We have to overcome the intermittent nature of renewables to master the energy transition. Harvesting renewable electricity is only part of the solution. Energy storage is another part and the main challenge of our time. Only with efficient storage solutions can big industries switch to renewables. Electricity storage is most efficient with batteries while industrial sites and synthetic fuel production require a sustained hydrogen input to drive the processes. The aim of this thesis was the research and development of storage solutions mainly based on earth-abundant iron to bridge intermittency. The following scientific questions were at the basis of the conducted research: 1. Combined battery and electrolyser: is it possible and reasonable to develop a device that serves two purposes? Do the materials endure and support this double functionality? 2. Multiple electrodes: nickel is required for electricity and oxygen storage; iron is required for electricity and hydrogen storage. Is it possible to store electricity, oxygen and hydrogen in one electrochemical cell? Is it possible to decouple the electricity input from the oxygen and hydrogen output? Can a single electrode be used for two purposes simultaneously? Are configurations with multiple electrodes scalable to larger arrays? 3. Fundamentals of iron electrodes: Which phases occur for the first and second iron discharge plateau? And why are iron electrodes less responsive to higher discharge rates? 4. Sustained hydrogen from intermittent sources: Decoupling of the electricity input and the hydrogen output is possible with an electrochemical cell consisting of at least three electrodes. Is more sustained hydrogen from intermittent sources also feasible with a standard electrochemical cell with two electrodes? 5. Doped iron electrodes: Iron electrodes can have a limited rechargeability and can show gas accumulation inside the electrode. Does the addition of dopants enhance the ability of the iron electrode to recharge? Do these dopants enhance performance and the material utilisation? Battolyser NiFe batteries are known to be practically indestructible. However, these NiFe batteries have the disadvantages of hydrogen and oxygen production and selfdischarge which makes them inefficient as a battery. In the battolyser we promote this “hydrogen-side-effect”, and obtain an energy-efficient device that produces hydrogen with excess energy with the potential to reduce undesirable renewable electricity curtailments. Such a device can be operational around the clock: either the surplus of electricity is used to charge the battery and to produce hydrogen or electricity is provided to consumers. The battolyser will supply hydrogen when overcharged, following an intermittent pattern of renewables availability. Therefore, downstream infrastructure needs the capability to handle an intermittent hydrogen input or requires a hydrogen storage infrastructure. Under these conditions the battolyser has the potential to become an essential single-combined tool for the energy transition since renewable electricity can be stored and excess electricity can be converted efficiently into hydrogen. Multi-Controlled (MC-)electrodes Then we demonstrated that we could supply a sustained hydrogen output from an intermittent energy input and that time shifting the hydrogen output comes at low energy costs. We accomplished that by creating electrochemical systems consisting of more than two electrodes within a single electrochemical cell. Here the storage electrodes can be charged/discharged while gas production, hydrogen and oxygen, can occur simultaneously and at independent rates. We also demonstrated that storage electrodes can serve two different processes at the same time. The proposed concept of MC-electrodes allows for controlling and scaling up multi-electrode configurations to larger arrays. Most importantly we used it for decoupling the electricity input from the hydrogen output by the combination of an iron storage electrode with two gas evolution electrodes, one for hydrogen evolution and one for oxygen evolution with two independent circuits. The position of the electrode phases in the Pourbaix diagram indicates that charging the iron electrode together with oxygen production requires most of the energy while little energy is required to generate hydrogen from previously charged iron electrodes. Independent operation of both circuits enables decoupling of the electricity input and the hydrogen output, and the iron storage electrode serves as an electrochemical storage reservoir. Time-shifting 50% of the hydrogen production requires only 5% of the energy while 95% of the required energy can be fed through a main controller when electricity is cheap and abundant. Moreover, hydrogen can later be provided from reduced iron electrodes with a substantial reduction of backup power. Compared to electrolysers, the electricity storage requirement is reduced by 85% to provide the same amount of hydrogen, using the previously reduced iron oxidation. In other words, seven times more hydrogen can now be provided from existing backup power, which can serve as a booster for delayed hydrogen generation. Half-cell used as Hydrogen Storage and Production cell (HSP-cell) We reduced the complexity of the system by combining the iron storage electrode with a bifunctional electrode for oxygen and hydrogen production which led to the concept of the HSP-cell. The HSP-cell is a simple half-cell, consisting of two electrodes which makes it easily scalable to larger bi-polar configurations. The HSP-cell can utilize the entire capacity of the iron electrode, comparable to the iron-air battery or battolyser, but delivers hydrogen instead of electricity. Both configurations can operate as a low-cost sink to store energy in reduced iron and both systems can use excess electricity for direct hydrogen generation to reduce undesirable curtailment of renewable power. The replacement of the nickel hydroxide battery electrode by a thin bifunctional nickel metal electrode provides space and allows to increase the storage density. Considering only the iron electrode (and omitting counter electrode, electrolyte, casing, valves or other parts), a storage density of 0.78 Ah/cm3 is currently feasible, equivalent to 29 kgH2/m3 or to a compressed hydrogen storage density of 500 bar. The stored hydrogen can be released easily and controlled by applying a current. This reduces the safety risk associated with the storage of compressed hydrogen gas. During electrochemical hydrogen release, only hydrogen is generated inside the cell, which offers an oxygen-free hydrogen gas output even at low discharge rates. The HSP-cells can be configured in a self-sustaining manner and in a way to provide a sustained hydrogen output from an intermittent input by simultaneous and phase-shifted operation of several units. The concept can provide sustained hydrogen to industrial processes or synthetic fuel production with an overall efficiency including storage and production which exceeds 80% when operated at 40 ◦C. Therefore, the HSP-cell has the potential to become an essential device to boost the energy transition. Doped Iron electrodes The iron electrode is the common part of all previously discussed configurations. Having an optimal iron electrode is essential since the iron electrode determines the rate capabilities and the efficiencies. In the battolyser thin iron electrodes suffice because the nickel electrode is capacity limiting. Thicker iron electrodes can be used in the iron-air battery/battolyser, in the MC-cell and in the HSPcell. We developed a strategy to produce sintered iron electrodes to study the phase behaviour of the electrode in operando by means of neutron diffraction. The study revealed that substantial amounts of iron hydroxide were inside the bulk of the sample which could not be reduced back to metallic iron upon charging. We concluded that the electrochemical circuit within the electrode must be interrupted, and it is our hypothesis that gas accumulation within the cell negatively affected the ionic conductivity. We assume that gas accumulation within the electrode replaces electrolyte which increases the ionic resistance for phase transition. As a consequence, the inserted charge shifts from battery charging with phase transition to hydrogen evolution. We wanted to improve the material utilization of the sintered iron electrodes and therefore needed to improve the ability of these electrodes to recharge. For this purpose, we added either zirconia oxide or alumina oxide to the electrodes. By adding metal-oxides to the electrode-composition we enhance the processability of the materials and the electrode performance. With the new synthesis strategy, we produced thick sintered iron electrodes which show a volumetric storage density of up to 0.8 Ah/cm3 and reach areal storage densities of up to 160mAh/cm2. These values are among the best reported values in literature for sintered iron electrodes. In the process we may create a sulphur free system which potentially reduces corrosion issues, and which potentially reduce the deterioration of air electrodes. Bridging intermittency with iron electrodes Summing up, the creation of an energy system based on renewables confronts us with the intermittent nature of renewable power generation. To bridge the intermittency we need storage solutions for electricity and hydrogen. With a sustained hydrogen output synthetic fuels based on renewables could be produced on a large scale. With the nickel-iron battolyser and the iron-air battolyser we can store electricity and we can convert excess electricity into hydrogen to overcome the curtailment-problem. With the concept of MC-electrodes and of the HSP-cell we can efficiently control, store, and postpone the hydrogen output, to provide a more sustained hydrogen output. The iron electrode is present in all configurations and recharging was the main challenge. We addressed the issue of rechargeability with a modified synthesis strategy for sintered iron electrodes doped with Zr and Al instead of sulphur. Electrodes produced with this strategy may have the potential to perform as effective sintered iron electrodes. With these new simple concepts and cost-efficient iron electrodes we offer new tools to support and accelerate the storage and conversion of renewable power, which is necessary for the energy transition and to overcome intermittency. We have to speed up the energy transition to limit the impact of climate change.@en

