The Paris Agreement aims to limit global warming to 2 degrees Celsius by reducing carbon dioxide emissions. Data centers are essential to the digital world and contribute approximately 1% to global electricity consumption. Major tech companies like Amazon, Microsoft, Google, and Meta are large contributors to this consumption. In countries like the Netherlands, this contribution is higher due to the Netherlands being a digital hub, accounting for 3% of the total national electricity consumption. Considering their significant energy usage, it is interesting to research how data centers can transition to using only renewable energy sources on an hourly basis so they can decrease their footprint. This thesis aims to identify a feasible and cost-effective configuration for a 100 MW data center that operates 100% on renewable energy on an hourly basis. The research will assess the environmental impacts, focusing on CO2 emissions (CO2e) and land usage, with the main goal of achieving the lowest Levelized Cost of Electricity (LCOE). First, an overview of potential energy storage technologies is provided to find suitable solutions. Based on different criteria, Lithium-ion (Li-ion), lead-acid, vanadium redox flow batteries (VRFB), zinc-bromide flow batteries (ZBFB), alkaline and PEM (proton exchange membrane) electrolyzers, storage tanks, and PEM fuel cells are chosen. With these technologies, eight different configurations are composed, combining various battery types and electrolyzer types with the storage tank and PEM fuel cell: Li-ion with alkaline (1), lead-acid with alkaline (2), Li-ion with PEM (3), lead-acid with PEM (4), VRFB with alkaline (5), ZBFB with alkaline (6), VRFB with PEM (7), and ZBFB with PEM (8). An energy model is developed to identify the most feasible and cost-effective solution, using solar, wind, and load profiles as input. The assets are modeled based on their technical parameters and limitations. A 10-year simulation of the hourly energy flows is executed. In this simulation, the constraints, degradation, and replacement of the assets are taken into account. The costs are determined based on Capital Expenditures (CAPEX), Operating Expenses (OPEX), replacement costs, and residual costs of the asset after 10-year. The connection costs are also considered. The configuration dimensions are optimized based on the LCOE using the particle swarm optimization method. The two configurations with the lowest LCOE are evaluated for environmental impacts, including CO2e and land usage. The study shows that combining a Li-ion battery or VRFB with an alkaline electrolyzer is the most cost-effective and feasible configuration for a data center. The LCOE is 0.243 and 0.241 €/kWh, respectively. Additionally, this reduces kg CO2e/kWh by 7-8 times compared to the current grid emissions in the Netherlands, which are approximately 0.037 and 0.032 kg CO2e/kWh. However, the required land usage is approximately 20 km², and the LCOE of the configuration is about 2.5 times higher than the current power purchase agreement (PPA) price for a data center, which is 0.10 €/kWh. Therefore, it is recommended to initially start with an 80% renewable scenario using only a Li-ion battery and renewable energy sources. This approach reduces the LCOE to 0.12 €/kWh, which is approximately the same as the current PPA price if an additional 0.01 €/kWh for the connection to the grid is included. Furthermore, kg CO2e/kWh is three times lower compared to the current situation, which is 0.08 kg CO2e/kWh. This strategy also mitigates the potential risks associated with the infancy of VRFB and the combination of intermittent renewable sources and an electrolyzer, as these are excluded in this scenario.

A transition towards renewable energy sources is necessary to globally achieve net-zero carbon emissions. To achieve reliable renewable energy production, storage of sustainable energy is key. The utilisation of hydrogen, whether for long-term or short-term storage, could provide a vital solution in achieving reliability for sustainable energy demand. However, producing green hydrogen (water electrolysis) is not cost-competitive with fossil fuels and other hydrogen production methods. An original alkaline electrolyser design has been created that can help with lowering the CAPEX and OPEX when optimized. In common alkaline electrolysers, electrodes are completely immersed in liquid. However, the electrode surfaces of the novel design apply wetted surfaces through the use of capillary forces to suck up electrolytes. Compared to immersed electrode surfaces, the wetted surface shows a reduction in (bubble) resistance leading to lower voltage losses. Additionally, the overpotential can be further reduced with, for example, the use of a PES membrane instead of a common Zirfon membrane, better contact with the anode by welding it to the current collector, and adding PTFE to the anode for better gas removal. Prior to testing the assembled cell design, the electrodes were tested individually to understand the mechanics of each electrode. Here, we report that the electrodes perform well individually in alkaline media.
However, our findings reveal that the assembled cell performs poorly, mostly because of significant overpotentials brought on by increasing resistance, along with other drawbacks including salt formation and exceeding the legal gas crossover limit. Some of these issues regarding performance are still poorly understood and therefore need further investigation.

