# 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
The voltage range is to be reduced from an upper limit of 1.5 V in 2020 to less than 1.48 V by 2050. The aim is to reduce the operating temperature from 700 to 850 \({}^{\circ}\)C to less than 600 \({}^{\circ}\)C. Cell pressure is to increase from 1 bar to over 20 bar. The load range is to be expanded from 30 to 125% to 0-200%. Hydrogen purity is to be improved from 99.9% to over 99.9999%. Voltage efficiency (LHV) and electrical efficiency (stack and system) both target improvements of more than 85% and reductions in energy consumption to less than 35 kWh kg\({}^{-1}\) H\({}_{2}\) and less than 40 kWh kg\({}^{-1}\) H\({}_{2}\), respectively. Stack life will increase significantly from less than 20,000 h to 80,000 h. Stack unit size is expected to be increased from 5 kW to 200 kW. R&D focus areas are identified for each parameter, with a particular emphasis on electrolytes, electrodes, catalysts, and the balance of plant. Additionally, Table 5 notes the importance of reducing capital costs for both the stack and the system, with targets of less than USD 200/kW for the stack and less than USD 300/kW for the system for a minimum size of 1 MW and 10 MW, respectively. Traditionally, SOEs use an oxygen-ion-conducting electrolyte with oxygen ions moving through the electrolyte from the cathode to the anode (O-SOEs), where they combine with electrons and hydrogen to form water and release hydrogen gas. However, recently there has been increasing research interest in an alternative prototype operating based on proton-conducting electrolyte solid oxide electrolyzers (H-SOEs) instead of O-SOEs, which could lower the operating temperature to approximately 400-700 \({}^{\circ}\)C and even below 400 \({}^{\circ}\)C due to the high proton mobility [67; 146; 147; 148; 149; 150; 151; 152; 153; 154]. In H-SOEs, protons are the charge carriers that move through the electrolyte from the anode to the cathode, where they combine with electrons from the electrical circuit to form hydrogen gas. Figure 13: Type of electrolyzer: (**a**) solid oxide electrolyzer (SOE) [145]; (**b**) principles of SOE [134]. Proton-conducting electrolytes in H-SOEs are typically made from materials that can conduct hydrogen ions at elevated temperatures [152; 153; 154]. These materials are often ceramic oxides that have been doped to create protonic defects, which allow for proton conduction. Recently, 3D printing technology has been proposed for the production of ceramic oxides for H-SOEs [155]. Figure 14 presents some of the most commonly used materials for proton-conducting electrolytes in H-SOEs. In H-SOEs, perovskite oxides with the general formula ABO\({}_{3}\) are mainly used, where A and B represent two different cations, and O represents oxygen anions [152; 153; 154; 155; 156; 157; 158; 159; 160]. A larger A-site cation is often an alkali metal, alkaline earth metal, or rare earth element, and a smaller B-site cation is typically a transition metal. The perovskite structure is characterized by a cubic or pseudocubic arrangement of oxygen anions, with the A cations occupying the larger interstitial sites and the B cations occupying the centers of the oxygen octahedra. The oxygen anions typically form a cubic close-packed (ccp) or face-centered cubic (fcc) arrangement, and the A and B cations occupy specific sites within this arrangement. There are several methods to improve the proton conductivity and chemical stability of perovskite oxides for H-SOEs by introducing defects into the crystal structure and modifying the electronic and ionic transport properties, including doping, cod  
Figure 14: Classification of materials for proton-conducting electrolytes (H-SOEs).