Current society is challenged with climate change and aims to adapt and improve to mitigate climate change. This is an extremely challenging transition that requires a paradigm shift from our current fossil fuel driven world to a primarily renewable electricity based one. Increasing the electricity generation alone would not be sufficient, as balance between supply and demand is required. Solar and wind power are intrinsically intermittent with strong diurnal and seasonal variations. 

Therefore, the need arises for electricity storage to compensate for intermittence. Within this thesis we focus on the combined system that can handle both situations, the Ni-Fe alkaline battery and electrolyser. Previously the Ni-Fe battery suffered from limited round-trip efficiency due to hydrogen and oxygen generation. However, when utilized within an integrated electrolysis application, the gas production is intentionally applied for long-term energy storage.
So, within chapter 4, we aimed at improving the Ni electrode by reducing the amount of required nickel, while also improving both electrolyser and battery properties. Depending on the amount of copper substitution, a significant improved electrochemical activity can be obtained. In addition, the Cu doped nickel hydroxide showed stability for over 1000 cycles with the amount of dopant reduced to Ni_0.95Cu_0.05(OH)₂. Overall, this would thus result in requiring 40% less nickel for the same observed capacity.
 
In addition to decarbonizing the electricity production, it is mandatory to replace current industrial processes that require fossil fuel and feedstock with renewable energy and feedstock based alternative processes. Within this Thesis we focused on the oxidation of ammonia to nitrite and nitrate. These products are mainly applied within the synthetic fertilizer production. Ammonia oxidation to nitrites and nitrates is currently performed in the Ostwald process with Pt/Rh gauze. As the ammonia electrooxidation is feasible in ammonia fuel cells, electrochemical oxidation could have potential for replacement of this thermal Ostwald process.
Therefore, in chapter 2, we focus on applying the doped nickel hydroxide materials to increase the performance for ammonia oxidation to nitrate and nitrite. Co, Mn and Cu as dopants showed promising results. Furthermore, it became clear that the reaction was dependent on e Ni(II)/Ni(III) equilibrium as the reaction also occurs through indirect oxidation via the charged NiOOH phase. Under continuous operation at 25 mA/cm² a high faradaic efficiency is obtained with a 97% NO₂^-:NO₃^- selectivity. 

The above sparks interest in further investigation of NiCu_0.2 as promising catalyst in chapter 3, while taking into account the significant of the setup layout. The setup contains a Nafion 117 membrane to keep the counter electrode and nitrite separate, as it would otherwise result in reduction of the nitrite to nitrogen gas or back to ammonia. The Ni_0.8Cu_0.2(OH)₂ can perform ammonia oxidation, with limited oxygen evolution, from 2.5 up to 400 mA/cm². At high initial ammonia concentration of 1 M, more than 75% was able to be converted in 3 hours at this 400 mA/cm² with a faradaic efficiency of 96%. Thus, this work reveals a potential approach for replacing the Ostwald process with electrochemistry within the near future.

The Netherlands has set ambitious climate targets, aiming to become climate neutral by 2050, which implies a 100% reduction of greenhouse gas emissions compared to 1990 levels. Solar and wind power are key to replacing fossil fuels in electricity generation, but their intermittent output creates a need for both short-term and seasonal energy storage. At the same time, sectors such as heavy industry, aviation and shipping remain challenging to electrify directly. Renewable hydrogen, also referred to as RFNBO-compliant hydrogen, is emerging as a clean and flexible energy carrier that helps fill the gaps where green electricity alone is not enough. It can be stored, transported, and used in applications where direct electrification is not feasible. Unlike grey hydrogen produced from fossil fuels, renewable hydrogen is made by using electricity from solar or wind to split water into hydrogen and oxygen, significantly reducing carbon emissions.
Producing renewable hydrogen remains expensive due to the high capital costs of electrolysers and the limited availability of cheap renewable electricity. Producing renewable hydrogen in a way that is both economically sustainable and scalable is important to decarbonize global industries and provide renewable seasonal storage. To evaluate and compare the economic performance of hydrogen production methods, the Levelised Cost of Hydrogen (LCOH) is used. LCOH represents the average cost of producing hydrogen over the lifetime of a facility, accounting for capital, operational, and energy costs. As the renewable hydrogen market is not yet completely developed, metrics such as Net Present Value (NPV) are less suitable for determining the competitiveness of the technologies. Therefore, it is important for renewable energy companies, like Eneco, to create renewable hydrogen with the lowest LCOH to be competitive in the hydrogen market.
One approach to lowering costs is to pair electrolysers with batteries, where revenues from storing electricity and selling it to the grid, known as arbitrage revenue, can help offset operational expenses. This can be achieved through two types of systems: the Battolyser, an innovative technology that combines battery storage and hydrogen production in a single device, and a system where a battery and electrolyser are co-located but operate as separate units. These systems have distinct advantages and disadvantages, which have not yet been evaluated and compared. Therefore, the main research question is: How does the Battolyser compare to a system with a separate battery and electrolyser in achieving the lowest levelised cost of hydrogen (LCOH) compliant to RFNBO (EU) standards?

