Water splitting is the general term for
a chemical reaction in which water is
separated into oxygen and hydrogen.
Efficient and economical water splitting
would be a key technology component of a
hydrogen economy. Various techniques for
water splitting have been issued in
water splitting patents in the United
States. In photosynthesis, water
splitting donates electrons to power the
electron transport chain in photosystem
II.
Electrolysis 
Electrolysis of water is the
decomposition of water into oxygen and
hydrogen gas due to an electric current
being passed through the water. In
chemistry and manufacturing,
electrolysis is a method of separating
chemically bonded elements and compounds
by passing an electric current through
them. One important use of electrolysis
of water or artificial photosynthesis is
to produce hydrogen. Recently,
researchers have shown that water
splitting can be broken into two
discrete steps using polyoxometalate
based redox mediators.
In power to gas the excess power or off
peak power generated by wind generators
or solar arrays is used for load
balancing in the energy grid by
injecting the hydrogen into the natural
gas grid using an electrolyser.
Production of hydrogen from water
requires large amounts of energy and is
uncompetitive with production from coal
or natural gas. Potential electrical
energy supplies include hydropower, wind
turbines, or photovoltaic cells.
Usually, the electricity consumed is
more valuable than the hydrogen produced
so this method has not been widely used.
Other potential energy supplies include
heat from nuclear reactors and light
from the sun. Hydrogen can also be used
to store renewable electricity when it
is not needed and then the hydrogen can
be used to meet power needs during the
day or fuel vehicles. This aspect helps
make hydrogen an enabler of the wider
use of renewables, and internal
combustion engines.
= High pressure electrolysis =
When the electrolysis is conducted at
high pressures, the produced hydrogen
gas is compressed at around 120–200 bar.
By pressurising the hydrogen in the
electrolyser the need for an external
hydrogen compressor is eliminated, the
average energy consumption for internal
compression is around 3%.
= High-temperature electrolysis =
When the energy supply is in the form of
heat, the best path to hydrogen is
through high-temperature electrolysis.
In contrast with low-temperature
electrolysis, HTE of water converts more
of the initial heat energy into chemical
energy, potentially doubling efficiency
to about 50%. Because some of the energy
in HTE is supplied in the form of heat,
less of the energy must be converted
twice, and so less energy is lost.
HTE processes are generally only
considered in combination with a nuclear
heat source, because the other
non-chemical form of high-temperature
heat is not consistent enough to bring
down the capital costs of the HTE
equipment. Research into HTE and
high-temperature nuclear reactors may
eventually lead to a hydrogen supply
that is cost-competitive with natural
gas steam reforming. HTE has been
demonstrated in a laboratory, but not at
a commercial scale.
Photoelectrochemical water splitting 
Using electricity produced by
photovoltaic systems potentially offers
the cleanest way to produce hydrogen.
Again, water is broken down into
hydrogen and oxygen by electrolysis, but
the electrical energy is obtained by a
photoelectrochemical cell process. The
system is also named artificial
photosynthesis.
Photocatalytic water splitting 
The conversion of solar energy to
hydrogen by means of water splitting
process is one of the most interesting
ways to achieve clean and renewable
energy systems. However if this process
is assisted by photocatalysts suspended
directly in water instead of using
photovoltaic and an electrolytic system
the reaction is in just one step,
therefore it can be more efficient.
Radiolysis 
Nuclear radiation routinely breaks water
bonds, in the Mponeng gold mine, South
Africa, researchers found in a naturally
high radiation zone, a community
dominated by a new phylotype of
Desulfotomaculum, feeding on primarily
radiolytically produced H2. Spent
nuclear fuel/"nuclear waste" is also
being looked at as a potential source of
hydrogen.
Photobiological water splitting 
Biological hydrogen can be produced in
an algae bioreactor. In the late 1990s
it was discovered that if the algae are
deprived of sulfur it will switch from
the production of oxygen, i.e. normal
photosynthesis, to the production of
hydrogen. It seems that the production
is now economically feasible by
surpassing the 7–10 percent energy
efficiency barrier. with a hydrogen
production rate of 10-12 ml per liter
culture per hour.
