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this process simply transforms electricity into chemical energy in the form
of hydrogen, and electric power expense has the largest share in the price of
electrolytic hydrogen, which is not a promising solution for energy sustain-
ability. Additionally, using platinum electrodes significantly increases the
cost of electrolyzers, and thus, the cost of producing hydrogen.
Alternatively, a number of methods have been developed to split water in
a clean and more cost-effective manner by using renewable solar energy [2].
For instance, an electrolyzer system can be powered by solar cells, such as
silicon-based solar cells or dye-sensitized solar cells. The solar cells can
harvest solar energy and provide photovoltages. Therefore, solar cells can be
connected in series to supply the required potential for electrolysis. Using a
combination of conventional electrolyzer and a commercially available solar
cell with 10-15% of conversion efficiency, a solar-to-hydrogen efficiency of
∼10% could be potentially achieved. Nevertheless, the relatively high cost
of solar cells and electrolyzers are major drawback of this approach. In this
regard, a photocatalytic or photoelectrochemical (PEC) system consisting of
semiconductor materials that can harvest light and use this energy directly
for splitting water is a more promising and cost-effective way for solar
hydrogen generation. In this chapter, we will highlight recent research prog-
ress in photocatalytic and PEC water splitting.
3.2 PHOTOCATALYIC METHODS
3.2.1 Background
With the rapid growth of global population, new energy resources should be
explored to address the continuously growing demand for energy. Renewable
solar energy is a promising solution to energy sustainability. In order to
obtain continuous and stable power supply, it requires efficient and cost
effective energy storage carriers or device to store solar energy during inter-
mittent sunlight irradiation [3]. Inspired by photosynthesis, which converts
solar energy into chemical energy that is stored in the form of carbohydrates,
great efforts have been made to mimic this natural process using man-made
materials for solar water splitting to generate hydrogen fuel [3, 4]. Ideally,
hydrogen gas can be continuously generated when photocatalyst powders are
dispersed in water under solar light illumination. However, the photocatalyst
must overcome the uphill Gibbs free energy change to drive this reaction [5].
The first demonstration of artificial photosynthesis was reported by Honda
and Fujishima in 1972 using semiconductor TiO 2 as photocatalyst [6]. When
the energy of incident light is larger than the bandgap energy of the semi-
conductors, photoexcited electrons and holes will be created in the conduc-
tion band and valence band, respectively. Water molecules will be reduced
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