# Sustainable Energy Solutions: Utilising UGS for Hydrogen Production by Electrolysis
## 1 Introduction
At the same time, a high share of renewable energy sources (RES) would be engaged [16]. Hydrogen can be produced using various processes. Possible processes for hydrogen production with their advantages and disadvantages, costs, and technical efficiencies are presented in [PERSON] et al. [17]. However, the two most common processes for hydrogen production are steam-methane reforming and electrolysis. Only 4% of today's global hydrogen production is derived from water electrolysis. While water splitting powered by renewable energy is considered the most environmentally friendly method for producing green hydrogen [18; 19; 20], renewable and low-carbon hydrogen types are not yet competitive compared with fossil-based hydrogen [15]. This means that the majority of hydrogen is currently produced by steam-methane reforming, producing around 830 million tonnes of carbon dioxide per year [21]. However, according to [PERSON] et al. [20], several challenges need to be overcome. These include controlling production costs; building a robust infrastructure for storage, distribution, and use in different sectors; and further developing innovative technologies. Specialised techniques are essential to prevent the leakage of hydrogen and to increase safety in storage, transport, and use. In addition, uncertainties remain regarding aspects of \"green\" hydrogen production, such as the impact on land use and greenhouse gas emissions over the entire life cycle [22]. These factors highlight the need for continued rigorous assessment and development to fully realise the potential of \"green\" hydrogen as a sustainable energy solution. Today, great efforts are being made to develop new materials, models, and storage methods, as evidenced by recent publications [23; 24; 25; 26; 27]. Nevertheless, achieving the required production capacities through water electrolysis necessitates enormous amounts of electricity [26]. In addition, the use of freshwater for large-scale hydrogen production is considered unsustainable [27]. On average, proton exchange membrane (PEM) electrolysis consumes the least water per kilogramme of hydrogen produced (17.5 L), followed by alkaline electrolysis (22.3 L/kg). Steam-methane reforming with carbon capture, utilisation, and storage (SMR-CCUS) and autothermal reforming (ATR)-CCUS consume even more at 24.2 L/kg and 32.2 L/kg, respectively. This means that around 2.2 billion m\({}^{3}\) of freshwater is currently used for hydrogen production, which corresponds to 0.6% of the total freshwater withdrawal in the energy sector. However, the projected increase in hydrogen production would lead to a threefold increase in freshwater withdrawals for global hydrogen production by 2040 and a sixfold increase by 2050, equivalent to 12.1 billion m\({}^{3}\). The use of seawater for hydrogen production and cooling is therefore an option for coastal areas affected by freshwater shortages [21]. Seawater, which constitutes nearly 97% of the world's total water resources, could be a viable alternative, but the desalination treatment required to make seawater suitable for electrolysis is not cost-effective and also has a significant environmental footprint. The hugest issue associated with direct seawater splitting is the chlorine evolution reaction, which takes place at the anode due to the presence of chloride anions and competes with the oxygen evolution reaction [28; 29; 30]. Utilising reservoir (formation) water for hydrogen production is an innovative approach that would fit into the circular economy as hydrogen is produced from the water produced during oil and gas extraction. However, various aspects such as water composition, treatment requirements, optimisation of the electrolysis process, and environmental aspects related to the use of formation water as a feedstock for hydrogen production need to be investigated and clarified. [PERSON] et al. [31] published a paper in which solar energy was used for the electrolysis of water produced during oil and gas activities in an oil production field in Libya.