# Technology for Green Hydrogen Production: Desk Analysis
## 4 Results and Discussion
### Current Technologies Used for Green Hydrogen Production
#### 4.2.4 Anion Exchange Membrane Electrolyzers  
AEM electrolyzers are a newer technology that aims to combine the advantages of AEs and PEM electrolyzers [15; 168; 169; 170; 171; 164; 172; 173; 166; 174; 175; 176; 177; 178; 179; 180; 170; 171; 172; 173]. AEM electrolyzers use a solid polymer electrolyte that conducts hydroxide ions from the cathode to the anode and can operate with less pure water than PEM electrolyzers. Figure 15a presents the AEM Flex 120, with a configuration from 70 to 480 kW, which enables the streamlined launch of green hydrogen in pilot projects from industrial process heat to refueling [173]. This modular AEM ensures reactivity to intermittent renewable energy, built-in redundancy, and easy scalability. A diagram of the AEM operating principles is shown in Figure 15b [134]. An AEM electrolyzer uses an anion exchange membrane as the electrolyte, which selectively conducts anions such as OH\({}^{-}\) ions while blocking cations. The AEM electrolyzer operates similarly to a PEM electrolyzer but with the charge carriers reversed. In an AEM electrolyzer, anions migrate from the cathode to the anode, while in a PEM electrolyzer, cations (protons) migrate from the anode to the cathode. In an AEM electrolyzer, hydroxide ions and electrons react to water and oxygen gas at the anode. The half-reaction is represented as Reaction (11):  
\[\text{Anode reaction (Oxidation): 2 OH}^{-}\rightarrow\text{H}_{2}\text{O}+\frac{1}{2} \text{O}_{2}+2\text{e}^{-}. \tag{11}\]  
At the cathode, water molecules are reduced to form hydrogen gas and hydroxide ions. The half-reaction is described by Reaction (12):  
\[\text{Cathode reaction (Reduction): 2H}_{2}\text{O}+2\text{e}^{-}\rightarrow\text{H}_{2}+2 \text{OH}^{-}. \tag{12}\]  
The electrons are supplied to the cathode by the external circuit, completing the electrical circuit. Combining the two half-reactions leads to Reaction (13) that represents the overall electrolysis reaction, showing that water is split into hydrogen and oxygen gases (Reaction (13)):  
\[\text{Overall reaction: 2H}_{2}\text{O}\to 2\text{H}_{2}+\text{O}_{2}. \tag{13}\]  
In an AEM electrolyzer, the anion exchange membrane plays a crucial role by allowing hydroxide ions to pass through while blocking other ions and gases. This ensures that the hydrogen produced at the cathode is pure and that the oxygen produced at the anode is kept separate, which is important for safety and efficiency. Figure 15: Type of electrolyzer: (**a**) Anion exchange membrane (AEM) electrolyzer [173]; (**b**) principles of AEMs [134]. Table 6 presents operating parameters for AEM technology and provides a comparison between the current (2020) and target (2050) operating conditions for AEM electrolyzers, highlighting the significant improvements needed to meet the climate objective [16]. In 2020, AEM electrolyzers operated with a nominal current density range of 0.2 to 2 A cm\({}^{-2}\), a voltage range of 1.4 to 2.0 V, and a temperature between 40 and 60 \({}^{\circ}\)C. The cell pressure was below 35 bar, with a load range of 5 to 100%, producing hydrogen with a purity between 99.9 and 99.999%. The voltage efficiency based on the LHV was 52 to 67%, and the electrical efficiency for the stack and system was 51.5 to 66 kWh kg\({}^{-1}\) H\({}_{2}\) and 57 to 69 kWh kg\({}^{-1}\) H\({}_{2}\), respectively. The stack lifetime exceeded 5000 h, with a unit size of 2.5 kW. By 2050, the target operating parameters aim to increase the nominal current density to over 2 A cm\({}^{-2}\), reduce the voltage to less than 2 V, and operate at a higher temperature of 80 \({}^{\circ}\)C and pressure above 70 bar. The load range is expected to expand to 5 to 200%, with hydrogen purity exceeding 99.9999%. The voltage efficiency is targeted to be over 75%, with electrical efficiency reducing the energy consumption to less than 42 kWh kg\({}^{-1}\) H\({}_{2}\) for the stack and less than 45 kWh kg\({}^{-1}\) H\({}_{2}\) for the system. The stack lifetime should reach 100,000 h, and the unit size is projected to be 2 MW.