Solar hydrogen production and CO recycling - Solar Energy Sciences and Engineering Applications - page 274

Figure 9.2.6 Components of a photochemical unit.
the electrons to a higher energy state. Secondly, two electrons are needed to form a
hydrogen molecule. However, the energy carried by a single photon in the solar spec-
trum can activate only one electron for the reduction of a proton. Therefore, another
reagent (catalyst) is needed to accumulate the activated electrons for the formation of
hydrogen molecules. In order to split water molecules rather than other substances,
the catalyst, or again one more catalyst, must also have the ability to capture electrons
from the negative valence oxygen atom of the water molecule (Kudo, 2007). As to the
formation of an oxygen molecule, four electrons must be extracted by the catalyst to
prepare for the oxidization process. The processes and reactions taking place on the
sensitizers and catalysts are shown in Equations 9.2.13-9.2.19.
Figure 9.2.6 shows the most basic components needed by a photochemical hydro-
gen production cell, including a sunlight window, sensitizer and at least one catalyst.
It can be found that a sensitizer and catalyst are preferably distributed uniformly in
the water so as to intercept more sunlight and approach sufficient contact with water.
As with particle-based PEC, hydrogen is not produced separately, leading to similar
challenges. It can also be found that the preparation and performance of the sensitizer
and catalyst may greatly influence the efficiency and economics. For example, if the
lifetime of the catalyst or sensitizer is not long, then it must be replenished frequently.
This may increase the operating cost and generate a waste stream, which then may
not be a strictly clean hydrogen production technology. There are many engineers and
scientists focusing on the development of low-cost, highly-efficient, and long-lifetime
sensitizers and catalysts (Grätzel, 2003; Hagiwara et al., 2006). By improving the