Environmental Engineering Reference
In-Depth Information
cycle is closed when the electrolyte anions generated at the counter-electrode diffuse
back to the surface of the semiconductor to recombine with holes.
2e
−
→
2OH
−
E
◦
H
2
O
/
H
2
Cathode: 2H
2
O
+
2H
2
+
=−
0
.
828 V
(10.2.2)
1
2
O
2
Anode: 2OH
−
+
2h
+
→
E
◦
O
2
/
OH
−
(10.2.3)
H
2
O
+
=
0
.
401 V
If an acid media is considered, instead of having hydroxyl anions traveling from the
counter electrode to the surface of the semiconductor we have hydrogen ions, as
described by the following equations:
Cathode: 2H
+
+
2e
−
→
E
◦
H
+
/
H
2
=
H
2
0
.
0V
(10.2.4)
1
2
O
2
E
◦
H
2
O
/
O
2
Anode: H
2
O
+
2h
+
→
2H
+
+
=
1
.
23 V
(10.2.5)
In both cases, i.e. for alkaline or for acid media, the overall PEC water-splitting reaction
can be written as follows:
1
2
O
2
H
2
O
+
2
hv
→
H
2
+
(10.2.6)
A similar phenomenon occurs when a p-type semiconductor is used. Nevertheless,
for this case the dominant (or the majority) charge carrier is holes, which will travel
through the external circuit towards the metal counter-electrode, working now as the
anode. On the other hand, electrons travel to the surface of the semiconductor in
contact with the electrolyte to reduce water (Nozik, 1978).
The minimum potential of
1.23 V at 25
◦
C is needed to electrolyze water. The
negative sign identifies the process as not being spontaneous and so the reaction cannot
occur without additional energy from an external electrical power source. This value
is obtained from the following relation:
−
G
o
n
e
F
E
o
=−
(10.2.7)
G
◦
mol
−
1
), representing a ther-
modynamic minimum for splitting water into the gaseous hydrogen and oxygen at
25
◦
C and 1 bar;
E
o
is the electric standard potential of the reaction.
For a direct photoelectrochemical water-split using a single-photon system, several
key criteria must be simultaneously fulfilled:
is the standard Gibbs free energy change (
+
237 kJ
·
i
)
The semiconductor system must generate sufficient voltage upon irradiation to
split water;
ii
)
The bulk bandgap must make efficient use of the solar spectrum;
iii
)
The band-edge potentials at the surface must straddle the hydrogen and oxygen
redox potentials according to the half-reactions described in Equations 10.2.2-
10.2.5;
iv
)
Low overpotentials;
v
)
The system must exhibit long-term stability in aqueous electrolytes;
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