode overpotentials are found. This observation is attributed to the formation of catalytically superior metal phosphates and nickel hydroxide on the cathode surface. Lastly, the root cause of all degradative processes is found to be the use of a selective membrane, which enables a gradual acidification of the anolyte and thus creates a highly corrosive environment at the stainless steel anode. The results of this thesis allow for a better understanding of the inherent limitations connected to the use of FO membranes in electrolysis cells. It also offers a wide range of opportunities for improving the system and cell design in future research. Specifically, it enables a more careful membrane selection for future cell designs. By highlighting the limited achievable water influx with the given system, it also emphasises the need for larger FO membranes, and thus a different system architecture, or alternatively the need for a switch to a different draw solution or electrolyte.

Nowadays, Hydrogen production is an important piece in energy transition system and holds a significant place in various industries. This gas is precursor for producing valuable compounds in the chemical industry and serves as a clean fuel, enabling efficient electricity generation when used with fuel cells. However, majority of its production still relies on reforming and gasification of non-renewable sources. A sustainable pathway for Green Hydrogen production through water electrolysis already exists. However, scaling up and commercializing this technology represents challenge in meeting global demand, which was reported to be around 95 million tonnes in 2022. Commercially available water electrolysis done in acidic environment is robust, however rather expensive. Anionic Exchange Membrane Water Electrolysis (AEMWE) is available alternative, but still not fully developed to be used on big industrial scales. AEMWE offers reduced cost as this technology does not require usage of expensive noble metal catalysts used in acidic electrolysis. Focus of the AEMWE research area is Anionic exchange membrane (AEM) as high values of conductivity of hydroxide ions could lead to fair technical competitiveness of alkaline and acidic electrolysis. However, the majority of currently available AEMs lack the desirable properties, such as mechanical/alkaline stability as well as anionic conductivity.
One of the promising novel techniques for membrane fabrication consists of membrane casting with employing DC electric field, which enhances charged polymer channel orientation. Reports have shown that polymer ion channels in random direction may cause slower migration and consequently lower values for conductivity. On the other hand, it has been proven that DC treated membranes can yield up to three times higher values for conductivity of OH− ions. Therefore, researching optimal DC values for casting significantly impacts electrochemical cell performance.
This thesis report focuses on fabrication, characterisation and performance evaluation of the cast membranes with DC empolyment, prepared from cationic polymer kindly provided from industrial collaborator. Furthermore, an attempt will be made to assess competitiveness between produced and commercially available membranes. Finally, future suggestions for research directions and alternative membrane fabrication techniques will be provided, as these could offer valuable insights for further exploration in this field.

