Chemistry Reference
In-Depth Information
performance indicators, is measured by recording the current-voltage be-
havior of the photovoltaic cell when the cell is irradiated under AM 1.5 G (air
mass 1.5 global) illumination and calculated according to the following
equations:
23
d
n
3
r
4
n
g
|
4
Photoconversion efficiency
ðÞ¼
FF
V
OC
J
SC
P
in
(8
:
1)
FF
¼
V
max
I
max
V
OC
I
SC
(8
:
2)
where FF is the fill factor, V
OC
(V) is the open-circuit voltage, J
SC
(mA cm
2
)is
the short-circuit current density, P
in
(mW cm
2
) is the intensity of the illu-
mination, V
max
(V) and I
max
(mA) are the voltage and current at the maximum
power output respectively.
8.3.2 PEC Cell for Fuel (Hydrogen) Generation
At least one of the electrodes of a PEC cell for hydrogen generation by water
splitting is a semiconductor. A conventional PEC cell is composed of an
n-type semiconductor as a photoanode and a metal (usually platinum)
electrode as a cathode immersed in an aqueous solution of a salt (liquid
electrolyte). Figure 8.3 shows the energy band structure of the PEC cells
demonstrating how the band level of the electrodes changes upon galvanic
contact, irradiation and applied bias.
24
Band bending occurs in electron
transfer between the semiconductor and the electrolyte so that equilibrium
is established between the Fermi levels. Upon irradiation with photon energy
equal to or greater than the band gap of the material, photogenerated car-
riers are separated by the applied external potential and/or the depletion
(space-charge) layer formed between the electrode and liquid electrolyte.
Depending on the band structure of the photoelectrode, external potential
may or may not be necessary.
Figures 8.4(a) and (b) show the energy band diagrams of PEC cells com-
posed of an n-type semiconductor electrode and a p-type semiconductor
electrode that acts as a photoanode and photocathode respectively. In the
photoanode, the holes accumulated on the semiconductor surface oxidize
water (O
2
evolution, 2H
2
O
þ
4h
1
.
4H
1
þ
O
2,
E
0
¼
1.23 V vs. NHE [normal
hydrogen electrode]) while reduction occurs (H
2
evolution, 4H
1
þ
4e
-
2H
2
,
E
0
¼
0.00 V vs. NHE) by the electrons transported to the metal counter
electrode and reacted with protons. In contrast, in the PEC cell with the
photocathode, H
2
evolves at the semiconductor surface while O
2
is produced
at the counter electrode. For ecient solar water splitting, the semi-
conductor photoanode should meet the following criteria:
14,25
(1) for prac-
tical applications, the bandgap should be at least 1.9 eV, which is
determined by the minimum potential for water oxidation (1.23 eV) plus the
thermodynamic losses (0.3-0.4 eV) and the overpotential required for suf-
ficiently fast reaction kinetics (0.4-0.6 eV), (2) the conduction band and the
-
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