23.2
1.
2.
3.
to use a good PV system with an overall efficiency of η PV = 18% we would have a total
maximal solar-to-hydrogen efficiency of
Photoelectrochemical (PEC) water splitting
Hydrogen can also be produced utilizing solar energy by using a photoelectrode, which
uses light to produce an electrochemical reaction, in which water is split into oxygen and
hydrogen. In this process, the photons reach the surface of the photoelectrode, which is
made of a photoactive semiconductor. As in any other semiconductor, photons with an
energy equal to or larger than the semiconductor bandgap energy will create an electronhole pair. The electrons and holes will be separated by an electric field, and both will be
used in the two half reactions involved in the overall water splitting process. To generate
the required electrical field, we need a voltage source, for example a solar cell. The
photoelectrode can be either an anode or a cathode.
If the solar-splitting device is made of a solar cell that is placed behind the
photoanode, the solar cell will receive the light transmitted through the photoanode. This
light creates another electron-hole pair and an electric field that brings the electrons to the
photoanode and the holes to the photocathode with enough potential to drive the redox
reaction in the electrolyte. As a result, the water molecule is split into oxygen and
hydrogen.
When the photoelectrode is made from an n-type semiconductor it acts as an electron
donor; if it is p type it acts as an electron acceptor. As an acceptor, the material will attract
more holes to the interface, which will enhance the reduction half reaction. Hence it acts
as the photocathode and produces hydrogen. If the semiconductor is n type, it acts as the
photoanode. Electrons are moved to the interface by the internal electric field, and those
electrons are involved in the oxidation half reaction, such that oxygen is produced.
The semiconductor material has to fulfil several requirements:
It has to absorb light incident on its surface.
Charge carrier transport inside the material and separation into the two electrodes
must be efficient.
A bandgap of 1.23 eV is not enough to drive the reaction because of the
overpotential already discussed above. It has been estimated that materials with
an energy bandgap close to 2.1 eV have the potential to split water.
1.
2.
3.
to use a good PV system with an overall efficiency of η PV = 18% we would have a total
maximal solar-to-hydrogen efficiency of
Photoelectrochemical (PEC) water splitting
Hydrogen can also be produced utilizing solar energy by using a photoelectrode, which
uses light to produce an electrochemical reaction, in which water is split into oxygen and
hydrogen. In this process, the photons reach the surface of the photoelectrode, which is
made of a photoactive semiconductor. As in any other semiconductor, photons with an
energy equal to or larger than the semiconductor bandgap energy will create an electronhole pair. The electrons and holes will be separated by an electric field, and both will be
used in the two half reactions involved in the overall water splitting process. To generate
the required electrical field, we need a voltage source, for example a solar cell. The
photoelectrode can be either an anode or a cathode.
If the solar-splitting device is made of a solar cell that is placed behind the
photoanode, the solar cell will receive the light transmitted through the photoanode. This
light creates another electron-hole pair and an electric field that brings the electrons to the
photoanode and the holes to the photocathode with enough potential to drive the redox
reaction in the electrolyte. As a result, the water molecule is split into oxygen and
hydrogen.
When the photoelectrode is made from an n-type semiconductor it acts as an electron
donor; if it is p type it acts as an electron acceptor. As an acceptor, the material will attract
more holes to the interface, which will enhance the reduction half reaction. Hence it acts
as the photocathode and produces hydrogen. If the semiconductor is n type, it acts as the
photoanode. Electrons are moved to the interface by the internal electric field, and those
electrons are involved in the oxidation half reaction, such that oxygen is produced.
The semiconductor material has to fulfil several requirements:
It has to absorb light incident on its surface.
Charge carrier transport inside the material and separation into the two electrodes
must be efficient.
A bandgap of 1.23 eV is not enough to drive the reaction because of the
overpotential already discussed above. It has been estimated that materials with
an energy bandgap close to 2.1 eV have the potential to split water.