To answer this question, this research analyses four different co-located system configurations. Each configuration combines either an alkaline or PEM electrolyser with an LFP lithium-ion battery or a vanadium redox flow battery, selected for their technological maturity and future potential. Together with the Battolyser, the five systems considered are:
• Battolyser
• Alkaline electrolyser + Lithium-ion battery
• PEM electrolyser + Lithium-ion battery
• Alkaline electrolyser + Redox flow battery
• PEM electrolyser + Redox flow battery

Financial and technological characteristics of the systems have been identified, taking the Dutch market into account. The five systems are simulated by using a novel techno-economic simulation model that has been created for this research. The model simulates the systems in a framework based on Dutch hourly electricity prices between 2030 and 2050.
The simulation model focuses on three main operational strategies: battery charging, battery discharging, and hydrogen production. The systems are simulated on an hourly basis for the period 2030–2050, using a combination of mixed onshore wind and solar power profiles contracted through a Power Purchase Agreement (PPA), together with grid electricity as power input in the base case simulations. The model uses a controller with a three-hour receding horizon to anticipate electricity price fluctuations. The simulation framework ensures an objective comparison by keeping operational strategies and regulatory compliance consistent while varying system-dependent parameters such as electrolyser efficiency, minimum stable load, and system costs.

The base case simulations showed that the Battolyser performs best with an LCOH of 9.96 €/kgH2. Among the four battery-electrolyser combinations, the system with a lithium-ion battery and a PEM electrolyser came closest to matching the performance of the Battolyser, followed by the alkaline and Li-ion system. Although redox flow battery systems have longer lifespans and flexible sizing, their higher costs result in poor LCOH performance, because the technical benefits do not compensate for this. The LCOH values lie close to each other, with a spread of 1.40 €/kgH2 when including redox flow systems, and just 0.40 €/kgH2 when excluding them.
Several analyses, including sensitivity and scenario analyses, were performed to understand the robustness of these findings. A local sensitivity analysis was performed by varying electricity prices by plus and minus 20% compared to the base case, to examine how changes in electricity costs impact LCOH performance. This analysis showed that the PEM + Li-ion system, followed by the ALK + Li-ion system, both perform better than the Battolyser when the electricity prices increase, as the systems can take full advantage of the flexibility by dynamically shifting operation. They can produce hydrogen at nominal capacity during one hour, switch to battery charging the next hour when electricity prices are low, and then sell electricity back to the grid during high-price periods. In contrast, the Battolyser performed significantly worse under these increased conditions, as it cannot separate battery operation from hydrogen production, which limits the system’s ability to fully exploit price fluctuation.
In the global sensitivity analysis, five key parameters (CAPEX, OPEX, electrolyser efficiency, battery capacity, minimum stable load) were varied to investigate the absolute effect on the LCOH, as well as their interaction with other parameters. This analysis shows that varying the electrolyser efficiency has the highest absolute effect on the LCOH. In addition, electrolyser efficiency is present in every statistically relevant interaction term. This shows that electrolyser efficiency is not only a significant factor on its own but also influences the impact of other parameters, making it the most important determinant of LCOH.
This is confirmed in the scenario analysis, where the systems are simulated with a more steady offshore wind power profile, in contrast to the mixed wind on land and solar power profile in the base case. The Battolyser and alkaline systems, both of which have better electrolyser efficiencies than the PEM system, perform better under these steady conditions. In contrast, the PEM-based systems show a decline in performance, requiring significantly lower offshore wind PPA prices to achieve LCOH levels similar to those in the base case. This indicates that under stable power conditions, electrolyser efficiency becomes a more dominant factor in contrast to operational flexibility.
Finally, the Battolyser and co-located systems were compared to standalone alkaline and PEM electrolysers. The standalone systems outperformed the hybrid configurations due to their lower cost structure and ability to channel all available power directly into hydrogen production. The alkaline electrolyser achieved a lower LCOH than the PEM system in the base case, the reduced electricity price scenario, and under an offshore wind profile. This advantage can be attributed to its higher electrolyser efficiency. These findings indicate that adding a battery to an electrolyser does not necessarily lead to lower LCOH. However, the standalone systems lacked operational flexibility, making them perform worse under scenarios with higher and more volatile electricity prices, where the PEM + Li-ion system performed better.