Nutrient recovery has lately been a concerning topic regarding the environmental friendliness of it and the high availability of technologies. Ammonia is one of the main compounds in reject water that could be recovered and utilized further in the agricultural sector. Several methods have been found, including conventional electrodialysis, in which anion and cation exchange membranes are being used and, with the application of electrical current, there is production of clean and desalinated water, creating at the same time a concentrated solution. As a further evolution of electrodialysis, bipolar membranes could be added in the configuration, leading to acid and base production. However, ammonium is not the only cation included in reject water, but also Na+, K+, Mg2+ and Ca2+ are present and affect the overall performance electrodialysis. Thus, the competition between the cations needs to be investigated further regarding the operational parameters of each configuration.
This study investigated the cation competition in electrodialysis and bipolar membrane configuration regarding the ammonia removal efficiency and the overall energy consumption. The research questions were focused on the effect of enriched solutions with cations on ED and BPC to the efficiency parameters, to the impact of cation composition in the feed solution when NH4+, Na+, K+, Mg2+ and Ca2+ are included in an ED and finally, the effect of municipal reject water cation molar ratios in a combined ED and BPC configuration. The experiments included batch mode systems, with several mass and molar ratios of NH4+ applied, the above-mentioned parameters were measured. More specifically, BPC and ED configurations were tested with mass ratios of other cations in an enriched NH4+ solution, while molar ratios were tested in case of an ED configuration with NH4+, Na+, K+, Mg2+ and Ca2+ be present in the feed solution. Finally, the two configurations were tested in a sequence batch, with ED to be the pretreatment step and BPC the final stage. The phenomena that were also investigated were proton production from bipolar membranes and EC pattern on the diluate solution in this case.
In ED removal efficiency was presented as a linear curve on time while in BPC the same value took a logarithmic trend, which is attributed to proton production and finally competition. During BPC operation, there was constant production of H+ through water dissociation that led to the acidic environment in the diluate solution but also to stabilization of EC when H+ presence was dominant. In addition, in molar ratio experiments with the application of ED, removal efficiency was higher for more challenging reject waters compositions such as molar ratios between 0.30 and 0.60. Considering 75% removal efficiency as an effective case, percent demineralization was also calculated. For removal efficiency below the effective case, percent demineralization presented a minimum for molar ratio of 0.60, while for higher removal efficiency the overall trend was slightly different, having a more exponential shape. Finally, energy consumption in molar ratio experiments, for removal efficiency of 75% presented a gradual decreasing linear trend with the increase of molar ratio.
Based on the results occurred in batch experiments, a sequence batch of ED to concentrate the feed solution was established, by applying the more challenging molar ratios of 0.30, 0.45 and 0.60 and the concentrate was then fed to a BPC to explore the proton effect in a concentrated solution. The percent demineralization and removal efficiency remained stable during the experimental phase while transport number had a notable increase with the increase of molar ratio, remaining approximately the same in every individual batch. Moreover, energy consumption had an important increase with the decrease of molar ratio due to the high membrane resistance and the observed scaling effect.

In this study, a model is developed in COMSOL to investigate the nature of energy losses in ion exchange membranes used in electrochemical energy conversion. By use of electrochemical impedance spectroscopy, the nature of resistance and reactance, and therefore the nature of Ohmic resistances and charge transfer resistances, are evaluated. This is done by first obtaining a model of the setup and the resulting impedance spectrum and then making adjustments in the model to observe the impact on the impedance spectrum as well as the resulting ion concentration profiles. In the used system, the diffusion effects were dominant compared to the double layer effects. Therefore, the impedance spectrum is mainly shaped by the diffusion boundary layer, especially at low frequencies. The ion concentration, diffusion coefficients, and membrane properties were found to have a significant influence on the impedance spectrum. Insights from this study can be used in the optimization of ion exchange membranes and the efficiency of energy conversion systems.

As a response to climate change, substantial efforts are being made to achieve global net-zero greenhouse gas emissions by the year of 2050, as established by the Paris agreement. To decrease reliance on fossil fuels, we are transitioning to renewable energies and electrifying various sectors. However, certain segments of the global supply chain will still require carbon-based chemicals and energy carriers. CO2 electrolysis allows the use renewable electricity to electrochemically reduce air-captured CO2, producing chemical building blocks such as CO, ethylene and formate. These chemicals can then be converted into larger hydrocarbons, e.g. into synthetic diesel using the Fischer-Tropsch process. In this way, CO2 electrolysis can aid in closing the carbon cycle by converting CO2 emissions into valuable chemicals and fuels. Despite its promise, a few hurdles still hamper the industrialization of CO2 electrolysis. These are the relatively low energy efficiency, salt deposition, the inefficient use of CO2 due to carbonate cross-over and the necessity of scarce iridium-based anode catalysts. Most of the mentioned challenges can potentially be solved by novel, or optimized ionexchange membranes (IEMs) - these have a pivotal role in the process since they provide a conductive medium to selectively transport ions between the electrodes. Increasing the IEM’s ionic conductivity and permselectivity can increase the energy efficiency of the process and decrease cation cross-over and therefore salt deposition. Furthermore, an OH– selective membrane which rejects other anions such as carbonate, can potentially solve the carbonate-cross over issue. In this way, the goal of this work is to develop novel IEMs to address each of the challenges tied with CO2 electrolysis...

