Environmental Engineering Reference
In-Depth Information
This device perhaps can be produced as a wide-area membrane with water on both
sides, generating oxygen on the one side and hydrogen on the other side. Unlike the
expensive semiconductor system discussed earlier, the oxide surfaces are inert with
respect to water exposure and do not involve single crystals or vacuum processing steps.
9.3.3
Possibilities for Dual-Purpose Thin-Film Tandem Cell Devices
One might imagine a monolithic dual-purpose self-regulating photoelectrochemical
device comprised of a multijunction tandem solar cell, electrolytic cell, and power
sharing circuitry. The tandemcell structure at illuminationwill be ef
cient inherently
by using several different gaps. Its series structure will give an open-circuit voltage
larger than that necessary to decompose water. The excess photovoltage with respect
to water splitting can be diverted to an external load. The cell will generate primarily
electric power under heavy load (small load resistance) and will generate primarily
hydrogen gas at light load (high load resistance). The structure might be built up as a
CIGS cell on aluminum foil, with, for example, a tungsten trioxide blue light cell
deposited on top. The upper surface of the tungsten trioxide cell could be metallized
with a transparent electrode, and fed through a load resistor to a second aluminum
foil below the original aluminum foil, the interspace between the two foils being
lled
with water to be electrolyzed.
9.4
The
Artificial Leaf
of Nocera
The type of cell shown in Figure 9.1 has been updated recently by Daniel Nocera [113]
(http://techtv.mit.edu/videos/633-daniel-nocera-describes-new-process-for-storing-
solar-energy) in three important ways. First, he has demonstrated that a commercial
tandem silicon solar cell, such as described in Figure 7.12, which can be made
inexpensively in
lm form, can replace the expensive GaAs multi-
junction cell. Second, he has demonstrated cobalt phosphate-based catalysts that
replace the expensive platinum catalyst
films described in Figure 9.1. Third, Nocera
described the photocatalytic device as an arti
cial leaf, pointing out its basic nature
and connection with photosynthesis. Photosynthesis includes water splitting in the
set of reactions that use CO
2
,H
2
O, and photons to produce sugar and carbohydrates.
To quote the author [113], TheMIT team spread its catalysts on opposite sides of
a silicon wafer (tandem cell). The silicon absorbs sunlight and passes energetic,
negatively charged electrons and positively charged electron vacancies to the
catalysts on opposite sides that use them to make H
2
and O
2
.Whenthedevice
is placed in a clear jar and exposed to sunlight, the setup converts 5.5% of the
energy in sunlight into hydrogen fuel. This almost allows water splitting on a small
scale to be implemented using off-the-shelf components, since the silicon tandem
cells have been made in large quantities for some time for application as rooftop
tiles. In addition, Nocera has taken steps toward commercializing this for rooftop
flexible thin-