In conclusion, this research explored whether combining battery storage with electrolysis could lower production costs through electricity market arbitrage. The Battolyser was compared with four battery-electrolyser systems for renewable hydrogen production. The results show a small spread in LCOH and that there is no single system that performs best across all situations. Among the hybrid systems, the Battolyser achieves the lowest LCOH in the base case, the reduced electricity price scenario, and under an offshore wind profile. Its strong performance can be attributed to its high electrolyser efficiency, which proves to be a significant factor in determining the LCOH. Nonetheless, standalone electrolysers outperform the Battolyser in these scenarios due to their lower cost structures and direct power use. However, when electricity prices rise and become more volatile, systems with separate batteries and electrolysers perform better, with the PEM + Li-ion configuration achieving the lowest LCOH.
It is recommended that Eneco prioritizes hydrogen production systems with high electrolyser efficiency as a strong option for RFNBO-compliant hydrogen. Standalone electrolysers should be considered when electricity prices are expected to follow patterns similar to the base case. However, Eneco should remain open to using separate battery and electrolyser setups in case when electricity prices are expected to rise. Since the LCOH differences between systems are relatively small, it is further recommended that the system selection should be guided by additional factors such as environmental impact and the availability of technologies within the European market.

While the system choice is important to provide a competitive edge in the hydrogen market, the results also highlight a broader challenge for renewable hydrogen. Eneco expects that the willingness-to-pay for renewable hydrogen will be approximately €9/kgH2. No system is able to produce renewable hydrogen below this threshold and thus a cost gap remains. Therefore, future research should investigate the renewable hydrogen market in the Netherlands and potential subsidy schemes. Moreover, it is recommended to expand the simulation model to allow a more detailed comparison between the systems as this simulation model has some limitations. Possible additions include incorporating optimization horizons that reflect the day-ahead electricity market, enabling dynamic power allocation, exploring participation in additional energy markets, and performing environmental life cycle assessments.

There is an urge for rapid change in our energy system as several deadlines for agreements on climate change mitigation goals are near, with the final deadline being 2050 to achieve the formidable task of realization of a CO2 neutral society. There are viable solutions to redesign our energy system through renewables, but the cost of such a redesign will only be more steep if carried out short before the deadline. From 2025 every year 4% of our energy system needs to be replaced using a simple linear approach, a percentage that will grow if we postpone it.
This work gives a clear insight in our current energy consumption behavior and the scale of our energy need. It provides possible improvements for specifically lithium batteries with liquid/polymerized liquid electrolytes. Currently it appears sodium battery chemistry is promising to be integrated in a completely renewable energy system. Fortunately the ‘lessons learned’ in this work are usable for research in sodium type of chemistry, because the fundamental mechanisms of intercalation, conductivity and redox activity are similar.

Energy storage technologies are critical to the global energy transition, given the need to reduce carbon emissions and the ever-increasing demand for energy. The need for energy storage is particularly crucial in the automotive and electricity generation sectors, where a shift from fossil fuels to renewable sources is imperative. Lithium-ion batteries are central contributors to this transition, powering electric vehicles (EVs) and supporting renewable energy integration into electricity grids. Historically, lithium-ion batteries evolved from using Li metal as a negative electrode to safer alternatives like graphite, and positive electrode materials such as LiNi0.8Mn0.1Co0.1O2 (NMC811) and LiFePO4 (LFP). However, despite these advancements, several challenges still need to be addressed.
First, increasing the energy density of lithium-ion batteries is a priority, particularly for EVs, where higher energy densities can extend the driving range and improve market adoption. Feasible solutions in this regard could be increasing the thickness of electrodes and switching to higher energy density materials such as Li metal. Second, safety remains a key concern, as current batteries use liquid electrolytes that are prone to thermal runaway, leading to fires or explosions. Solid-state electrolytes offer a safer alternative with higher thermal stability. However, they also present their own challenges at the electrode/electrolyte interfaces. Third, there is a growing emphasis on developing environmentally friendly batteries, as many of the materials and manufacturing processes for current lithium-ion batteries are not sustainable. Efforts are being made to eliminate fluorinated compounds and other toxic components from batteries, but this needs to be done without compromising performance. The optimization of the electrode-electrolyte interfaces in batteries holds the key to enabling batteries of the future. Optimizing Li morphology during plating/stripping, improving interfacial stability, and ensuring sufficient ion and electron percolation in cathode composites are key to improving battery capacity, lifetime, safety, and efficiency.

