## 1 Introduction  
To achieve global greenhouse gas reduction targets, it is necessary to force a rapid transition from fossil fuels to a renewable-energy-based energy supply. However, when increasing the rate of utilization of renewable energies from wind and solar, their weather and seasonal dependencies do represent a technical challenge [1, 2]. Their temporal variations in electricity production and consumption therefore raise the need for effective, affordable, and safe storage systems. One emission-free opportunity to store surpluses of renewable electricity is to transform it into green hydrogen. To this aim, surplus renewable electrical energy can be utilized to split water into hydrogen and oxygen by an electrolyser. Green hydrogen can thus be stored and reused as soon as the energy demand increases. In a fuel cell or a hydrogen combustion engine connected to a generator, it can be converted back into electricity and heat. Hydrogen, as a cross-sector energy carrier, can also be used to decarbonize the transport and chemical sectors [3, 4]. The storage of hydrogen at reasonable energy densities provides economic and technical challenges, because of its low density. Indeed, at atmospheric pressure conditions,hydrogen has a low energy density of 0.0033 kWh 1\({}^{-1}\). The current state-of-the-art methodologies to increase its volumetric energy density are to compress or liquefy hydrogen. At compressions of 350 and 700 bar, a volumetric energy density of 0.792 and 1.32 kWh1\({}^{-1}\) can be reached, respectively. The second form of hydrogen storage technology is in the form of liquefied hydrogen. To liquify hydrogen, it has to be cooled down to about \(-253\,^{\circ}\)C, which represents the boiling point of hydrogen (\(-252.9\,^{\circ}\)C or 20.3 K). The simplest liquefaction process is the so-called Linde cycle, in which pure hydrogen is compressed and cooled down, e.g., by liquid nitrogen. The compressed, cold, and non-condensed gas is forced to pass through an adiabatic throttling valve. The result is a saturated liquid-vapour mixture, which is typically formed at a temperature of \(-253\,^{\circ}\)C. \"The liquid hydrogen is collected as the desired product, and the vapor is routed through a heat exchanger to cool the compressed gas on its way to the throttling valve\" [5]. The obtained liquefied hydrogen is then stored in vacuum-insulated cryogenic tanks. In the case of liquid H\({}_{2}\), the volumetric storage density amounts to about 2.3 kWh 1\({}^{-1}\)[4; 5; 6; 7; 8]. In addition to the above-mentioned physical storage technologies, hydrogen can also be chemically stored. Currently, a way to store hydrogen chemically in liquid form is to use liquid organic hydrogen carriers (LOHC). In the exothermic charging phase (hydrogen), unsaturated organic compounds are hydrated at an elevated temperature and pressure. The endothermic dehydrogenation of the saturated, hydrogen-rich compounds (discharging) also requires an elevated temperature and a slightly increased pressure. Due to the low boiling point of LOHC materials, in most cases, the hydrogen has to be purified after dehydrogenation. In addition, typically, a catalyst is used in both charging and discharging. The process conditions and the storage density strongly depend on the used organic carrier material. In the case of N-ethylcarbazole [9; 10], the hydrogenation typically takes places at a temperature between 130-160 \({}^{\circ}\)C and a pressure of about 70 bar. To release the hydrogen, a temperature of 200-230 \({}^{\circ}\)C and a slight overpressure is usually needed. With this N-ethylcarbazole, a storage density up to 5.8 wt.-% can be achieved [11; 12]. Hydrogen can also be bonded onto solid materials. Due to their capability to strongly bind hydrogen, metals are often used as carrier materials (metal hydrides). In the exothermic charging phase, the adsorption of hydrogen into the metal lattice takes place. Due to the exothermic reaction, the temperature increases during charging.