Environmental Engineering Reference
In-Depth Information
Hydrogen is a well-known, potentially highly efficient and environmentally clean
fuel since the chemical energy stored in the H-H bond can be easily released when
the molecule reacts with oxygen to yield only water as a by-product (i.e., com-
bustion). Moreover, hydrogen is a light gas (0.08988 g/L) with a higher energy
density compared to any other fuel such as gasoline. For example, one gram of
hydrogen can provide about 140 kJ of energy, which is almost four times that
of methane (33 kJ/g) [
22
]. Therefore, large-scale and cost-effective generation of
hydrogen, preferably using renewable and carbon-free resources, is highly
attractive. Currently, hydrogen is produced from a variety of primary sources such
as natural gas, heavy oil, methanol, biomass, wastes, coal, solar, wind, and nuclear
power. Among these sources, hydrogen production from photocatalytic water
splitting in the presence of semiconductor photocatalysts using solar irradiation
represents one of the most promising approaches and has garnered attention
because of its direct use of sunlight. In so doing, the process avoids the ineffi-
ciencies due to thermal transformation or electrolysis with the conversion of solar
energy to electricity [
23
]. Since the pioneering work by Fujishima and Honda in
1972 on a photoelectrochemical cell (PEC) using a TiO
2
photoelectrode, water
splitting under sunlight has made remarkable progress in the past 40 years [
24
,
25
].
Water splitting is a thermodynamically uphill or endothermic process with a
significantly positive change in Gibbs free energy (DG
o
= +237.2 kJ/mol, 1.23 eV
per electron), and a minimum potential of 1.23 eV is needed for the reaction to
proceed. Taking the recombination of excited electron-hole pairs and losses from
devices such as contacts and electrode resistances into consideration, the optimal
energy required for water splitting is around 2 eV [
22
]. In a PEC, the key com-
ponents are the electrodes (i.e., cathode and anode) on which redox chemical
reactions involving electron transfer take place. Typically, a PEC cell is composed
of a semiconductor photoanode and a Pt counter electrode in an electrolyte solution.
As shown in Fig.
2
a, incident light irradiation with the photon energy matching or
greater than the forbidden band gap energy (E
g
) of the semiconductor generates
electron-hole pairs and the photo-excited electrons are then promoted from the
valence band (VB) to the unoccupied conduction band (CB), which then migrate to
the cathode and react with protons to generate hydrogen (2H
+
+ 2e
-
? H
2
).
Concurrently, the holes accumulate on the surface of the photoanode and split water
molecules to produce oxygen (H
2
O + 2h
+
? 2H
+
+ 1/2O
2
)[
26
].
To effectively split water for hydrogen generation, the match of the band gap
and the potential of the conduction and valence bands are important, that is, the E
g
of the semiconductor should be larger than 1.23 eV (k \ 1000 nm) to realize
water splitting. However, when using visible light, E
g
should be less than 3.0 eV
(k [ 400 nm) [
27
]. In addition, the semiconductor photoanode with a conduction
band edge more negative than the H
2
evolution potential and a valence band edge
more positive than the O
2
evolution potential is also required. Other requirements
include stability under light irradiation and in aqueous solution, excellent
absorption in the solar spectrum region, high-quality structure for effective charge
transport, and low production cost [
28
]. Unfortunately, no such material has been
found that can satisfy all the requirements simultaneously.
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