Sulfide-based solid-state batteries (SSBs) are emerging as a top contender for next-generation rechargeable batteries with improved safety and outstanding energy densities. SSBs incorporate non-flammable solid-electrolytes (SEs), eliminating safety hazards for batteries in electric vehicles and electronic devices. In addition, the development of SSBs with SEs allows the conventional graphite anode to be replaced by a lithium metal anode, theoretically surpassing the energy densities of lithium-ion batteries (LIBs). However, SSBs with high nickel cathode materials such as LiNi0.8Mn0.1Co0.1O2 (NMC), exhibit several cathode interface-related issues preventing the large-scale adoption of this technology. Specifically, the problems include challenges such as mechanical and chemical instability, which results in particle cracking, contact loss, and decomposition of the solid-electrolyte.
To overcome these challenges, this study used polymeric interlayers at the surface of the NMC to buffer volume changes and passivate chemical side reactions. Herein, the Ni-rich layered oxides polycrystalline NMC811 (PC-NMC) and single-crystal NMC Ni82 (SC-NMC) were coated with PDDA-TFSI, poly(diallyldimethy-lammonium) bis(trifluoro-methanesulfonyl)imide, using a microencapsulation method. Additionally, this study aimed to maximize the utilization of cathode active material with the use of the conductive additive carbon nanofibers (CNFs) and the addition of lithium salt (LiTFSI) in the polymeric coating. Full lab-scale SSBs consisted of a coated NMC-based cathode, a Li-In alloy anode, and Li6PS5Cl solid-electrolyte.
The conductive carbon additive severely increased SSB degradation, which was associated with the formation of decomposition elements sulfates/sulfites (SOx ), polysulfides (P2Sx ), phosphates (POx ), and lithium phosphate phases (P/LixP). The polymeric coating on PC-NMC slightly improved cycle stability. Here, improvements were associated with the significant reduction of contact loss between NMC and Li6PS5Cl particles. The cathode with SC-NMC with a PDDA-(Li)TFSI polymeric coating demonstrated exceptional performance improvements in SSBs, mitigating chemical and mechanical degradation. It showcased improved cycle stability with a capacity retention of 95% observed after 100 cycles at 0.2C, compared to a capacity retention of 84% for SSBs without a cathode interface coating. Furthermore, a significantly improved initial capacity of 155 mAh g−1 at 0.2C was established, compared to an initial
capacity of 144 mAh g−1 for uncoated SSBs. Overall, the results highlight the performance-enhancing effect of a polymeric coating with added lithium salts in Li6PS5Cl-based SSBs.

Ammonia is a precursor in fertilizer production and a potential carbon-free energy carrier, which is essential for the energy transition to more renewable energy sources. To that end, the current fossil fuel based method of industrial ammonia production through the Haber-Bosch process should be replaced by electrochemical ammonia synthesis. However, research into this topic faces several challenges, including a strong dinitrogen bond, competition from the hydrogen evolution reaction and low solubility of nitrogen in aqueous electrolytes, limiting the availability of nitrogen at the reaction sites on the electrode surface. In this study, in order to better understand these limitations to electrochemical nitrogen reduction to ammonia, a porous iron electrode is used in a cell design allowing both aqueous electrolyte and nitrogen gas access to the surface, which is successfully reduced electrochemically to serve as a nitrogen dissociation catalyst. Careful control of the applied potential limits the competition from the hydrogen evolution reaction. Nuclear magnetic resonance spectroscopy and ion chromatography measurements of the electrolyte were performed to check for possible contamination by nitrogen containing species. Small amounts of ammonia were measured using gas chromatography-mass spectroscopy on the gas output, but only with a high residence time of the nitrogen gas inside the cell. Since reduced iron surfaces are known to be good nitrogen dissociation catalysts, even at ambient temperature and pressure, the cause of the low ammonia production rate reported here is the abundant presence of water, which blocks nitrogen from the surface adsorption sites even when the catalytic surface is not submerged in the electrolyte. These results show that creating a surface with sufficient catalytic activity to break the dinitrogen bond is very well possible, but the main challenge is keeping the surface clean to prevent blocking of the reaction sites. This gives additional insight for future efforts in developing electrochemical ammonia synthesis methods.

Ammonia is essential for global food production as a component of fertiliser, and a potential energy carrier in the energy transition. Electrochemical ammonia synthesis faces numerous challenges as an alternative to the carbon intensive Haber-Bosch process, including competition from hydrogen evolution, and mass transport limitations. An unconventional cell design with a hydrogen permeable electrode could help to address these problems. The reaction mechanism and performance of ammonia synthesis using hydrogen permeable electrodes was investigated at elevated temperatures and pressures of up to 120 °C and 8 bar. Furthermore, a facile method for enhancing the electrochemical surface area of the electrode was developed and tested. Operation at elevated pressure resulted in a moderate increase in cell performance. However, replenishment of the nitrogen vacancies in the nickel nitride catalyst through dinitrogen adsorption is identified as the elementary step that limits activity and stability. The amount of pre-deposited N is found to significantly influence the ammonia production rate and its stability. Dynamics of ammonia desorption could also play a role in nitride regeneration. In future studies, the presence of a decomposition reaction of the nitride should be investigated. Usage of more stable nitride species or a combination of host nitride and dopants is recommended to improve stability and simultaneously promote dinitrogen activation and hydrogenation to ammonia. These findings expand the understanding of the mechanisms underlying the nitrogen reduction reaction, paving the way for the development of a more efficient green ammonia synthesis process.

The battolyser™, which is based on a nickel iron battery, functions both as a battery, and when overcharged, as an alkaline water electrolyser for the production of H2. In times of renewable energy oversupply, i.e. low energy prices, the battery electrodes are charged and subsequently produce H2 and O2. Conversely, in times of undersupply of renewables, i.e. high energy prices, energy stored in the battery electrodes can be discharged back to the grid. This flexibility in operation results in a high utilisation factor despite the fluctuating energy output of renewables. However, the electrodes employed in conventional nickel-iron batteries are not designed for this dual application. The goal of this thesis is the development of hybrid battery-electrolyser nickel and iron electrodes that provide both a high areal battery storage capacity and allow for efficient electrolysis at industrially relevant current densities...

