## 4 Discussion  
Based on the results presented (Tables 1-9) in Section 3, the following discussion and insights can be synthesized:  
* Conventional H\({}_{2}\) production via SMR has the advantage of being low-cost (around USD 1-2/kg H\({}_{2}\)) and having a high TRL. However, SMR technology without CCS suffers from a high global warming impact of around 11 kg CO\({}_{2}\)/kg H\({}_{2}\), as well as its nonrenewable feedstock requirement of natural gas. * SMR with CCS and ATR with CCS result in lower global warming impacts, but they still produce around 2-8 kg CO\({}_{2}\)/kg H\({}_{2}\), which is higher than the emissions of the majority of H\({}_{2}\) production via water electrolysis. Also, the energy requirements (MJ/kg H\({}_{2}\)) of blue H\({}_{2}\) production technologies are generally higher than those of gray \begin{table}
\begin{tabular}{l c c c c c} \hline \hline \multirow{2}{*}{**Hydrogen Production Technology**} & \multirow{2}{*}{**TRL**} & \multicolumn{3}{c}{**Supply Chain Material Availability**} & \multirow{2}{*}{**Comments**} \\ \cline{3-3} \cline{5-6}  & & **L** & **M** & **H** & \\ \hline \# **Greener H\({}_{2}\) (water electrolysis):** & & & & & \\ \hline \# **Wind + AKE** & 9 & 1 & & & \\ \hline \# **Wind + PEME** & 6-8[1, 2, 1] & & & & Nickel, Culper \\ \hline \# **Wind + PCE** & 6-8[1, 2, 1] & & & & Platinum, Iridium, titanium \\ \hline \# **Wind + SOE** & 5[9], 6-2[1, 2, 1] & & & & Scandium \\ \hline \# **Wind + AEME** & 2-3[0], 4-5[23], 6[3] & & & & Platinum, Nickel, Transition \\ \hline \# **Solar PV + AKE** & 9 & 1 & & & Metal Catalysts (CoO\({}_{2}\)–La\({}_{2}\)O)* \\ \hline \# **Solar PV + PEME** & 6-8[1, 2, 1] & & & & Nickel, Culper \\ \hline \# **Solar PV + SOE** & 5[9], 6-7[23], 8[1] & & & & Platinum, Iridium, titanium \\ \hline \# **Solar PV + AEME** & 2-3[0], 4-5[23], 6[3] & & & & & Platinum, Nickel, Transition \\ \hline \# **Hydropower + electrolyzers** & 9 & 1 & & & Metal Catalysts (CoO\({}_{2}\)–La\({}_{2}\)O)* \\ \hline \# **Grid mix + electrolyzers** & 9 & 1 & & & \\ \hline \# **Nuclear + SOE** & 5[9], 8[15] & & & & Scandium \\ \hline \# **Nuclear power + PEME** & 6-8[0, 9, 1] & & & & Platinum, Iridium, titanium \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Color-coded technology readiness level (TRL) and supply chain material availability of H\({}_{2}\) production via various water electrolysis technologies with different energy sources. \begin{table}
\begin{tabular}{l c c c c} \hline \hline \multicolumn{2}{c}{**Hydrogen Production Technology**} & \multirow{2}{*}{**TRL**} & \multicolumn{3}{c}{**Supply Chain Material Availability**} & \multirow{2}{*}{**Comments**} \\ \cline{3-3} \cline{5-6}  & & **L** & **M** & **H** & \\ \hline \# **Emerging technologies** & & & & & \\ \hline \# **Biomethane pyrolysis** & 3-7[12, 7] & & & & \\ \hline \# **ESMR** & 4[23] & & & & \\ \hline \# **SMC** & 3-5[6] & & & & \\ \hline \# **Solar thermal + thermochemical decomposition** & 3-4[0, 8] & & & & ZnO/Zn cycle \\ \hline \# **Nuclear + thermochemical decomposition** & & & & \\ \hline \# **S-1** & 3-4[0, 15] & & & & \\ \hline \# **HyS** & 3-4[0, 25] & & & & \\ \hline \# **Cu-Cl** & 3-4[0, 25] & & & \\ \hline \# **Mg-Cl** & 3-4[0, 26] & & & \\ \hline \# **Bio-photolysis** & 4[57] & & & & \\ \hline \# **Ammonia thermal decomposition via fixed bed reactors** & 9[1, 10] & & & & Precious metal catalyst \\ \hline \# **Ammonia thermal decomposition with PSA-membrane separation** & 4[45] & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Color-coded technology readiness level (TRL) and supply chain material availability of emerging clean H\({}_{2}\) production technologies. H\({}_{2}\) production technologies. The emerging carbon capture, utilization, and storage (CCUS) is expected to add additional environmental and economic values that go beyond CCS only. * Assessment of waste valorization as an alternative to the conventional methods for H\({}_{2}\) production reveals that its most robust advantage is its high supply chain material availability due to using waste as the feedstock.