CO2 electrolysis to form CO with the use of electricity from renewable sources is a promising technique in closing the carbon cycle. Via the electrochemical reduction CO2 can be converted into different hydrocarbon products. These value-added products are for example CO, HCOOH, CH3OH or C2H4. The direct electrochemical route of CO2 reduction is often tested on three metrics: the faradaic efficiency, the current density and the energy efficiency. The implementation of CO2 reduction has some technical challenges. One of which is controlling the local pH and carbonate formation inside the electrochemical cell producing hydrocarbon products. Controlling the pH inside the electrochemical cell becomes an issue because of the production of hydroxide anions that increase the pH. A high faradaic efficiency is affected by the local pH. With an increasing pH CO2 is consumed to form carbonate and Hydrogen Evolution Reaction (HER) is favored. Mass transfer limitations result in the pH increasing near the electrode. The local pH is dependent on the electrochemical reaction, buffering reactions inside the electrolyte and mass transfer within the electrolyte flow. Previous studies have used numerical modelling to obtain a 2D transport model to present concentration gradient along the cathode or inside the cathode. Other studies that show the pH experimentally do this using scanning probe techniques or measure the intensity of dye sensitive to pH.
In this project the effects on the local pH are studied with the use of Fluorescence Lifetime Imaging Microscopy (FLIM). This method is able to image the local pH with the use of a fluorescent dye that has a lifetime dependent on the local surroundings. This thesis researched the effects of three process parameters: electrolyte anion type and concentration, the catholyte flow rate and the current density. The electrolytes studied are: 0.1 M KHCO3, 1 M KHCO3 and 0.4 M K2SO4. The catholyte flow rates studied are corresponding to Reynolds number 0.8, 8 and 47. The different current densities are −1, −5, −10 and −50 mA cm-2 . The effect of these parameters was studied with performing electrochemical tests and studying the cell potential.
This study obtains a fluorescence lifetime-pH calibration curve inside an electrochemical cell. From this calibration curve could be concluded that we have a clear trend above pH 9 to the phase-shift fluorescence lifetime. We also concluded that the used salt KHCO3 is likely to have an effect on the performance of the alpha dye. An unexpected effect of using FLIM onto the spatial resolution in an electrolyser was due to the presence of bubbles. We suggest a more elaborate study into the effects of the settings used in the FLIM method.
The results show that the characteristics affected by the anion type and concentration of electrolytes can be described in the buffer capacity and conductivity of the electrolytes. With increasing buffer capacity and bicarbonate concentration the pH difference between the bulk and near the cathode decreased. The study also found that the overall cell potential was increasing with increasing conductivity of the electrolyte. We cannot distinguish a clear effect of the concentration overpotential as an effect of the buffer capacity onto the overall cell potential.
Additionally, the study found that a higher Reynolds number leads to a decrease in potential due to lower concentration overpotential and better gas removal. From a study on the current density and its effects could be concluded that even though we expect laminar flow with these Reynolds number mass transfer is occurring perpendicular to the flow of the electrolyte. This is an effect of the formation of hydrogen or carbon monoxide bubbles inside the electrolyte that improve mixing. This was also shown in the decrease of pH increase near the cathode with increasing Reynolds number. For the highest current density an overall increase of the bulk pH was linked to the improved mixing due to gas bubbles. The effect of gas bubbles onto the local pH and the cell potential should not be underestimated. Improvement of gas removal with a higher Reynolds number of the electrolyte flow rate or in other ways remains a recommended field of research. We suggest the use of Particle Image Velocimetry alongside of FLIM to obtain more insights into the flow profile inside the catholyte affected by the formation and removal of gas bubbles. Finally, we recommend to research the system with the implementation of the gas channel and different types of membranes.

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.

