The electrochemical reduction of carbon dioxide, also known as electrolysis of carbon dioxide, is the conversion of

carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...

() to more reduced chemical species using electrical energy. It is one possible step in the broad scheme of carbon capture and utilization, nevertheless it is deemed to be one of the most promising approaches. Electrochemical reduction of carbon dioxide represents a possible means of producing chemicals or fuels, converting carbon dioxide () to organic feedstocks such as formic acid (HCOOH), carbon monoxide (CO), methane (CH₄), ethylene (C₂H₄) and ethanol (C₂H₅OH). Among the more selective metallic catalysts in this field are

tin Tin is a chemical element with the symbol Sn (from la, stannum) and atomic number 50. Tin is a silvery-coloured metal. Tin is soft enough to be cut with little force and a bar of tin can be bent by hand with little effort. When bent, t ...

for formic acid,

silver Silver is a chemical element with the symbol Ag (from the Latin ', derived from the Proto-Indo-European ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, white, lustrous transition metal, it exhibits the highest electrical ...

for carbon monoxide and

copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...

for methane, ethylene or ethanol. Methanol, propanol and 1-butanol have also been produced via CO₂ electrochemical reduction, albeit in small quantities. The first examples of electrochemical reduction of carbon dioxide are from the 19th century, when carbon dioxide was reduced to

carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...

using a

zinc Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...

cathode A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. A conventional current describes the direction in wh ...

. Research in this field intensified in the 1980s following the

oil embargo An oil embargo is an economic situation wherein entities engage in an embargo to limit the transport of petroleum to or from an area, in order to exact some desired outcome. One commentator states, " oil embargo is not a common commercial practice; ...

es of the 1970s. As of 2021, pilot-scale carbon dioxide electrochemical reduction is being developed by several companies, including Siemens,

Dioxide Materials Dioxide Materials was founded in 2009 in Champaign, Illinois, and is now headquartered in Boca Raton, Florida. Its main business is to develop technology to lower the world's carbon footprint. Dioxide Materials is developing technology to co ...

, Twelve an
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The techno-economic analysis was recently conducted to assess the key technical gaps and commercial potentials of the carbon dioxide electrolysis technology at near ambient conditions.

Chemicals from carbon dioxide

In carbon fixation, plants convert carbon dioxide into sugars, from which many biosynthetic pathways originate. The catalyst responsible for this conversion,

RuBisCO Ribulose-1,5-bisphosphate carboxylase-oxygenase, commonly known by the abbreviations RuBisCo, rubisco, RuBPCase, or RuBPco, is an enzyme () involved in the first major step of carbon fixation, a process by which atmospheric carbon dioxide is con ...

, is the most common protein. Some anaerobic organisms employ enzymes to convert CO₂ to

, from which fatty acids can be made. In industry, a few products are made from CO₂, including

urea Urea, also known as carbamide, is an organic compound with chemical formula . This amide has two amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest amide of carbamic acid. Urea serves an important ...

, salicylic acid, methanol, and certain inorganic and organic carbonates. In the laboratory, carbon dioxide is sometimes used to prepare carboxylic acids in a process known as

carboxylation Carboxylation is a chemical reaction in which a carboxylic acid is produced by treating a substrate with carbon dioxide. The opposite reaction is decarboxylation. In chemistry, the term carbonation is sometimes used synonymously with carboxylatio ...

. No electrochemical CO₂ electrolyzer that operates at room temperature has been commercialized. Elevated temperature

solid oxide electrolyzer cell A solid oxide electrolyzer cell (SOEC) is a solid oxide fuel cell that runs in regenerative mode to achieve the electrolysis of water (and/or carbon dioxide) by using a solid oxide, or ceramic, electrolyte to produce hydrogen gas (and/or carbon ...

s (SOECs) for CO₂ reduction to CO are commercially available. For example, Haldor Topsoe offers SOECs for CO₂ reduction with a reported 6-8 kWh per Nm³Normal Cubic Meter - the quantity of gas that occupies one cubic meter at standard temperature and pressure. CO produced and purity up to 99.999% CO.

Hydrosilane Hydrosilanes are tetravalent silicon compounds containing one or more Si-H bond. The parent hydrosilane is silane (SiH4). Commonly, hydrosilane refers to organosilicon derivatives. Examples include phenylsilane (PhSiH3) and triethoxysilane ((C2 ...

s reduce

(to

methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Ea ...

): Unfortunately such reactions are stoichiometric and exclusively of academic interest. :

Electrocatalysis

The electrochemical reduction of carbon dioxide to various products is usually described as: The redox potentials for these reactions are similar to that for hydrogen evolution in aqueous electrolytes, thus electrochemical reduction of CO₂ is usually competitive with

hydrogen evolution reaction Hydrogen evolution reaction (HER) is a chemical reaction that yields H2. The conversion of protons to H2 requires reducing equivalents and usually a catalyst. In nature, HER is catalyzed by hydrogenase enzymes. Commercial electrolyzers typically em ...

. Electrochemical methods have gained significant attention: 1) at ambient pressure and room temperature; 2) in connection with renewable energy sources (see also solar fuel) 3) competitive controllability, modularity and scale-up are relatively simple. The electrochemical reduction or electrocatalytic conversion of CO₂ can produce value-added chemicals such methane, ethylene, ethanol, etc., and the products are mainly dependent on the selected catalysts and operating potentials (applying reduction voltage). A variety of homogeneous and

heterogeneous catalyst In chemistry, heterogeneous catalysis is catalysis where the phase of catalysts differs from that of the reactants or products. The process contrasts with homogeneous catalysis where the reactants, products and catalyst exist in the same phase. P ...

s have been evaluated. Many such processes are assumed to operate via the intermediacy of metal carbon dioxide complexes. Many processes suffer from high overpotential, low current efficiency, low selectivity, slow kinetics, and/or poor catalyst stability. The composition of the electrolyte can be decisive. Gas-diffusion electrodes are beneficial.

Chemicals from carbon dioxide

Electrocatalysis

See also

Notes

References

Further reading