Water splitting
Encyclopedia
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 of water
is the decomposition of water
(H2O) into oxygen
(O2) and hydrogen
gas (H2) 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
(photoelectrolysis
in a photoelectrochemical cell
) is to produce hydrogen.
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 (like the wind blowing at night) 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. (See hydrogen economy
.)
at around 120–200 bar
(1740–2900 psi
). 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%.
(HTE). In contrast with low-temperature electrolysis, HTE of water converts more of the initial heat
energy into chemical energy (hydrogen), 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 (from heat to electricity, and then to chemical form), 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 (concentrating solar thermal) 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.
(PEC) process. The system is also named artificial photosynthesis
.
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 (the conversion of sunlight into hydrogen) barrier.
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. However, catalysts can accelerate the dissociation of the water molecules at lower temperatures.
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 catalytic and two step processes.
s 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 (HTGR) would cost $1.53/kg
. In 2003, steam reforming of natural gas yielded hydrogen at $1.40/kg. At 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. However, Generation IV reactor
s are not expected until 2030 and it is uncertain if they can compete by then in safety and supply with the distributed generation
concept.
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 the megawatt range by multiplying the available reactor units and by connecting the plant to heliostat
fields (fields of sun-tracking mirrors) 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.
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 (1) consumes sodium hydroxide and produces both hydrogen gas and an aluminate
byproduct. Upon reaching its saturation limit, the aluminate compound decomposes (2) into sodium hydroxide and a crystalline precipitate of aluminum hydroxide. This process is similar to the reactions inside an aluminium battery
.
Overall:
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 (or 11.1%) 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.
, 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
(also HTE or steam 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 reaction
s 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
(S-I cycle) is a series of thermochemical
processes used to produce hydrogen
. The S-I cycle consists of three chemical reaction
s 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 and 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.
Research is concentrated on the following cycles :
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...
in which water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
is separated into oxygen and hydrogen. Efficient and economical water splitting would be a key technology component of a hydrogen economy
Hydrogen economy
The hydrogen economy is a proposed system of delivering energy using hydrogen. The term hydrogen economy was coined by John Bockris during a talk he gave in 1970 at General Motors Technical Center....
. Various techniques for water splitting have been issued in water splitting patents in the United States. In photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
, water splitting donates electrons to power the electron transport chain in photosystem II
Photosystem II
Photosystem II is the first protein complex in the Light-dependent reactions. It is located in the thylakoid membrane of plants, algae, and cyanobacteria. The enzyme uses photons of light to energize electrons that are then transferred through a variety of coenzymes and cofactors to reduce...
.
Electrolysis
Electrolysis of water
Electrolysis of water is the decomposition of water into oxygen and hydrogen gas due to an electric current being passed through the water.-Principle:...
is the decomposition of water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
(H2O) into oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
(O2) and hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
gas (H2) due to an electric current
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...
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
Electrolysis of water
Electrolysis of water is the decomposition of water into oxygen and hydrogen gas due to an electric current being passed through the water.-Principle:...
or artificial photosynthesis
Artificial photosynthesis
Artificial photosynthesis is a chemical process that replicates the natural process of photosynthesis, a process that converts sunlight, water, and carbon dioxide into carbohydrates and oxygen. The term is commonly used to refer to any scheme for capturing and storing the energy from sunlight in...
(photoelectrolysis
Photoelectrolysis
Photoelectrolysis occurs in a photoelectrochemical cell when light is used for electrolysis. In other words, photoelectrolysis is the conversion of light into a current, and then the division of a molecule using that current....
in a photoelectrochemical cell
Photoelectrochemical cell
Photoelectrochemical cells or PECs are solar cells which generate electrical energy from light, including visible light. Some photoelectrochemical cells simply produce electrical energy, while others produce hydrogen in a process similar to the electrolysis of water.-Photogeneration cell:In this...
) is to produce hydrogen.
Hydrogen economy
The hydrogen economy is a proposed system of delivering energy using hydrogen. The term hydrogen economy was coined by John Bockris during a talk he gave in 1970 at General Motors Technical Center....
