The large concentrations of anthropogenic CO2 emissions present in our atmosphere are causing a severe thread on the world as we know it. It is therefore highly necessary to move away from fossil fuels to sustainable alternatives. In order to do so, lots of research has focused already on finding new ways to create renewable energy, through for example solar and wind energy. These renewable energy sources will be able to replace the energy that we need to warm our houses and drive our cars. However, these technologies are not able to replace the chemicals that we use in our daily lives that are made through fossil-based processes. For example, plastic bottles and packing, synthetic fabrics such as polyester and nylon, cosmetics, and detergent are all made from fossil-based chemicals. An sustainable alternative process to make these chemicals is through electrochemical CO2 reduction, where CO2 is converted into chemicals by applying electricity. This involves capturing CO2 from the atmosphere and transforming it into a value-added chemical. In this way, CO2 becomes a resource instead of a waste gas that is vented off into the atmosphere and a so called circular carbon economy can be established.

Typical CO2 capture systems use a thermal step to remove the captured CO2 from the capture solvent and regenerate the solution such that it can be recycled back to the capture step. This thermal process is highly energy intensive and therefore a costly step in the CO2 capture process. However, regeneration of the capture solvent can potentially also be achieved by an electrochemical process. The CO2 rich solvent is sent directly to the electrolyser in which the CO2 is converted into carbon products and simultaneously creates a CO2 lean solvent at the outlet that is suitable for a new CO2 capture cycle. This dissertation studies the feasibility of integrating CO2 capture with electrochemical conversion. This is done by looking at two different pathways using two different solvents. The first pathway investigates the use of an organic solvent and the second pathway uses a (bi)carbonate solvent. This dissertation addresses different challenges related to the effective electrochemical CO2 conversion for these two different solvents and provides a perspective on the feasibility of integrating CO2 with electrochemical conversion.

The severe effects of climate change, along with the rising global energy demand, have driven extensive research efforts into the development of sustainable technologies for energy generation, conversion, storage, distribution, and CO2 removal from various industrial sectors. Electrocatalysis is expected to play a pivotal role in achieving these goals, as it can utilize intermitent renewable energy sources such as wind, geothermal, hydropower and solar energy, together with CO2 directly captured from the air or from flue gas, and H2O, to store energy into chemical building blocks. Meanwhile, the catalyst is indispensable in these electrochemical conversions, as it enables the reduction of the reaction energy barrier, thereby lowering the electrochemical overpotential required to initiate reactions. Moreover, it facilitates the direction of reactions along specific pathways without itself being consumed in the process, thereby enhancing reaction rates and improving the efficiency. This thesis focuses on the electrocatalysts used for CO2 reduction and water spli􀆫ng, and uses atomic layer deposition (ALD) and molecular layer deposition (MLD) to precisely control the catalyst structure and protect the catalysts from degradation and poisoning...

The increasing dependence on fossil fuels for energy and chemicals has caused a significant rise in atmospheric CO2 concentrations, leading to global warming and ecological imbalances. Electrochemical CO2 reduction (CO2R) has emerged as a promising technology to mitigate CO2 emissions while converting it into valuable chemicals and fuels, such as ethylene, ethanol, and acetic acid. With its compatibility with renewable energy sources, moderate operating conditions, and potential for high selectivity, CO2R is positioned as a key player in the transition toward a carbon-neutral economy. However, challenges such as mass transfer limitations, impurities in industrial CO2 feedstocks, and economic feasibility hinder its large-scale implementation. This thesis aims to address these challenges through experimental studies, process design, and techno-economic analysis. The combined findings also reveal key limitations that must be addressed for large-scale deployment.

As global warming proceeds with increasing consequences, new and improved solutions that can mitigate the increasing CO2 emissions are becoming ever more important. Renewable energy technologies are advancing and along with carbon capture technologies, they promise to lower global emissions and help combat climate change. Renewable energy sources can be used to form value-added chemicals from captured CO2 through a method known as CO2 electrolysis. A novel approach to CO2 electrolysis is bicarbonate electrolysis, which uses carbon capture solutions directly to make products. By integrating the capture and conversion process this way, effectively bypassing the energy intensive steps of CO2 recovery required for conventional gas fed operation, CO2 conversion to products can become even more sustainable.

