## 2 Materials and Devices
In addition, carbon capture efficiencies are expected to reach 85-95% at best, which means that 5-15% of the CO\({}_{2}\) will still be emitted and consequently these high capture rates have yet to be achieved [14]. In sum, the carbon emissions from H\({}_{2}\) generation could be reduced by CCS but not eliminated. For these reasons, blue H\({}_{2}\) should be seen only as a short-term transition to facilitate the uptake of green H\({}_{2}\) on the path to net-zero emissions. An alternative could be \"turquoise\" hydrogen, which combines the use of natural gas as feedstock with no CO\({}_{2}\) production. This is achieved through the pyrolysis of methane, with solid carbon (known as carbon black) as a by-product, which can be more easily stored than gaseous CO\({}_{2}\). At the moment, turquoise H\({}_{2}\) is still at the pilot stage; however, a market for carbon black already exists, which can provide an additional revenue stream and foster its development and deployment [14]. Among the different shades of hydrogen, \"green\" hydrogen--meaning H\({}_{2}\) produced from renewable energy--is the most suitable for a fully sustainable energy transition and is more consistent with the net-zero route. Thus, the most established technology option for producing green H\({}_{2}\) is water electrolysis using renewable electricity, obtained from natural sources, such as the sun, water, and wind, as this allows the exploitation of synergies from sector coupling, thus decreasing technology costs and providing flexibility to the power system. In this way, increased interest in the production of green hydrogen, namely from water electrolysis, is boosted. In fact, even though low-emission H\({}_{2}\) production was less than 1 Mt (0.7%) in 2021, almost all was from fossil fuels with CCS, only (35 kt H\({}_{2}\)) being from electricity via water electrolysis. This very small amount, however, reflects an increase of nearly 20% compared to 2020 [24]. It is important to note that hydrogen is not a source of energy, but is an energy carrier/vector--just like electricity. Hence, the realization of a sustainable hydrogen energy system requires the further development of alternative means of hydrogen production, using renewable energy and/or materials, together with effective networks of storage, distribution, and use. It is specifically a fuel to be used in fuel cells, given that these electrochemical devices are some of the cleanest (CO\({}_{2}\)-free) and most efficient technologies for generating electricity, independently of the grid. One must keep in mind that, apart from the intermittence in the energy production associated with the variability of the renewable energy sources, another problem is the difficulty of storing the electricity produced in a feasible way, due to the limited storage capacity of current technology [18]. Consequently, a way to circumvent this problem is by producing green H\({}_{2}\) that can be used anytime and everywhere in fuel cells' technology [18; 32]. Further details regarding hydrogen fuel cells and water electrolyzers powered by renewables are explored in the following sections, given their relevance to and positive impact on the decarbonization of the power supply. The goal is to achieve a carbon neutral  
Figure 2: Selected shades of hydrogen according to IRENA. Reproduced with permission from [14]. energy scenario along with net-zero carbon and the reduction of GHG emissions into the atmosphere, by taking advantage of the contribution from green H\({}_{2}\) and fighting against climate change, as well as developing a green H\({}_{2}\) economy [18; 32; 33; 34; 35; 36; 37]. The ultimate goal is the economic feasibility of hydrogen production and the security of the energy supply at a large scale--using renewable sources readily available--to substitute the current power economy based on fossil fuels.