Electric vehicles have become the focal point to reduce GHG in the quest for cleaner and more efficient means of transportation. At the core of this transition lies the key component of energy storage, the battery pack. The battery pack has electrodes manufactured with the binder PVDF. Using fluorine-rich compounds makes processing and recycling a more difficult process and it is also harmful for the environment. Therefore, alternative binders should be explored to adhere to the principles of sustainability. In this thesis, a comparative study is presented between electrodes fabricated with fluorine free polymeric binders and PVDF. Moreover, a battery cell solely constituted of fluorine free components is fabricated and tested. By using the analysis techniques electrochemical performance testing, electrochemical impedance spectroscopy and scanning electron microscopy the effect of the binders and electrolytes on the electrode performance is explored. The findings reveal that during rate performance cycling, graphite half cells based on the binder PAA demonstrated the highest discharge capacities at C-rates up to 4C with a capacity retention of 39%. NMC811 half cells based on a composite binder CMC/SBR achieved the highest capacity retention after 100 cycles, being 24%. All the fabricated full cells suffered from a 30-40% initial capacity depletion after the first charge, presumably originating from oxidative degradation of the electrolyte. Consequently, the highest capacity retention after 150 cycles was 14% for a PVDF based full cell. Fluorine free cells using a PAA anode and CMC/SBR cathode exhibited expected capacities with a higher overpotential.

Carbon sequestration involves the conversion of carbon dioxide gas into carbonates through reactions with magnesium or calcium bearing minerals, presenting a potential method to mitigate CO2 emissions and reduce their release into the atmosphere. This process can be classified into two categories: in-situ (subsurface) and ex-situ (surface). CO2 mineralization has demonstrated the ability to permanently store substantial quantities of CO2 without the need for extensive post-monitoring efforts, while also offering potential business model benefits for the generated end products. However, the field of mineralization still faces substantial technical and economic challenges, including high cost projections and slow carbonation kinetics, impeding further technological advancements in the field. Additionally, logistical challenges related to plant design and associated emissions contribute to the complexity of finding solutions to these problems.

In this particular study, the investigation focused on ex-situ carbon dioxide sequestration primarily through direct aqueous mineral carbonation, incorporating a direct air capture element. This approach was chosen due to the flexibility provided by direct air capture in terms of CO2 supply and the desirable characteristics of direct aqueous carbonation. Various models were developed using Aspen Plus software and compared, leading to the selection of the direct aqueous carbonation system as the optimal choice for CO2 carbonation. Further enhancements were implemented in the system, including recycled streams and heat integration, to reduce overall energy consumption. Optimization steps were also undertaken to determine the appropriate sizing of key equipment that make up the majority of the system's overall costs and to improve system efficiency.

An economic analysis was then performed, revealing that plant scales of 50 ktons/year yielded a positive Net Present Value (NPV), indicating profitability. Conversely, smaller-scale plants of 0.5 ktons/year and 5 ktons/year did not generate positive revenue, even with a high carbon credit price. This was followed by a sensitivity analysis that showcased the relevant parameters that holds significant effects on the economic performance of the system.

Moreover, business model cases were also explored, and it was concluded that utilizing the end products in building materials and road construction could potentially generate additional revenue for the mineralization system beyond storage options.

Overall, this investigation highlights the potential of ex-situ carbon sequestration through direct aqueous mineral carbonation, considering direct air capture, and emphasizes the importance of economic viability and revenue diversification in the successful implementation of mineralization systems to mitigate the effects of global warming.

Dutch offshore wind farms are primarily located near the coast and connected to the grid, but as offshore wind farms continue to expand and occupy more space, developers are turning to deeper waters. However, installing traditional monopile or jacket structures in these deeper waters can be expensive. One potential solution is the Dogger Bank, a shallow sand bank located 275 km from the Dutch coast, which allows for inexpensive wind turbine installation and high capacity factors. For far offshore locations like the Dogger Bank, electricity transport can be costly, making hydrogen transport via the existing gas infrastructure a more efficient solution. This could enable hydrogen production far offshore and its use in the decarbonisation of hard-to-abate sectors.

