Photoelectrolysis of Water

Photoelectrolysis of Water:

Hydrogen is a very good source of energy and it has an advantage over other fuels like petrol, cooking gas or coal because its combustion does not produce CO, CO₂ or some other poisonous by-product and is, therefore free from pollution. Since water is an inexhaustible source of energy and efforts are being made to split up H₂O into H₂ and O₂ using solar energy. One of the most important such methods is known as “Photoelectrolysis of Water“. It involves the electrolysis of water using Photosensitive semiconductor electrodes (Cathode and Anode) in a Photoelectrolysis Cell which make use of one electrode or both the electrodes capable of providing electrons for reduction (act as a cathode) or remove electrons for oxidation (act as an Anode) instead of Voltaic Cell. The two commonly used Photoelectrolysis Cells are-

Cells having both Semi-Conductor Photo Cathode and Semi-Conductor Photo Anode:

In this cell, an aqueous solution of NaOH or Na₂SO₃ is used as the electrolyte, the cathode is a P-type photosensitive semi-conductor like GaP (Gallium phosphide) and Anode is an N-type photosensitive semi-conductor like TiO₂. In some photoelectrolysis cells, the cathode and anode are made of the same semiconductor material, one side of which is dropped with N-type impurity and the other with P-type. Example- Pellets of Fe₂O₃ dopped with Si⁴⁺ would become N-type semiconductor and dopped with Mg²⁺ will become P-type semiconductor. Fe₂O₃ dopped with Mg²⁺ and Si⁴⁺ ions and cemented back to back becomes such an electrode system. When electrodes in such cells are exposed to radiations, H₂O is oxidized at the Anode evolving O₂ gas and reduced at the cathode with the evolution of H₂ gas.

Cells having one Photosensitive N or P-type Semi-Conductor electrode and the other metal counter electrode:

In such cells, a suitable aqueous solution of an electrolyte (NaOH) is used, the Anode is a photosensitive semiconductor electrode like N-TiO₂ and the cathode is Pt. In such a cell, it is necessary to apply external voltage in addition to normal solar energy exposure. But if SrTiO₃ (Strontium Titanate) is used as a photoanode (in place of TiO₂) no external voltage is required and electrolysis occurs only by light energy.

Maximum efficiency of 20-25% has been achieved in Photoelectrolysis by using oxygen-deficient N-TiO₂ or alloying with VO₂ and using radiation of 335 nm.