Solar hydrogen production and CO recycling - Solar Energy Sciences and Engineering Applications - page 272

the cycle. No external wiring is required as all components are internally connected.
Short electron pathways and large surface areas result in improved efficiencies. The
major drawback of this design is the generation of both H 2 and O 2 at the same loca-
tion. Therefore, hydrogen is not produced separately in the reactor, although each
hydrogen molecule is formed separately if viewed from the level of a single molecule.
The mixture of hydrogen and oxygen is quite sensitive to sparks that may lead to an
explosion. The separation of hydrogen and oxygen is a major engineering challenge.
It is difficult to directly compare the photoelectrolysis and water electrolysis
because the range of technologies is very broad, so only a few general conclusions
are made here on the basis of approximate similarity. Currently, the band gaps that
can be provided by photoelectrode materials are still large, e.g., greater than 3.2 eV,
although there are some materials capable of providing smaller band gaps. This makes
the solar irradiance of larger wavelengths such as infrared less available, hence the sun-
light usage efficiency is low (Walter et al., 2010; Conibeer et al., 2007; Currao, 2007;
Prakasam, 2008; Grimes et al., 2007). Therefore, the overall solar-to-hydrogen effi-
ciency of current photoelectrolysis or photoelectrochemical hydrogen production units
rarely reaches higher than 16% (Solarska et al., 2012; Yang et al., 2011; Prakasam
2008; Licht et al., 2000; Mohapatra et al., 2007). Also, in comparison with water
electrolysis, it is more challenging for the photoelectrolysis or photoelectrochemical
unit to efficiently track the sun because of the structure and operating complexity of
the equipment to simultaneously process hydrogen, oxygen, water, and sunlight win-
dow. For example, the contact between an electrode and water may be changed when
the equipment is tilted for efficient sunlight tracking. In addition, the auxiliary com-
ponents of the system may occupy a large portion of the sunlight projection area or
shade the sunlight in the sunlight tracking operation.
The lack of a combination of a stable, efficient light absorption system consisting
of suitable photoelectrodes and light windows partly accounts for the low efficiency.
No reliance on the external power source may bring some advantages, including sim-
plicity of system design because of the elimination of the auxiliary components required
by the electrolyzer, and potentially a large photoanode and photocathode surface with
nanosize materials (Yang et al., 2011; Walter et al., 2010; Conibeer et al., 2007;
Currao, 2007). Also, photoelectrolysis or photoelectrochemical hydrogen production
plants can be more readily distributed in hydrogen fueling stations or remote geo-
graphic areas to avoid building an expensive power transmission and distribution grid
for otherwise using water electrolysis.
9.2.6 Photochemical, photocatalytic, photodissociation,
photodecomposition, and photolysis
In addition to using concentrated solar thermal energy or electricity, there is another
way to use the irradiance for the water splitting. As shown in Equations 9.2.3 and
9.2.5, for a single water molecule, if the photons are directly trapped by some auxiliary
substances (sensitizers and catalysts) to activate the electrons to a higher energy state,
then the water molecules can capture the activated electrons from auxiliary substances.
As a result, the water molecules are activated to a high energy state, preparing for
further formation of hydrogen and oxygen atoms (Hagiwara et al., 2006). This series