Photoelectrochemical cells for hydrogen production from solar energy - Solar Energy Sciences and Engineering Applications - page 333

there is no recombination in the space charge region and at the interface (O'Hayre
et al., 2006).
In 1989 Reichman improved this model introducing more appropriated boundary
conditions for the total valence band photocurrent in an n-type semiconductor (Krol
and Schoonman, 2008):

j 0 exp −

kT

eη

j G −

j p = (10.4.42)

j p exp − kT

j 0 eη

1 +

where j p is the hole transfer rate at the semiconductor/electrolyte interface, and j 0 is
the saturation current density, i.e. the hole current in the valence band at x

= W when

I 0 = 0. η is the overvoltage, which is defined as the difference between the applied

potential and the open-circuit potential under illumination. The Reichman derivation
is of special interest since it includes the possibility of recombination in the space
charge region, and it can be used to model the effect of the slow hole transfer kinetics
that is often observed in oxides such as

-Fe 2 O 3 (Reichman, 1980). These models,
which describe only the minority carriers process, cannot be used under near-flatband
conditions since at this point also the majority charge carriers contribute to the overall
current.

α

10.5 PEC CELLS BOTTLENECKS AND FUTURE PROSPECTS

In past few years, photoelectrochemical technology for hydrogen production from
solar energy has been growing fast, not only due to the opportunity of having a storable
form of energy, but also because this kind of energy is totally produced in a clean and
environmentally friendly way. Photoelectrochemical hydrogen production is not yet a
commercial solution for our energy problems; in fact for considering it as an alternative
there are some key challenges that should be addressed. Indeed material improvements
are strongly envisaged, closely followed by the implementation of robust and cost-
effective systems. Thus, an effective way would be to use a tandem system where the
voltage difference to split water is given by a photovoltaic arrangement. In fact, with
materials capable of producing photocurrents of 8 mA cm − 2 at 100 mW cm − 2 with a
0.8 V voltage difference would open the opportunity of exploitation PEC systems with
a cost/m 2 of around $80, excluding the PV bias cost with lifetimes higher than 10
years (Krol and Grätzel, 2012). Thus, it is expected that this process for producing
a chemical fuel would be affordable with no big fluctuations in the prices, as often
happens with the oil market.

Nomenclature
A - Cell area, m 2
b - Cell thickness, m
C i - Concentration of species i , mol

m − 3
D i - Diffusion coefficient of species i ,m 2

·

s − 1 ·

D i - Diffusion coefficient of species i in the electrolyte bulk, m 2 s − 1

·

D i - Diffusion coefficient of species i in the electrolyte in the pores, m 2 s − 1

·