This study focuses on the development of a stand-alone offshore hydrogen production system, incorporating battolyser technology, on the Dogger Bank. The battolyser, which combines a Ni-Fe battery and an alkaline electrolyser, is able to store energy and produce hydrogen efficiently and flexibly. However, as the battolyser is not yet commercially available, the proposed systems are analysed for the years 2030 and 2050. The study examines three system configurations: an electrolyser-battolyser, battolyser-only, and electrolyser-battery configuration. For the electrolysers, alkaline and PEM electrolysis is considered. For the battery a vanadium redox flow battery (VRFB).

To ensure autonomous operation, the battolyser serves as a backup power source for supplying
the system’s auxiliary energy demand. This demand is used to determine the system’s sizing, and a MATLAB/Simulink model is developed to calculate the hydrogen production based on future estimates of technical parameters. The hourly wind data from offshore platforms near the Dogger Bank is used as input for the model. To project future capital expenditures (CAPEX) for the model components, a learning curve method is employed, which assumes a specific cost reduction rate with every doubling of cumulative installed capacity.

This study demonstrates the feasibility of an offshore wind-hydrogen system with battolyser
technology, both technically and economically. Battolyser CAPEX are used as an input variable to estimate a range of possible levelized cost of hydrogen (LCOH) values for different systems. The study findings indicate that LCOH values of less than 1.55 C/kg could be achieved by 2030, and by 2050, these values could drop to less than 1.18 C/kg.

In the current world, fossil fuels and non-renewable energy sources are being phased out and renewable energies are taking their place. Electrical energy from these renewable energies are more difficult to store than fossil fuels. Transportation and feedstock for industries can also not always be supplied through electricity. Here, hydrogen has the potential to be a good alternative, clean energy source. However, hydrogen is currently either not produced in a renewable manner (grey hydrogen), or renewable but too expensive (green hydrogen). Grey hydrogen is less expensive at a cost of around 1.50 €/kg, while green hydrogen has a cost of 2.50 - 5.00 €/kg. A new method to produce green hydrogen at a lower cost is the hydrogen turbine developed by HYGRO. This method uses an integrated electrolyser system in an existing wind turbine to produce green hydrogen and has the potential to drop the cost of green hydrogen to a more competitive level. Since this new method changes design questions of the wind turbine, a potential increase in specific power, along with a decrease in cost of hydrogen (expressed in levelised cost of hydrogen, or LCoH) is expected. In order to research these effects, this thesis investigated the effects of an increased specific power on the cost of the components of the turbine and its design, as well as the amount of hydrogen it was able to produce. This investigation was done through a model along with a case study to determine the results for a specific case. The increase in specific power was achieved by increasing the power rating of the turbine, but maintaining a constant rotor radius and maximal rotor speed. The case study performed investigates the effects based on a 15 MW reference turbine, which has a specific power of 331 W/m2. Its power rating was increased from 15 MW up to 35 MW and the resulting costs and production amount were compared to determine an optimal point were the resulting LCoH would be lowest. The result of the case study suggests that if the 15 MW reference turbine was converted to a hydrogen turbine, the optimal power rating would be in the range of 25-30 MW, where an LCoH of around 1.69 €/kg was achieved. These power ratings are equal to a specific power range of 553 - 663 W/m2 or 66 - 100 % higher than the reference. Through an error analysis, the results were validated and determined to have a reasonable degree of certainty. The main component of the model that could be improved upon to increase its robustness is the wake loss model, which was not precisely modelled but does have a significant effect on the results.
The increase in specific power suggests that hydrogen turbines have the potential to perform at higher power ratings than the wind turbine it is based on. This would allow the green hydrogen cost to drop significantly, down to a value were it is nearly competitive with grey hydrogen.

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.

In the search for new large-scale battery technologies that are cheap, safe and durable, in this research it is found that the state of the art semi-solid redox flow battery (“SSRFB”) technology shows perspective. Therefore, a novel, cheap and safe aqueous SSRFB design is introduced based on iron- and manganese hydroxide materials. This design was subsequently tested in the lab through which it was concluded that it is not possible to effectively make a SSRFB based on FeOOH and Mn(OH)2 semi-solid electrode (“SSE”) suspensions. Systematic proof is provided on how this can be attributed to the specific combination of the active materials in the highly alkaline environment, but that in itself, both the tested SSRFB configuration and SSE suspensions are functional. On the one hand, a complex chemical system of the active species consisting of multiple thermodynamic equilibria had the consequence that the open circuit voltage (“OCV”) had a very low value of 0.2 V instead of the expected 0.63 V. On the other hand, it was shown that both the FeOOH and the Mn(OH)2 SSE suspension could be separately cycled versus respectively a solid zinc and copper electrode and that a cheap microfiltration membrane suffices in the design of a SSRFB (on the condition that ionic active species are not apparent in the system). In sum, this research demonstrates that the SSRFB by its very nature is a suitable battery technology in which cheap and safe materials can be used for large-scale energy storage. However, that the combination and optimization of the right active materials in the right environment proves to be fundamental for proper operation and is in fact a complex balance between electric and ionic conductivity, colloidal stability, viscosity and (electro)chemical stability.