Iron is a promising, earth-abundant material for future energy applications. In this study, we use a neutron diffractometer to investigate the properties of an iron electrode in an alkaline environment. As neutrons penetrate deeply into materials, neutron scattering gives us a unique insight into what is happening inside the electrode. We made our measurements while the electrode was charging or discharging. Our key questions are: Which phases occur for the first and second discharge plateaus? And why are iron electrodes less responsive at higher discharge rates? We conclude that metallic iron and iron hydroxide form the redox pair for the first discharge plateau. For the second discharge plateau, we found a phase similar to feroxyhyte but with symmetrical and equally spaced arrangement of hydrogen atoms. The data suggest that no other iron oxide or iron (oxy)hydroxide formed. Remarkable findings include the following: (1) substantial amounts of iron hydroxide are always present inside the electrode. (2) Passivation is mostly caused by iron hydroxide that is unable to recharge. (3) Iron fractions change as expected, while iron hydroxide fractions are delayed, resulting in substantial amounts of
amorphous, undetectable iron phases. About 40% of the participating iron of the first plateau and about 55% of the participating iron for the second plateau are undetectable. (4) Massive and unexpected precipitation of iron hydroxide occurs in the transition from discharging to charging. (2), (3), and (4) together cause accumulation of iron hydroxide inside the electrode.@en

Grid scale electricity storage on daily and seasonal time scales is required to accommodate increasing amounts of renewable electricity from wind and solar power. We have developed for the first time an integrated battery-electrolyser ('battolyser') that efficiently stores electricity as a nickel-iron battery and can split water into hydrogen and oxygen as an alkaline electrolyser. During charge insertion the Ni(OH)₂ and Fe(OH)₂ electrodes form nanostructured NiOOH and reduced Fe, which act as efficient oxygen and hydrogen evolution catalysts respectively. The charged electrodes use all excess electricity for efficient electrolysis, while they can be discharged at any time to provide electricity when needed. Our results demonstrate a remarkable constant and a high overall energy efficiency (80-90%), enhanced electrode storage density, fast current switching capabilities, and a general stable performance. The battolyser may enable efficient and robust short-term electricity storage and long-term electricity storage through production of hydrogen as a fuel and feedstock within a single, scalable, abundant element based device.

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