Environmental Engineering Reference
In-Depth Information
3.3 PHOTOELECTROCHEMICAL METHODS
3.3.1 Background
Solar-driven photoelectrochemical (PEC) water splitting represents a sus-
tainable and environmentally friendly method to produce hydrogen. A PEC
cell consists of at least one semiconductor photoelectrode, which can harvest
solar light [31].
n
- and
p
-type semiconductors are preferred for the photo-
anode and photocathode, respectively. Under light irradiation with photon
energy equal to or exceeding the bandgap energy of the semiconductor
photoelectrode, the electrons are excited from the valence band to the
unoccupied conduction band. The band bending at the semiconductor-
electrolyte interface or an applied bias will facilitate the separation of
photogenerated electrons and holes. The electrons will transfer to
the cathode-electrolyte interface to reduce protons to generate hydrogen
(2H
+
+ 2e
−
→ H
2
), while the holes will oxidize water molecules to produce
oxygen (H
2
O + 2h
+
→ 2H
+
+ ½ O
2
) at the anode-electrolyte interface.
Depending on the band edge positions of electrode materials, additional
external potential may be required to achieve water splitting.
The development of high performance photoelectrodes (anode and
cathode) represents a major challenge for PEC water splitting. A good pho-
toelectrode should have favorable bandgap for efficient visible light absorp-
tion and band edge positions that straddle the redox potentials of water
electrolysis. Moreover, it must be highly resistant to photocorrosion, electro-
chemically stable in aqueous solution in reductive (cathode) and oxidative
(anode) environment, and have good electrical conductivity. And finally it
must be inexpensive. In the meantime, for PEC devices with semiconductor
anode and cathode, it is also critical that the photocurrent of anode and
cathode are matched and the device can be operated at zero external bias as
shown in Figure 3.4 [31].
3.3.2 Photocathode for Water Reduction
Cuprous oxide, Cu
2
O, is an attractive
p
-type metal oxide for PEC hydrogen
production with a direct bandgap of 2 eV, for which a theoretical photocur-
rent density of 14.7 mA cm
−2
and a solar to hydrogen conversion efficiency
of 18% are predicted [32]. Cu
2
O also possesses favorable energy band posi-
tions, with the conduction band lying 0.7 V negative of the hydrogen evolu-
tion potential and the valence band lying just positive of the oxygen
evolution potential. However, the redox potentials for the reduction and
oxidation of Cu
2
O lie within its bandgap, resulting in its poor electrochemi-
cal stability. Cu(I) is more prone to be reduced to Cu metal, rather than
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