300
Water for Energy and Fuel Production
They studied the effects of increasing operating pressure up to several hundred bars
for direct storage of hydrogen in a pressurized vessel. Their study showed that while
PEM water electrolyzers operating at pressures up to 70  bar can be used to produce hydrogen and oxygen of electrolytic grade with high efficiencies; an increase
in cross-permeation at higher pressure, can cause the hydrogen and oxygen mixture
concentration to reach the critical level of explosive mixtures. The cross-permeation
can be reduced by surface modification of solid electrolytes using low-permeability
protective layers of coating. Contaminant concentration in the produced gases can
also be reduced by adding catalyst gas recombiners either directly in the electrolytic
cells or along the production line. Fateev et al. (2012, pers. comm.) showed that by
using gas recombiners inside the electrolysis cell, it was possible to maintain the
hydrogen content below 2 vol% at an operating pressure of 30 bar, with Nafion 117
as the solid electrolyte.
11.2.4 PhoToeleCTrolySiS
In this process, hydrogen and oxygen are separated in a light-driven electrolysis cell.
Thus, the reactions that occur at the p-type cathode involve the evolution of hydrogen and those that occur at the n-type anode involve the evolution of oxygen. No
external battery is used in the electrolysis process. While ideally the current between
electrodes can be used as electricity and hydrogen produced from the process can be
used as fuels, the efficiency of the overall process is about 1% [22–26] (Rajeshwar,
2012, pers. comm.). The progress in photoelectrolysis faces three major barriers:
(1) There are no valid and significant theoretical analysis on the subject. The
works of Scaife et al. [25], Scaife [26], and Ohashi et al. [22] appear to have some
deficiencies. (2) The assumption made for years that Fermi level in solution as an
important aspect of the conditions under which cells would work is proven not to
be true. (3) The corrosion of semiconductor surfaces in contact with solution can
be considerable. The corrosion is caused by heat as well as by photoelectrochemical
reactions. Photoelectrochemical reaction efficiency is currently the same as that of
photosynthesis. In the recent years, the increase in efficiency by photoelectrocatalysis has been achieved. Numerous metals such as TiO 2 /pGaP, SrTiO 2 /GaP, tin oxide,
and other coatings of TiO 2 and CdS [22–26] (Rajeshwar, 2012, pers. comm.) on
electrodes have been tested to improve the efficiency and life of the photoelectrolytic
cell. The work of Szklarczyk and Bockris [23,24] showed that photoelectrocatalysis
is directly related to electrocatalysis. The rate-determining step in photoelectrocatalysis is dependent on the transfer of charge at the metal–solution interface and not
at the semiconductor–solution interface.
11.2.5 PhoTo-Aided eleCTrolySiS
One method to improve efficiency is to have light falling upon an electrode. This
can be achieved by applying a potential from an outside power source to the concerned electrodes. A 30%–40% efficiency in this case is not very impressive because
the overall efficiency includes efficiencies for both light and electricity to hydrogen
and not of light alone. The efficiency in this case can be improved by about 3%–4%