.)
High pressure electrolysis
When the electrolysis is conducted at high pressures, the produced hydrogen gas is compressedCompressed hydrogen
Compressed hydrogen is the gaseous state of the element hydrogen kept under pressure. Compressed hydrogen in hydrogen tanks at 350 bar and 700 bar is used for mobile hydrogen storage in hydrogen vehicles...
at around 120–200 bar
Bar (unit)
The bar is a unit of pressure equal to 100 kilopascals, and roughly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the megabar , kilobar , decibar , centibar , and millibar...
(1740–2900 psi
Pounds per square inch
The pound per square inch or, more accurately, pound-force per square inch is a unit of pressure or of stress based on avoirdupois units...
). By pressurising the hydrogen in the electrolyser the need for an external hydrogen compressor
Hydrogen compressor
A hydrogen compressor is a device that increases the pressure of hydrogen by reducing its volume. Compression of hydrogen gas naturally increases its temperature, due to Charles' Law....
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 (solar thermal, or nuclear), the best path to hydrogen is through high-temperature electrolysisHigh-temperature electrolysis
High-temperature electrolysis is a method currently being investigated for the production of hydrogen from water with oxygen as a by-product.-Efficiency:...
(HTE). In contrast with low-temperature electrolysis, HTE of water converts more of the initial heat
Heat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...
energy into chemical energy (hydrogen), potentially doubling efficiency
Fuel efficiency
Fuel efficiency is a form of thermal efficiency, meaning the efficiency of a process that converts chemical potential energy contained in a carrier fuel into kinetic energy or work. Overall fuel efficiency may vary per device, which in turn may vary per application, and this spectrum of variance is...
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 (from heat to electricity, and then to chemical form), 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 (concentrating solar thermal) 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 cellPhotoelectrochemical cell
Photoelectrochemical cells or PECs are solar cells which generate electrical energy from light, including visible light. Some photoelectrochemical cells simply produce electrical energy, while others produce hydrogen in a process similar to the electrolysis of water.-Photogeneration cell:In this...
(PEC) process. The system is also named artificial photosynthesis
Artificial photosynthesis
Artificial photosynthesis is a chemical process that replicates the natural process of photosynthesis, a process that converts sunlight, water, and carbon dioxide into carbohydrates and oxygen. The term is commonly used to refer to any scheme for capturing and storing the energy from sunlight in...
.
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.Photobiological water splitting
Biological hydrogen can be produced in an algae
Algae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...
bioreactor
Bioreactor
A bioreactor may refer to any manufactured or engineered device or system that supports a biologically active environment. In one case, a bioreactor is a vessel in which a chemical process is carried out which involves organisms or biochemically active substances derived from such organisms. This...
. In the late 1990s it was discovered that if the algae are deprived of sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
it will switch from the production of oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
, i.e. normal photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
, to the production of hydrogen. It seems that the production is now economically feasible by surpassing the 7–10 percent energy efficiency (the conversion of sunlight into hydrogen) barrier.
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 hydrogenHydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
and oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
. 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. However, catalysts can accelerate the dissociation of the water molecules at lower temperatures.
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 catalytic and two step processes.
Nuclear-thermal
Some prototype Generation IV reactorGeneration IV reactor
Generation IV reactors are a set of theoretical nuclear reactor designs currently being researched. Most of these designs are generally not expected to be available for commercial construction before 2030...
s operate at 850 to 1000 degrees Celsius, considerably hotter than existing commercial nuclear power
Nuclear power
Nuclear power is the use of sustained nuclear fission to generate heat and electricity. Nuclear power plants provide about 6% of the world's energy and 13–14% of the world's electricity, with the U.S., France, and Japan together accounting for about 50% of nuclear generated electricity...
plants. General Atomics
General Atomics
General Atomics is a nuclear physics and defense contractor headquartered in San Diego, California. General Atomics’ research into fission and fusion matured into competencies in related technologies, allowing the company to expand into other fields of research...
predicts that hydrogen produced in a High Temperature Gas Cooled Reactor (HTGR) would cost $1.53/kg
Kilogram
The kilogram or kilogramme , also known as the kilo, is the base unit of mass in the International System of Units and is defined as being equal to the mass of the International Prototype Kilogram , which is almost exactly equal to the mass of one liter of water...