The objective of this thesis is to explore ways to improve the selectivity of carbon monoxide (CO) in a bicarbonate electrolyser. In this experimental study, focus is also placed on the stability of the process, characterising the selectivity over time. Causes of selectivity decline are examined as well as methods of improvement. Furthermore, the effect of pH on CO selectivity and stability is given special consideration. Literature in the field of (bi)carbonate electrolysis was reviewed to gather understanding on the process, to clarify recent advances made and to find areas for improvement. Based on the findings from the literature review the experimental study was designed.

Experiments were conducted in a membrane electrode assembly (MEA) flow-cell in constant current fashion, applying a current of 100 mA/cm2. The membrane chosen was a bipolar membrane (BPM) as it offers the possibility of operating with distinct electrolyte environments, separating the 3M bicarbonate catholyte from the 1M potassium hydroxide anolyte. Gas diffusion electrodes (GDE) were prepared by spray-coating silver nanoparticles on the surface, using Nafion ionomer as binding material. An interdigitated catholyte flow plate was used which ensured the bicarbonate would pass through the GDE due to its discontinuous channels forcing the flow through.

By introducing a catalyst-membrane gap through inserting a hydrophilic porous spacer between the GDE and the BPM, CO selectivity was improved from 50% to 78% in peak production, recording 55%
averaged over 3 hour operation. This enhancement in selectivity can be explained by the defined pH gradient resulting from the gap, permitting a low pH at the BPM for protons to react with the bicarbonate, liberating i-CO2; and a higher pH at the catalyst for CO2 conversion to CO while suppressing the hydrogen evolution reaction (HER). An optimum gap was found to be 135 - 270 μm. These results compare with the previously highest reported CO selectivity values from the literature at ambient conditions and 100 mA/cm2. While improving the selectivity, the stability of CO was not improved by the catalyst-membrane gap. The pH was found to affect both the selectivity and stability of CO, with higher bicarbonate pH leading to reduced selectivity but improved stability. This behaviour is explained by the reduced i-CO2 liberation and increased carbonation reactions taking place at higher pH levels…

Rising global CO2 levels underscore the urgent need for effective carbon capture and utilization (CCU) technologies to support a circular carbon economy. This study evaluates the techno-economic per- formance of a novel integrated CCU system that combines a K2CO3-based capture column with a bicarbonate electrolyser for syngas production, specifically targeting applications in the steel industry. An ASPEN PLUS model of the capture column was developed and integrated with a pH-dependent Faradaic Efficiency (FE) model of the electrolyser in Excel. Five cases were defined: (I) 90 wt% CO2 capture, (II) syngas production with a 2:1 H2/CO ratio for the Fischer-Tropsch process, (III) electrolyser operation with FECO > 50%, (IV) syngas composition suited as feedstock for electric arc furnaces (EAF) in the Energiron III process, and (V) an intermediate pH step. A techno-economic analysis (TEA) was conducted across worst, base, and best-case scenarios for each case.  Key findings reveal a trade-off between achieving high FECO at low pH levels and maximizing CO2 capture efficiency at high pH levels. Systems operating with large pH steps demonstrated a lower Lev- elized Cost of Syngas normalized to the Lower Heating Value (LCOSLHV ), due to increased hydrogen output. In contrast, systems with smaller and narrower pH steps incurred higher LCOSLHV due to their output’s lower LHV. The techno-economic analysis (TEA) indicates that the operational expenditure (OPEX) for the integrated CCU system is currently too high to be cost-competitive with alternative solu- tions. Sensitivity analysis reveals that the integrated CCU system is competitive with other electrolysis methods only under best-case conditions. Electricity costs and a low CO2 utilization ratio are identified as the primary drivers of OPEX. Improvements in these areas result in the most significant reduction in LCOSLHV, making them critical enablers for the integrated CCU system. Additionally, the cost per kilogram of CO2 saved is high compared to EU CO2 Emission Trading System (ETS) prices.  Current bicarbonate electrolysers are more costly than gas-fed CO2RR systems in terms of Unit Capital Cost (UCC) per kilogram of CO produced, largely due to reduced performance at higher current den- sities (>100 mA/cm2). Achieving CAPEX parity with gas-fed CO2RR systems would require increasing current densities while maintaining high FECO and sustaining these efficiencies at alkaline pH levels.  Future work should prioritize reducing both OPEX and CAPEX for the system, with a particular focus on improving the technical performance of the bicarbonate electrolyser. Key objectives include increasing current density while maintaining high FECO at alkaline pH levels, improving the CO2 utilization ratio, and enhancing the stability of the electrolyser.  Keywords: Carbon Capture and Utilization, Bicarbonate Electrolysis, K2CO3-based CO2 Capture, Ben- field Process, Integrated CCU System, Techno-economic Analysis

