Environmental Engineering Reference
In-Depth Information
overpotential on anode and cathode electrodes, practical potential needed to drive
water splitting is larger than 1.8 V. A promising solution is using renewable
energy to split water, for example photoelectrolysis [ 11 ]. There are two approaches
to utilize solar energy for electrolysis. For instance, electrolyzer can be coupled
with photovoltaic (PV) cells, which provide photovoltage for water electrolysis
[ 11 ]. However, the relative high cost of PV cells and electrolyzer could limit its
practical application. Alternatively, water splitting can be achieved in a photo-
electrochemical (PEC) cell [ 11 ]. PEC cells are consisted of semiconductor pho-
toelectrodes for harvesting solar light. The photogenerated electrons and holes can
reduce and oxidize water to produce hydrogen and oxygen gas, respectively. In
order to compete with PV/electrolyzer system, the key of this approach is to use
low-cost semiconductor materials as photoelectrodes.
The concept of PEC water splitting was first demonstrated by Honda and
Fujishima in 1972 [ 17 ]. Figure 1 shows the schematic diagram of PEC cell that
consists of an n type semiconductor as photoanode and counter electrode [ 23 ].
Upon light illumination, photoexcited electron-hole pairs will be generated in the
photoelectrode. The electron-hole pairs will be separated due to the band bending
at the electrolyte/semiconductor interface and/or the application of external bias.
The holes and electrons will oxidize and reduce water to produce oxygen and
hydrogen, respectively. The reaction can be described by the following equations:
Photanode: H 2 O+2h þ ! 2H þ + 1/2O 2
E ox ¼ 1 : 23 V
Cathode: 2H þ +2e ! H 2
E red ¼0V :
According to the Nernst equation, a minimum energy of 1.23 V is needed to drive
water electrolysis. Therefore, in order to achieve hydrogen evolution and oxygen
evolution reactions simultaneously under light illumination, the band-gap energy
(E g ) of the semiconductor photoelectrode must be larger than 1.23 eV and the
band-edge potentials (conduction band and valence band) should straddle the
electrochemical potentials of E o (H + /H 2 ) and E o (O 2 /H 2 O). Moreover, a favorable
semiconductor photoelectrode should able to absorb a significant portion of solar
light, and has low kinetic overpotential for water oxidation and reduction, good
corrosion resistivity, and electrochemical stability in aqueous solution [ 92 ].
In order to properly evaluate the performance of photoelectrodes, it is critical to
define the parameters to characterize solar to hydrogen (STH) conversion effi-
ciency. The PEC properties of photoelectrodes are typically studied in a three
electrode electrochemical cell system, which consists of a working electrode
(photoelectrode), a reference electrode, and a counter electrode. For the reference
electrode, the internationally accepted primary reference is the standard hydrogen
electrode (SHE) or reversible hydrogen electrode (RHE), which has all compo-
nents at unit activity [(Pt/H 2 (a = 1)/H + (a = 1, aqueous)]. However, this kind of
reference electrode is impractical, and therefore other reference electrodes such as
saturated Ag/AgCl or saturated calomel electrode (SCE) are normally used in the
measurement. In this case, the experimentally obtained potentials should be
Search WWH ::




Custom Search