# On Green Hydrogen Generation Technologies: A Bibliometric Review
## 4 Discussion
Expansion of these facilities is essential to take advantage of the abundance of renewable sources and meet the needs of diverse applications, from mobility to heavy industry. On the other hand, the efficiency of electrolysis should also be analyzed (Table 7). Electrolysis is one of the simplest processes for creating hydrogen from water. However, since it is an endothermic reaction, it requires a high energy input of electricity to be \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Challenge**} & \multirow{2}{*}{**Description**} & \multirow{2}{*}{**Added Search Field**} & **Number Results** & **References** \\  & & & & **(\%)** \\ \hline \multirow{4}{*}{Limited infrastructure} & In the various key stages of & & & \\  & green hydrogen—production, & & & \\  & storage, and distribution—high & & & \\  & financial investments are needed & & & \\  & to develop an infrastructure that & & & \\  & is currently in an early stage & & & \\  & of development. & & & \\ \hline \multirow{4}{*}{Low electrolysis efficiency} & Although it has improved, & & & \\  & electrolysis faces major & & & \\  & challenges in terms of energy & & & \\  & efficiency. Improving the & & & \\  & effectiveness of electrolyzers is & & & \\  & critical to making green & & & \\  & hydrogen production & & & \\  & more sustainable. & & & \\ \hline \multirow{4}{*}{High technical complexity in storage and transport} & Hydrogen is a difficult gas to & & & \\  & store and transport efficiently. & & & \\  & Advances in advanced storage & & & \\  & technologies and safe and & & & \\  & efficient transportation systems & & & \\  & are needed. & & & \\ \hline \multirow{4}{*}{Economic viability} & Although production costs have & & & \\  & decreased, they are still & & & \\  & relatively high compared to & & & \\  & other technologies. Green & & & \\  & hydrogen must be positioned & & & \\  & from the point of view of & & & \\  & economic profitability compared & & & \\  & to other technologies such as & & & \\  & gray hydrogen. & & & \\ \hline \multirow{4}{*}{Environmental impact} & The development of this & & & \\  & technology implies a certain & & & \\ \cline{1-1}  & environmental impact, as well as & & & \\ \cline{1-1}  & the consumption of sometimes & & & \\ \cline{1-1}  & scarce material resources. & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Main challenges facing large-scale green hydrogen generation. produced. It is a process in which electrical energy is converted into chemical energy in the form of H\({}_{2}\) and O\({}_{2}\) as a by-product. Two reactions take place at each electrode (anode and cathode) [58]. Water splits when electricity is applied, producing H\({}_{2}\) at the cathode and O\({}_{2}\) at the anode through the following reactions:  
\[2\text{H}_{2}\text{O}\to 2\text{H}_{2}+2\text{O} \tag{1}\]  
According to this equation, two H\({}_{2}\) molecules are needed for each H\({}_{2}\)O molecule. Therefore, the ideal molar ratio of hydrogen to water in electrolysis is 2:1. However, in practice, the efficiency of electrolysis can vary depending on different factors. The molar mass of H\({}_{2}\)O is 18 g/mol and that of H\({}_{2}\) is 1 g/mol, and 1 mole of H\({}_{2}\)O is expected to produce 2 moles of H\({}_{2}\). In terms of mass, 18 g of H\({}_{2}\)O could be expected to produce approximately 2 g of H\({}_{2}\) (considering that the density of hydrogen is approximately 0.0899 g/L at standard conditions). Therefore, under ideal conditions, 39 kWh of electricity and 8.9 L of water (H\({}_{2}\)O) are required to produce 1 kg of hydrogen H\({}_{2}\) at 25 C and 1 atmosphere pressure [59]. If a hydrogen fuel cell consumes about 0.9 kg/100 km [60], about 48 L of water (H\({}_{2}\)O) would be needed to cover 600 km under ideal conditions. Considering that it would take around 36-60 L of fossil fuels to cover this distance, it is very unlikely at this stage that hydrogen can replace fossil fuels as a vehicle fuel. A separator between the anode and cathode electrodes ensures that the products remain segregated.