The tremendous economic development of the 20^th century was largely driven by the availability of abundant, cheap fossil fuels. However, there is now a consensus within the scientific community that CO₂ emissions due to fossil fuel combustion are directly correlated with the ongoing climate change [1] . In order to slow down climate change and to prevent the resulting dramatic consequences, there is a need for new, renewable energy sources. The transition from a fossil fuel―to a renewable energy based economy―is already happening in certain countries. In Germany, the contribution of solar electricity to the total electrical consumption increased from less than 0.5% in 2005 to 7.5% in 2015 [2] . However, there is one important drawback of renewable energy sources such as wind and solar: these types of energy sources cannot be controlled and are poorly predictable. Thus, new energy storage capacities have to be built to match the energy supply and demand both timely and geographically. Hydrogen has emerged as a possible energy vector to bridge that gap. Hydrogen could be used directly as an energy vector or as a feedstock for the production of synthetic hydrocarbons via carbon dioxide reduction [3] [4] . Both approaches are shown graphically in Figure 1. In the first case (left), hydrogen is produced by using renewable electricity sources through the electrolysis process. The oxygen is released in the atmosphere, and the hydrogen is stored. Upon demand, hydrogen is combusted in order to recover energy, for instance in a fuel cell or in a conventional combustion engine. The synthetic fuel cycle (right) is similar. However, the key difference is that CO₂ is included in that cycle. Therefore, synthetic hydrocarbons are produced. The strong advantage of this cycle is the fact that the distribution channels and users’ customs do not need to be changed. The challenge lies in the addition of a relatively complex step in the process, i.e. the capture of CO₂ from ambient air. In both cases, the advantage is the absence of net CO₂ emissions.

Regardless of the approach chosen, the production of hydrogen by electrolysis is a key step of the process. There exist three main types of electrolysers, classified based on the ion transferred: Acidic (PEM) electrolysers, alkaline (AEL) and solid oxide electrolysers (SOEC).

Electrolysis is based on the splitting of water by means of an electrical potential (dissociation of the water molecule in an electrical field). Hydrogen is evolved on the cathode (−) and oxygen on the anode (+). Between the electrodes is an electrolyte, which acts as an electrical insulator and ionic conductor. The ions transferred between the electrodes are, either H⁺, OH⁻ or O 2 − and the corresponding electrolysers are called polymer electrolyte membrane, acidic (PEM), alkaline or solid oxide electrolyte. The schematic principle of the three different types of electrolysers is summarized in Figure 2. A membrane separates the evolved gases H₂ and O₂ between the electrodes. The membrane has to fulfill several

requirements, e.g. stability under operating conditions, separation of the gases, mechanical separation of the electrodes, ion conduction and mechanical support for pressure differences between the two sides in the cell.

The electrolysers are operated under different conditions such as pressure and temperature. Typically, PEM and AEL devices operate at moderate temperatures (<80˚C and <220˚C, respectively) while solid oxide electrolysers operate at elevated temperatures (>600˚C). A summary of the typical key operating parameters for the three types of technologies is shown in Table 1.

In general, PEM electrolysers have a low hydrogen production capacity (<30 Nm³/h) and a moderate efficiency while AEL electrolysers have larger production capacity and a higher efficiency. The energy required to produce 1 Nm³ of hydrogen is plotted versus the hydrogen production rate for several installations in Figure 3. The lower horizontal axis shows the energy content of the produced hydrogen stream based on the Higher Heating Value (HHV)―i.e. the minimal power required to produce that quantity of energy at a 100% conversion efficiency. Thereby, the difference is made between the PEM electrolyser efficiency of the full system or the stack only, which is sometimes reported in the documentation.