Electrochemical CO₂ reduction may present a solution to close the carbon cycle and to utilise CO₂ emissions. However, for this technology to have a significant impact, it has to be successfully implemented on an industrial scale. Numerical simulations can aid with the study of process parameters and reactor design.

The overall aim of this project is to develop and utilise a numerical model that can describe phenomena arising in the CO₂ electrolyser inside the flooded catalyst layer (CL). First, the general operation of the electrolyser is addressed, and this is extended for the effect of liquid flow rate, electrolyser length, and operating pressure. To investigate the performance and limitations arising at the large-scale, the model is scaled-up to describe a one meter long electrolyser. The study is concluded with two considerations that could improve the electrolyser performance. These points were addressed by developing a 2D numerical model of a gas diffusion-based CO₂ electrolyser in COMSOL Multiphysics. We assessed the performance of the electrolyser in terms of current density, reflecting rate of species formation, and of faradaic efficiency for CO (FE), reflecting selectivity towards the desired product.

Investigating the small-scale electrolyser we find that at high current density (200 mA cm^-2), the pH in the CL immediately increases by 3 units and further diagonally increases from pH 10.1the inlet to 12.2 around the outlet. When operating the electrolyser with excess of CO₂ supply, we find the CL to perform the best near the gas phase boundary (311 mA cm^-2, 95% FE), while the regions close to the electrolyte are underperforming (250 mA cm^-2, 89% FE). This shows that the performance in certain regions of the CL needs to be improved.

When scaling-up the electrolyser to a length of one meter we find that the performance does not change dramatically when operating at excess of gas supply. However, if a high CO₂ conversion should be achieved, the long electrolyser shows a 10% decrease in FE, and CO₂ conversion compared to a small-scale electrolyser, at the same level of current density (115 mA cm^-2).  Analysing the current density locally, we find that difference between the inlet and outlet can be as large as 100 mA cm^-2. Next, we find that FE can fall to almost 50% around the outlet. This shows that when adding extra length to the long electrolyser, this extra length only adds a fraction of its potential performance.

The uneven utilization of the catalyst can be improved by varying the catalyst loading along the electrolyser length. This improves the FE by around 5% while using 40% less catalyst. We also find that while the current density is slightly lower, the amount of product generated per mass of catalyst has significantly increased. This shows that carefully engineering the catalyst loading can save the amount of catalyst needed and could potentially improve the cost-effectiveness of the CO₂ electrolyser.

In all cases the performance over the CL is unevenly distributed. To achieve a higher performance, research needs to find ways how to enhance the performance also in the poorly utilised regions of the CL. Scaling-up the electrolyser just by extending its length proves inefficient and inevitably leads to a lower performance. The beneficial buffering effect provided by the electrolyte at a small-scale does not translate to a large-scale. At this moment, performance of large-scale CO₂ electrolysers seems satisfactory only when operating at very low CO₂ conversion. From the investigated parameters that address the performance issues, higher operating pressure and smart catalyst loading seem only promising options, however, other options should be found to speed up the development.

To mitigate climate change, carbon capture is necessary. In addition to the energy transition towards renewable sources and green house gasses emission reduction, CO2 capture from flue gas and its sinks, including air and the ocean, must be promoted. By 2030, in less than 8 years, the global carbon capture capacity must increase 100 × (from the current ca. 40 MtCO2 yr−1 to 4 GtCO2 yr−1). To meet the net zero carbon goals of 2050, sustainable, scalable, inexpensive technologies that fit in an electrified industry and have a small footprint are needed for carbon capture. Currently, such technologies do not exist. In the framework of the necessary carbon capture, and the opportunities for electrochemical (ocean) CO2 capture, five research questions are defined and addressed in this thesis...