. In 2003, steam reforming of natural gas yielded hydrogen at $1.40/kg. At 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
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...
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
Grid energy storage
Grid energy storage refers to the methods used to store electricity on a large scale within an electrical power grid. Electrical energy is stored during times when production exceeds consumption and the stores are used at times when consumption exceeds production...
schemes. What is more, there is sufficient hydrogen demand in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
that all daily peak generation could be handled by such plants. However, Generation IV reactor
Generation IV reactor
Generation IV reactors are a set of theoretical nuclear reactor designs currently being researched. Most of these designs are generally not expected to be available for commercial construction before 2030...
s are not expected until 2030 and it is uncertain if they can compete by then in safety and supply with the distributed generation
Distributed generation
Distributed generation, also called on-site generation, dispersed generation, embedded generation, decentralized generation, decentralized energy or distributed energy, generates electricity from many small energy sources....
concept.
Solar-thermal
The high temperatures necessary to split water can be achieved through the use of concentrating solar power. Hydrosol-2Hydrosol-2
HYDROSOL is series of European Union funded projects for the promotion of renewable energy...
is a 100-kilowatt pilot plant at the Plataforma Solar de Almería
Plataforma Solar de Almería
The Plataforma Solar de Almería is a center for the exploration of the solar energy, situated in the Province of Almería.-History:It was founded in the early 1980s and run by the centro de Investigaciones Energéticas, Medioambientales y Tecnológicas , its location is on the edge of the Tabernas...
in Spain
Spain
Spain , officially the Kingdom of Spain languages]] under the European Charter for Regional or Minority Languages. In each of these, Spain's official name is as follows:;;;;;;), is a country and member state of the European Union located in southwestern Europe on the Iberian Peninsula...
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 the megawatt range by multiplying the available reactor units and by connecting the plant to heliostat
Heliostat
A heliostat is a device that includes a mirror, usually a plane mirror, which turns so as to keep reflecting sunlight toward a predetermined target, compensating for the sun's apparent motions in the sky. The target may be a physical object, distant from the heliostat, or a direction in space...
fields (fields of sun-tracking mirrors) 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 (which is also a "chemical method"), 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 zincZinc
Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...
metal in Kipp's apparatus
Kipp's apparatus
Kipp's apparatus, also called Kipp generator, is an apparatus designed for preparation of small volumes of gases. It was invented around 1860 by the Dutch pharmacist Petrus Jacobus Kipp and widely used in chemical laboratories and for demonstrations in schools into the second half of the 20th...
.
In the presence of sodium hydroxide, aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
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
Sodium borohydride
Sodium borohydride, also known as sodium tetrahydridoborate, is an inorganic compound with the formula NaBH4. This white solid, usually encountered as a powder, is a versatile reducing agent that finds wide application in chemistry, both in the laboratory and on a technical scale. Large amounts are...
.
The initial reaction (1) consumes sodium hydroxide and produces both hydrogen gas and an aluminate
Aluminate
An aluminate is a compound containing aluminium and oxygen. When precipitated from solution, the anion condenses with more electropositive elements. The generally accepted form of the aluminate is a mononuclear tetrahedral complex that is negatively charged, Al4− or AlO2−...
byproduct. Upon reaching its saturation limit, the aluminate compound decomposes (2) into sodium hydroxide and a crystalline precipitate of aluminum hydroxide. This process is similar to the reactions inside an aluminium battery
Aluminium battery
Aluminium–air batteries or Al–air batteries produce electricity from the reaction of oxygen in the air with aluminium. They have one of the highest energy densities of all batteries, but they are not widely used because of previous problems with cost, shelf-life, start-up time, and byproduct...
.
