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technology of manufacturing the sensitizer and catalyst, for example with nanotech-
nology, then photochemical hydrogen production has the potential to be improved
significantly.
Recent advancement in the creation of supramolecular catalysts has combined the
sensitizer and the catalyst into a single unit. These units are designed to be either
to be a Hydrogen Evolving Reaction (HER) (Vayssieres, 2009) or Oxygen Evolving
Reaction (OER) (Crabtree, 2010). Each of these reactions operates as half cells. The
HER requires an influx of electrons and light, then reduces water to produce hydrogen
gas and OH ions. The OER requires light and oxidizes water to produce oxygen
gas, H + ions, and an excess of electrons. By coupling the two half cells together using
electrodes and a proton exchange membrane (PEM), a complete reactor can be built
(Zamfirescu et al., 2011). Alternatively, the OER could instead use light to oxidize the
OH produced from the HER to produce oxygen gas, water and an excess of electrons.
Overall, this is a lower energy pathway than the OER presented above, but there are
larger challenges in finding a suitable membrane. Even though electrodes are present,
this is not classified as electrolysis or photoelectrolysis as the reactions do not occur
at the electrodes but at the catalysts. The electrodes are only present to complete the
electron circuit. Unlike the previously described method, the hydrogen and oxygen
are produced separately, creating similar engineering challenges to those discussed in
photoelectrolysis.
The advantages of photoelectrolysis over conventional electrolysis are also
expected for photochemical water splitting, such as the elimination of a power source
and auxiliary components of the electrolyzer. In addition, photochemical processes
can be implemented in homogeneous catalytic compounds promoting the HER (Wang
et al., 2011). Hence, the processing of catalysts can be greatly simplified to the pro-
cessing of a fluid in engineering. Also, greater tunability is possible with modular
architectures and precise details of molecular scale transformations are more accessi-
ble for the research (Teets et al., 2011). By comparison, the photonelectrode cannot
be homogeneous even if the size is at the nanoscale, otherwise the band gap won't be
satisfied and the photovoltage cannot be created.
Similar to photoelectrolysis, it is challenging for the photochemical unit to effi-
ciently make use of the solar irradiance of all wavelengths due to the wavelength
selectivity of catalysts (Maeda et al., 2006, 2010; Li et al., 2011). The solar-
to-hydrogen efficiency of an operational small pilot-scale photocatalytic hydrogen
production demonstration of 1.88 liters per hour is even below 1% (Jing et al., 2010).
Therefore, the overall efficiency of current photochemical hydrogen production units
rarely reaches 10%, although the quantum efficiency at a specific wavelength could
reach 56% (Li et al., 2011; Maeda, 2010, 2011; Kudo, 2009). Considering the
sunlight tracking challenges due to the structure and operating complexity of the
equipment to simultaneously process hydrogen, oxygen, water, and a sunlight window,
it can be concluded that much further research and development is needed towards
commercialization of the photocatalytic hydrogen production.
9.2.7 Hybrid and other hydrogen production methods
Two or more of the technologies presented in the previous sections can be combined
together for the production of hydrogen, for a hybrid production technology possessing
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