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...

In the last century, the indiscriminate use of fossil energy to power the industrial revolution and technological progress of humankind, has led to the depletion of limited natural resources and, most importantly, the emission and accumulation of alarming levels of pollutants and greenhouse gases (GHG) in the atmosphere. One of the major consequences of these emissions is the climate crisis that we are currently facing. As our society is in constant need for energy to live and progress, we are urged to find more sustainable and renewable energy sources, and to decrease the environmental impact of industrial processes. Electrochemistry can be used to temporary store intermittent renewable electricity, to then be reconverted back to electrons, or it can be used to produce chemicals. As such, the electrification of the chemical industry offers the opportunity to reduce its GHG footprint. The variable supply of renewable electricity can be used by the chemical industry to generate artificial fuel and feedstock. In this way, the synergy between the chemical industry and the energy sector can boost market access, scale and competitiveness.
In particular, this thesis focuses on one of the largest processes in chemical industry, i.e. the ammonia production. An introduction on the topic is given in Chapter 1. Ammonia is produced at large scale (178 million tons per year) and it is a commodity essential for the fertiliser and food sector. The current production of ammonia, via the Haber-Bosch process, relies on fossil fuels and hydrogen derived from steam-methane reforming. Consequently the sector is responsible for releasing 1.4 % of the global CO2 emissions. The implementation of a fully renewable powered Haber-Bosch process is limited by its large reactor scale and its continuous and steady operation, which clashes with the intrinsic intermittency of sources such as solar and wind. This is one of the reasons why a direct electrochemical route for ammonia synthesis has recently attracted significant attention in the scientific and industrial communities. The concept entails the direct synthesis of ammonia from water, dinitrogen and renewable electricity. Moreover, the possibility of producing ammonia in a sustainable manner may enable a new scenario where ammonia can also be used as carbon free energy carrier, thus playing a key role in a decarbonised energy landscape powered by renewables. However, the lack of a selective catalyst and the arduous competition with side reactions, as the hydrogen evolution reaction, make this process extremely challenging.
The aim of this thesis is to expand the current understanding of the nitrogen reduction reaction at near ambient conditions, addressing both fundamental and practical challenges. The first part of this thesis (Chapter 2-4) provides insights on the implementation of reliable electrochemical nitrogen reduction experiments and sensitive operando ammonia detection. Chapter 2 provides a fast and reliable ammonia detection method to speed-up catalyst screening and development of novel sustainable ammonia evolution devices, as it requires significantly less sample handling and preparation compared to other reported methods. The proposed method is based on a gas chromatography technique, and it allows for in situ monitoring of ammonia evolution, down to 150 ppb, from -but not limited to- electrochemical devices. Chapter 3 presents an isotope sensitive gas chromatography-mass spectrometry method for the quantification of NH3 at low concentration level, typically encountered in electrochemical ammonia synthesis applications. This method allows the discrimination of 15/14NH3, necessary for the required 15N2 isotope labelling control experiments. Additionally, this method can directly and simultaneously measure other species in the analyte, thus it allows researchers to directly assess reaction selectivity by measuring reaction by-products, as well as the presence of gaseous/volatile contaminants in the experimental setup. Chapter 4 investigates the impact of contaminations on electrochemical nitrogen reduction experiments, with the aid of multiple analytical techniques and instrumentation, such as ion chromatography, gas chromatography, mass spectrometry, NOx chemiluminescence analyser and UV-Vis spectrophotometry. This chapter not only provides a comprehensive identification and quantification of the contaminations, but it also critically analyses the effectiveness of different cleaning strategies, establishing a series of guidelines to perform reliable experiments.
The second part of this thesis (Chapter 5-7) investigates the room temperature spontaneous dinitrogen activation on selected metallic surfaces and its hydrogenation to ammonia via electrochemical atomic hydrogen permeation, using a solid metallic hydrogen permeable membrane electrode. Chapter 5 demonstrates a novel strategy for ambient condition ammonia synthesis from water and dinitrogen, designed to limit the competition between nitrogen activation and other competing adsorbates at the catalytic surface. As such, a hydrogen permeable nickel membrane electrode is used to spatially separate the electrolyte and the hydrogen activation side from the nitrogen activation and hydrogenation sites. With this approach, ammonia is produced catalytically directly in the gas phase and in the absence of electrolyte. Gaseous nitrogen activation at the nickel electrode is confirmed with 15N isotope labelling control experiments and it is attributed to a Mars-van Krevelen mechanism enabled by the formation of N-vacancies upon hydrogenation of surface nitrides. Chapter 6 reports on the interactions of adsorbing N and permeating H at the catalytic interface of nickel, iron and ruthenium based hydrogen permeable electrodes during electrolytic ammonia synthesis. In situ near ambient pressure X-ray photoelectron spectroscopy (XPS) is used to measure modifications in the surface electronic structure of the catalyst and the nature of the adsorbed molecules. This chapter shows that permeating atomic hydrogen reduces surface Ni oxide and hydroxide species, under conditions at which gaseous H2 does not. Moreover, the results demonstrate that the availability of surface Ni0 sites is a primary requirement for the chemisorption of gaseous N2. In situ XPS measurements reveal that nitrogen gas chemisorbs on the generated metallic sites, followed by hydrogenation via permeating H, as adsorbed N and NH3 are found on the Ni surface. Our findings indicate that the first hydrogenation step to NH and the last NH3 desorption step might be limiting at the utilised operating conditions. Finally, the study was then extended to Fe and Ru surfaces. However, the formation of surface iron oxide and nitride species on iron blocks the H permeation and prevents the reaction to advance; while on ruthenium the stronger Ru-N bond might favour the recombination of permeating hydrogen to H2 over the hydrogenation of adsorbed nitrogen. Chapter 7 provides a systematic investigation of the effect of operating temperature (in the range 25 to 120 °C) and H permeation flux on the N2 reduction reaction on Ni, leading to a considerably improved NH3 synthesis process. At 120 °C a stable operation was achieved for over 12 h with a 10 times higher cumulative NH3 production and almost 40-fold increase in faradaic efficiency compared to the room temperature operation reported in chapter 5. The results obtained in this chapter indicate that increasing operating temperatures enhances nitrogen adsorption and NH3 desorption, maintaining a steady N surface coverage throughout the NH3 synthesis cycle. Moreover, to operate the nitrogen reduction reaction in a stable and efficient manner, the control over the population of N, NHx and H species at the catalyst surface is critical, as well as the capability of oxides to be reduced by permeating H. As such, the adoption of H permeable electrodes allows to independently control the N activation and H permeation, by a large extent.

