# 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 electrolyte must be compatible with the electrodes to form stable interfaces that allow for efficient charge transfer. Research into perovskite oxides for proton-conducting electrolytes in H-SOEs is an active area of study [152; 153]. Scientists are exploring new compositions, doping strategies, and processing techniques to further enhance their proton conductivity and stability under operating conditions. The goal is to develop materials that can improve the efficiency and cost-effectiveness of hydrogen production through electrolysis. The choice of material depends on the desired operating temperature, chemical stability, mechanical properties, and cost. Advances in materials science, processing techniques, and understanding of proton conduction mechanisms are expected to further enhance the viability of these materials for large-scale hydrogen production [161]. Brownmillerite oxides are another class of materials that have garnered interest for their potential use as proton-conducting electrolytes in H-SOEs (Figure 14). These oxides have a general formula of A\({}_{2}\)B\({}_{2}\)O\({}_{5}\), where A and B are typically metal cations, with A being larger and occupying eight-coordinated sites, and B being smaller and occupying four- and six-coordinated sites [152; 153; 156; 157; 158; 159; 162; 163; 164]. A is typically an alkali metal, alkaline earth metal, or rare earth element, and B is a transition metal. The structure of brownmillerite oxides consists of layers of corner-shared BO\({}_{6}\) octahedra and BO\({}_{4}\) tetrahedra, with the A cations located between these layers. Doping of brownmillerite oxides can occur at the A-site (acceptor doping) or B-site (donor doping). A-site doping with alkaline earth metals creates oxygen vacancies, facilitating proton transport via the Grothuss mechanism [161]. B-site doping with trivalent cations enhances proton uptake and conductivity. Codoping at both sites optimizes the concentration of oxygen vacancies and excess ions, improving conductivity and stability. Hydrogenation of brownmillerite oxides introduces hydroxyl groups into the structure, acting as proton donors and enhancing conductivity. Surface modification with catalytic materials like Pt or Pd improves proton exchange kinetics. Grain boundary engineering reduces resistance, and microstructural engineering, such as creatingcomposite materials with other proton conductors, provides additional transport pathways. Brownmillerite oxides must have good sinterability and adequate mechanical strength to withstand thermal cycling and operating stresses [152; 153; 156]. The electrolyte must be compatible with the electrode materials to ensure good contact and minimize interfacial resistance. This includes thermal expansion match and chemical compatibility. The cost and availability of the constituent elements are important considerations for the commercial viability of brownmillerite oxides as electrolyte materials. Examples of brownmillerite oxides that have been recently investigated for proton conductivity include materials such as Ca\({}_{2}\)Fe\({}_{2}\)O5 [162], Ba\({}_{2}\)In\({}_{2}\)O\({}_{5}\)(H\({}_{2}\)O)\({}_{x}\) (\(x\) = 0.30 and 0.92) [163], and Ba\({}_{2}\)In\({}_{1.8}\)Si\({}_{0.2}\)O\({}_{5.1}\)[164]. These materials have shown promising proton conductivity at intermediate temperatures, making them potential candidates for use in H-SOEs. To enhance the chemical stability of brownmillerite oxides, the choice of dopants and the doping level are critical. Dopants that do not significantly disrupt the brownmillerite structure and maintain a high tolerance factor are preferred. Additionally, doping strategies that minimize the formation of secondary phases and maintain structural integrity under operating conditions are essential for achieving high chemical stability. Research into brownmillerite oxides for proton-conducting electrolytes is ongoing, with efforts focused on optimizing their composition, microstructure, and processing conditions to enhance their performance and stability [162; 163; 164].