Environmental Engineering Reference
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overpotential on anode and cathode electrodes, practical potential needed to drive
water splitting is larger than 1.8 V. A promising solution is using renewable
energy to split water, for example photoelectrolysis [
11
]. There are two approaches
to utilize solar energy for electrolysis. For instance, electrolyzer can be coupled
with photovoltaic (PV) cells, which provide photovoltage for water electrolysis
[
11
]. However, the relative high cost of PV cells and electrolyzer could limit its
practical application. Alternatively, water splitting can be achieved in a photo-
electrochemical (PEC) cell [
11
]. PEC cells are consisted of semiconductor pho-
toelectrodes for harvesting solar light. The photogenerated electrons and holes can
reduce and oxidize water to produce hydrogen and oxygen gas, respectively. In
order to compete with PV/electrolyzer system, the key of this approach is to use
low-cost semiconductor materials as photoelectrodes.
The concept of PEC water splitting was first demonstrated by Honda and
Fujishima in 1972 [
17
]. Figure
1
shows the schematic diagram of PEC cell that
consists of an n type semiconductor as photoanode and counter electrode [
23
].
Upon light illumination, photoexcited electron-hole pairs will be generated in the
photoelectrode. The electron-hole pairs will be separated due to the band bending
at the electrolyte/semiconductor interface and/or the application of external bias.
The holes and electrons will oxidize and reduce water to produce oxygen and
hydrogen, respectively. The reaction can be described by the following equations:
Photanode: H
2
O+2h
þ
!
2H
þ
+ 1/2O
2
E
ox
¼
ffi
1
:
23 V
Cathode: 2H
þ
+2e
ffi
!
H
2
E
red
¼0V
:
According to the Nernst equation, a minimum energy of 1.23 V is needed to drive
water electrolysis. Therefore, in order to achieve hydrogen evolution and oxygen
evolution reactions simultaneously under light illumination, the band-gap energy
(E
g
) of the semiconductor photoelectrode must be larger than 1.23 eV and the
band-edge potentials (conduction band and valence band) should straddle the
electrochemical potentials of E
o
(H
+
/H
2
) and E
o
(O
2
/H
2
O). Moreover, a favorable
semiconductor photoelectrode should able to absorb a significant portion of solar
light, and has low kinetic overpotential for water oxidation and reduction, good
corrosion resistivity, and electrochemical stability in aqueous solution [
92
].
In order to properly evaluate the performance of photoelectrodes, it is critical to
define the parameters to characterize solar to hydrogen (STH) conversion effi-
ciency. The PEC properties of photoelectrodes are typically studied in a three
electrode electrochemical cell system, which consists of a working electrode
(photoelectrode), a reference electrode, and a counter electrode. For the reference
electrode, the internationally accepted primary reference is the standard hydrogen
electrode (SHE) or reversible hydrogen electrode (RHE), which has all compo-
nents at unit activity [(Pt/H
2
(a = 1)/H
+
(a = 1, aqueous)]. However, this kind of
reference electrode is impractical, and therefore other reference electrodes such as
saturated Ag/AgCl or saturated calomel electrode (SCE) are normally used in the
measurement. In this case, the experimentally obtained potentials should be
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