Thermal decomposition of water 
Thermal decomposition, also called
thermolysis, is defined as a chemical
reaction whereby a chemical substance
breaks up into at least two chemical
substances when heated. At elevated
temperatures water molecules split into
their atomic components hydrogen and
oxygen. For example at 2200 °C about
three percent of all H2O molecules are
dissociated into various combinations of
hydrogen and oxygen atoms, mostly H, H2,
O, O2, and OH. Other reaction products
like H2O2 or HO2 remain minor. At the
very high temperature of 3000 °C more
than half of the water molecules are
decomposed, but at ambient temperatures
only one molecule in 100 trillion
dissociates by the effect of heat.
Thermal water splitting has been
investigated for hydrogen production
since the 1960s. The high temperatures
needed to obtain substantial amounts of
hydrogen impose severe requirements on
the materials used in any thermal water
splitting device. For industrial or
commercial application, the material
constraints have limited the success of
applications for hydrogen production
from direct thermal water splitting and
with few exceptions most recent
developments are in the area of the
catalysis and thermochemical cycles.
= Nuclear-thermal =
Some prototype Generation IV reactors,
such as the HTTR, operate at 850 to 1000
degrees Celsius, considerably hotter
than existing commercial nuclear power
plants. General Atomics predicts that
hydrogen produced in a High Temperature
Gas Cooled Reactor would cost $1.53/kg.
In 2003, steam reforming of natural gas
yielded hydrogen at $1.40/kg. At 2005
gas prices, hydrogen cost $2.70/kg.
Hence, just within the United States, a
savings of tens of billions of dollars
per year is possible with a
nuclear-powered supply. Much of this
savings would translate into reduced oil
and natural gas imports.
One side benefit of a nuclear reactor
that produces both electricity and
hydrogen is that it can shift production
between the two. For instance, the plant
might produce electricity during the day
and hydrogen at night, matching its
electrical generation profile to the
daily variation in demand. If the
hydrogen can be produced economically,
this scheme would compete favorably with
existing grid energy storage schemes.
What is more, there is sufficient
hydrogen demand in the United States
that all daily peak generation could be
handled by such plants.
Recent research on the hybrid
thermoelectric Copper-chlorine cycle has
focused on a cogeneration system using
the waste heat from nuclear reactors,
specifically the CANDU supercritical
water reactor.
= Solar-thermal =
The high temperatures necessary to split
water can be achieved through the use of
concentrating solar power. Hydrosol-2 is
a 100-kilowatt pilot plant at the
Plataforma Solar de Almería in Spain
which uses sunlight to obtain the
required 800 to 1,200 °C to split water.
Hydrosol II has been in operation since
2008. The design of this 100-kilowatt
pilot plant is based on a modular
concept. As a result, it may be possible
that this technology could be readily
scaled up to megawatt range by
multiplying the available reactor units
and by connecting the plant to heliostat
fields of a suitable size.
An interesting approach to solar thermal
hydrogen production is proposed by H2
Power Systems. Material constraints due
to the required high temperatures above
2200 °C are reduced by the design of a
membrane reactor with simultaneous
extraction of hydrogen and oxygen that
exploits a defined thermal gradient and
the fast diffusion of hydrogen. With
concentrated sunlight as heat source and
only water in the reaction chamber, the
produced gases are very clean with the
only possible contaminant being water. A
"Solar Water Cracker" with a
concentrator of about 100 m² can produce
almost one kilogram of hydrogen per
sunshine hour.
Chemical production 
A variety of materials react with water
or acids to release hydrogen. Such
methods are non-sustainable. In terms of
stoichiometry, these methods resemble
the steam reforming process. The great
difference between such chemical methods
and steam reforming, is that the
necessary reduced metals do not exist
naturally and require considerable
energy for their production. For
example, in the laboratory strong acids
react with zinc metal in Kipp's
apparatus.
In the presence of sodium hydroxide,
aluminium and its alloys react with
water to generate hydrogen gas.
Unfortunately, due to its energetic
inefficiency, aluminium is expensive and
usable only for low volume hydrogen
generation. Also high amounts of waste
heats must be disposed.
