The production of hydrogen from solar sources has numerous advantages, including its environmental friendliness, high energy density and relative abundance when derived from water. Traditional methods of hydrogen production from renewable sources involves using electrical energy to electrochemically separate water into hydrogen and oxygen using an electrolyser. However, this method has numerous disadvantages including additional production stages that add to cost and complexity, and transmission losses. Therefore, the direct production of hydrogen from solar photons is preferable. Our research in this area focuses on the direct photoelectochemical production of hydrogen from water and hydrogen sulphide.
Principles of photo-electrochemistry
Photo-electrochemical devices employ semiconductor materials, such as Fe2O3 (EG ≈ 2.2 eV; λ < ca. 550 nm), which can perform one or several functions in the process of converting solar photon energy into chemical energy. Semiconductors absorb photons with energies greater than their band gaps, generating an electron - hole pair for every photon absorbed. The electrons (e-), which are promoted into the semiconductor conduction band (CB) with the energy delivered by the photon, and holes or 'electron vacancies' (h+), which remain behind in the valence band (VB), become the free charge carriers that ultimately enhance the rate of the water splitting process:
Photo-reduction of H2O where into hydrogen is performed by the photo-generated electrons in the semiconductor conduction band:
while the photo-oxidation of water is performed by the photo-generated holes in the valence band:
The photo- reduction and oxidation processes are possible if the potential of the conduction band edge is negative relative to the reversible potential for water reduction and the potential of the valence band edge is positive relative to the reversible potential for water oxidation, respectively. For a fully spontaneous water splitting process both conditions must be satisfied simultaneously.
Our research is focused on two types of systems: one for photo-assisted and the other for spontaneous water splitting. Our photo-assisted system utilises an n-type hematite photo-anode and a metallic cathode, immersed in pH 14 sodium hydroxide solution. Our system for spontaneous solar water splitting utilises a photo-cathode in the place of a metallic cathode.
The requirements for materials, which theoretically enable spontaneous solar water splitting in a two-electrode system comprising a photo-anode and a photo-cathode are:
- The absorption of the semiconductors is well matched to the solar spectrum;
- The potential of the conduction band edge of the photo-cathode lies negative of the reversible potential for water reduction to hydrogen at the chosen pH;
- The potential of the valence band edge of the photo-anode lies positive of the reversible potential for water oxidation to oxygen at the chosen pH;
- Both photo-electrodes are stable to decomposition by e- and h+ over a wide potential range;
- Each photo-electrode is a good catalyst for the reduction/oxidation reaction;
- Easily fabricated on a large scale at low cost.