Environmental Engineering Reference
In-Depth Information
Figure 9.2 Schematic band diagram [111]
(electron energy increasing vertically) of
illuminated monolithic
photovoltaic
oxidation (release of oxygen) are sketched on the
right side of diagram. An alignment is needed
betweenconductionbandedgeandwater
reduction level, at the semiconductor interface.
Alignment is also needed between the lower
water oxidation energy and a level providing
holes from the anode. So, on the right, the metal
Fermi level (dashed line on the right) has to be
pushed down by 1.23 eV þ 2
g
to allow a hole to
be injected into the water (i.e., platinum anode
accepts an electron from the water) and thus to
release oxygen.
photoelectrochemical device for
hydrogen production by water splitting, with
12.4%efficiency. (This is the same device as shown
in Figure 9.1, but the platinum anode has been
moved to the right side of the picture.) In this
device, electrons flow to the illuminated
semiconductor
-
electrolyte interface, where they
act to release hydrogen gas. Energy levels for water
reduction (release of hydrogen) and water
-
device here described is expensive and not a candidate for any practical water splitting
approach. In the next sections, we will
find examples of cheaper tandem cell
possibilities.) One can infer that adding an additional junction in the cell to make
a three or more junction tandem cell could result in excess cell output voltage beyond
that needed to release hydrogen. Such a cell might be con
gured to simultaneously
split water and supply power to an external load.
9.3.1
Tandem Cell as Water Splitter
The situation (Figure 9.3) in curve 2 (zero photocurrent at zero applied voltage, that is,
the water splitter does not work) is usually the case for a single junction. The reason is
that the conduction and valence band energies must be separated by 1.2 eV or so for
ef
cient absorption of solar spectrum photons. At the same time, the conduction
band edge has to be above the water reduction energy level and the valence band edge
has to be below the water oxidation energy level (these two levels, separated by
1.23 eV
1.6 eV, are shown in Figure 9.2 on the right). It is common to reduce
the overvoltage losses
g
at the semiconductor/electrolyte interfaces by depositing
platinum particles as catalysts. (We will see below that recently progress has been
þ
2
g
