# Technology for Green Hydrogen Production: Desk Analysis
## 2 Background of Analysis
### Water Electrolysis Process  
The production of green hydrogen through electrolysis involves the use of an electrolyzer, where water is split into its constituent hydrogen and oxygen gases by passing an electric current through it [95; 99; 100]. In the electrolyzer, water is fed into the system, and an electric current is applied to two electrodes--the anode and the cathode--which are separated by a membrane, as shown in Figure 6[101]. At the cathode, electrons from the electric current cause water molecules to gain electrons (reduction) and form hydrogen gas. At the anode, water molecules lose electrons (oxidation) and produce oxygen gas and positively charged hydrogen ions. The hydrogen ions migrate through the membrane to the cathode, where they combine with electrons to form hydrogen gas. The efficiency of different electrolyzer technologies for green hydrogen production can vary depending on the design of the electrolyzer, the operating conditions, the energy source used, the electrical energy input, the conductivity of the electrolyte, and, to a large extent, the electrode materials used for the HER at the cathode and the OER at the anode. Electrolyzers play a pivotal role in green hydrogen production, as they are the devices responsible for splitting water [16; 67; 69; 51]. They aim to maximize the efficiency of the electrolysis process, minimizing energy losses and ensuring that a significant portion of the electrical energy input is converted into chemical energy in the form of hydrogen. Advanced electrode materials and electrolyzer designs help reduce overpotential, which is the additional voltage required beyond the thermodynamic minimum to drive the electrolysis reactions [16]. Lower overpotential means less energy is wasted as heat. Electrolyzers can be scaled up or down to meet different production capacities, making them suitable for both small-scale and large-scale hydrogen production facilities [16; 62; 72]. Modular electrolyzer systems allow for easy expansion or reduction in capacity by adding or removing modules, providing flexibility in response to changing demand or availability of renewable energy. The materials used in electrolyzers, including electrodes, electrolytes, and membranes, must be stable under operating conditions to ensure long-term durability and performance [16; 24; 43; 57]. Regular maintenance and timely replacement of components can extend the lifetime of electrolyzers, ensuring consistent and reliable hydrogen production. Electrolyzers can be designed to operate flexibly, adjusting their power consumption based on the availability of renewable energy. This capability is crucial for maximizing the use of intermittent RESs like solar and wind [74; 75; 76]. Electrolyzers can also serve as a form of energy storage, converting excess renewable energy into hydrogen, which can be stored and used later when demand exceeds supply. Large-scale production and deployment of electrolyzers can benefit from economies of scale, reducing the cost per unit of hydrogen produced [16; 72]. The selection of electrode materials for green hydrogen production through electrolysis is critical for achieving high efficiency, durability, and cost-effectiveness. Key factors that should be considered when choosing electrode materials include catalytic activity, stability and durability, cost and availability, compatibility with electrolytes, scalability and manufacturing, environmental impact, and operational conditions [16; 24; 43; 57; 62; 72]. The electrode material should have high catalytic activity for the HER and OER to ensure high current densities and fast reaction kinetics [13; 14; 15; 18; 20; 24; 25; 26; 27; 28; 29; 30; 50; 97]. Low overpotential is desirable to minimize the energy required for electrolysis. Noble metals like Pt for HER [16; 19; 22] and Ir or ruthenium Ru for OER are known for their low overpotentials but are expensive [16; 27; 30].