While the potential of CO2R towards C2+ products is widely recognised, the technique struggles with industrial scale-up. Within this report, an elaborated analysis is performed on the industrial potential of CO2 reduction, more specifically on cells operating in a strongly acidic medium. These type of cells boast not only a high CO2 conversion of over 70% but also potentially enable the extraction of liquids at concentrations required for efficient separation as opposed to anion exchange membrane (AEM) based cells. Although immature the acidic cell has been proven to achieve excellent C2+ product formation with a faradaic efficiency above 75%. A currently not surpassed barrier is the scale-up of acidic cells above 1 cm2 as most utilize a non-conductive polytetrafluoroethylene (PTFE) membrane. Within this research an alternate approach was examined adopting the conductivity of a modified commercial carbon gas diffusion layer (GDL). Using the hydrophobicity of PTFE beneath, within and on top of the catalyst layer, a COR promoting environment is created. Within this research, one of the largest highly acidic (pH<1) cells to date has been devised while yielding a respectable faradaic efficiency towards C2+ products of 53%.

Over the coming decades, human society has to transition from being dependent on fossil fuels to renewable energy sources. However, renewable energy sources bring with them several inherent problems that need to be solved to integrate them into our society. The power supply of renewable energy sources is intermittent and does not match the global energy demand, necessitating the need for energy storage to bridge the gap. Additionally, the chemical industry relies heavily upon the usage of fossil fuels as chemical feedstocks that cannot be directly replaced by (electrical) renewable energy. Electrochemical CO2 reduction to these synthetic fuels and chemicals provides a promising approach to both these problems. However, finding a suitable catalyst for this electrochemical reaction has proven difficult. So far, among the monometallic transition metals, only copper has been shown to actively reduce CO2 into the desired synthetic fuels and chemicals. Unfortunately, these reactions take place at high overpotentials and unselectively. Alloying different metals together provides an elegant way to find new promising catalyst materials for the electrochemical reduction of CO2 to synthetic fuels and chemicals. This thesis investigates different aspects of bimetallic electrochemical CO2 reduction to synthetic fuels and chemicals....

Ammonia can be used as a global energy carrier to connect the geographically divided landscape of renewable energy sources. Unfortunately, the current ammonia production process of the century old fossil-fuel based Haber-Bosch process is not sustainable and is responsible for approximately 1.2% of the global anthropogenic CO2 emissions. The most polluting part of the process is the hydrogen generation step by either coal gasification or the more common steam methane reforming. The majority of the emissions can be cut down by replacing this step by water electrolysis o en referred to as the electrified Haber- Bosch. An alternative technology for sustainable ammonia production, which is still in its infancy, is ammonia synthesis via the electrochemical reduction of nitrogen (NRR), requiring a proton source and electrons from renewable electricity. The following NRR approaches are prominently reported in the literature: (i) NRR in aqueous based electrolytes at ambient conditions (aqueous NRR), (ii) NRR at elevated temperatures with a solid oxide electrolyte, (iii) Li-mediated NRR in non-aqueous electrolytes at room temperature (Li-NRR). The main aim of this thesis is to identify and understand which of the above-mentioned electrochemical ammonia routes are the most promising for future application...