The transition from stable fossil fuel energy production to intermittent renewable sources poses a huge challenge to our generation. The mismatch between electricity supply and demand will become an increasingly
pressing problem, due to the growing share of renewable energy production and the electrification of the automotive industry. The upcoming field of Solar Redox Flow Batteries (SRFB) proposes a possible solution in the search for feasible energy storage. Integrating a photovoltaic module with a redox flow battery results in a highly reversible storage system, which has both the advantage of flexibility in capacity and charge/discharge power, as well as the direct and efficient coupling to solar energy generation. However, due to its early stage in development, the SRFB has several challenges to overcome, like low charging efficiencies and energy densities. To improve SRFB efficiency, this work provides a performance estimation model, to identify general sensitivities of SRFB efficiency with respect to the loss mechanisms associated with the device, such as (but not limited to) kinetic overpotentials and resistances, to establish device design principles. This model is also extended to simulate realistic environments, using real (daily/seasonally dependent) solar radiation and ambient temperature data. Moreover, a fit tool is realized to identify losses in a specific system, by varying parameters and mimicking experimental linear scan voltammetry data. Experimentally, several conducting layers are tested for their kinetics and conductivity and impact on device performance. A thin film of platinum is found to result in the best kinetics and lowest resistance. However, carbon-coated electrodes also show promising results and could be a feasible and cheaper alternative to platinum, when applying layer-optimization. Based on the design principles derived from the modeled and experimental results, a single-junction siliconbased SRFB is developed, based on a ferri-/ferrocyanide electrolyte, coupled with Cu2+=+, yielding high solar-to-chemical charging efficiency of 9.4%. This efficiency is close to the simulated efficiency, while multiple other SRFB photo-charging experiments follow modeled trends, validating the model.

Up until now, ammonia has primarily served the fertilizer and chemical industry with limited regards to carbon emissions. The hydrogen within the ammonia molecule is commonly produced using fossil fuels; this process accounts for 1,2 % of the global CO2 emissions. Rather than using fossils fuels, electrolysis with renewable energy could be used to produce “green” carbon-free hydrogen and ammonia. The transition could decarbonize the ammonia industry and reduce carbon emissions drastically. Considering future emission restrictions and emerging new technologies, the techno-economic cost of green ammonia is expected to decrease. Accordingly, the the research question to be answered was: What are the effects of techno-economic induced cost reductions on the potential for green ammonia in the current and future global ammonia market? This report investigates the production and import costs over several global-oriented supply chains using a supply chain model. Rather than predicting the future, the aim is to gain an insight into the future of the global ammonia market. The scope of this thesis considers world scale ammonia production facilities with a capacity of 2200 ton NH3 per day. Due to their potential to become a producer or consumer of green and grey ammonia, the following countries are examined in more detail: Australia, Brazil, Chile, China, India, Japan, Morocco, Mexico, the Netherlands, Norway, Oman, South Korea, Trinidad & Tobago, United Kingdom, United States. The supply chain model distinguishes between the “grey” (with hydrogen from natural gas through SMR) and “green” (with hydrogen from electrolysis on renewable energy) production process. The supply chain model that is used to gain insight into the future global ammonia market models a green ammonia production process. The following components are included in the model: a 1,5 GW electrolyser (78 % efficient, 2020), 100 MW Haber-Bosch reactor, 57 MW air separation unit, and a 94 MW fuel cell (60 % efficient) for a location that has 5000 FLH. The modelled production plant is sized according to the full load hours in a given country and implemented electrolyser efficiency according following a learning curve (75 % in 2020 to more than 82 % from 2030. In future research, it would be more exact to optimize the production plant’s exact sizing using the available resources at one specific location. Results show that up until 2030 importing grey ammonia is cheaper than domestically producing green ammonia. In addition, due to the sensitivity of the ammonia industry to carbon leakage, a strong CO2 policy is essential to the transition. The main cost drivers for green ammonia are the hydrogen buffer and firm up power of the Haber-Bosch reactor and air separation unit; both are directly related to the cost of the renewable energy, which in turn is strongly dependent on the weather conditions. Currently, Morocco, Chile and China are considering the domestic production of green energy as it is expected to become cost-competitive with domestic grey production of ammonia by 2020. The analyses show that in addition to these countries, Australia, the United States of America and Oman show optimal conditions for domestic production of green ammonia as well.

