## 2 Materials and Devices  
Hydrogen (H) is the simplest and most abundant element in the universe. However, due to its reactivity, on Earth it rarely exists as a gas (H\({}_{2}\)), rather existing in combination with other elements, notably oxygen as water (H\({}_{2}\)O), and many others such as ammonia (NH\({}_{3}\)) and methane (CH\({}_{4}\)). Thus, H\({}_{2}\) can be obtained readily from water by applying electrochemical and thermochemical processes. Moreover, unlike conventional fossil fuels that generate CO\({}_{2}\) upon combustion, H\({}_{2}\) is a clean fuel that only produces H\({}_{2}\)O as a by product when combusted or reacted with oxygen (O\({}_{2}\)), namely in fuel cell devices. As shown in Figure 1, hydrogen has the highest energy per mass of any traditionally employed fuels (120 MJ/kg, nearly three times greater than that of gasoline 44 MJ/kg). In contrast, its low ambient temperature density results in a low energy per unit volume. Therefore, typically, hydrogen is stored in the gaseous form, requiring safe and large-volume systems. This increases costs and raises significant challenges regarding high density hydrogen storage, i.e., to pack hydrogen as close as possible, using as little additional material and energy as possible [27]. However, this is out of the scope of the present work; for the interested reader, more details can be found in [28; 29; 30]. Presently, H\({}_{2}\) production is primarily based on fossil fuel technologies. More specifically, according to the International Energy Agency (IEA) \"Global Hydrogen Review 2022\" report [24], in 2021 the total global H\({}_{2}\) production was 94 million tons (Mt)--with associated emissions of more than 900 Mt CO\({}_{2}\)--obtained from reforming natural gas (a mixture of mostly methane with other gases) (62%) and from coal gasification (19%, mainly in China), as well as as a by-product of naphtha reforming at refineries (18%); it is then used for other refinery processes (e.g., hydrocarbons, desulphurization). It is worth noting that the production of low-emission H\({}_{2}\) was around 0.6 Mt in 2021 (less than 1% of global hydrogen production), as explained in more detail subsequently. Although H\({}_{2}\) is an invisible gas, it is commonly described in the literature with color-coding to distinguish between different production methods and to facilitate cleanness level discussion [14; 31], as can be seen in Figure 2. Therefore, \"grey\" hydrogen is produced using fossil fuels--i.e., using steam methane reforming (SMR) or coal gasification--which in this last case can also be denominated \"black\" or \"brown\" H\({}_{2}\) depending on the type bituminous (black) or lignite (brown) coal used. In any case, substantial CO\({}_{2}\) emissions occur, given the high carbon content of the fuel, thus making these hydrogen technologies unsuitable as a route toward net-zero emissions. However, when the \"grey\" hydrogen production is combined with carbon capture and storage (CCS), \"blue\" hydrogen is obtained, which, during the early stages of the energy transition, is considered to facilitate the growth of a hydrogen market. Given that, retrofitting the existing natural gas infrastructure with CCS would allow its continued use to produce H\({}_{2}\) with lower GHG emissions, while reducing pressure on the renewable energy capacity installation rate to produce green H\({}_{2}\). Nonetheless, blue H\({}_{2}\) has limitations that have so far restricted its deployment: it uses finite resources, is exposed to fossil fuel price fluctuations, and does not support the goals of  
Figure 1: Gravimetric and volumetric density of hydrogen and other fuels. Reproduced with permission from [27]. energy security. Moreover, it faces social acceptance issues, such as those associated with additional costs for CO\({}_{2}\) transport, storage, and monitoring.