Water splitting

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

Water splitting is the chemical reaction in which water is broken down into oxygen and hydrogen: [1]

Contents

2 H2O → 2 H2 + O2

Efficient and economical water splitting would be a technological breakthrough that could underpin a hydrogen economy. A version of water splitting occurs in photosynthesis, but hydrogen is not produced. The reverse of water splitting is the basis of the hydrogen fuel cell. Water splitting using solar radiation has not been commercialized.

Electrolysis

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 radiant energy US Patent 28793.png
Atmospheric electricity utilization for the chemical reaction in which water is separated into oxygen and hydrogen. (Image via: Vion, US patent 28793. June 1860.)
Electrolyser front with electrical panel in foreground Electrolyser, front, B.jpg
Electrolyser front with electrical panel in foreground

Electrolysis of water is the decomposition of water (H2O) into oxygen (O2) and hydrogen (H2): [2]

Water electrolysis ship Hydrogen Challenger Hydrogen-challenger hg.jpg
Water electrolysis ship Hydrogen Challenger

Production of hydrogen from water is energy intensive. Usually, the electricity consumed is more valuable than the hydrogen produced, so this method has not been widely used. In contrast with low-temperature electrolysis, high-temperature electrolysis (HTE) of water converts more of the initial heat energy into chemical energy (hydrogen), potentially doubling efficiency to about 50%.[ citation needed ] 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 the process is more efficient.[ citation needed ]

energy efficiency for electrolytic water splitting was 60%–70% in 2020. [3]

High-temperature electrolysis (also HTE or steam electrolysis) is a method for the production of hydrogen from water with oxygen as a by-product. brary.unt.edu/ark:/67531/metadc270782/}}</ref>

Water splitting in photosynthesis

A version of water splitting occurs in photosynthesis but the electrons are shunted, not to protons, but to the electron transport chain in photosystem II. The electrons are used to reduce carbon dioxide, which eventually becomes incorporated into sugars.

Photo-excitation o photosystem I initiates electron transfer to a series of electron acceptors, eventually reducing NADP + to NADPH. The oxidized photosystem I captures electrons from photosystem II through a series of steps involving plastoquinone, cytochromes, and plastocyanin. Oxidized photosystem II oxidizes the oxygen-evolving complex (OEC), which converts water into O2 and protons. [4] [5] Since the active site of the OEC contains manganese, much research has aimed at synthetic Mn compounds as catalysts for water oxidation. [6]

An algae bioreactor for hydrogen production. Algae hydrogen production.jpg
An algae bioreactor for hydrogen production.

In biological hydrogen production, the electrons produced by the photosystem are shunted not to a chemical synthesis apparatus but to hydrogenases, resulting in formation of H2. This biohydrogen is produced in a bioreactor. [7]

Photoelectrochemical water splitting

Using electricity produced by photovoltaic systems potentially offers the cleanest way to produce hydrogen, other than nuclear, wind, geothermal, and hydroelectric. 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. [8] [9] [10] [11]

Catalysis and proton-relay membranes are often the focus on development. [12]

Photocatalytic water splitting

The conversion of solar energy into hydrogen by means of water splitting process might be more efficient if it is assisted by photocatalysts suspended in water rather than a photovoltaic or an electrolytic system, so that the reaction takes place in one step. [13] [14]

Radiolysis

Energetic nuclear radiation can break the chemical bonds of a water molecule. 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. [15]

Thermal decomposition of water

In thermolysis, water molecules split into hydrogen and oxygen. For example, at 2,200 °C (2,470 K; 3,990 °F) about three percent of all H2O 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 3,000 °C (3,270 K; 5,430 °F) more than half of the water molecules are decomposed. At ambient temperatures only one molecule in 100 trillion dissociates by the effect of heat. [16] The high temperatures and material constraints have limited the applications of this approach.

