# 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 goal is to develop materials that can operate efficiently at lower temperatures, thereby reducing the cost and improving the durability of H-SOEs for green hydrogen production. In addition to perovskite and brownmillerite oxides, several other types of oxides have been explored for their potential as proton-conducting electrolytes in H-SOEs (Figure 14). These materials include inter alia NASICON (Na Super Ionic Conductor)-type oxides with the general formula AM\({}_{2}\)(XO\({}_{4}\))\({}_{3}\), where the symbols A and M represent different cations that occupy specific sites within the crystal structure [165; 166; 167]. A is a monovalent cation, which can be a proton in the case of proton-conducting electrolytes. M is a divalent cation, often a transition metal such as Zr, Ti, Ge, or Hf. X is a trivalent cation, such as P or S. O is oxygen and xH\({}_{2}\)O represents the presence of water molecules that can contribute to proton conductivity through the formation of hydroxyl groups within the structure. The structure of NASICON-type oxides is based on a three-dimensional network of MO\({}_{6}\) octahedra (where M is the divalent cation) and XO\({}_{4}\) tetrahedra (where X is the trivalent cation), which creates a framework with interstitial sites that can accommodate the monovalent cations (A). These materials are known for their ability to conduct ions (such as Na+ or Li+) through these interstitial sites, making them useful for applications that require fast ion transport. The NASICON structure can accommodate protonic defects, such as hydroxyl groups or water molecules, which are essential for proton conductivity. The presence of these defects can facilitate proton transport through mechanisms such as the Grothuss mechanism [161]. NASICON-type oxides can absorb water from the atmosphere or an operating environment, leading to the formation of hydroxyl groups or water molecules within the structure. This hydration can enhance proton conductivity. Under humid conditions and at intermediate temperatures (\(\sim\)100-300 \({}^{\circ}\)C), NASICON-type oxides can exhibit significant proton conductivity. The proton conductivity is often attributed to the mobility of protonic defects within the crystal structure. NASICON-type oxides can exhibit good chemical stability under the operating conditions of SOEs, which is crucial for the long-term performance and durability of the electrolyzer. One of the most well-known NASICON-type oxides is Na\({}_{3}\)Zr\({}_{2}\)Si\({}_{2}\)PO\({}_{12}\), where A = Na, M = Zr, and X = Si, P [165]. Another recent example of a proton-conducting NASICON-type oxide with superionic conduction is Li\({}_{3}\)Zr\({}_{2}\)Si\({}_{2}\)PO\({}_{12}\)[165; 166] and proton conductor NASICON-structure Li\({}_{1+x}\)Cd\({}_{x/2}\)Zr\({}_{2-x/2}\)(PO\({}_{4}\)) [167]. Doping can occur at the A-site (acceptor doping) or M-site (donor doping). A-site doping involves introducing alkaline earth metals to create oxygen vacancies, facilitating proton transport via the Grothuss mechanism [161]. M-site doping includes adding trivalent cations to enhance proton uptake and conductivity. Codoping, combining both strategies, optimizes oxygen vacancies and excess ions for improved conductivity and stability. Incorporation of proton donors, such as through hydrogenation, introduces hydroxyl groups into the structure, acting as proton donors. Surface modification with catalytic materials enhances proton exchange kinetics. Microstructural engineering, including grain boundary engineering, reduces resistance and improves conductivity. Composite electrolytes, combining NASICON-type oxides with other proton-conducting materials, provide additional transport pathways for protons. To enhance chemical stability, the choice of dopants and the doping level are critical. Dopants that do not significantly disrupt the NASICON structure and maintain a high tolerance factor are preferred.