Photoelectrolysis of Water
Breaking up of water into hydrogen and oxygen by exposing it to radiation of suitable wavelength in the presence of a photosensitive semiconductor electrode(s), is termed photoelectrolysis of water.The photoelectrolysis is caried out in a specially designed cell, called photoelectrolysis cell.
These cells can be constructed in many ways, viz.,
(i) By using a semiconductor (nor p-type) photoelectrode, and another a metal counter-electrode.
For example in a cell, one semiconductor electrode may be TiO2 (n-type semiconductor described as n-TiO2), and the other platinum counter electrode. A suitable electrolyte is used. In this cell external voltage is applied in addition to the normal solar energy exposure. However, if an electrode of strontium titanate (SrTiO3) is used as a photo anode (in place of TiO2), then no external voltage is needed, and electrolysis occurs only by the light energy.
(ii) By using both the electrodes as semiconductor photoelectrodes – one n-type and the other p-type.
For example, a cell may consist of TiO2 (n-type) and GaP (p-type) photo electrodes. The photoelectrolysis cell of p –n type can be constructed using two electrodes of the same Semiconductor material, one doped with p-type and the other with n-type material, or with two different (n-and p-) semiconductors.
A cell consisting of pellets of Fe2O3 doped with Mg2+ and Si4+ ions, cemented back to back and dipped in an aqueous solution of NaOH or Na2SO3 is an example. In p – n cell, both the electrodes are illuminated. These types of cells are more efficient, and are used for carrying out other photo-reactions (other than water) as well.
Photoelectrolysis of water is the most attractive reaction for solar energy conversion, but it has not been possible to achieve this efficiently. Recent work by some scientists has shown that an efficiency of about 21 per cent can be achieved, if n-TiO2 is made oxygen deficient, or is alloyed with VO2 and a radiation of 335 nm (ultraviolet) are used.
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