Solar Hydrogen Production From WaterNaimeh Naseri
The alarming depletion rate of reserved fossil fuel associated with rapid increase in environmental pollution has caused an urgent need to develop efficient clean and renewable energy resources. In this regard, many different approaches have been followed up. The sun is a free, clean, sustainable and easy access energy source, and the solar produced hydrogen, which can be used in fuel cells to generate electricity or changed directly in combustion engines, makes no pollution except water. Hence, to obtain hydrogen as a clean energy carrier, the scenario of a renewable hydrogen economy has attracted much attention from researchers recently. Because of low operation temperature and strong synergies with contemporary researches in the field of photovoltaic and nanomaterials, photoelectrochemical (PEC) water splitting is an emerging technology for the future world hydrogen generation. The concept of this method is based on a semiconductor photoelectrode device which excites with sunlight irradiation, oxidizes/reduces H2O molecules by generated h+/e− pair and finally converted them to chemical energy (H2 gas). For efficient PEC reaction, the selected semiconductor photoanodes should exhibit chemical stability, suitable band edge positions for absorbing sunlight and also participating in water oxidation/reduction, high charge carrier mobility and also variety of low cost synthesis methods.
Photoanodes based on n-type metal oxides such as TiO2,WO3, ZnO, and Fe2O3 have been extensively investigated due to their chemical and physical stability in aqueous solution under evolving oxygen, ease of fabrication, and reasonably high incident light to current generation when operated in a PEC cell. Different approaches have been followed to prohibit charge recombination and improve visible response of these photoanodes. For example, size-controlled noble metal nanoparticles can enhance visible light absorption of the semiconductor photoanodes as a result of their surface plasmonic effects. Moreover, junctions formed between metal nanoparticles and semiconductor nanomaterials can facilitate interfacial charge transfer processes and, hence, cause retardation of electron−hole recombination. Many interesting reports and reviews are available in this context which present recent strategies and trends to improve solar hydrogen production systems in PEC, EC or PC approaches.