# Systematic Analysis of the Hydrogen Value Chain from Production to Utilization
## 2 Hydrogen Production Technologies
### Photolytic Processes  
Solar radiation can be used to split water into hydrogen and oxygen. Depending on the configuration selected to use solar energy in the water decomposition process, hydrogen production from solar-based water splitting can be divided into direct and indirect methods [50].  
In the direct method category, the direct thermal method stands out as the most common. This method produces hydrogen and oxygen through the decomposition of water subjected to a great amount of heat in a single process step. The direct thermal technique is characterized by having low conversion efficiency, requiring high temperatures that result in high heat dissipation, and presenting big challenges in disassociating the obtained gases. Due to all these drawbacks, indirect methods have gained greater relevance as the go-to photolytic group of processes to generate hydrogen [50].  
Photochemical water splitting is a solar-based hydrogen production method through water decomposition. This process has a similar working principle to the type of processes previously studied (electrolysis), but the power supply battery is replaced by a semiconductor material that generates an electron-hole pair when subjected to solar radiation equal or greater than the semiconductor bandgap (the gap between the valence and conduction bands) [50].  
Depending on the system components that carry out the process, frequently referred to as reactors, photochemical water splitting can be divided into photocatalytic (PC) water splitting and photoelectrochemical (PEC) water splitting [51]. The main difference between the two is that powder semiconductors are used in PC water splitting, while semiconductor electrodes are used in PEC water splitting.  
The PC water splitting process is composed of three main steps [51]. In the first step, photons are absorbed by the powder semiconductors, generating electron-hole pairs when the absorbed energy is higher than the bandgap. In the second step, electrons and holes are separated and migrate to the reaction sites on different sides of the surface of the material. In the third step, water is oxidized by the holes to produce oxygen and hydrogen ions, at the material surface, and the produced hydrogen ions are reduced by the electrodes, producing hydrogen at the material surface. A schematic illustration of the process operating principle can be found in Figure 12a. Depending on the reactor type, the nanoparticle semiconductors suspended in the electrolyte can be disposed of in a single-bed colloidal or in a dual-bed colloidal, where one size carries out the oxidation reaction and the other one performs the reduction reaction.  
Figure 12: (**a**) Schematic representations of photocatalytic water splitting (type I reactor); (**b**) schematic representations of photoelectrochemical water splitting (type III reactor) (adapted from [52]).  
PEC water splitting has a similar working principle to PC, but in this process, planar photovoltaic cells are placed in a reservoir with an electrolyte [51]. Each cell has two photoelectrodes, taking the role of the anode and the cathode. The oxidation reaction is carried out on the anode side, while the reduction reaction is accomplished on the cathode side, as in Figure 12b.