Ammonia (NH3) is a bulk commodity chemical known for its large production volumes and application in the global fertiliser industry, and is more recently being explored for its role as a sustainable energy vector. The industry standard ammonia production method is the century-old Haber-Bosch (HB) process, which is power by fossil fuels, and is accompanied with high energy intensity and large carbon-dioxide emissions. In this research, conceptual processes for the electrochemical synthesis of ammonia via the direct electrochemical nitrogen reduction reaction (e-NRR) from air and water were developed to assess their technical and economic viability compared to the HB benchmark. Different scales (91, 544 and 2055 t d-1) and electrolyzer cell configurations (alkaline electrolyzer (AEL), gas-diffusion electrode flow cell (GDE) and solid oxide electrolyzer (SOEL)) were considered. The results showed that small-scale production is more feasible for e-NRR NH3 synthesis due to the economies of scale of the HB benchmark. Among different electrolyzer types, the gas-diffusion electrode flow cell was found to be the most practical and economical for e-NRR NH3 synthesis. A sensitivity analysis showed that the electricity price is the most important parameter for the feasibility of e-NRR, and should ideally be as low as possible. Performance parameters of the electrolyzer, such as stack cost, operational current density, and faradaic efficiency, were optimized for minimal NH3 production cost, but were challenging to estimate due to the early stage of e-NRR technology. An optimized case was presented that demonstrated e-NRR NH3 can reach HB-parity, but the validity of the optimised parameters was difficult. It is advised that reliable laboratory-scale demonstrations are needed for an accurate assessment of the commercial feasibility of electrochemical NH3 synthesis.

Electrochemical cells and systems have been around for a few centuries. Lately, these technologies have been attracting attention. Although the technology to generate electricity from renewable sources is well developed and widely available -such as photovoltaic and wind energy- this is not always available. Because of this, it is necessary to store produced surplus electricity to be able to use it at moments when the sun is not shining or the wind is not blowing. Many different electrochemical technologies can be used to store electricity or transform it to a useful energy carrier- such as hydrogen. However, the energy transition will also need to address the optimal usage of critical materials. Integrating functionalities and optimizing energy storage can help bridge the gap between electricity production and consumption using only a limited amount of critical materials. New innovative technologies that use less critical materials will be key to sustainably transition to a fossil-fuel free future. It will be necessary to move forward and upscale technologies at a quick pace. A combined modeling and experimental approach can help move through the TRL development stages quickly, optimizing the use of resources and experimental work required. The battolyser is a new integrated battery and electrolyser system that provides flexibility in energy storage. During periods of high availability of renewable energy it can be charged indefinitely, filling up the battery capacity first and producing hydrogen from there on out. A battolyser system can be used to guarantee access to cheap electricity and green hydrogen, all in one device and using the materials required for one device. Modeling the electrochemical reactions of the battolyser and optimizing the cell design parameters when moving towards an upscaled system is a tool that can be used for the continuous development of a better prototype and scaling up. Chapter 3 describes the modeling studies performed on the battolyser system, including the relevant experimental validation. Here, a 1D COMSOL model was developed to study the cell parameters and understand the effect of electrode and gap thickness, electrode porosity, and electrolyte conductivity. Testing experimentally at larger scales is challenging and often not done. Highly alkaline KOH electrolytes are usually not tested in lab conditions, and therefore the effect of higher concentrations than 5M KOH is unknown on new electrode material developments. To optimize an integrated device, the effect on both the electrolysis function and the battery function need to be reconciled and designed for the specific application. In Chapter 4, extensive lab scale experiments on the electrolyte concentration are described, including different alkali metal cation concentrations. To optimize for different functionalities of the battolyser, different cations can be used at specific concentrations. A flow cell was designed and built, and different flow configurations were tested. 3D printing technology allows for quick iterations and modifications of the design, however the proprietary resins are usually not tested at highly alkaline conditions which could potentially cause degradation of the cell components. Working with higher than 5MKOH concentrations results in practical difficulties that will only scale with plant capacity. In Chapter 5, the preliminary results of a flow cell configuration are included. The results of this work can be applied directly to predict the optimal design and operating parameters of an up-scaled battolyser cell. This will allow for quicker iterations of up-scaled designs to further develop the prototype technology. For this, it is important to verify simulation results with experimental data. Using a combined approach including simulations and experimental work allows testing various setups and optimizing the energetic efficiency of the device. 3D printing manufacturing technology can also help speed up this iterative process to generate design modifications and quickly manufacture experimental setups to validate the simulation data.