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.

Ammonia is an important chemical used in the production of fertilizers and has the potential to become a green energy carrier. It is synthesized through the Haber-Bosch (HB) process, which involves nitrogen and hydrogen reacting at high temperature and pressure and consumes approximately 2% of the global energy supply. An alternative to this energy intensive process is the electrocatalytic nitrogen reduction reaction (NRR). The main challenge of this approach is the competing hydrogen evolution reaction (HER) that consumes most of the applied electricity, making the ammonia production inefficient. Based on the Sabatier principle, an ideal catalyst for the NRR would bind nitrogen and ammonia optimally, while hydrogen would be bound too strongly or too weakly, suppressing the HER. So far, no such catalyst has been found. A strategy that could lead to the discovery of a suitable catalyst is alloying, since materials with new adsorption properties can be created this way. The main goal of this project is to design an electrochemical cell that can be used for the screening of large numbers of bimetallic catalysts, increasing the chance of finding a suitable NRR catalyst. For this so-called high-throughput approach an electrochemical cell was designed that allows the screening of up to 16 catalysts in parallel. Furthermore, methods for sample preparation, characterisation and detection were developed that are compatible with a high-throughput technique

As the variable renewable energy share keep on increasing throughout the energy transition, power systems require more and different flexibility measures. Battery energy storage can provide these essential services that enable the energy system to carbonise and transform on short time scales. Energy arbitrage, the trading of electricity on the electricity markets, can create economic value for batteries. This research assesses the energy arbitrage opportunities for utility-scale battery energy storage in the Netherlands. It compares five different battery technologies and defines the optimal size and power capacity with a linear mixed-integer Matlab optimization model. Furthermore, the operation of for the most optimal technology is simulated for one year with a Model Predictive Control algorithm to be able to compare the effects of forecasting accuracy of the electricity prices on the estimated profits. The results show that according to optimized planning and dispatch in the operation of a battery system can lead to financial opportunities in the Netherlands regarding energy arbitrage for flow batteries. If not today, then in the future. It has been shown that towards 2030 the business case for battery energy storage will significantly increase due to decreasing capital costs and increasing price volatility with the rise of the VRES and phase-out of fossil energy sources. With this research, a contribution is delivered towards the realisation of commercially operating an independent utility-scale battery energy storage facility. Insights are provided on what battery configuration is best suited for energy arbitrage purposes and the effectiveness of simulating the optimized operation of a battery facility with a model predictive control method has been shown. This has lead to clearer insights into the profitability of operating a BESF in the Netherlands.

