The electrochemical reduction of carbon dioxide, also known as CO2RR, is the conversion of carbon dioxide (CO2) to more reduced chemical species using electrical energy. It represents one potential step in the broad scheme of carbon capture and utilization. [1]
CO2RR can produce diverse compounds including formate (HCOO−), carbon monoxide (CO), methane (CH4), ethylene (C2H4), and ethanol (C2H5OH). [2] The main challenges are the relatively high cost of electricity (vs petroleum) and that CO2 is often contaminated with O2 and must be purified before reduction.
The first examples of CO2RR are from the 19th century, when carbon dioxide was reduced to carbon monoxide using a zinc cathode. Research in this field intensified in the 1980s following the oil embargoes of the 1970s. As of 2021, pilot-scale carbon dioxide electrochemical reduction is being developed by several companies, including Siemens, [3] Dioxide Materials, [4] [5] Twelve and GIGKarasek. 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. [6] [7]
CO2RR electrolyzers have been developed to reduce other forms of CO2 including [bi]carbonates sourced from CO2 captured directly from the air using strong alkalis like KOH [8] or carbamates sourced from flue gas effluents using alkali or amine-based absorbents like MEA or DEA. [9] While the techno-economics of these systems are not yet feasible, they provide a near net carbon neutral pathway to produce commodity chemicals like ethylene at industrially relavant scales. [10]
In carbon fixation, plants convert carbon dioxide into sugars, from which many biosynthetic pathways originate. The catalyst responsible for this conversion, RuBisCO, is the most common protein. Some anaerobic organisms employ enzymes to convert CO2 to carbon monoxide, from which fatty acids can be made. [11]
In industry, a few products are made from CO2, including urea, salicylic acid, methanol, and certain inorganic and organic carbonates. [12] 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 has not yet been commercialized. 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 [note 1] CO produced and purity up to 99.999% CO. [13]
The electrochemical reduction of carbon dioxide to various products is usually described as:
Reaction | Reduction potential Eo (V) at pH = 7 vs SHE [14] |
---|---|
CO2 + 2 H+ + 2 e− → CO + H2O | −0.52 |
CO2 + 2 H+ + 2 e− → HCOOH | −0.61 |
CO2 + 8 H+ + 8 e− → CH4 + 2 H2O | −0.24 |
2 CO2 + 12 H+ + 12 e− → C2H4 + 4 H2O | −0.34 |
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. [2]
Electrochemical methods have gained significant attention:
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 [16] have been evaluated. [17] [2]
Many such processes are assumed to operate via the intermediacy of metal carbon dioxide complexes. [18] Many processes suffer from high overpotential, low current efficiency, low selectivity, slow kinetics, and/or poor catalyst stability. [19]
The composition of the electrolyte can be decisive. [20] [21] [22] Gas-diffusion electrodes are beneficial. [23] [24] [25]
Catalysts can be grouped by their primary products. [17] [26] [27] Several metal are unfit for CO2RR because they promote to perform hydrogen evolution instead. [28] 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. [29]
Three common products are carbon monoxide, formate, or higher order carbon products (two or more carbons). [30]
Carbon monoxide can be produced from CO2RR over various precious metal catalysts. [31] Steel has proven to be one such catalyst., [32] or hydrogen. [33]
Mechanistically, carbon monoxide arises from the metal bonded to the carbon of CO2 (see metallacarboxylic acid). Oxygen is lost as water. [34]
Formic acid is produced as a primary product from CO2RR over diverse catalysts. [35]
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. [34] Indium catalysts promote formate production because the Indium-Oxygen binding energy is stronger than the Indium-Carbon binding energy. [36] This promotes the production of formate instead of Carbon Monoxide.
Copper electrocatalysts produce multicarbon compounds from CO2. These include C2 products (ethylene, ethanol, acetate, etc.) and even C3 products (propanol, acetone, etc.) [37] These products are more valuable than C1 products, but the current efficiencies are low. [38]
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."
Formic acid, systematically named methanoic acid, is the simplest carboxylic acid, and has the chemical formula HCOOH and structure H−C(=O)−O−H. It is an important intermediate in chemical synthesis and occurs naturally, most notably in some ants. Esters, salts and the anion derived from formic acid are called formates. Industrially, formic acid is produced from methanol.
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, referring to any scheme for capturing and then storing energy from sunlight by producing a fuel, specifically a solar fuel. An advantage of artificial photosynthesis would be that the solar energy could 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, but it has never been demonstrated in any practical sense. The economics of artificial photosynthesis are noncompetitive.
The water–gas shift reaction (WGSR) describes the reaction of carbon monoxide and water vapor to form carbon dioxide and hydrogen:
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.
Hydrogen gas is produced by several industrial methods. Nearly all of the world's current supply of hydrogen is created from fossil fuels. Most hydrogen is gray hydrogen made through steam methane reforming. In this process, hydrogen is produced from a chemical reaction between steam and methane, the main component of natural gas. Producing one tonne of hydrogen through this process emits 6.6–9.3 tonnes of carbon dioxide. When carbon capture and storage is used to remove a large fraction of these emissions, the product is known as blue hydrogen.
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An electrocatalyst is a catalyst that participates in electrochemical reactions. Electrocatalysts are a specific form of catalysts that function at electrode surfaces or, most commonly, may be the electrode surface itself. An electrocatalyst can be heterogeneous such as a platinized electrode. Homogeneous electrocatalysts, which are soluble, assist in transferring electrons between the electrode and reactants, and/or facilitate an intermediate chemical transformation described by an overall half reaction. Major challenges in electrocatalysts focus on fuel cells.
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Photoelectrochemical reduction of carbon dioxide, also known as photoelectrolysis of carbon dioxide, is a chemical process whereby carbon dioxide is reduced to carbon monoxide or hydrocarbons by the energy of incident light. This process requires catalysts, most of which are semiconducting materials. The feasibility of this chemical reaction was first theorised by Giacomo Luigi Ciamician, an Italian photochemist. Already in 1912 he stated that "[b]y using suitable catalyzers, it should be possible to transform the mixture of water and carbon dioxide into oxygen and methane, or to cause other endo-energetic processes."
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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 convert carbon dioxide, water and renewable energy into carbon-neutral gasoline (petrol) or jet fuel. Applications include CO2 recycling, sustainable fuels production and reducing curtailment of renewable energy(i.e. renewable energy that could not be used by the grid).
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Elod Lajos Gyenge is a professor of Chemical and Biological Engineering at the faculty of Applied Science in University of British Columbia in Vancouver, BC, Canada. He is also an associate member of the Clean Energy Research Center of UBC Vancouver campus. Gyenge has been nominated for several teaching and research awards including Japanese Society for Promotion of Science (JSPS) Fellowship at Osaka University and the recipient of the distignshuied Elisabeth and Leslie Gould Endowed Professorship at UBC from 2007 to 2014. His research has been toward development of electrochemical systems such as fuel cells, batteries and electrosynthesis systems. He is also an appointed professor in the engineering school of Osaka University in Japan.
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