To reduce greenhouse gas emissions and to limit global warming, fossil fuel based energy technologies need to be replaced by clean energy technologies. Renewable energy sources are dependent on weather conditions therefore security of energy supply is not ensured and the need for energy storage is growing. Hydrogen is considered a clean energy carrier that can be used for energy storage. Water electrolysis that uses renewable energy is a sustainable method for the production of green hydrogen. Water electrolysis is a process by which water is split into hydrogen and oxygen by using direct current to drive the reaction. During the production of green hydrogen heat is released to the environment. Little research has been conducted on the amount of heat released during green hydrogen production and on the temperature of the released heat. It is unclear if the released heat could be used.
This research serves to answer the question "Is it possible to use the heat released during the production of green hydrogen using alkaline water electrolysis?". To answer this question a model has been developed in ASPEN Plus. This model represents an alkaline electrolyser, consisting of an electrochemical model, a thermal model and a cooling system. To validate the output of the model, the hydrogen production output has been compared with an alkaline electrolyser developed by the company Nel hydrogen. The thermal efficiency of the model has been calculated with and without using the waste heat. To use the waste heat the system first needs to be cooled. To determine which heat exchanger is best to use for recovering the heat of the alkaline electrolyser, three different designs have been implemented in the ASPEN model. The designs have been analyzed and the best option is implemented in the model. The amount of heat that can be recovered has been investigated as well as the options for the use of the recovered heat. The results are discussed and recommendations are made for follow-up research.

One of the greatest concerns of this century is climate change due to rising greenhouse gas emissions. The ammonia industry is responsible for 1.4% of the global CO2 emissions, thereby having a negative climate impact. However, ammonia is an essential ingredient in nitrogen fertilisers and is also considered as a potential energy carrier. Ammonia is currently produced in the energy intensive Haber-Bosch process, where natural gas, coal or oil are used as the hydrogen and energy source. For these reasons, it is necessary to investigate more sustainable alternatives. One of the alternatives is electrochemical ammonia synthesis, where ammonia is formed out of nitrogen and water, and renewable energy sources fuel the process. The goal of this thesis is to give insight into the required performance metrics of the electrolyser and its production process in order for this technology to become technologically feasible and competitive with a Haber-Bosch process. This is done by investigating different options for the pre-treatment, electrochemical ammonia synthesis and separation steps, and comparing their energy requirements. The final product of this thesis is a process design for an electrochemical ammonia synthesis plant with a production capacity of 1,500 tonnes per day. Cryogenic distillation and pressure swing adsorption are researched and modeled as nitrogen generators. An adsorption column for the pressure swing adsorption unit is modeled in Matlab, while Aspen is used for modeling of the distillation column. Subsequently, an alkaline and a proton exchange membrane electrolyser are considered as ammonia synthesisers. The electrochemical cells are modeled as black boxes, operating at 353 K and 1 bar and 30 bar respectively. Next, a distillation column and a flash drum are modeled in Aspen as ammonia separators. Finally, four different process diagrams are created, two that are based on an alkaline electrolyser and two based on a proton exchange membrane electrolyser. Their overall energy consumptions are analysed and for both types of electrolysers the most optimal route for ammonia production is found. The results of this thesis point out that cryogenic distillation is preferred over adsorption for the generation of nitrogen, with an energy consumption of 0.56 kWh/kg nitrogen for compression, cooling and distillation. Adsorption can be competitive with cryogenic distillation when the nitrogen recovery rate is increased, or at lower production capacities. In a reasonable case for future electrolysers, operating at a cell voltage of 1.77 V and a Faradaic efficiency of 100%, the energy consumption for an alkaline electrolyser amounts to 12.00 kWh/kg ammonia and to 11.95 kWh/kg ammonia for a proton exchange membrane electrolyser. A distillation column was considered as utility for the separation of ammonia from the KOH solute product stream from an alkaline electrolyser. For the cathodic product stream from a proton exchange membrane electrolyser, containing only ammonia, hydrogen and nitrogen, flash separation was determined to be the best separation technology. For both separation options, it was found that an ammonia concentration of at least 10 mol% in the cell’s product stream is required for an efficient separation. Ultimately, with electrolysers operating at 1.77 V and a Faradaic efficiency of 70%, the total energy consumption of an alkaline based process is equal to 17.30 kWh/ kg ammonia and 15.45 kWh/kg ammonia for a proton exchange membrane based process, with overall energy efficiencies of 30% and 33% respectively. Based on the energy consumption of their respective pre¬treatment and separation steps, a proton exchange membrane electrolyser is favoured over an alkaline electrolyser. In order for an alkaline based process to be advantageous, its electrochemical cell should consume at least 1.80 kWh/kg ammonia less than a proton exchange membrane electrolyser. Finally, currently it is not possible for AEL or PEMEL based ammonia synthesis processes to be competitive with the Haber-Bosch production process in terms of energy consumption. However, with Faradaic efficiencies of 100% and minimal overpotentials, a PEMEL based process does come close to reaching this objective.

