# Water splitting

Diagram of the chemical equation of the electrolysis of water, a form of water splitting.

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.[1] In photosynthesis, water splitting donates electrons to power the electron transport chain in photosystem II.

## Electrolysis

Main article: Electrolysis of water
Electrolyser front with electrical panel in foreground

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.

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.

Electrolysis of water ship Hydrogen Challenger

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,[2] and internal combustion engines. (See hydrogen economy.)

### High pressure electrolysis

When the electrolysis is conducted at high pressures, the produced hydrogen gas is compressed at around 120–200 bar (1740–2900 psi).[3] 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%.[4]

### 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 electrolysis (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.

## 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 (PEC) process. The system is also named artificial photosynthesis.[5][6][7][8]

## 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.[9][10]

## Photobiological water splitting

An algae bioreactor for hydrogen production.

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.[11] with a hydrogen production rate of 10-12 ml per liter culture per hour.[12]

## Thermal decomposition of water

Main article: Thermochemical cycle

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. 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.[13] 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 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 2005 gas prices, hydrogen cost \$2.70/kg.[citation needed] 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.[14] However, Generation IV reactors are not expected until 2030 and it is uncertain if they can compete by then in safety and supply with the distributed generation concept.[original research?]

### 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 (fields of sun-tracking mirrors) of a suitable size.[15]

An interesting approach to solar thermal hydrogen production is proposed by H2 Power Systems.[16] 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 zinc metal in Kipp's apparatus.

In the presence of sodium hydroxide, aluminium and its alloys react with water to generate hydrogen gas.[17][18] 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[19] 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.

(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 (or 11.1%) from water. When employed in a fuel cell, that hydrogen can also produce electricity, recovering half of the water previously consumed.[20] The U.S. Department of Energy has outlined its goals for a compact hydrogen storage device[21] and researchers are trying many approaches, such as by using a combination of aluminum and NaBH4, to achieve these goals.[22]

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,[23] 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,[24] 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.[25]

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.[26]

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 (S-I 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.,[27][28] These cycles promise to produce hydrogen oxygen from water and heat without using electricity.[29] 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:

$2 \text{ } H_2 O \text{ } \stackrel {Heat} {\rightleftharpoons} \text{ } 2 \text{ } H_2 + \text{ } O_2$

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.[30][31]

Recently Metal-Organic Framework (MOF)-based materials have been shown to be a highly promising candidate for water splitting with cheap, first row transition metals.;[32][33]

Research is concentrated on the following cycles:[29]

Thermochemical cycle LHV Efficiency Temperature (°C/F)
Cerium(IV) oxide-cerium(III) oxide cycle (CeO2/Ce2O3) ? % 2,000 °C (3,630 °F)
Hybrid sulfur cycle (HyS) 43% 900 °C (1,650 °F)
Sulfur iodine cycle (S-I cycle) 38% 900 °C (1,650 °F)
Cadmium sulfate cycle 46% 1,000 °C (1,830 °F)
Barium sulfate cycle 39% 1,000 °C (1,830 °F)
Manganese sulfate cycle 35% 1,100 °C (2,010 °F)
Zinc zinc-oxide cycle (Zn/ZnO) 44% 1,900 °C (3,450 °F)
Hybrid cadmium cycle 42% 1,600 °C (2,910 °F)
Cadmium carbonate cycle 43% 1,600 °C (2,910 °F)
Iron oxide cycle (Fe3O4/FeO) 42% 2,200 °C (3,990 °F)
Sodium manganese cycle 49% 1,560 °C (2,840 °F)
Nickel manganese ferrite cycle 43% 1,800 °C (3,270 °F)
Zinc manganese ferrite cycle 43% 1,800 °C (3,270 °F)
Copper-chlorine cycle (Cu-Cl) 41% 550 °C (1,022 °F)

## Patents

Atmospheric electricity utilization for the chemical reaction in which water is separated into oxygen and hydrogen. (Image via: Vion, US patent 28793. June 1860.)
• Vion, U.S. Patent 28,793, "Improved method of using atmospheric electricity", June 1860.

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