# Technology for Green Hydrogen Production: Desk Analysis
## 4 Results and Discussion
### Current Technologies Used for Green Hydrogen Production  
#### 4.2.1 Alkaline Electrolyzers  
AEs are the most mature and commonly used electrolyzers for industrial-scale hydrogen production [12; 16; 19; 20; 21; 22; 23; 132; 133; 134; 135]. Figure 11a shows an alkaline electrolyzer and water isolated [136]. The operating principles of AEs are schematically illustrated in Figure 11b [134]. The operating parameters of AEs are summarized in Table 3. AEs are characterized by their straightforward design and ease of production, with electrode areas up to 3 m\({}^{2}\). They utilize a concentrated KOH solution as the electrolyte, zirconium dioxide-based diaphragms, and nickel-coated stainless-steel electrodes. The OH\({}^{-}\) ion serves as the ionic charge carrier, facilitating the electrochemical reaction. The electrochemical reactions in an AE involve the transfer of electrons and the exchange of ions at the electrodes. At the anode, hydroxide ions react and form water and oxygen gas. The half-reaction releases electrons to the anode, which then flow through an external circuit to the cathode as described in Reaction (2):  
\[\text{Anode reaction (Oxidation): 2 OH}^{-}\rightarrow\text{H}_{2}\text{O}+\frac{1}{2}\, \text{O}_{2}+2\text{e}^{-}. \tag{2}\]  
At the cathode, the protons that have been produced at the anode, along with electrons from the external circuit, combine to form hydrogen gas. The electrons are supplied to the cathode by the external circuit, completing the electrical circuit according to the half-reaction given in Reaction (3):  
\[\text{Cathode reaction (Reduction): 4H}^{+}+4\text{e}^{-}\to 2\text{H}_{2}. \tag{3}\]  
Combining the two half-reactions gives the overall Reaction (4), which shows that water is split into hydrogen and oxygen gases:  
\[\text{Overall reaction: 2H}_{2}\text{O}\to 2\text{H}_{2}+\text{O}_{2}. \tag{4}\]  
In AEs, the alkaline electrolyte facilitates the movement of ions between the electrodes. The hydroxide ions from the electrolyte participate in the reactions at the electrodes. At the anode, hydroxide ions are oxidized to form oxygen gas and water, while at the cathode, they combine with the protons and electrons to form water and hydrogen gas. Table 3 outlines the operating parameters and cost considerations for the AE technology used for green hydrogen production [16]. In 2020, AE operated with a nominal current density range of 0.2 to 0.8 A cm\({}^{-2}\), a voltage range of 1.4 to 3 V, and a temperature range of 70 to 90 \({}^{\circ}\)C. The cell pressure was below 30 bar, with a load range of 15 to 100% and hydrogen purity between 99.9 and 99.9998%. The voltage efficiency based on the lower heating value (LHV) was 50 to 68%, with electrical efficiency for the stack and system at  
Figure 11: Type of electrolyzer: (**a**) alkaline electrolyzer (AE) and water isolated [136]; (**b**) principles of AE [134]. 47 to 66 kWh kg\({}^{-1}\) H\({}_{2}\) and 50 to 78 kWh kg\({}^{-1}\) H\({}_{2}\), respectively. The stack lifetime was approximately 60,000 h. By 2050, the targets for AE include a nominal current density exceeding 2 A cm\({}^{-2}\), a voltage below 1.7 V, an operating temperature above 90 \({}^{\circ}\)C, and a cell pressure greater than 70 bar. The load range is expected to be 5 to 300%, with hydrogen purity exceeding 99.9999%. The voltage efficiency is targeted to be over 70%, with electrical efficiency for the stack and system reducing to less than 42 kWh kg\({}^{-1}\) H\({}_{2}\) and 45 kWh kg\({}^{-1}\) H\({}_{2}\), respectively. The stack lifetime is projected to increase to 100,000 h. R&D efforts are focused on various components, including the diaphragm, catalysts, cell frames, balance of plant components, and electrodes. The minimum size for a stack unit is 1 MW, with an electrode area of 10,000 to 30,000 cm\({}^{2}\). The time to reach nominal load from a cold start should be less than 50 min for a 1 MW unit and less than 30 min for a 10 MW unit.