Environmental Engineering Reference
In-Depth Information
This device [111] can be regarded as a water electrolysis cell in series with a tandem
solar cell, to supply the needed voltage to split water. Crudely, one can think of the
semiconductor layers as a (light-activated) battery, the right-hand cathode terminal is
in the electrolyte and the left-hand terminal (ohmic contact) is insulated from the
electrolyte, but connected to the platinum (foil or gauze) anode through the external
ammeter. The electrolyte in the cell is 3 molar H
2
SO
4
, a strongly acidic solution
conductive by H
þ
ions, also known as solvated protons or hydronium ions H
3
O
þ
.
Consider
first the electrolytic cell aspects. The cathode was coated with a thin layer
of platinum particles, in the nature of platinum black, by an electroplating
procedure.
The cathode reaction (reduction) is
2H
þ
ð
2e
!
aq
:
Þþ
H
2
ð
gas
Þ
The anode reaction (oxidation) is
4H
þ
ð
4e
ð
:
Þ!
O
2
ð
Þþ
:
Þþ
2H
2
O
liq
gas
aq
Taking twice the
first reaction and adding to the second reaction, we get
2H
2
O
ð
liq
:
Þ!
O
2
ð
gas
Þþ
2H
2
ð
gas
Þ
The standard potential for this reaction is 1.23 V. To make the reaction work, a larger
voltage is needed, the excesses being termed cathodic and anodic overvoltages.
Solar cell output voltages are limited by the bandgaps of the underlying semi-
conductors, which are 1.83 eV on the right and 1.42 eV on the left, in Figure 9.1. The
sum of these voltages provides an upper limit of voltage from the tandem cell, which
well exceeds the voltage, nominally 1.23 V, to decompose water.
The photocurrent flows along the wire at the top of the diagram (ammeter was
shown in the previous diagram) and the Fermi levels of the ohmic contact and the
platinumelectrode are aligned. So, the voltage developed in the semiconductor layers
is dropped across the electrolye, driving the decomposition of water. In the two
semiconductor layers, it is seen that the Fermi level is
flat (no voltage drop) across the
connecting tunnel diode interconnect, so that the photovoltages of the two junc-
tions are additive. The light enters on the right, and the right-hand junction has the
larger bandgap, so that lower energy photons that are not absorbed on the right pass to
the left to the second junction where they are absorbed.
In such a tandem cell, the electrical current has to be the same in each junction, so
that the photocurrent of the series tandem cell is less than the short-circuit current of
the weaker junction. The voltages add directly, allowing a higher conversion effi-
-
ciency than a single-junction photocell. The ef
ciency was calculated as the chemical
energy in the resulting hydrogen divided by the radiation power input. The chemical
energy was calculated from the measured current multiplied by the voltage 1.23 V
that is related to the hydrogen molecule.
The ef
ciency of water splittingwas calculated fromcurve 1, withno external voltage
applied, as power-out divided by power-in, or 0.12 A
0.124, for
1cm
2
(Figure 9.3). The light intensity was about 11 suns for this experiment. (The
1.23 V/(1.190W)
¼