Lithium ion batteries are currently the most attractive choice for mobile energy storage and power sources [4] in terms of energy density. However, it is unlikely that current battery chemistries will reach the energy density desired for future applications [8]. Li-metal, which has the highest reduction potential of all metals and the highest achievable capacity per weight unit, could significantly increase the energy density when used as an Anode. Unfortunately, the dendritic growth of Li-metal is a safety hazard, and the low electrochemical potential of Li-metal
is outside the stability window of many electrolytes leading to capacity degradation.
It has been suggested that safe Li-metal Anodes may be possible in combination with solid electrolytes, as the higher shear modulo of solid over liquid electrolytes may be able to suppress dendrite formation [21].
To study the stability of the interface between Li-metal and the solid electrolyte Li6PS5Cl, electrochemical measurements of the cells, ex-situ x-ray diffraction, and solid state Nuclear Magnetic Resonance measurements were made. It was not possible to confirm the decomposition products that were proposed in literature by density functional theory calculations and in-situ XPS. The XRD-pattern of the cycled electrolyte did not show additional phases. The NMR spectra developed broader peaks and one additional phosphorous environment upon cycling. As a tool to study dendritic growth through Li-ion concentration profiles, a cell for in-situ neutron depth profiling (NDP) was developed. The electrochemical performance is comparable to regular cells, except for a voltage drop due to contact problems during the break after Li-metal stripping. A thin layer of Li-metal between the window and the electrolyte pellet, as well as a collimator plate to put some pressure on the window, could be the solution
to go towards operando measurements.
The current standard data analysis method was implemented and its sensitivity towards gaussian broadening, due to energy straggling (from small angle scattering in the material and the stochastic nature of energy loss), was
investigated. It is shown that the standard deviation of energy straggling is larger than that of energy broadening due to the detector resolution, and that the error on the depth scale is in the order of micrometers.

This report aims to evaluate the feasibility of a small pilot plant placed near Broome, Australia to produce 3 tons of hydrogen per day using photovoltaic (PV) energy. Using PVsyst, a PV modelling software, the maximum power operating points were determined for a test layout. This was then transformed to other layouts which allowed the optimum layout to be found for a system connected to electrolyzers using maximum power point tracking technology. A second scenario using a battery was also modelled and optimized. Finally, the third scenario used direct coupling, meaning that the PV panels were not operated at their maximum power point but rather at where the electrolyzer and PV current and voltage lines crossed. This resulted in a lower power yield but also lower costs. To find the current and voltage curve for the PV field, the data from PVsyst analyzed to find the short circuit current and the open circuit voltage. This allowed the full current and voltage curve to be determined, which allowed the intersection point with the (experimental) electrolyzer current and voltage curve to be determined.

In all simulations piping storage was used to remove the intermittency of the hydrogen production by having a capacity of 3 tons of gaseous H2. This ensures a constant stream of hydrogen to the liquefaction system.

Using preliminary results an electrolyzer degradation simulation was carried out, to find how the electrolyzer would behave after 20 years of use. Although the influence of intermittency could not be found in literature, it was shown that the electrolyzer produces, on average during its 20 year lifetime, approximately 5.7% less hydrogen than it would without any degradation. This has been included in all financial analyses carried out in this report, along with a 7% weighted average cost of capital.

The financial framework has been based off of a number of different sources and forecasts. Due to the limitations of publicly available data some forecasts were replaced with constant prices which do not evolve throughout the years.

Using the hydrogen production models and financial frameworks it is possible to compare the different scenarios. From this a price of 4.16$/kg was found for a MPP coupled system, 4.39$/kg for a MPP coupled system including a battery, and 4.02$/kg for a direct coupled system. The third scenario was furthermore looked at in terms of physical layout; it was found that the decentralized layout consisting of smaller subplots was slightly more expensive than the standard centralized layout (4.09$/kg). For this pilot plant it is advised to use a decentralized topology, combined with a decentralized layout consisting of multiple smaller plots. Although the centralized layout is slightly cheaper, it contains more system critical components which could cause a large portion of the system to be inactive if they are broken.

