## 3 Further Discussion  
It is clear that green H\({}_{2}\) production (coupling renewable energy systems with fuel cells and water electrolyzers) is witnessing an exponential increase. According to the recent joint report from the Hydrogen Council and the World Bank, hydrogen needs to grow seven-fold to support the global energy transition, eventually accounting for 10% of total energy consumption by 2050 [72]. However, these technologies are heavily dependent upon the materials used, i.e., catalysts, electrolytes, separators, and on the working operation conditions, such as temperature and pressure. Presently, as previously mentioned, hydrogen production via electrolysis is more expensive than other methods, due to the capital costs and dependence on electricity costs, which hinder its commercial viability. One strategy can be to improve the electrolyzer system's efficiency to reduce the costs and electricity consumed, along with their large-scale deployment. Consequently, as a \"rule of thumb\", new materials that are low-cost, highly performing, and durable, with a particular focus on thinner membranes (electrolytes), more active and durable catalysts, and less critical raw materials, are needed [61]. As pointed out by the previously mentioned World Bank report [72], the scale of the raw materials' impact on hydrogen and fuel cell technologies is small relative to the scale of the current demand for critical raw materials. However, there are some materials for which production levels could be a key challenge, namely those from the platinum-group metals: platinum and iridium (see Figure 6). Notably, it is considered that the demand for iridium could reach over 160% of current production in the 2040s. Consequently, in order to overcome these challenges, incorporating sustainable practices and policies for mining and processing materials that can help minimize environmental impacts is encouraged and is of paramount importance. Key among these approaches is increasing the supply from above-ground stocks (such as recovery from premium spark-plugs and tailings), recycling and designed-in circularity (innovations in design in order to reduce material intensities), and stimulating material substitution whenever possible. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Alkaline** & **PEM** & **AEM** \\ \hline \multirow{4}{*}{Electrolyte} & \multirow{4}{*}{\
\begin{tabular}{c} Aqueous KOH \\ (20–40 wt\%) \\ \end{tabular} } & \multirow{4}{*}{
\begin{tabular}{c} Proton exchange \\ ionomer \\ (e.g., AS-4) + \\ \end{tabular} } \\  & & & (e.g., Nafion) \\ \cline{1-1}  & & & caustic solution \\ Cathode & Ni, Ni–Mo alloys & Pt, Pt–Pd & Ni and Ni alloys \\ Anode & Ni, Ni–Co alloys & RuO\({}_{2}\), IrO\({}_{2}\) & Ni, Fe, Co oxides \\ Half-cell separation & Diphragm & Nafion 117 & AEM \\ \cline{1-1}  & (Zirton Perf 500 \(\mu\)m) & (e.g., 180 \(\mu\)m) & (20–100 \(\mu\)m) \\ Current density (A cm\({}^{-2}\)) & 0.2–0.4 & 0.6–2.0 & 0.2–1.0 \\ Cell voltage (V) & 1.8–2.4 & 1.8–2.2 & 1.8–2.2 \\ Cell area (m\({}^{2}\)) & \textless{}4 & \textless{}3 & Lab testing cells \\ Operating temperature (\({}^{\circ}\)C) & 60–80 & 50–80 & 50–60 \\ Operating pressure (bar) & 1–30 & 30–76 & 1–30 \\ Production rate (N m\({}^{3}\) h\({}^{-1}\)) & \textless{}760 & \textless{}40 & \textless{}1 \\ Gas purity (vol\%) & \textgreater{}99.5 & \textgreater{}99.999 & \textgreater{}99.99 \\ System response & Seconds & Milliseconds & na \\ Stack lifetime (h) & 60 k to 100 k & 20–60 k & na \\ Technology status & Mature & Commercial & R\&D \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Main characteristics of alkaline, PEM, and AEM water electrolysis devices. Reproduced with permission from [65]. The adoption of such a framework (the core of the World Bank's Climate-Smart Mining principles) can help with managing the use of critical raw materials in green H\({}_{2}\) and fuel cells'/water electrolyzers' deployment and providing the security of the supply of the key critical raw materials needed for global hydrogen deployment.