Photoelectrochemical cells for hydrogen production from solar energy - Solar Energy Sciences and Engineering Applications - page 295

unfavorable climatic conditions such as partial shading, haze or cloudiness may ulti-
mately interrupt the photoelectrolysis. Similar problems are observed when a PV cell is
used as external bias for a PEC cell to promote water-splitting. In this case, the electric
current generated by the PV cell goes directly to the PEC cell instead of feeding an elec-
trolyzer, resulting in a cheaper but not necessarily more efficient embodiment (Grimes
et al., 2008; Minggu et al., 2010). Still, two separated parts must be considered when
estimating the initial and operating costs. Moreover, the available area for solar expo-
sure must be substantially increased since both PV and PEC cells have to be illuminated
(Minggu et al., 2010; Conibeer and Richards, 2007). In this sense, a more effective
approach would be to “merge'' the PV cell with an electrolyzer to make photoelec-
trochemical devices with a semiconductor-liquid junction. Thus, several efforts have
been made to design a monolithic system to partially avoid the previously mentioned
technological and economic drawbacks (Minggu et al., 2010).

10.2.1.2 Single devices

Single water-splitting devices (Figure 10.2.1) can be divided into biased and zero biased
systems. Concerning biased systems, there are chemically biased photo-assisted pho-
toelectrolysis cells and tandem devices (Grimes et al., 2008; Minggu et al., 2010). In
the first case, the bias is achieved using two different electrolytes (e.g. acid and basic
electrolytes) placed in two separated half-cells. However, this configuration is not self-
sufficient, relying not only on sunlight but also on additional input of chemicals to
stabilize the electrolyte solutions (Minggu et al., 2010). In the tandem approach, the
cell is normally characterized by layered stacked or hybrid structures involving several
different semiconductor films placed on top of each other. In this configuration, at
least one of the substructures must work as a bias source. These internal biased pho-
toelectrode tandem structures can be subdivided into: i ) PV/PEC (Miller et al., 2005);
ii ) PV/PV (Khaselev et al., 2001) and; iii ) PEC/PEC (Grätzel and Augustynski, 2001).
The use of PV/PEC systems has an advantage over PV/PV systems because the PEC
face (layer) can replace the face conductor grids that partially obscure the PV layer.
Consequently, PEC panels are able to reduce some cost components and improve pho-
ton capture of PV layer (James et al., 2009). Recently a new multiphoton combination
of a PEC cell and two dye cells (tandem arrangement) was proposed (Brillet, 2010).
Three different architectures were suggested. The authors found that the “trilevel'' tan-
dem architecture (hematite/squaraine dye/black dye) produces the highest operating
current density. The expected highest solar-to-hydrogen efficiency was about 1.36%.
However, this value is far below the expected 3.3% that should be possible with the
nanostructured hematite photoanodes used (Brillet, 2010).
Photoelectrochemical devices with no additional bias represent a prospective
pathway to overcome the complexity of biased systems. No-bias photoelectrochem-
ical devices comprise single and multiple photo-system arrangements. The possible
arrangements of single photo-system are:

i ) n-type semiconductor photoanode and a metal counter-electrode (Figure 10.2.2a)

(Kay et al., 2006);

ii ) p-type semiconductor photocathode and a metal counter-electrode (Figure

10.2.2b) (Chandra et al., 1985);