Other research includes thermolysis on defective carbon substrates, thus making hydrogen production possible at temperatures just under 1,000 °C (1,270 K; 1,830 °F). [17]

One side benefit of a nuclear reactor that produces both electricity and hydrogen is that it can shift production between the two. For instance, a nuclear 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. As of 2005, there was sufficient hydrogen demand in the United States that all daily peak generation could be handled by such plants. [18]

The hybrid thermoelectric Copper-chlorine cycle is a cogeneration system using the waste heat from nuclear reactors, specifically the CANDU supercritical water reactor. [19]

Solar-thermal

Concentrating solar power can achieve the high temperatures necessary to split water. 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 (1,070 to 1,470 K; 1,470 to 2,190 °F) 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. [20]

Material constraints due to the required high temperatures 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. [21]

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 by thermolysis. [22] These cycles promise to produce hydrogen and oxygen from water and heat without using electricity. [23] 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: [23]


Thermochemical cycle LHV EfficiencyTemperature (°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)

Related Research Articles

<span class="mw-page-title-main">Hydrogen</span> Chemical element, symbol H and atomic number 1

Hydrogen is a chemical element; it has symbol H and atomic number 1. It is the lightest element and, at standard conditions, is a gas of diatomic molecules with the formula H2, sometimes called dihydrogen, but more commonly called hydrogen gas, molecular hydrogen or simply hydrogen. It is colorless, odorless, tasteless, non-toxic, and highly combustible. Constituting approximately 75% of all normal matter, hydrogen is the most abundant chemical substance in the universe. Stars, including the Sun, primarily consist of hydrogen in a plasma state, while on Earth, hydrogen is found in water, organic compounds, and other molecular forms. The most common isotope of hydrogen consists of one proton, one electron, and no neutrons.

<span class="mw-page-title-main">Electrolysis</span> Technique in chemistry and manufacturing

In chemistry and manufacturing, electrolysis is a technique that uses direct electric current (DC) to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially important as a stage in the separation of elements from naturally occurring sources such as ores using an electrolytic cell. The voltage that is needed for electrolysis to occur is called the decomposition potential. The word "lysis" means to separate or break, so in terms, electrolysis would mean "breakdown via electricity."

<span class="mw-page-title-main">High-temperature electrolysis</span> Technique for producing hydrogen from water

High-temperature electrolysis is a technology for producing hydrogen from water at high temperatures or other products, such as iron or carbon nanomaterials as higher energy lowers needed electricity to split molecule and opens up new, potentially better electrolytes like molten salts or hydroxides.

<span class="mw-page-title-main">Sabatier reaction</span> Methanation process of carbon dioxide with hydrogen

The Sabatier reaction or Sabatier process produces methane and water from a reaction of hydrogen with carbon dioxide at elevated temperatures and pressures in the presence of a nickel catalyst. It was discovered by the French chemists Paul Sabatier and Jean-Baptiste Senderens in 1897. Optionally, ruthenium on alumina makes a more efficient catalyst. It is described by the following exothermic reaction:

Artificial photosynthesis is a chemical process that biomimics the natural process of photosynthesis. The term artificial photosynthesis is used loosely, refer to any scheme for capturing and storing energy from sunlight by producing a fuel, specifically a solar fuel. An advantage of artificial photosynthesis is that the solar energy can be immediately converted and stored. By contrast, using photovoltaic cells, sunlight is converted into electricity and then converted again into chemical energy for storage, with some necessary losses of energy associated with the second conversion. The byproducts of these reactions are environmentally friendly. Artificially photosynthesized fuel would be a carbon-neutral source of energy, which could be used for transportation or homes. The economics of artificial photosynthesis are not competitive.

<span class="mw-page-title-main">Sulfur–iodine cycle</span> Thermochemical process used to produce hydrogen

The sulfur–iodine cycle is a three-step thermochemical cycle used to produce hydrogen.

<span class="mw-page-title-main">Electrolysis of water</span> Electricity-induced chemical reaction

Electrolysis of water is using electricity to split water into oxygen and hydrogen gas by electrolysis. Hydrogen gas released in this way can be used as hydrogen fuel, but must be kept apart from the oxygen as the mixture would be extremely explosive. Separately pressurised into convenient 'tanks' or 'gas bottles', hydrogen can be used for oxyhydrogen welding and other applications, as the hydrogen / oxygen flame can reach approximately 2,800°C.

<span class="mw-page-title-main">In situ resource utilization</span> Astronautical use of materials harvested in outer space

In space exploration, in situ resource utilization (ISRU) is the practice of collection, processing, storing and use of materials found or manufactured on other astronomical objects that replace materials that would otherwise be brought from Earth.

Hydrogen gas is produced by several industrial methods. In 2022 less than 1% of hydrogen production was low-carbon. Fossil fuels are the dominant source of hydrogen, for example by steam reforming of natural gas. Other methods of hydrogen production include biomass gasification and methane pyrolysis. Methane pyrolysis and water electrolysis can use any source of electricity including renewable energy. Underground hydrogen is extracted.

