Page 81 - Multifunctional Photocatalytic Materials for Energy

70 Multifunctional Photocatalytic Materials for Energy

simultaneously meet the following key criteria for direct water splitting: [15,38–41] (i)
the semiconductor must have a band gap greater than 1.6–2.0 eV to split water; (ii) the
semiconductor should harvest and utilize most fractions of the sunlight spectrum; (iii)
the band-edge potentials of the semiconductor should straddle the water red-ox (H 2
and O 2 ) potentials; and (iv) the photoinduced charge carriers must be highly selective
for OER and HER. So far no materials satisfy all these technical requirements. Efforts
are ongoing to design multifunctional systems that consist of several functional parts
to work together for overall water splitting or CO 2 reduction, that is, the multifunc-
tional systems, including tandem cells and Z-scheme (Fig. 4.13).
Advantageous to a single-junction cell, the light absorption in multijunction and
tandem cells can be tuned and extended by coupling each individual junction [6,38,40].
The geometry of a multijunction cell to align each junction unit must be carefully de-
signed to avoid light losses caused by the light absorption overlap between absorber
layers or by the protective layer and even the co-catalyst layer. In addition to the ex-
tended light absorption, the summed overall photovoltage from each individual junc-
tion offers the possibility for unbiased solar fuel generation by splitting water or CO 2
reduction with the energy input from only sunlight. It is worth noting, however, that
the overall conversion efficiency of a multijunction cell is determined by the minimum
photocurrent among all the individual junctions. So of great importance is optimizing
each junction unit to achieve the best performance for a multijunction cell. Tandem
PEC cells with a multijunction design consist of either a photoelectrode/photovoltaic
configuration (Fig. 4.13A) or a photoanode/photocathode configuration (Fig. 4.13B).
The world record efficiency of up to 18% for solar water splitting is still held by III–V/
Si-based multifunctional PEC cells [41]. However, high cost and instability diminish
its practical application. A PV/PEC tandem cell with Fe 2 O 3 or WO 3 as the photoanode
and dye-TiO 2 cell as the PV has been constructed with an STH efficiency of 3.1%, as
shown in Fig. 4.13A [40]. In such a design, the dye-TiO 2 PV part is meant to provide
an external voltage for water splitting, and a counter electrode, generally noble metal
Pt, is required for water reduction reaction. A more desirable concept is to couple an n-
type photoanode and a p-type photocathode in a PEC tandem cell (Fig. 4.13B), where
water oxidation and reduction take place separately on each photoelectrode [38].
As shown in Fig. 4.13C, a Z-scheme system consists of two semiconductors bridged
with a red-ox mediator [6,97]. Similar to a multijunction system, in a Z-scheme sys-
tem, the light absorption capability could be tuned and extended because several semi-
conductors couple together. The difference for a Z-scheme is that it works only in
a particulate configuration suspended in a reaction solution (Fig. 4.4A), not in an
electrode configuration (Fig. 4.4B) because of the thermodynamic requirement for
charge transfer (Fig. 4.13C). One characteristic of a Z-scheme system is the presence
of a red-ox mediator, either liquid or solid, that consumes the photoelectrons from
the conduction band of semiconductor I and the photoholes from the valence band of
semiconductor II, and enables the OER and HER to take place on separated semicon-
ductors. In this way, each semiconductor drives only one half-reaction, which provides
more options for choosing semiconductors with proper band-edge energy levels, and
also a larger driving force for each responsible reaction [6,97].