Photoelectrochemical cells for hydrogen production from solar energy - Solar Energy Sciences and Engineering Applications - page 306

semiconductor material. Actually, none of the semiconductors presented in Figure
10.2.8 have their Fermi level edges positioned as shown in Figure 10.2.7a, meaning
that they are cathodically and/or anodically unstable. However, some of these semicon-
ductors show cathodic and/or anodic stability because the reaction kinetics can help in
preventing crystal decompositions where the charge transfer of photogenerated carriers
in the interface is faster compared to the crystal decomposition (Gadgil, 1970).
The stability of a semiconductor in contact with an electrolyte solution strongly
depends on the competition between anodic dissolution and redox reaction, which are
controlled by thermodynamic and kinetic parameters, respectively (Memming, 2001).
Thus, even if the semiconductor oxides are not thermodynamically stable, following
Gerischer's approach, their stabilities can only be achieved in the presence of a suitable
redox system for kinetic reasons (Sinn et al., 1990). For instance, even if the metal
oxides are thermodynamically stable, based on the n E d and p E d positions, towards
cathodic photocorrosion, most of them are unstable towards anodic photocorrosion.
Nevertheless, there are some n-type semiconductors, such as -Fe 2 O 3 and TiO 2 , which

are sufficiently stable in aqueous electrolytes because their decomposition is controlled
by very slow corrosion reaction kinetics (Krol and Schoonman, 2008). As suggested by
Krol, if the material has a tendency for photodecomposition, this may be prevented by
adding a suitable co-catalyst to favor the water oxidation route (Krol and Schoonman,
2008).

α

10.2.5 PEC reactors
A photoelectrochemical cell combines the harvesting of solar energy and the electrol-
ysis of water process in a single device. Thus, when a semiconductor with the ideal
set of properties is immersed in an aqueous electrolyte and illuminated, the corre-
sponding photon energy is directly used to split water into hydrogen and oxygen (i.e.
in a chemical energy). The basic setup for water-splitting comprises two electrodes
immersed in an aqueous electrolyte solution, where one or both electrodes are pho-
toactive. The electrolyte container must be transparent, or have at least a transparent
window for allowing light to strike the photoelectrode; then, water-splitting occurs
when the energetic requirements are met (Minggu et al., 2010). In a laboratory setup,
to measure the PEC cell efficiency, a three-electrode configuration is normally used,
the third electrode being a reference one. However, to simulate a real PEC cell applica-
tion the two-electrode configuration is preferable. The more common PEC cell design
is the conventional electrochemical cell used for corrosion studies, with an optically
transparent window, as reported by Chen et al. in 2010 (Chen et al., 2010). The
optically transparent window is very important for PEC cells to work properly, for
instance, a normal soda lime glass cuts off the transmission for wavelengths lower
than 350 nm, while a quartz window will normally have a transmittance higher than
90% from 250 nm. Nevertheless, a cheaper material can be used with similar perfor-
mance; fused silica (amorphous silica) allows transmission values higher than 90%
and shows an excellent stability in both acid and alkaline aqueous solutions (except
for fluoridric acid) (Krol and Schoonman, 2008). Normally, the research laboratories
in water photosplitting usually manufacture their own PEC cells, e.g. simple cubic
or cylindrical open vessels, closed vessels equipped with an ion exchange membrane
separating hydrogen from oxygen evolutions, H-type PEC cells, sandwich assembly,