Electroreduction of CO2 into high-valued chemicals is a promising way to reduce CO2 emissions while simultaneously producing bulk chemicals currently produced from fossil-fuel feedstocks. The downside of this process is that conversion rates are low, meaning the resulting product stream is a complex gas mixture consisting primarily of reactants and by-products and a relatively small amount of product. This study focuses on the development of a new downstream separation process to capture ethylene from a mock-up reaction mixture (mole fractions C2H4/CO2/CO/H2/H2O : 20/55/15/15/5), based on low driving forces and suitable for application in a 100kW test case within the e-Refinery. An extensive literature study of numerous separation techniques for gases was conducted and adsorption was chosen as the most suitable option. After screening of various adsorbents, active carbon was selected as the most potential sorbent. Based on a selectivity analysis, the primary focus was on the behaviour of C2H4/CO2 on active carbon. Using a simple, custom-build set-up, transient breakthrough experiments were performed for this gas mixture and the resulting selectivity for an equivolume feed, yielded a lower separation performance than expected based on the ideal adsorption solution theory, respectively a selectivity of 1.5–1.7 versus 3.2–3.5. Additionally a theoretical model was developed using MATLAB, which described the velocity profile inside the adsorber column and could qualitatively predict breakthrough behaviour. Further analysis led to the conclusion that for a more accurate quantitative match between experimental and numerical results, isotherm parameters should be obtained from the same type of active carbon. Ultimately this technique could be used to increase the ethylene content in a CO2-bearing stream and pave the way for a new, energy-efficient method to obtain hydrocarbons, ethylene in this case, from an electrolyzer cell.

An energy transition is needed in order to combat climate change. With the rise of intermittent renewable energy, a need for energy storage is also inevitable. Carbon dioxide electrolysis is a potential solution as CO₂ emissions can be recycled and subsequently converted for energy storage. However, the technology is rather new and research has yet to be conducted in this field. An important aspect is the temperature within an electrochemical cell, especially when scaling up. An increase in temperature can benefit the performance of the cell, but it also has downsides. Hot-spot formation with non-uniform reaction kinetics and thermal sensible components can have a great influence on the life-time of the cell.



For that reason, a modeling study on the heat generation within carbon dioxide electrolysis systems is done. Different volumetric gas flow rates of carbon dioxide have been considered for two geometries: the membrane electrode assembly (MEA) and the gas diffusion electrode (GDE). The model considers three separate models: a mass model, electrochemical model and thermal model, and operates at a fixed current. The finite difference method is applied using Python 3.0 to solve the relevant conservation equations. Furthermore, the model includes different material layers, where the materials and dimensions are based on recently done experiments.



The model showed that irreversible losses caused by the activation overpotentials are the biggest contributor to the total heat generation. Reversible heat also contributes to the heat generation, where heat is required in the anode and heat is generated in the cathode. Furthermore, within the cathodic catalyst layer most heat is generated. Joule heating caused by ohmic losses has proven to have negligibly impact on the total heat generation. As a result, the hot-spot is located within the cathodic catalyst layer for both geometries. Due to the additional electrolyte in the GDE, the hot-spot does not reach the membrane, in contrast to the MEA. Besides, different results in the y-direction are observed for the volumetric flow rates. For both geometries, the hot-spot is located at the inlet for 10 ml/min and in the middle for 100 ml/min. From the analysis, the GDE is more favorable as less heat is expected and the hot-spot does not reach the membrane. The sensitivity analysis showed that the thermal conductivity is of great importance.

