Photocatalytic water splitting

The Photocatalytic water splitting is the process in which photons are directly used to unravel water into its components, hydrogen and oxygen. The reaction can be simplified described as follows:

Methods for obtaining hydrogen will win in a transition to a renewable energy -based hydrogen economy in importance. The advantage of the photo-catalysis to other techniques such as electrolysis, consists in the fact that charge separation and cleavage of the water can be carried out by a material at the same interface, whereby transmission loss can be minimized, and material can be saved.

Since the discovery of photocatalytic water splitting on TiO2 semiconductor in 1972 by Akira Fujishima and Kenichi Honda, a large number of scientists in the development of suitable materials tried.

Current research is attempting the necessary energy of the photons that were at A. Fujishima and K. Honda in the UV region to move in the range of visible light. This was mainly due to losses are reduced, resulting in the interfaces between the catalyst and water. Another big problem represents the decomposition of the catalyst under illumination

Types of photocatalysis cells

The water splitting by sunlight can be realized through different cellular structures. In this case, a voltage of 1.23 V between anode and cathode must be established. Which corresponds to the energy that must be expended in order to separate a hydrogen atom of an oxygen atom. In practice, however, the required voltage is higher, typically in the range of 1.6-2.4 V, which can be attributed to a strong bond during the reaction on the catalyst, as well as line losses.

N- type photoanode

In this photocatalytic cell the necessary voltage is generated by the excitation of electron-hole pair due to photons in the band gap of n-type semiconductor. By adjusting the Fermi level of the n-type semiconductor with the quasi- Fermi level of the electrolyte band bending occurs, which causes the active charge separation of the electron and hole. The hole is used for the reaction of H2O to O2. In order to raise the Fermi level of the metal over the potential reacts from the H to H2, an external voltage must be applied in this cell assembly under certain circumstances.

P / n- type photoanode / photocathode

In this cell configuration, a p-type semiconductor is connected with an n-type semiconductor on an ohmic contact. In contrast to the n-type photo- anode charge separation of the electron and the hole takes place both in the photo anode and the photocathode. The band bending of the p-type semiconductor leads to this is that the electrons travel to the interface, where the reaction of H to H2 drives. The holes are driven to the boundary surface to the n-type semiconductor where they recombine with the electrons. For the generation of an electron and a hole for the reaction that is, two photons are required. However, these can each have a lower energy than that of a one-photon process, whereby the spectrum of the sun may be better utilized.