<span class="mw-page-title-main">Biohydrogen</span> Hydrogen that is produced biologically

Biohydrogen is H2 that is produced biologically. Interest is high in this technology because H2 is a clean fuel and can be readily produced from certain kinds of biomass, including biological waste. Furthermore some photosynthetic microorganisms are capable to produce H2 directly from water splitting using light as energy source.

Oxygenevolution is the process of generating molecular oxygen (O2) by a chemical reaction, usually from water. Oxygen evolution from water is effected by oxygenic photosynthesis, electrolysis of water, and thermal decomposition of various oxides. The biological process supports aerobic life. When relatively pure oxygen is required industrially, it is isolated by distilling liquefied air.

<span class="mw-page-title-main">Copper–chlorine cycle</span>

The copper–chlorine cycle is a four-step thermochemical cycle for the production of hydrogen. The Cu–Cl cycle is a hybrid process that employs both thermochemical and electrolysis steps. It has a maximum temperature requirement of about 530 degrees Celsius.

<span class="mw-page-title-main">Solid oxide electrolyzer cell</span> Type of fuel cell

A solid oxide electrolyzer cell (SOEC) is a solid oxide fuel cell that runs in regenerative mode to achieve the electrolysis of water by using a solid oxide, or ceramic, electrolyte to produce hydrogen gas and oxygen. The production of pure hydrogen is compelling because it is a clean fuel that can be stored, making it a potential alternative to batteries, methane, and other energy sources. Electrolysis is currently the most promising method of hydrogen production from water due to high efficiency of conversion and relatively low required energy input when compared to thermochemical and photocatalytic methods.

<span class="mw-page-title-main">Zinc–zinc oxide cycle</span>

For chemical reactions, the zinc–zinc oxide cycle or Zn–ZnO cycle is a two step thermochemical cycle based on zinc and zinc oxide for hydrogen production with a typical efficiency around 40%.

<span class="mw-page-title-main">Cerium(IV) oxide–cerium(III) oxide cycle</span> Chemical reaction

The cerium(IV) oxide–cerium(III) oxide cycle or CeO2/Ce2O3 cycle is a two-step thermochemical process that employs cerium(IV) oxide and cerium(III) oxide for hydrogen production. The cerium-based cycle allows the separation of H2 and O2 in two steps, making high-temperature gas separation redundant.

Photocatalytic water splitting is a process that uses photocatalysis for the dissociation of water (H2O) into hydrogen (H
2) and oxygen (O
2). The inputs are light energy (photons), water, and a catalyst(s). The process is inspired by Photosynthesis, which converts water and carbon dioxide into oxygen and carbohydrates. Water splitting using solar radiation has not been commercialized. Photocatalytic water splitting is done by dispersing photocatalyst particles in water or depositing them on a substrate, unlike Photoelectrochemical cell, which are assembled into a cell with a photoelectrode. Hydrogen fuel production using water and light (photocatalytic water splitting), instead of petroleum, is an important renewable energy strategy.

Thermochemical cycles combine solely heat sources (thermo) with chemical reactions to split water into its hydrogen and oxygen components. The term cycle is used because aside of water, hydrogen and oxygen, the chemical compounds used in these processes are continuously recycled.

A solar fuel is a synthetic chemical fuel produced from solar energy. Solar fuels can be produced through photochemical, photobiological, and electrochemical reactions.

<span class="mw-page-title-main">Proton exchange membrane electrolysis</span> Technology for splitting water molecules

Proton exchange membrane(PEM) electrolysis is the electrolysis of water in a cell equipped with a solid polymer electrolyte (SPE) that is responsible for the conduction of protons, separation of product gases, and electrical insulation of the electrodes. The PEM electrolyzer was introduced to overcome the issues of partial load, low current density, and low pressure operation currently plaguing the alkaline electrolyzer. It involves a proton-exchange membrane.

<span class="mw-page-title-main">Reversible solid oxide cell</span>

A reversible solid oxide cell (rSOC) is a solid-state electrochemical device that is operated alternatively as a solid oxide fuel cell (SOFC) and a solid oxide electrolysis cell (SOEC). Similarly to SOFCs, rSOCs are made of a dense electrolyte sandwiched between two porous electrodes. Their operating temperature ranges from 600°C to 900°C, hence they benefit from enhanced kinetics of the reactions and increased efficiency with respect to low-temperature electrochemical technologies.

References

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