Although other metals can perform the
same reaction, aluminium is among the
most promising materials for future
development because it is safer, cheaper
and easier to transport than some other
hydrogen storage materials like sodium
borohydride.
The initial reaction consumes sodium
hydroxide and produces both hydrogen gas
and an aluminate byproduct. Upon
reaching its saturation limit, the
aluminate compound decomposes into
sodium hydroxide and a crystalline
precipitate of aluminum hydroxide. This
process is similar to the reactions
inside an aluminium battery.
(1) Al + 3 H2O + NaOH → NaAl(OH)4 + 1.5
H2
(2) NaAl(OH)4 → NaOH + Al(OH)3
Overall:
Al + 3 H2O → Al(OH)3 + 1.5 H2
In this process, aluminium functions as
a compact hydrogen storage material
because 1 kg of aluminum can produce up
to 0.111 kg of hydrogen from water. When
employed in a fuel cell, that hydrogen
can also produce electricity, recovering
half of the water previously consumed.
The U.S. Department of Energy has
outlined its goals for a compact
hydrogen storage device and researchers
are trying many approaches, such as by
using a combination of aluminum and
NaBH4, to achieve these goals.
Since the oxidation of aluminum is
exothermic, these reactions can operate
under mild temperatures and pressures,
providing a stable and compact source of
hydrogen. This chemical reduction
process is specially suitable for
back-up, remote or marine applications.
While the passivation of aluminum would
normally slow this reaction
considerably, its negative effects can
be minimized by changing several
experimental parameters such as
temperature, alkali concentration,
physical form of the aluminum, and
solution composition.
Research 
Research is being conducted over
photocatalysis, the acceleration of a
photoreaction in the presence of a
catalyst. Its comprehension has been
made possible ever since the discovery
of water electrolysis by means of the
titanium dioxide. Artificial
photosynthesis is a research field that
attempts to replicate the natural
process of photosynthesis, converting
sunlight, water and carbon dioxide into
carbohydrates and oxygen. Recently, this
has been successful in splitting water
into hydrogen and oxygen using an
artificial compound called Nafion.
High-temperature electrolysis is a
method currently being investigated for
the production of hydrogen from water
with oxygen as a by-product. Other
research includes thermolysis on
defective carbon substrates, thus making
hydrogen production possible at
temperatures just under 1000 °C.
The iron oxide cycle is a series of
thermochemical processes used to produce
hydrogen. The iron oxide cycle consists
of two chemical reactions whose net
reactant is water and whose net products
are hydrogen and oxygen. All other
chemicals are recycled. The iron oxide
process requires an efficient source of
heat.
The sulfur-iodine cycle is a series of
thermochemical processes used to produce
hydrogen. The S-I cycle consists of
three chemical reactions whose net
reactant is water and whose net products
are hydrogen and oxygen. All other
chemicals are recycled. The S-I process
requires an efficient source of heat.
More than 352 thermochemical cycles have
been described for water splitting or
thermolysis., These cycles promise to
produce hydrogen oxygen from water and
heat without using electricity. Since
all the input energy for such processes
is heat, they can be more efficient than
high-temperature electrolysis. This is
because the efficiency of electricity
production is inherently limited.
Thermochemical production of hydrogen
using chemical energy from coal or
natural gas is generally not considered,
because the direct chemical path is more
efficient.
For all the thermochemical processes,
the summary reaction is that of the
decomposition of water:
All other reagents are recycled. None of
the thermochemical hydrogen production
processes have been demonstrated at
production levels, although several have
been demonstrated in laboratories.
There is also research into the
viability of nanoparticles and catalysts
to lower the temperature at which water
splits.
Recently Metal-Organic Framework-based
materials have been shown to be a highly
promising candidate for water splitting
with cheap, first row transition
metals.;
Research is concentrated on the
following cycles:
Patents 
Vion, U.S. Patent 28,793, "Improved
method of using atmospheric
electricity", June 1860.
See also 
Photocatalytic water splitting
Water gas shift reaction
References 
External links 
JEAC
