Figure 34 Photoelectrolysis cellFUELS FROM WATER AND SUNLIGHT: NEW PHOTOELECTRODES
FOR EFFICIENT PHOTOELECTROLYSIS
Photoelectrochemical systems have been demonstrated to robustly form hydrogen from water
using sunlight. The known light absorbers, however, are either too inefficient (1–2%) in sunlight
or too unstable in the field for practical implementation. New electrodes or electrode
combinations, developed by a close coupling between theory and experiment, are needed to
enable a robust, efficient system for direct solar-induced water splitting.
EXECUTIVE SUMMARY
Photoelectrochemical water splitting for
hydrogen production, also known as
photoelectrolysis, represents an advanced
alternative to combining photovoltaic cells
with an electrolysis system (Bard and Fox
1995; Khaselev and Turner 1998; Memming
2001; Nozik 1978; Nozik and Memming
1996; Licht 2002). The major advantage is
that energy capture, conversion, and storage
are combined in a single system. The solar
energy, absorbed in a semiconductor
electrode immersed in an aqueous solution,
is used to produce storable fuels such as
hydrogen (see Figure 34). In operation, the
semiconductor collects the light energy,
then produces and directs the photogenerated carriers to a catalyst on the surface of the
semiconductor where, depending on the semiconductor, either hydrogen, oxygen, or other
photoproducts are produced. Other products are produced at a separate electrode that is either a
metal or another illuminated semiconductor electrode. The water-splitting process has
demonstrated high solar-to-hydrogen conversion efficiencies (>10%), but lifetime and cost issues
remain to be solved.
RESEARCH DIRECTIONS
Discovery of Photoelectrodes via Conventional Synthetic, Combinatorial, and
Computational Methods
Discovery of semiconductors that have appropriate light absorption characteristics and are stable
in aqueous solutions is a key issue. Efficient photoelectrolysis of water has been achieved by
using expensive single-crystal III-V multijunction electrodes. However, concerns about the long-
term stability of these systems and their high cost are significant issues. New theoretical and
experimental approaches to discover photoelectrodes capable of photoelectrolysis reactions are
needed.