Water Dissociation Technologies for Hydrogen
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not significantly improve the rate and the yield of algal photobiological hydrogen
production. Solar-to-hydrogen energy conversion using algae has efficiency <0.1%
[83]. The rate and yield of algal photobiological hydrogen production is limited by
(1) proton gradient accumulation across the algal thylakoid membrane, (2) competition from carbon dioxide fixation, (3) requirement for bicarbonate binding at photosystem II (PSII) for efficient photosynthetic activity, and (4) competitive drainage of
electrons by molecular oxygen. Recently, Lee [84–86] has outlined two inventions
for more efficient and robust photobiological production of hydrogen from water:
(1) designer proton channel algae and (2) designer switchable PSII algae. These
two new inventions eliminate not only the four problems mentioned earlier but
also oxygen sensitivity of algal hydrogenase and H 2 –O 2 gas separation and safety
issue. More work in this area is needed. The details of the two new inventions are
described by Lee [49].
11.3.3 PlASmA-induCed PhoTolySiS
It has been suggested that plasma [87] can be used to produce photons of appropriate energy so that water can be dissociated in the gas phase. Thus, in a hypothetical fusion of hydrogen, it would be possible to produce a light in the region of
1800–950A by the addition of aluminum to the plasma [11,88]. The main gain from
this method is that the thermal energy absorbed would be converted to electricity
in a heat engine at about 30% efficiency. A gain in efficiency is obtained because
hydrogen will be produced by both photolysis and electrolysis. At the present time,
however, the production of high energy protons is only possible by the injections
of aluminum into plasmas. The possibility of obtaining very high efficiency (up to
90% which is possible for electrolysis) is unlikely. Furthermore, the recombination of
hydrogen and oxygen could be a major drawback of this process [11].
11.4 thermal and thermOChemiCal
deCOmPOsitiOn OF Water
The direct thermal dissociation has been examined since 1960s [1,11,89–140]
(Funk, 2011, pers. comm.; Bamberger, 2011, pers. comm.). In direct thermal
decomposition, the energy needed to decompose water is supplied by heat only.
This requires a minimum temperature of at least 2200°C (even for partial decomposition) and as high as about 4700°C, and this makes the process somewhat unrealistic. At this temperature, about 3% of all water molecules are dissociated as
H, H 2 , O, O 2 , and OH. Other reaction products like H 2 O 2 or HO 2 remain minor.
At about 3200°C, about half of the water molecules are dissociated. It is well known
that an initiation of thermal splitting of water even at low pressure requires 2000 K
(about 1730°C). As mentioned above, at an atmospheric pressure, 50% dissociation requires about 3500 K. This temperature can be reduced to less than 3000 K
(about 2730°C) at 0.01 atm pressure. As will be discussed later, the catalysts can
accelerate the dissociation at lower temperature. The lower total pressure favors
the higher partial pressure of hydrogen, which makes the reactor to operate at pressures below an atmospheric pressure very difficult [1,11].