Environmental Engineering Reference
In-Depth Information
Table 10.2.2 Examples of n-p photoelectrochemical cells for water splitting (FIGURE 10.2.3a).
Energy conversion
n-SC/p-SC
Electrolyte
Efficiency, η
Reference
n-TiO 2 /p-GaP
0.2M H 2 SO 4
0.25%
Nozik, 1976
n-SrTiO 3 /p-GaP
1M NaOH
0.67%
Ohashi et al., 1977
n-Fe 2 O 3 /p-Fe 2 O 3
0.1MH 2 SO 4
0.10%
Ingler et al., 2006
10.2.4 Stability issues - photocorrosion
In photoelectrochemical cells, stability issues are one of the major problems to be solved
(Krol et al., 2008). Usually, when a semiconductor electrode is placed in contact with
an electrolyte solution some reactions may occur, for instance ionic oxidation or reduc-
tion of the semiconductor with simultaneous reduction or oxidation of a component
(Gadgil, 1990). The electrolytic reduction of a semiconductor is often associated with
the electrons in the valence band, while the electrolytic oxidation reaction is related
to holes in the conduction band as electronic reactants (Gadgil, 1990). Following the
Gerischer's derivations, it is possible to formulate the simplest type of decomposition
reaction involving a binary semiconductor MX and the solvation (complexing) of the
elements (labeled hereafter as “solv'') as stated next (Memming, 2001):
X z
solv
z e +
MX
+
solv
M
+
(10.2.8)
for a cathodic reaction, and
M z +
z h + +
MX
+
solv
solv +
X
(10.2.9)
for an anodic reaction. Using H + /H 2 standard potentials as reference, the correspond-
ing reaction for hydrogen may be written as:
1
2 z H 2
z H solv +
z e
+
solv
(10.2.10)
The addition of Equation 10.2.10 to Equation 10.2.8 or to Equation 10.2.9 yields the
corresponding equations for the free energy values, n G sH and p G sH , respectively.
The decomposition potentials equations are:
p E decomp = p G sH /z
(10.2.11)
for the oxidation, and
n E decomp =− n G sH /z
(10.2.12)
for the reduction of the semiconductor (Memming, 2001).
 
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