Within the electricity driven conversion methods, electrocatalysis has been assessed to be the closest to commercialisation. This can only be realised, if the system efficiency is increased in terms of the reaction rate, onset potential and/or selectivity. Current research has shown that, apart from the more widely investigated routes to improve these factors (GDEs, catalyst, etc.), cascade electrode systems and high pressure reactors could prove to be effective for increasing the system efficiency. By splitting the CO2 reduction into two separate steps, CO2 reduction to CO and the sequential reduction to C2+ products, both steps can be optimised in terms of operating conditions. Applying a cascade system has, therefore, shown to increase the selectivity and reaction rates in the system. Additionally, the advantage of the high pressure reactor originates from the fact that increasing the pressure, will result in an increase in the solubility of the reactant. By increasing the solubility, the mass transport to the electrode surface will subsequently be enhanced. The low solubility of reactants is often identified as a main limiting factor in system efficiency, therefore, increasing solubility has proven to increase reactant transport (current density) and selectivity in high pressure systems. Even though both of these advancements have shown promising results, technoeconomic studies indicate that their feasibility is still too low to become commercially attractive at this point. Therefore, this research proposes to combine both technologies to increase the overall system efficiency in terms of: increasing the current density, increasing the Faradaic efficiency and decreasing the overpotential losses for the production of C2+ products. Since this combination has not been investigated before, and both technologies are still rather new, there will be a lot to investigate in order to demonstrate the potential of this new combination. Therefore, in addition to an extensive literature study to uncover the relevant unanswered research questions regarding this field of research, a mathematical model was developed. A model is a valuable resource in determining the potential for a novel system, as it enables instantaneous control over system parameters and, therefore, can provide a lot of insight into its relations and limitations. However, as the accuracy of a model strongly depends on the quality of its input data, this research will also provide a design approach leading to a novel high pressure cascade reactor design. Eventually, the model can, therefore, provide the insight required for extensive experimental research, while the design can simultaneously aid in the improvement of the model. The results evaluated by the model demonstrate both sequential reduction steps are positively affected by increasing the pressure. In addition, the otherwise poor CO solubility, can be dramatically increased by applying the combined system. The reaction rates are also evaluated to increase with the higher reactant concentration of CO and CO2. In addition, since the hydrogen evolution reaction is not affected by the pressure, as it does not present mass transfer limitations, the selectivity has been shown to also increase with increasing the system pressures. Additionally, the presence of the high CO concentration in the reduction towards C2+ products affects the selectivity of the system as well. The CO reduction reactions possess different behaviour from the CO2 reduction reactions. Therefore, by indicating the share of both separate reduction reactions in the generated products, operating conditions for the maximum C2+ selectivity can be identified. This way, the combination of high pressure on a cascade system, has been demonstrated to possess a lot of potential for increasing the system efficiency towards C2+ products.

The growing global energy demand and correlated rise in carbon emissions is forcing us to increase the use of renewable sources. The residential sector represents a large part of the total energy consumption, and European governments are investing in distributed PV to increase the renewable share in this sector. However, on top of the solar power variability, residential systems are also characterized by very unstable load profiles. This issue can be solved by incorporating energy storage, that has many technical and economic benefits for the prosumer, especially if a long-term seasonal storage technology is used. Among all the available storage types, after an extensive literature study, some developing technologies proved to be suitable for this purpose: redox flow batteries (all-vanadium, hydrogen-bromide, zinc-bromide) and hydrogen systems.

For this work, a model was built in MATLAB Simulink to study and reproduce the behaviour of these storage systems in a grid-connected residential environment, and an optimization was set up to find the optimal sizing of the components and investigate the economic feasibility of the whole system. The models results proved that in the present scenario, storage integration is still too expensive with these technologies. However, future projections with different incentive scenarios demonstrate the potential of vanadium and zinc-bromine batteries, and highlight a dramatic cost reduction for hydrogen systems.

