## 2 Materials and Devices
Regardless, it is recognized that designing the next generation of PEMFCs requires improving their power densities/efficiency, reducing costs, and increasing the durability, in order to promote large-scale commercialization and thus compete with energy systems based on fossil fuels [56]. In their essence, the opposite of hydrogen fuel cells are water electrolyzer devices, meaning that in the electrolyzers, electricity is used to split water (H\({}_{2}\)O) into H\({}_{2}\) at the cathode, while O\({}_{2}\) is generated at the anode, without harmful emissions if enough energy is provided. Thereby, when powered by renewable energy sources (such as solar and wind), green H\({}_{2}\) (with 99.9% purity) is produced onsite [57]. However, an energy input (theoretical/thermodynamic voltage of 1.23 V) needs to be applied, since water splitting is a non-spontaneous redox process. In practice, due to the sluggish kinetics of the oxygen evolution/generation reaction and other inefficiencies, voltage > 1.8 V is commonly applied [19]. Moreover, the pure O\({}_{2}\) also produced is normally vented, while it could be useful for hospitals, space, or submarine missions, or the chemical industry. Thus, although water electrolysis has already been a well-established H\({}_{2}\) production technology for almost a century, these aspects have hindered the deployment of the green H\({}_{2}\) production by water electrolysis, with the cost being a major drawback [58]. However,  
Figure 4: Schemes and electrochemical reactions equations for PEMFCs and AEMFCs. Reproduced with permission from [44]. initiatives such as the U.S. Department of Energy's (DOE's) Hydrogen Shot, launched in 2021 [59], are targeting a production cost of ($1/kg), lower than using conventional fossil fuels means (such as SMR), within the next decade. Additionally, an economically viable (minimum) production scale for green H\({}_{2}\), produced from water electrolysis using renewable electricity, will require scaling-up the electrolyzers to the basic units of renewable electricity generation, i.e., from kW to a few MW. Such small-scale fossil parity is considered to provide an important paradigm shift, given that it allows renewables to be harvested anywhere and used directly for the local production and consumption of green H\({}_{2}\) in a decentralized manner. Therefore, water electrolyzers have the potential to support the energy system both by producing hydrogen for use elsewhere and at other times and by providing a controllable load for the grid (reducing or stopping production when electricity prices are high and increasing production up to the maximum load when electricity prices are low, instead of operating at all times). Nevertheless, further intensification of the water electrolysis process is still required [60]. A review regarding the most recent water electrolysis details, covering from textbook knowledge to the latest scientific strategies and industrial developments being tested in large-scale applications, can be found in [61]. This also includes a discussion regarding the basic mechanisms of the hydrogen and oxygen evolution reactions (HER and OER), along with general findings in recent advances in developing strategies to improve noble and non-noble metal-based electrocatalysts and their alloys. This work, supported by almost 2000 references, clearly points out that, as an endeavor to lower GHG emissions, water electrolyzers will become sufficiently technologically-advanced and economically-viable to be deployed at large. Herein, given the scope of this work, the focus will be on the two main (most advanced and already commercial) water electrolysis technologies that produce H\({}_{2}\) at low temperatures [62]. These can be distinguished by the electrolyte used in the devices, namely liquid alkaline solution (KOH and/or NaOH aqueous solution) and proton exchange membrane (PEM/Nafion(r)). Along with the alkaline anion exchange membranes (AEMs), now at laboratory scale under research and development efforts [63].