-
- (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
Hydrogen storage
Hydrogen storage describes the methods for storing H2 for subsequent use. The methods span many approaches, including high pressures, cryogenics, and chemical compounds that reversibly release H2 upon heating...
material because 1 kg of aluminum can produce up to 0.111 kg of hydrogen (or 11.1%) from water. When employed in a fuel cell
Fuel cell
A fuel cell is a device that converts the chemical energy from a fuel into electricity through a chemical reaction with oxygen or another oxidizing agent. Hydrogen is the most common fuel, but hydrocarbons such as natural gas and alcohols like methanol are sometimes used...
, that hydrogen can also produce electricity, recovering half of the water previously consumed. The U.S. Department of Energy
United States Department of Energy
The United States Department of Energy is a Cabinet-level department of the United States government concerned with the United States' policies regarding energy and safety in handling nuclear material...
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
Sodium borohydride
Sodium borohydride, also known as sodium tetrahydridoborate, is an inorganic compound with the formula NaBH4. This white solid, usually encountered as a powder, is a versatile reducing agent that finds wide application in chemistry, both in the laboratory and on a technical scale. Large amounts are...
, 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
Passivation
Passivation is the process of making a material "passive", and thus less reactive with surrounding air, water, or other gases or liquids. The goal is to inhibit corrosion, whether for structural or cosmetic reasons. Passivation of metals is usually achieved by the deposition of a layer of oxide...
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 photocatalysisPhotocatalysis
In chemistry, photocatalysis is the acceleration of a photoreaction in the presence of a catalyst. In catalysed photolysis, light is absorbed by an adsorbed substrate. In photogenerated catalysis, the photocatalytic activity depends on the ability of the catalyst to create electron–hole pairs,...
, 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
Artificial photosynthesis
Artificial photosynthesis is a chemical process that replicates the natural process of photosynthesis, a process that converts sunlight, water, and carbon dioxide into carbohydrates and oxygen. The term is commonly used to refer to any scheme for capturing and storing the energy from sunlight in...
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
Nafion
Nafion is a sulfonated tetrafluoroethylene based fluoropolymer-copolymer discovered in the late 1960s by Walther Grot of DuPont. It is the first of a class of synthetic polymers with ionic properties which are called ionomers...
.
High-temperature electrolysis
High-temperature electrolysis
High-temperature electrolysis is a method currently being investigated for the production of hydrogen from water with oxygen as a by-product.-Efficiency:...
(also HTE or steam 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
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
substrates, thus making hydrogen production possible at temperatures just under 1000°C.
The iron oxide cycle
Iron oxide cycle
The iron oxide cycle is a two-step thermochemical cycle proposed for use for hydrogen production.-Process description:The thermochemical two-step water splitting process uses redox systems...
is a series of thermochemical
Thermochemistry
Thermochemistry is the study of the energy and heat associated with chemical reactions and/or physical transformations. A reaction may release or absorb energy, and a phase change may do the same, such as in melting and boiling. Thermochemistry focuses on these energy changes, particularly on the...
processes used to produce hydrogen
Hydrogen production
Hydrogen production is the family of industrial methods for generating hydrogen. Currently the dominant technology for direct production is steam reforming from hydrocarbons. Many other methods are known including electrolysis and thermolysis...
. The iron oxide cycle consists of two chemical reaction
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...
s whose net reactant is water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
and whose net products are hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
and oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
. All other chemicals are recycled. The iron oxide process requires an efficient source of heat.
The sulfur-iodine cycle
Sulfur-iodine cycle
The sulfur–iodine cycle is a three-step thermochemical cycle 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...
(S-I cycle) is a series of thermochemical
Thermochemistry
Thermochemistry is the study of the energy and heat associated with chemical reactions and/or physical transformations. A reaction may release or absorb energy, and a phase change may do the same, such as in melting and boiling. Thermochemistry focuses on these energy changes, particularly on the...
processes used to produce hydrogen
Hydrogen production
Hydrogen production is the family of industrial methods for generating hydrogen. Currently the dominant technology for direct production is steam reforming from hydrocarbons. Many other methods are known including electrolysis and thermolysis...
