# Technology for Green Hydrogen Production: Desk Analysis
## 4 Results and Discussion
### Current Technologies Used for Green Hydrogen Production
#### 4.2.2 Proton Exchange Membrane Electrolyzers
By 2050, the load range is expected to increase to 5-300%, with hydrogen purity maintained, voltage efficiency exceeding 80%, and electrical efficiency dropping below 42 kWh kg\({}^{-1}\) H\({}_{2}\) for the stack and 45 kWh kg\({}^{-1}\) H\({}_{2}\) for the system. The stack lifetime is projected to extend to 100,000-120,000 h, with larger unit sizes of 10 MW and an electrode area greater than 10,000 cm\({}^{2}\). Capital costs for the electrolyzer stack and system are also highlighted in Table 4. In 2020, the minimum capital cost for a 1 MW stack was USD 400/kW, with a target of less than 5 min for a cold start to nominal load. For a 10 MW system, the capital costs ranged \begin{table}
\begin{tabular}{l l l l} \hline \hline
**Parameter** & **2020** & **Target 2050** & **R\&D Focus** \\ \hline Nominal current density & 1–2 A cm\({}^{-2}\) & 4–6 A cm\({}^{-2}\) & Design, membrane \\ \hline Voltage range (limits) & 1.4–2.5 V & \textless{}1.7 V & Catalyst, membrane \\ \hline Operating temperature & 50–80 \({}^{\circ}\)C & 80 \({}^{\circ}\)C & Effect on durability \\ \hline Cell pressure & \textless{}30 bar & \textgreater{}70 bar & Membrane, reconversion catalysts \\ \hline Load range & 5–120\% & 5–300\% & Membrane \\ \hline Hydrogen purity & 99.9–99.9999\% & Same & Membrane \\ \hline Voltage efficiency (LHV) & 50–68\% & \textgreater{}80\% & Catalysts \\ \hline Electrical efficiency (stack) & 47–66 kWh kg\({}^{-1}\) H\({}_{2}\) & \textless{}42 kWh kg\({}^{-1}\) H\({}_{2}\) & Catalysts/membrane \\ \hline Electrical efficiency (system) & 50–83 kWh kg\({}^{-1}\) H\({}_{2}\) & \textless{}45 kWh kg\({}^{-1}\) H\({}_{2}\) & Balance of plant \\ \hline Lifetime (stack) & 50,000–80,000 h & 100,000–120,000 h & Membrane, catalysts, PTLs \\ \hline Stack unit size & 1 MW & 10 MW & MEA, PTL \\ \hline Electrode area & 1500 cm\({}^{2}\) & \textgreater{}10,000 cm\({}^{2}\) & MEA, PTL \\ \hline Cold start (to nominal load) & \textless{}20 min & \textless{}5 min & Insulation (design) \\ \hline Capital costs (stack) minimum 1 MW & USD 400/kW & \textless{}USD 100/kW & MEA, PTLs, BPs \\ \hline Capital costs (system) minimum 10 MW & USD 700–1400/kW & \textless{}USD 200/kW & Rectifier, water purification \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Operating parameters of PEMs, where MEA—membrane electrode assembly, PTLs—porous transport layers, BPs—bipolar plates [21]. from USD 700-1400/kW. Table 4 sets future targets of less than USD 100/kW for the stack and less than USD 200/kW for the system, indicating a significant reduction in costs by 2050. Further insights and developments in PEM electrolyzer technology are discussed more specifically in [16]. PEM electrolyzers operate at low temperatures and can respond more quickly to changes in power supply, making them well-suited for integration with variable RESs like solar and wind [137, 138, 139, 140, 141, 134, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153  
Cathode reaction (Reduction): \(\mathrm{H_{2}O}+2\mathrm{e^{-}}\rightarrow\mathrm{H_{2}}+\mathrm{O^{2-}}\). At the anode, the oxygen ions reaching the anode release their electrons to the anode, completing the electrical circuit. These \(\mathrm{OH^{-}}\) ions then combine to form oxygen gas. The half-reaction related to oxidation at the anode is represented by Reaction (9):  
\[\mathrm{Anode\ reaction}\ (\mathrm{Oxidation}):4\mathrm{O^{2-}}\rightarrow 2\mathrm{O_{2}}+8\mathrm{e^{-}}. \tag{9}\]  
Reaction (10) shows the overall electrolysis reaction in an SOE combining the reactions at the cathode and anode:  
\[\mathrm{Overall\ reaction}\ \mathrm{:2H_{2}O}\rightarrow\mathrm{2H_{2}}+ \mathrm{O_{2}}. \tag{10}\]  
This reaction shows that water is split into hydrogen and oxygen gases. Table 5 outlines the specifications and targets for SOEs for the production of green hydrogen, comparing the parameters in 2020 with the projected targets for 2050 [16]. Based on key parameters, it can be stated that nominal current density is expected to increase from 0.3 to 1 A cm\({}^{-2}\) in 2020 to over 2 cm\({}^{-2}\) by 2050.