In a bid to battle global warming the Dutch government has set ambitious targets to cut CO2 emissions in the Dutch Climate Agreement [1]. Aside from this state-driven initiative in line with the Paris agreement, several Dutch cities have presented their own climate initiatives. The city of Amsterdam has pledged to reduce its CO2 emissions by 55% in 2030 compared to 1990 levels which surpasses the national goal by 5% [2]. The capital has laid out a detailed plan to achieve these goals and it is clear that the electrification of mobility and heating, along with the roll-out of solar energy across the city, will play an important role in the future. Amsterdam is planning to install 1 million solar panels by 2022, aims to ban non-electric cars from the city by 2030 [3] and wants all buildings to be heated gas-free by 2040. While all of these steps are very helpful regarding the reduction of greenhouse gasses, they pose a big challenge to local grid operator Liander to ensure sufficient grid capacity [4]. The spokesperson of Alliander (parent of Liander) estimates that in 2050, peak power demand in Amsterdam could be 2.5 to 6 times higher than now. From the 25 substations in the city, which transform high-voltage electricity to middle-voltage, a minimum of 11 will not have sufficient capacity by 2030. The problem lies with an increase in both electricity supply and demand. On the supply side, as more and more Renewable Energy Sources (RES) have entered the Dutch energy mix in the past decade the strain on the electricity grid has increased. While wind turbines have largely been built offshore and feed into the high voltage grid, on-land turbines and especially large scale solar generation are mainly connected to the middle-voltage (MV) grid which is primarily designed to distribute electricity, not to accept it. On a sub-level, solar panels installed on private roofs deliver their electricity to the low-voltage (LV) grid. The capacities of cables and transformers in residential areas are sized to deliver electricity from generator to consumer, and not the other way around. Liander has indicated that capacity is sufficient to deal with privately owned solar for now, but that the significant increase envisioned by the city of Amsterdam will pose a problem in the future.

High temperature aquifer thermal energy storage (HT-ATES) can potentially solve the mismatch between heat supply and demand. It can provide a large scale seasonal heat storage solution. Thereby it enables an increase in full load hours of the base heat source, which can benefit project performance on both costs and emissions. However, the limited number of successful pilot projects indicates the technology has not escaped its state of infancy. There is a gap from concept to implementation, which is signified by the disagreement of experts on performance drivers and barriers of HT-ATES. This research aims to narrow the described knowledge gap, by improving identification of HT-ATES performance drivers and barriers. Thereby it strives to improve decision making of HT-ATES implementation, and further enhance future HT-ATES application in heating projects. The broad scope of research demands both a diagnostic and design-orientated approach, and fits seamlessly with a multi-criteria decision analysis. The analysis entails the stages of creating, evaluating, comparing and ranking of case-specific scenarios. Parametric variation changes the conditions for HT-ATES implementation across the scenarios. A simulation model is developed and connected to a groundwater model to apply the parametric variation, to create the different scenarios, and consequently to produce the quantitative information for further evaluation. During the stages of creating, evaluating, comparing and ranking, the methodology systematically produces new results on the opportunities and risks introduced by HT-ATES, and additionally on the HT-ATES performance drivers and barriers. The results show that HT-ATES enables the opportunity of improving project performance with respect to the internal rate of return and emissions. Groundwater impact remains the greatest risk, but it can be minimised with smart decision making. To support the decision maker and to overcome the risk of groundwater impact, the research proposes several performance-enhancing, non-explicit guidelines. The guidelines focus on realising an HT-ATES implementation, where project performance with respect to internal rate of return, emissions and groundwater impact are balanced. Thereby they explain the major HT-ATES performance drivers and barriers. The guidelines are summarised below. The decision maker is recommended to .. 1. .. minimise the uncertainty, through thorough subsurface characterization before implementation. Secondly, to focus on aquifers with a minimum depth of 200 [m] and a minimum hydraulic conductivity of 5 [m/d] 2. .. assure network return temperatures during peak demand are below expected storage temperatures 3. .. not consider project life-times exceeding 20 years 4. .. assure yearly maximum base source heat production is always lower than yearly consumer heat demand 5. .. to strive for a flat demand curve and apply peak-shaving, by means of, for example, variable heat prices Currently, the guidelines have the purpose of giving direction to the decision maker, but they will become more explicit once the methodology is improved, and the uncertainty and number of assumptions in the model is decreased.

In connection with attempts to increase the Solar Redox Flow Battery (SRFB) viability, an investigation was done into the charge carrier transport mechanisms entailed at the photoelectrode interlayer interfaces. On the basis of a preliminary study, the investigation focused on the development of a suitable conducting layer at the electrolyte/electrode interface as well as the development and control of a hole extraction layer, based on the device electronic structure and energy levels matching. Research was conducted into the properties of various prospective materials that could improve charge carrier transport at the electrolyte/photoelectrode interface. Once the optimal configuration is determined, the photoelectrode and redox couples are prepared and characterized and the photocharging performance of the device is optimized. Furthermore, the control of the formation conditions of a hole extraction layer took place, in order to yield improved internal charge carrier separation and transport. Various hole transport layers were assessed in SRFB system in relation to the photo-charging performance. The described process resulted in the development of a conducting layer through the growth of well distributed islands, which led to a high solar-to-chemical conversion of 9.4%. Through the control of the formation conditions of the hole extraction layer, better spatial separation of the photogenerated charge carriers was achieved, leading to improved device performance. Despite the enhanced viability of each device in terms of efficiency, further research into the long-term stability is essential for its practical implementation. This mainly relates to electrolyte degradation to form solid precipitates and photocorrosion of the absorber semiconductor materials in acidic conditions.