. The S-I cycle consists of three chemical reaction
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...
s whose net reactant is water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
and whose net products are hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
and oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
. All other chemicals are recycled. The S-I process requires an efficient source of heat.
More than 352 thermochemical
Thermochemistry
Thermochemistry is the study of the energy and heat associated with chemical reactions and/or physical transformations. A reaction may release or absorb energy, and a phase change may do the same, such as in melting and boiling. Thermochemistry focuses on these energy changes, particularly on the...
cycles have been described for water splitting or thermolysis., These cycles promise to produce hydrogen and 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.
Research is concentrated on the following cycles :
| Thermochemical Thermochemistry Thermochemistry is the study of the energy and heat associated with chemical reactions and/or physical transformations. A reaction may release or absorb energy, and a phase change may do the same, such as in melting and boiling. Thermochemistry focuses on these energy changes, particularly on the... cycle | LHV Efficiency | Temperature (°C/F) |
|---|---|---|
| Cerium(IV) oxide-cerium(III) oxide cycle Cerium(IV) oxide-cerium(III) oxide cycle The cerium oxide–cerium oxide cycle or CeO2/Ce2O3 cycle is a two step thermochemical process based on cerium oxide and cerium oxide for hydrogen production... (CeO2/Ce2O3) |
? % | 2000 °C (3,632 °F) |
| Hybrid sulfur cycle Hybrid sulfur cycle The hybrid sulfur cycle is a two-step water-splitting process intended to be used for hydrogen production. Based on sulfur oxidation and reduction, it is classified as a hybrid thermochemical cycle because it uses an electrochemical reaction for one of the two steps... (HyS) |
43 % | 900 °C (1,652 °F) |
| Sulfur iodine cycle (S-I cycle) | 38 % | 900 °C (1,652 °F) |
| Cadmium sulfate cycle | 46 % | 1000 °C (1,832 °F) |
| Barium sulfate cycle | 39 % | 1000 °C (1,832 °F) |
| Manganese sulfate cycle | 35 % | 1100 °C (2,012 °F) |
| Zinc zinc-oxide cycle (Zn/ZnO) | 44 % | 1900 °C (3,452 °F) |
| Hybrid cadmium cycle | 42 % | 1600 °C (2,912 °F) |
| Cadmium carbonate cycle | 43 % | 1600 °C (2,912 °F) |
| Iron oxide cycle Iron oxide cycle The iron oxide cycle is a two-step thermochemical cycle proposed for use for hydrogen production.-Process description:The thermochemical two-step water splitting process uses redox systems... (Fe3O4/FeO) |
42 % | 2200 °C (3,992 °F) |
| Sodium manganese cycle | 49 % | 1560 °C (2,840 °F) |
| Nickel manganese ferrite cycle | 43 % | 1800 °C (3,272 °F) |
| Zinc manganese ferrite cycle | 43 % | 1800 °C (3,272 °F) |
| Copper-chlorine cycle Copper-chlorine cycle The copper–chlorine cycle is a four-step thermochemical cycle. It has a maximum temperature requirement of about 530 degrees Celsius. The Cu–Cl cycle is one of the prominent thermochemical cycles under development within the Generation IV International Forum... (Cu-Cl) |
41 % | 550 °C (1,022 °F) |
Patents
- Vion, , "Improved method of using atmospheric electricity", June 1860.
See also
- Photocatalytic water splittingPhotocatalytic water splittingPhotocatalytic water splitting is the term for the production of hydrogen and oxygen from water by directly utilizing the energy from light. Hydrogen fuel production has gained increasing attention as oil and other nonrenewable fuels become increasingly depleted and expensive...
- Water gas shift reactionWater gas shift reactionThe water-gas shift reaction is a chemical reaction in which carbon monoxide reacts with water vapor to form carbon dioxide and hydrogen:The water-gas shift reaction is an important industrial reaction. It is often used in conjunction with steam reforming of methane or other hydrocarbons, which is...