# Technology for Green Hydrogen Production: Desk Analysis
## 4 Results and Discussion  
### Tasks and Challenges of Hydrogen Economy towards Green Hydrogen  
The main tasks of the HE towards green hydrogen are presented in Figure 9[99; 112; 113; 114]. R&D research conducted within the functional stages of the HE concerns green hydrogen production (stage I), green hydrogen purification and transportation (stage II), green hydrogen storage (stage III), and conversion of chemical energy of green hydrogen into electricity using fuel cells (stage IV) [8; 9; 91]. Stage I primarily involves the development of low-emission or emission-free methods of hydrogen production [92; 93; 94; 95]. A concept of green hydrogen production through water electrolysis supporting net zero emissions in the future is shown in Figure 9a[99]. Currently, mainly prototype solutions and technologies are used in hydrogen energy. Their costs must be significantly reduced in widespread use and mass production for hydrogen energy to be competitive with conventional fossil fuel-based energy [91]. Hydrogen can be transferred using gas pipelines as in the case of natural gas (stage II) [115; 116; 117; 118; 119], or transported under pressure using tankers (stage III) [120; 121; 122; 123; 105]. An exemplary pipeline in modern style for green hydrogen long-distance transport is shown in Figure 9b[112]. Current challenges related to the HE focus on the search for new methods of green hydrogen storage (stage III) [96; 124; 125; 126; 106] as Figure 9c shows [113], and converting green hydrogen (stage IV) [124; 125; 126] by fuel cells (Figure 9d) [114]. Hydrogen storage is the most difficult technical barrier to overcome, hindering the implementation of hydrogen technologies on an industrial scale. Present  
Figure 9: Hydrogen economy (HE) tasks towards green hydrogen: (**a**) a concept of green hydrogen production through water electrolysis (stage I) [99]; (**b**) green hydrogen purification and transmission (stage II) [112]; (**c**) green hydrogen storage (stage III) [113]; (**d**) green hydrogen conversion by fuel cells (stage IV) [114]. vessels and less durable cryogenic tanks are used to store hydrogen. Addressing the challenges of storage efficiency, safety, and cost for large-scale hydrogen applications is crucial for the widespread adoption of hydrogen as a clean energy carrier. Several innovative solutions and strategies are being developed and implemented to overcome these challenges [96; 105; 106; 110; 120; 121]. The compressed hydrogen storage strategy under development is based on high-pressure tanks and metal hydrides. The liquid hydrogen storage strategy is based on cryogenic storage and insulated tanks. Vacuum-insulated cryogenic tanks are used to minimize heat transfer and boil-off. The underground storage strategy uses salt caverns and geological formations. The chemical storage strategy includes Liquid Organic Hydrogen Carriers (LOHC) and ammonia [127]. A solid-state storage strategy is developed based on Metal-Organic Frameworks (MOFs) and chemical hydrides [96; 106; 120; 128]. Safety considerations concern leak detection and monitoring, fire and explosion prevention, and regulatory compliance. Advanced sensors and monitoring systems are crucial for detecting hydrogen leaks and ensuring the safety of storage facilities. Hydrogen's wide flammability range and high diffusion rate require stringent safety measures, including inert gas purging and explosion-proof designs. Adherence to international standards and regulations for hydrogen storage and handling is essential to ensure safety and gain public acceptance. Cost reduction strategies are focused on economies of scale, R&D, and policy and incentives [105; 110; 121]. Large-scale production and storage of hydrogen can benefit from economies of scale, reducing the cost per unit of hydrogen stored. A combination of technological innovations, material advancements, and policy support is needed to address the challenges of storage efficiency, safety, and cost for large-scale hydrogen applications.