The abundant presence of nitrogen in natural waters can be a threat to both humans and environment. Therefore, municipal and industrial wastewater streams need to be treated before their disposal in the environment. Currently used biological and physical-chemical treatment methods have drawbacks such as high greenhouse gas emissions, high energy use and high use of chemicals. This research used bipolar membrane electrodialysis (BPMED) as a technology for the removal of ammonium sulfate from industrial wastewaters with extreme characteristics. The industrial stripper/scrubber system removing nitrogen from industrial waters creates an ammonium sulfate rich water stream with a pH of 2, a temperature of 70 degrees Celsius and concentrations up to 250 g/L. The study investigated the influence of a low pH, high concentration and high temperature on the removal of ammonium and sulfate from wastewater and in situ generation of sulfuric acid and production of ammonium hydroxide. Key performance parameters to measure the influence of these extreme conditions were the efficiency of nitrogen removal in the form of ammonium current efficiency and ammonium removal efficiency}) and sulfate removal removal efficiency. Other key performance parameters of interest were the consumed energy for this removal in the form of electrochemical energy consumption, the purity of the generated acid and base and the final ammonium concentration in the base and the sulfate concentration in the acid. Model wastewater with a pH between 2 and 10, a concentration between 50 - 250 g/L ammonium sulfate and with a temperature between 20 degrees Celsius and 40 degrees Celsius was treated with BPMED. The influence of pH, concentration and temperature was researched independently in experiments of 180 minutes, after which the removal efficiency and the quality of the produced acid and base were assessed.

Electrolyzers for CO2 reduction can be used to synthesize renewable fuels, as a route to replace fossil fuels in our everyday economy with the purpose of minimizing the effects of climate change. For a high-scale implementation of electrolyzers, high operation current densities and low overpotentials are essential. Currently available options do not offer this and are limited by the low mass transfer of CO2 in the system. We envision using catalyst-coated capacitive particles (slurry electrodes) to be a solution: a system where the reaction site can be brought to the reagent, instead of the other way around. This thesis is a first step in the direction of a slurry electrode flow cell and aims to produce silver nanoparticles deposited on activated carbon particles to function as catalysts in such a system. The goal of the synthesis procedures was to produce a capacitive powder (activated carbon) with efficient deposition of silver nanoparticles spheres) ranging 5-20 nm in diameter. Based on literature, the most interesting methods for synthesis were selected to be: solution-based methods with only activated carbon, with added polyvinylpyrrolidone, with added sucrose, electrodeposition and impregnation. The resulting powders were analyzed using ICP-OES and TEM, showing that only impregnation and electrodeposition can yield the desired powders with a near-100% silver deposition efficiency and particles < 20 nm in size. Next, the impregnation and electrodepostion samples were prepared into a 15 wt% slurry, and a third slurry was made with bare AC as a blank. The catalysis experiments were executed in a flow cell (electrode area = 4 cm2), with currents ranging from -5 to -15 mA/cm2. Within this range of currents, extreme potentials up to -10 V (measured over working electrode versus counter electrode) were measured. The potentials on the anode and cathode side of the cell were also measured individually using reference electrodes, showing that the anodic side of the cell reached +5 V, whereas the cathodic side of this cell reached only -1 V. Additionally, CV scans before and after the experiments confirm that the cell’s conductivity decreased within an experiment, however, this degradation is reversed for a new experiment. These observations combined lead us to believe that the cause of the cell problems is located on the anodic side of the system, and is most likely resistance from oxygen bubbles, which needs to be solved to allow cell operation at higher current densities. Since the aim of the project was CO2 reduction on the slurry electrodes, the outgoing gases from the cathode were measured in a GC. The blank was included to demonstrate the catalytic effect of the silver nanoparticles, yet the highest CO flow (around 0.05 ml/min) was measured at the lowest current density during this blank experiment. Calculating the Faradaic efficiency (FE) suggests that 33% of the electrons in that experiment was used for this conversion. The other measurements, at higher currents or with silver present, did not reach an FE of 10%. This combination leads to the conclusion that the observed CO is most likely not produced by CO2 reduction. A hypothesis was made that this CO was already present at the powder’s surface and released during the experiment, however XPS data showed that there were no noticeable differences in the content of oxygen-bound carbon between the coated samples and the bare sample. Thus, the source of the CO is at this stage unknown. Future work towards proving CO2 taking place on the slurry electrode should first investigate the source of the observed CO, which requires testing the influence of the different carbon sources present in the system. Additionally, the blank experiment should be improved: using a bare carbon slurry that underwent the same procedure as the coated slurry (without the precursor present) eliminates any differences in the powder.

