Hydrogen Sources - Hydrogen and Fuel Cells: Advances in Transportation and Power - page 114

The main reaction product is water vapor. The water splitting reaction is
endothermic and the energy required for a significant hydrogen produc-
tion rate is high.
Solar energy is available in abundant supply and nonpolluting. It is
an extremely attractive means to convert solar energy to chemical energy.
The photocatalytic process uses semiconducting catalysts or elec-
trodes in a photoreactor to convert optical energy into chemical energy.
A semiconductor surface is used to absorb solar energy and to act as an
electrode for splitting water. The technology is still at an early stage of de-
velopment. The most stable photoelectrode is TiO 2 but this material has a
conversion efficiency of less than 1%. New materials, which require no ex-
ternal electricity still need to be found. In order to reduce corrosion, ultra
thin layers of protective material on the semiconducting surface may be
used. Research is also directed in the areas of low cost systems, multiple
layers of organic dyes, and thin film semiconductors.
Hydrogen production can also be based on a water-splitting thermo-
chemical cycle using metal oxides. The simplest thermochemical process
to split water involves heating it to a high temperature and separating the
hydrogen from the equilibrium mixture. The decomposition of water does
not progress well until the temperature is about 4700 degrees K. The prob-
lems with materials and separations at such high temperatures makes di-
rect decomposition not practicable at this time. A two-step water-splitting
cycle, based on metal oxides redox pairs bypasses the separation obstacle.
Multi-step thermochemical cycles allow the use of more moderate operat-
ing temperatures, but their overall efficiency is limited by an irreversibil-
ity associated with heat transfer and product separation.
Hydrogen production by a 2-step water splitting thermochemical cy-
cle based on metal oxides redox pairs is being investigated in the areas of
the thermodynamics, technical feasibility, and cost. A lower-valence met-
al oxide is able to split water with a partial reduction of the metal oxide
without the use of a reducing agent. Hydrogen and oxygen are derived in
different steps, without the need for high temperature gas separation. Pos-
sible redox pairs include the following: Fe 3 O 4 /FeO, ZnO/Zn, TiO 2 /TiO x
(with X < 2), Mn 3 O 4 /MnO and Co 3 O 4 /CoO.
Hydrogen has a unique role in a secure energy future for the United
States. The U.S. could be energy self-sufficient with hydrogen, ensuring
our national stability from an energy security, supply, and economic per-
spective.
Hydrogen can also be produced from resources that are renewable,