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. @en

The effect of different alkali metal cations on the oxygen evolution activity and battery capacity of nickel electrodes has recently been studied in low concentration alkali hydroxide electrolytes. As high concentration hydroxide electrolytes are favored in applications due to their high conductivity, we investigate if the cation effects observed in low concentration electrolytes translate to more industrially relevant conditions for both alkaline water electrolysis and nickel iron batteries. We investigate the alkali metal cation effect on the electrochemical properties of nickel electrodes in highly concentrated hydroxide electrolytes by adding Li⁺, Na⁺, Cs⁺ and Rb⁺ cations to a 6.5 M KOH electrolyte, while keeping the hydroxide concentration constant. For OER we find a trend in activity similar to that at low concentrations Rb⁺>Cs⁺>K⁺>Na⁺>Li⁺, where especially larger additions of Rb⁺ and Cs⁺ (1 M or 0.5 M) cause a significant decrease in OER potential. Smaller cations interact with the layered hydroxide structures in NiOOH to stabilize the α/γ phases and increase the potential for OER. The intercalation of small cations also causes an increase in battery electrode capacity because of the higher average valence of the Ni(OH)2/NiOOH α/γ pair. Small concentrations of Li⁺ added to a concentrated KOH electrolyte can therefore be beneficial for the nickel electrode battery functionality and for an integrated battery and electrolyser system, where it increases the battery capacity without a significant increase in OER onset potential.

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Both daily and seasonal fluctuations of renewable power sources will require large-scale energy storage technologies. A recently developed integrated battery and electrolyzer system, called battolyser, fulfills both time-scale requirements. Here, we develop a macroscopic COMSOL Multiphysics model to quantify the energetic efficiency of the battolyser prototype that, for the first time, integrates the functionality of a nickel-iron battery and an alkaline electrolyzer. The current prototype has a rated capacity of 5 Ah, and to develop a larger, enhanced system, it is necessary to characterize the processes occurring within the battolyser and to optimize the individual components of the battolyser. Therefore, there is a need for a model that can provide a fast screening on how the properties of individual components influence the overall energy efficiency of the battolyser prototype. The model is validated using experimental results, and new configurations are compared, and the energy efficiency is optimized for the scale-up of this lab-scale device. Based on the modeling work, we find an optimum electrode thickness for the nickel electrode of 3 and 2.25 mm for the iron electrode with optimal electrode porosities in the range of void fraction of 0.15-0.35. Additionally, electrolyte conductivity and the gap thickness are found to have a small effect on the overall efficiency of the device.

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One of the main problems of renewable energies is storage of the energy carrier. For long-term storage, solar fuels seem to be a good option. Direct solar water splitting could play an important role in the production of these solar fuels. One of the main challenges of this process is the charge separation and collection at the interfaces. The knowledge on photovoltaic (PV) junctions can be used to tackle this challenge. In this work, the use of doped layers to enhance the electric field in an a-SiC:H photocathode, and the use of thin-film silicon multijunction devices to achieve a stand-alone solar water splitting device are discussed. Using a p-i-n structure as a-SiC:H photocathode, a current density of 10mA/cm² is achievable. The p-i-n structure proposed also indicates the suitability of traditional PV structures for solar water splitting. In addition, hybrid devices, including a silicon heterojunction PV device, are proposed. A combination of the a-SiC:H photocathode with a nc-Si:H/c-Si is demonstrated and potential STH efficiencies of 7.9% have been achieved. Furthermore, a purely PV approach such as a triple junction a-Si:H/nc-Si:H/nc-Si:H solar cell is demonstrated, with solar-to-hydrogen (STH) efficiencies of 9.8%

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Amorphous silicon carbide (a-SiC:H) is a promising material for photoelectrochemical water splitting owing to its relatively small band-gap energy and high chemical and optoelectrical stability. This work studies the interplay between charge-carrier separation and collection, and their injection into the electrolyte, when modifying the semiconductor/electrolyte interface. By introducing an n-doped nanocrystaline silicon oxide layer into a p-doped/intrinsic a-SiC:H photocathode, the photovoltage and photocurrent of the device can be significantly improved, reaching values higher than 0.8V. This results from enhancing the internal electric field of the photocathode, reducing the Shockley-Read-Hall recombination at the crucial interfaces because of better charge-carrier separation. In addition, the charge-carrier injection into the electrolyte is enhanced by introducing a TiO₂ protective layer owing to better band alignment at the interface. Finally, the photocurrent was further enhanced by tuning the absorber layer thickness, arriving at a thickness of 150nm, after which the current saturates to 10mAcm^-2 at 0V vs. the reversible hydrogen electrode in a 0.2m aqueous potassium hydrogen phthalate (KPH) electrolyte at pH4.

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One of the main problems of renewable energies is storage of the energy carrier. For long-term storage, solar fuels seem to be a good option. Direct solar water splitting could play an important role in the production of these solar fuels. One of the main challenges of this process is the charge separation and collection at the interfaces. The knowledge on photovoltaic (PV) junctions can be used to tackle this challenge. In this work, the use of doped layers to enhance the electric field in an a-SiC:H photocathode, and the use of thin-film silicon multijunction devices to achieve a stand-alone solar water splitting device are discussed. Using a p-i-n structure as a-SiC:H photocathode, a current density of 10mA/cm2 is achievable. The p-i-n structure proposed also indicates the suitability of traditional PV structures for solar water splitting. In addition, hybrid devices, including a silicon heterojunction PV device, are proposed. A combination of the a-SiC:H photocathode with a nc-Si:H/c-Si is demonstrated and potential STH efficiencies of 7.9% have been achieved. Furthermore, a purely PV approach such as a triple junction a-Si:H/nc-Si:H/nc-Si:H solar cell is demonstrated, with solar-to-hydrogen (STH) efficiencies of 9.8%.@en