Reducing carbon emissions in the power generation sector can be done by generating energy from renewable sources such as wind and sun. However these sources alone cannot provide a reliable electricity system and therefore an energy storage system is needed. Power-to-gas is a concept in which surplus renewable electricity is used for the production a gas fuel. The gas can be stored and is used for electricity production when there is a deficit in renewable electricity. When CO2 exhaust gases form reactants for new production of gas, the system has no net CO2 emissions. The gas functions as an energy carrier for electrical energy. Methane could be an interesting gas for large scale energy storage as there is much knowledge about methane transport, storage and combustion and the gas is easier to store than hydrogen. Power-to-gas-to-power conversions come with great electricity losses. Therefore it is interesting to investigate what waste heat streams can be extracted from the process to use for external purposes. The aim of this thesis is to map the input and output energy streams of a power-to-methane-to-power system operating in 2030 to find the efficiency of the system and to see how efficiency could be maximized by using waste heat streams for external purposes. Also, a power-to-methane-to-power system requires a lot of gas storage capacity. Therefore it is useful to estimate the capacity of the gas storage facility to find if the system is technically feasible and to see what gas storage does with the efficiency of the system. Last, since the aim is to reduce carbon emissions the (small) CO2 emissions of the system are determined and analyzed. The system is scaled up to a scenario in which it provides a fully renewable electricity grid in the year 2050, to explore the feasibility in terms of carbon emissions and required gas storage capacity. The system is also compared to a power-to-hydrogen-to-power system to find the most feasible solution. The round trip energy efficiency of the system in 2030 is 88.0%, which is the sum of an electrical efficiency of 30.0% and a thermal efficiency of 57.1% The total energy efficiency can only be achieved when streams modeled as usable heat output streams can actually be used. This depends highly on the location of the system. The carbons emissions of a system with a 44MW output in 2030 are 56.3kt and the required gas storage capacity is 5.09 x 10^5 m3 of underground salt caverns. When scaling up the system to provide a fully renewable electricity grid, the total gas storage capacity is 9.34 x 10^7 m3, which is 5.5% of the total potential salt cavern volume in the Netherlands. The carbon emissions are 2.78 kt/y, which is 0.0056% of the current annual carbon emissions caused by the Dutch power generation sector. Compared to a hydrogen system the methane system performs worse in term of electrical efficiency, gas storage capacity and carbon emissions.

The so-called “hydrogen economy” became one of the scientific targets among the different renewable energies alternatives, as a result of the efforts to transition from fossil fuels to environmentally-friendly energy sources. In this context, various options to transport and store hydrogen are being explored. Gaztransport & Technigaz (GTT) company, intending to be part of this challenge, is exploring the possibility to transport liquid hydrogen (LH2) in pre-existent ship’s containers initially designed for liquid natural gas (LNG) transportation. This project is about the study of the surface effect of the interaction between hydrogen with iron-based alloys in the case of 304L stainless steel (uncoated and coated with TiO2) and Invar alloy.The methodology consisted of electrochemical induced hydrogen evolution on an iron-based austenitic metal cathode taking advantage of the intermediate adsorbates (atomic hydrogen) generated during the reaction to study the electrochemical adsorption efficiency. Characterisation of the materials, by techniques like XRF, XRD, optical microscopy, and SEM, is conducted before and after hydrogen exposureso that it was possible to evaluate the effect of hydrogen ingress.The results showed that the chemistry of the surfaces is irreversible changed after the electrochemical induced hydrogen sorption/desorption process due to the formation of oxides. The amounts of hydrogen desorbed were quantified after different H2 loading times. In all cases, the amount of hydrogen desorbed showed a maximum after which the hydrogen desorbed decreased significantly. The maximum for uncoated 304L stainless steel was after 24 h, 90 min for the coated 304L, and 2 hfor Invar. The welds are the most vulnerable sections to hydrogen ingress in both cases. XRD results before hydrogen exposure revealed that 304L consists of an austenitic matrix with around 5% of ferrite. An increment of the austenitic volume fraction of 2.2% was observed after the H2 sorption/desorption process. Invar is a purely austenitic phase, and no changes in the phase composition were observed after the H2 sorption/desorption process.