Due to the electricity storage facilities required for a future powered on renewable energy, and due to the high performance batteries necessary for electric vehicles and mobile electronics, battery research and development is more urgent than ever. In this thesis battery technology was investigated both on the material-, as on the electrode-level. This research was carried out to elucidate working principles of new battery materials and to develop new fabrication methods as to contribute to improving Li-ion batteries....

The battolyser concept is based on an old fashioned nickel iron battery, consisting of a positive nickel(oxy) hydroxide electrode and negative iron (hydroxide) electrode. Research has been performed on the structural changes of the nickel and iron electrode upon charge and discharge. A lot of research can be found for the nickel electrode, but crystal structure studies are few. Much less research is performed on the iron electrode. Therefore the structural changes of the nickel(oxy) hydroxide and iron (hydroxide) electrodes during operation are further researched. Ni(OH)2 exists in the form of an α- or β-modification and NiOOH in a β- or γ-modification. The structure of the nickel(oxy)hydroxide phases has layers of edge-sharing NiO2 octahedra, where in between guest atoms can situate. Conventional nickel electrodes operate mainly between β-Ni(OH)2 and β-NiOOH, so most research focussed on this area. Upon overcharge the γ-NiOOH is formed and when discharging a direct reduction of β-NiOOH and γ-NiOOH into β-Ni(OH)2 occurs. The research on the nickel electrode published in this thesis is compared with a previous discharge study performed by Morishita et al. In their Rietveld refinement of the β-phases an ideal and fault phase model are assumed and the weight fraction of all phases present in the samples at 0, 50, 100 and 150% of state of charge (SOC) are determined. For the iron electrode four phases can be distinguished. The pure Fe in the electrode upon discharge becomes Fe(OH)2, under deep discharge this transforms even further towards FeOOH. For the deactivation process the reaction moves towards magnetite (Fe3O4). First the battery electrodes from the shelf are analysed by SEM-edx , X-ray diffraction and neutron diffraction. Then neutron powder and X-ray diffraction analysis have been performed to study the structural changes of the electrodes during charge and discharge. The nickel electrodes are measured at various states of charge from 0 to 100% and iron electrodes 0 to 90%. Preliminary experiments for the in-situ neutron diffraction test setup are performed. Different thicknesses of quartz glass, several sizes of nickel foam current collectors are tested for the amount of their background noise. In the nickel electrode are next to the carbon, 3 phases present. In the comparison with Morishita et al. significant differences are found. The transition towards β-NiOOH and subsequently γ-NiOOH occurs faster. Already at 50% of SOC the weight fraction of β-NiOOH is 58 wt% and at 100% of SOC the γ-NiOOH is 57 wt%. The c-parameter of γ-NiOOH is in agreement with Morishita et al. with a lattice distance of 20.8 Å. Morishita et al. did not specify how they determined the state of charge, so maybe they calculated the SOC in a different way. In the iron electrode three phases are identified, namely iron (Fe), goethite (FeOOH) and magnetite (Fe3O4). This is a strange result, because where Fe(OH)2 is expected, FeOOH is found. Probably discharged too far, moving the reaction into the second discharge plateau. The weight fraction of Fe increases and FeOOH decreases with increasing state of charge. The hydrogen content is decreasing with increasing SOC proportional to the background function for both the nickel and iron electrode.

Modern life is moving towards a mobile and sustainable energy economy, in which rechargeable batteries play an essential role as a power supply. The current battery of choice is Li ion battery that is dominating the market but faces great challenges for future use mainly due to the demand for higher capacities and target for cost reduction. Next-generation rechargeable batteries such as Li-O₂, Li-S and Na ion batteries, which offers higher capacities and cost-effectiveness, are being intensively researched as potential solutions to meet the future energy storage demand. 
This thesis focuses on the search of high-performance anode materials for both Li and Na ion batteries, including metallic Li and Na, Si, MgH₂, and black P and Sn₄P₃ based composites. Various methods are involved to synthesize the active materials and electrodes in a cost-effective manner; and comprehensive characterization on the physico-chemical and electrochemical properties has been performed to provide fundamental understanding and insights into the electrochemical processes. This work has achieved long-lifespan and safe Li and Na metal anodes by suppressing the hazardous dendrite growth. The Si, P and MgH₂ anodes presented in this work also exhibit high and stable electrochemical performance for Li and Na ion storage. Notably, the Na ion uptake in Si and MgH₂ has been, for the first time, realized in experiments. This research shows great promise towards the commercial introduction of these anodes in next-generation high energy density Li and Na ion batteries.