This thesis provided an overview of the achievable ammonium concentrations, and the energy used to achieve these concentrations, by ion exchange, electrodialysis and reverse osmosis. The need for this overview arose during research for the N2kWh project, where one step in the process focused on increasing the ammonium concentration of (anaerobic digestion) reject water. The research presented some knowledge gaps. As available reports focused on the ammonium removal from the solution, the concentration in the residual stream was barely mentioned. Even when concentrations were reported, the conditions between the discussed technologies made it hard to make a comparison between the three technologies. Even less was reported on the energy that was needed to achieve the various concentrations.

To compare the three technologies, a 1.5 g NH4+/L solution was used as feed water for all three technologies. The technologies used different techniques to increase the ammonium concentration (using zeolites, electrical current over a membrane or high pressure through a membrane). This made comparing the technologies, very difficult. By using the same feed water for all experiments, the achieved concentrations and the energy that was used in the process could be used to compare the three technologies.

The highest ammonium concentration that was achieved during the experiments (7.1 g NH4+/L) was produced by Electrodialysis. Ion exchange achieved a concentration of 6.0 g NH4+/L, while a laboratory scale Reverse Osmosis achieved 4.5 g NH4+/L. The amount of energy used by the three technologies to produce 6 g NH4+/L was: ion exchange = 0.24 kWh, electrodialysis = 6 kWh, reverse osmosis = 4 kWh (per m3 of produced concentrated NH4+ solution). ROSA software was used to estimate energy use for Reverse Osmosis to achieve 6 g NH4+/L (as the laboratory scale Reverse Osmosis was unable to provide sufficient pressure to achieve this concentration). So ion exchange was the most energy efficient, but the maximum achievable ammonium concentration was the lowest of the three technologies. Electrodialysis was able to achieve the highest ammonium concentration but was less energy efficient and used more energy in the process.

Energy demand is constantly increasing and the use of fossil fuels causes an accumulation of carbon dioxide (CO2) which is an important environmental problem that needs to be solved. A promising solution to this problem would be the electrochemical reduction of CO2 to useful products, using the surplus of electricity from renewable sources. This would be a way of storing this excess of electricity in chemical bonds. However, this has not reached high efficiency and selectivity needed for establishing its use. For this process, the CO2 is usually dissolved in an aqueous electrolyte. This thesis proposes the new approach, using Deep Eutectic Solvents (DES) which would capture a higher concentration of CO2 helping to make the whole process more efficient. However, not all salt mixtures reach the eutectic point and therefore, the solvents formed are a low-transition temperature mixtures (LTTMs) of the selected salts.

Experimental work was performed to find out if these solvents can be used for electrochemical carbon dioxide reduction. Non-reported LTTMs were synthetized; they were formed with mixtures of the hydrogen bond donor citric acid (CA), fructose (F) and diethanolamine (DEA) with the hydrogen bond acceptor tetrabutylammonium chloride (TBA-Cl) and different quantities of water, which was found necessary to carry out electrochemistry since the solvents formed were too viscous, and this water has an important effect in the electrocatalytic reduction of CO2 as proton source and charge carrier.

These solvents were characterized electrochemically, performing studies to find out on which metallic surface they behave better (wider working potential window, low degree of decomposition and adsorption) and how different water quantities affect this process. These studies were performed using cyclic voltammetry. From this, it was seen that the solvents are more stable during electrochemical process in presence of copper. Moreover, the solvent formed by DEA:TBA-Cl:H2O in the molar proportions 1:1:1,375 was found to have interesting features that resemble CO2 reduction. As result, this solvent was further analysed using electrochemical in-situ Fourier-Transformed Infrared (FTIR) spectroscopy in surface-enhanced attenuated total reflectance configuration. With this, it was seen that the carbon dioxide was captured by the solvent and that there are visible changes when applying potential. Remarkably, the data shows the formation of a dimer OCCO on the Cu electrode surface, stabilized by the solvent.

This project shows for the first time the electrochemical reduction of CO2 in LTTMs, evaluating different solvents, metals and water quantities. Determining that the carbon dioxide reduction is possible with the solvent DEA:TBA-Cl:H2O in the molar proportions 1:1:1,375 in a copper polycrystalline electrode.