## 2 Different Methods of Hydrogen Generation
### Commercial Technologies of Hydrogen Generation
#### 2.2.2 _Partial Oxidation_  
In the process of producing hydrogen by the partial burning of hydrocarbons (natural gas) with the equation for oxygen gas (3), the reaction is exothermic, meaning that heat is released during the reaction. This heat can be utilized to maintain the reactor's temperature, eliminating the need for external heating. The carbon monoxide generated during the partial oxidation process can then be further transformed into hydrogen gas using another reaction, which is represented by Equation (2). The coefficients \"n\" and \"m\" represent the stoichiometric coefficients, indicating the number of molecules of each component involved in the reaction.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline
**Technology/** & **Feed Stocks** & \begin{tabular}{c} **Operating** \\ **Temperature** \\ **(\({}^{\circ}\)C)** \\ \end{tabular} & \begin{tabular}{c} **CO\({}_{2}\)** \\ **Emissions** \\ \end{tabular} & \begin{tabular}{c} **Energy** \\ **Source** \\ \end{tabular} & \begin{tabular}{c} **Efficiency** \\ **(\%)** \\ \end{tabular} & \begin{tabular}{c} **Hydrogen** \\ **Color** \\ \end{tabular} & **Scope** & **Ref** \\ \hline Partial oxidation/ commercial & Heavy & 523–593 & \begin{tabular}{c} Unclear with \\ natural gas \\ \end{tabular} & Heat & 95 & Gray & \begin{tabular}{c} Hydrogen \\ production via \\ \end{tabular} & \begin{tabular}{c} methanol \\ reforming using \\ (Cu/ZnO/Al\({}_{2}\)O\({}_{3}\) \\ catalyst) \\ \end{tabular} & [23, 24, 25] \\ PEM/ commercial & Water & 20–200 & \
\begin{tabular}{c} Clean with \\ no emissions \\ \end{tabular} & Electric & 67–82 & Green &
\begin{tabular}{c} Hydrogen \\ production via \\ solid polymeric \\ -Anode: IrO\({}_{2}\) \\ -Cathode: Pt/C \\ \end{tabular} & [23, 24, 25] \\ \hline \end{tabular}
\end{table}
Table 1: Cont.  
Equation (3): Partial oxidation of hydrocarbons:  
\[C_{n}H_{m}+\frac{n}{2}\,O_{2}\to\ nCO+\frac{m}{2}H_{2}+heat \tag{3}\]  
_Proton exchange membrane (PEM) electrolysis_  
In PEM electrolyzers, an acidic polymer membrane acts as the electrolyte, separating hydrogen from oxygen in water through the application of an electric current. The process involves a negatively charged cathode and a positively charged anode. The reactions involved in PEM electrolysis are as follows.  
Equation (4): Anode reaction [32]:  
\[H_{2}O\to\frac{1}{2}O_{2}+2H^{+}+2e^{-} \tag{4}\]  
At the anode, water is split into oxygen gas, hydrogen ions (protons), and electrons. The protons are released into the electrolyte, while the electrons flow through an external circuit.  
Equation (5): Cathode reaction:  
\[2H^{+}+2e^{-}\to H_{2} \tag{5}\]  
At the cathode, the protons from the anode combine with the electrons that have traveled through the external circuit. This recombination produces hydrogen gas. These reactions demonstrate the overall process of PEM electrolysis, where water is split into oxygen and hydrogen gas by utilizing electricity. This method allows for the production of \"green hydrogen\" using renewable energy sources. Although hydrogen produces zero carbon emissions at the point of use, its cleanliness depends on the production pathway and the energy sources used in the production process. Therefore, it is crucial to ensure the origin of hydrogen to consider it as a clean energy.  
In recent years, there has been a lot of interest in the various technologies of hydrogen generation to play a significant role in integrating future energy systems and bridging the transition from fossil-based energy to renewable energy. The methods of hydrogen generation can influence its cleanness, cost-effectiveness, and efficacy. All of this is dependent on the feedstocks used to extract the hydrogen, the yield of production, and the available energy resources [33].