# 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  
PEM electrolyzers use a solid polymer electrolyte to conduct protons from the anode to the cathode while keeping the electronic conductivity low to prevent short-circuiting [13; 16; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41]. The electrochemical reactions in PEM electrolyzers are similar to those in AEs but are facilitated by the proton exchange membrane, which plays a crucial role. The membrane also serves to keep the hydrogen and oxygen gases separated to prevent recombination, which is important for safety reasons. A minimalist and photorealistic image of a PEM electrolyzer, showcasing technological sophistication and commercial potential, is shown in Figure 12a [142]. Figure 12b illustrates the operating principles of PEMs [134]. In a PEM electrolyzer, the oxidation reaction occurs at the anode, in which water is oxidized to produce oxygen gas, protons, and electrons in the half-reaction represented as Reaction (5):  
\[\text{Anode reaction (Oxidation): 2H}_{2}\text{O}\rightarrow\text{O}_{2}+4\text{H}^{+}+4 \text{e}^{-}. \tag{5}\]  
Reaction (5) releases electrons to the anode, which then flow through an external circuit to the cathode. At the cathode, the reduction reaction takes place, where the protons that have been produced at the anode, along with electrons from the external circuit, combine to form hydrogen gas. In the half-reaction, the electrons are supplied to the cathode by the external circuit, completing the electrical circuit as described in Reaction (6):  
\[\text{Cathode reaction (Reduction): 4H}^{+}+4\text{e}^{-}\to 2\text{H}_{2}. \tag{6}\]  
Reaction (7) represents the overall electrolysis reaction showing the water splitting into hydrogen and oxygen gases as a result of combining the two half-reactions:  
\[\text{Overall reaction: 2H}_{2}\text{O}\to 2\text{H}_{2}+\text{O}_{2}. \tag{7}\]  
Figure 12: Type of electrolyzer: (**a**) proton exchange membrane (PEM) electrolyzer [142]; (**b**) principles of PEM [134]. Table 4 presents the operating parameters of PEM electrolyzers, a key technology for green hydrogen production, as outlined by the International Renewable Energy Agency in their 2020 report [16]. Table 4 compares the nominal current density, voltage range, operating temperature, and cell pressure between the year 2020 and the target for 2050, indicating a significant improvement in efficiency and operational parameters by the latter year. In 2020, PEM electrolyzers operated with a nominal current density of 1-2 A cm\({}^{-2}\), a voltage range of 1.4-2.5 V, a temperature between 50 and 80 \({}^{\circ}\)C, and a cell pressure below 30 bar. By 2050, the target is to achieve a higher current density of 4-6 A cm\({}^{-2}\), a reduced voltage of less than 1.7 V, an increased operating temperature of 80 \({}^{\circ}\)C, and a higher cell pressure exceeding 70 bar. These advancements are expected to enhance the durability and efficiency of the electrolyzers. R&D efforts are focused on various aspects such as design improvements, membrane and catalyst technologies, and the impact on durability (Table 4). Specifically, the R&D aims to improve the membrane, reconversion catalysts, and the balance of plant components like the membrane electrode assembly (MEA) and platinum group metal (PGM) catalysts. Table 4 also outlines the load range, hydrogen purity, voltage efficiency, electrical efficiency, and lifetime of the electrolyzer stack and system. In 2020, the load range was 5-120%, with hydrogen purity varying from 99.9 to 99.9999%. The voltage efficiency was 50-68%, and the electrical efficiency ranged from 47 to 66 kWh kg\({}^{-1}\) H\({}_{2}\) for the stack and 50-83 kWh kg\({}^{-1}\) H\({}_{2}\) for the system. The stack lifetime was between 50,000 and 80,000 h, with a unit size of 1 MW and an electrode area of 1500 cm\({}^{2}\).