Electrochemical reduction of carbon dioxide

In 1870, French chemist Ernest Pierre Royer first reported the electrochemical reduction of "gaseous carbonic acid" (carbon dioxide) to formic acid using a zinc cathode. In 1904, German chemists Alfred Coehn and Stefan Jahn studied the process in more detail, testing a range of cathodes and pH ranges, noting a strong preference for zinc amalgam cathodes and non-acidic electrolytes. These conditions quickly became the standard for research in the nascent field. Experiments in the 1940s and 1950s, especially by Pierre Van Rysselberghe, studied the reaction mechanism, notably proving that carbon dioxide (rather than the carbonate or bicarbonate ions) is the electroactive species. == In industry ==

While some chemicals are made industrially from CO2, including urea, salicylic acid, methanol, and certain inorganic and organic carbonates, these are standard chemical, rather than electrochemical, processes. However, since the early 2020s, a number of companies have invested in the development of commercial-scale processes for the electrochemical reduction of carbon dioxide. Twelve, a California-based start-up, aims to reduce carbon dioxide to carbon monoxide, which can in turn be converted to hydrocarbons via the Fischer–Tropsch process. Dioxycle, a French-American start-up, produces ethylene, a widely-used raw material a wide range of chemicals and plastics. In the laboratory, carbon dioxide is sometimes used to prepare carboxylic acids in a process known as carboxylation. An electrochemical CO2 electrolyzer that operates at room temperature at an industrial scale cell size (15,000cm2) was announced by OCOchem in April 2024 as part of an R&D contract issued by the US Army. The CO2 electrolyzer was reported as the largest in the world with a cathode surface area of 15,000cm2, 650% larger than nearest alternative, and achieving a sustained 85% Faradaic efficiency. Elevated temperature solid oxide electrolyzer cells (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 Nm3 CO produced and purity up to 99.999% CO. ==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. 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 catalysts have been evaluated. 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. == Catalysts ==

Catalysts can be grouped by their primary products. Several metal are unfit for CO2RR because they promote to perform hydrogen evolution instead. Electrocatalysts selective for one particular organic compound include tin or bismuth for formate and silver or gold for carbon monoxide. Copper produces multiple reduced products such as methane, ethylene or ethanol, while methanol, propanol and 1-butanol have also been produced in minute quantities. Three common products are carbon monoxide, formate, or higher order carbon products (two or more carbons). Carbon monoxide-producing Carbon monoxide can be produced from CO2RR over various precious metal catalysts. Gold is known to be the most active, however Ag is also highly active. It is known that the stepped facets of both Au and Ag crystals (e.g. 110 and 211) are over an order of magnitude more active than the planar facets (e.g. 111 and 100). Single site catalysts, usually based on Ni in a graphitic lattice have also shown to be highly selective towards carbon monoxide. Mechanistically, catalysts that convert CO2RR to carbon monoxide do not bind strongly to carbon monoxide allowing it to escape from the catalyst. The rate limiting step for CO2RR to carbon monoxide is the first electron transfer step and this is heavily influenced by the electric field. Formate/formic acid-producing Formic acid is produced as a primary product from CO2RR over diverse catalysts. Catalysts that promote Formic Acid production from CO2 operate by strongly binding to both oxygen atoms of CO2, allowing protons to attack the central carbon. After attacking the central carbon, one proton attaching to an oxygen results in the creation of formate. This promotes the production of formate instead of Carbon Monoxide. C>1-producing catalysts Copper electrocatalysts produce multicarbon compounds from CO2. These include C2 products (ethylene, ethanol, acetate, etc.) and even C3 products (propanol, acetone, etc.) These products are more valuable than C1 products, but the current efficiencies are low. == See also ==