Oceanic CO2 capture technology can be used as a negative emission technology, or pre-treatment to reduce inorganic fouling (i.e., scaling) potential when further processing seawater. In this work CO2 from (synthetic) seawater was captured electrochemically via bipolar membrane electrodialysis. Although previous studies showed promising results regarding energy consumption (kJ/mole CaCO3), inorganic fouling is a drawback, which is why an inorganic fouling control and removal study was done. The effect of applied current density and flowrate on inorganic fouling build-up and dissolved organic carbon removal was investigated for 2 cell pair configurations. For inorganic fouling removal 5 methods were
investigated. This research was a proof of concept.

Zero Emission Fuels B.V. (ZEF) is developing an integrated micro-plant to produce methanol from captured CO2 and green Hydrogen produced via alkaline water electrolysis. The electrolysis unit represents one of the largest financial and energetic costs in the system and is required to operate at 166 mA/cm2 under 2 V. In order to decrease the costs, ZEF is looking to synthesize highly efficient nickel-based electrodes to avoid using expensive noble-metal ones.
The objective of this work was the synthesis of efficient and stable nickel-based electrodes. Two electrodes were synthesized through electrodeposition (Raney nickel and NiFe) and one was synthesized through hydrothermal treatment (NiFe-LDH) and were compared to a RuO2-containing (Permascand) electrode and to a smooth nickel electrode. On/off cycles for 2.5 hours were carried out to measure the stability of the electrodes. Additionally, performance tests in a pressure range of 1-5 bar were carried out to measure the effect of increasing pressure. The experiments were carried out in an in-house designed and built zero-gap alkaline electrolysis cell.
Raney nickel, with a measured roughness factor of 150 was the best performing of the synthesized electrodes. After the stability tests, it was able to produce 166 mA/cm2 at 1.9 V (1.73 and 2.12 V for Permascand and smooth nickel, respectively). Raney nickel as anode material presented significant degradation. Raney nickel is the most promising material for HER, none of the synthesized materials presented significant stability for OER.
From the nickel-iron electrodes, NiFe with a roughness factor of 19.5 presented the best performance of the synthesized materials with 2.04 V for the mentioned current density. NiFe presented significant degradation, especially as anode material. NiFe presented a relative high performance considering its low electrochemical active surface area attributed to the presence of highly efficient active sites. NiFe-LDH was quickly degraded during the tests as cathode and anode material.
The pressure tests showed an inverse relationship between voltage and pressure. This hints that a decrease in bubble size with pressure is the cause behind the decrease in voltage. A simple model, based on experimental data and thermodynamic considerations, estimated that operating the cell at 50 bar reduces the voltage by 0.25 V compared to operation at 1 bar. The estimated reduction in voltage at a pressure of 50 bar would allow to operate the electrolysis cell under 2 V even with smooth nickel mesh as electrode.

Municipal and industrial wastewater contains a significant amount of dissolved nitrogen. This is the results of the organic protein degradation and of the large employment of nitrogen (N), usually in the form of ammonia (NH₃), in the industry. Currently, nitrogen is removed in wastewater treatment plant (WWTP) by means of biological treatments. Globally, the most applied treatment consists in the combination of nitrification and denitrification. This technique is characterized by a high energy demand (44.3 MJ per Kg_NH4⁺ removed)  which accounts for 70% of the total energy consumption in the WWTP. A less energetically intensive alternative treatment is the Anammox process. However, this also presents significant limitations, especially when dealing with extremely polluted stream and in the flexibility of operation.
An alternative to reduce the energy cost of nitrogen removal by valorizing the content of residual streams is explored by the N2kWh project. The novel concept underlying the N2kWh project aims to use NH₃ from waste streams as a fuel source for a solid oxide fuel cell (SOFC). In this way, the energy cost for biological N-removal processes is cut, and, ideally, energy can be even produced. WWTP digested reject water was recognized as potential N-source for energy recovery as a result of its relatively high total ammonia nitrogen (TAN) concentrations (up to 1.5 g − N ⋅ L^-1). A (selective) concentration step and pH regulation are needed in order to convert the TAN in the reject water, mostly present in form of ammonium bicarbonate (NH₄HCO₃), into NH₃ gas which can be stripped and directly fed in theSOFC. Previous studies proved electrodialysis (ED) to be the most energetically efficient technology for the removal and concentration of ammonium bicarbonate. However, due to the high alkalinity in the obtained concentrate, a large amount of chemicals is required for the pH regulation. An interesting opportunity to avoid this chemical addition is the employment of a bipolar membrane electrodialysis (BPMED). Through this unique technology, it is possible to simultaneously achieve the concentration of TAN, as well as the regulation of pH without chemicals.
The main objective of this work was to evaluate the potential of BPMED for the recovery of NH₃ in the boundaries of the N2kWh project. The energy performance of BPMED was compared with the alternative use of conventional ED plus sodium hydroxide for pH control. In order to obtain valuable information on the operation, three setups were employed: regular ED, BPMED and BPMED coupled with two membrane vacuum stripping devices (VMS). For this purpose, a synthetic solution obtained by dissolving 6.6 g ⋅ L^-1 NH₄HCO₃ salt in demiwater was used to simulate the digested reject water. Experiments were designed to characterize the BPMED operation and clarify which processes influence the performance.
Experimental laboratory results showed that, in terms of energy consumption for the NH₄⁺ removal, the BPMED used more energy compared to the ED. The removal of 90% of the initial NH₄⁺ is achieved by using an average of 13.2±0.1 MJ ⋅ Kg_NH4^-1. This value was 3.3 times higher than that achieved with regular ED. This discrepancy was explained by the extra-elements in the stack and the water dissociation process, responsible for the higher voltage measured during BPMED operation. The energy consumption was proven to be higher also as a consequence of the lower current efficiency for salt transport (73% compared to 95% of conventional ED). This was due to the more severe undesired diffusion processes, mainly gas diffusion and hydroxide leakage from the alkaline stream to the diluate, taking place within the BPMED stack. However, the energy consumption for ammonium removal in BPMED was still more than three times lower than the energy consumed by nitrification-denitrification and comparable to what used by anammox. 
The implementation of the membrane vacuum stripping modules, in series with the BPMED, only slightly increased the overall current efficiency (up to 3.4 % current efficiency gain). The benefit in current efficiency was less significant for higher concentrations and pH in the alkaline stream. Besides that, the tested technology tandem (BPMED+VMSs) was capable of stripping NH₃ and, consequently, reduce the gas diffusion over the stack. The energy consumed for the production of NH₃ and for the ED operation to reach a pre-defined pH and TAN concentration was estimated to be equal to 82.55 Wℎ per L of concentrate. This value was substantially higher than what consumed by BPMED (48.7 Wℎ) to achieve the same result in the alkaline stream.
To conclude, the studied BPMED system was demonstrated to be superior in terms of energy consumption for the recovery of NH₃ gas when compared to the combination of ED and chemical addition. BPMED showed also crucial advantages for the environment, design and safety of the treatment facility. The energy consumption for the removal of nitrogen with BPMED was proven to be lower than what used in the competitive biological technology. Finally, neither the technology nor the operation methods employed were specifically designed/optimized for this application. This made the use of BPMED for the N2kWh purpose even more interesting and promising.

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.