Electrochemical reduction of carbon dioxide

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]

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, is the most common protein. Some anaerobic organisms employ enzymes to convert CO₂ to carbon monoxide, from which fatty acids can be made.^[8]

In industry, a few products are made from CO₂, including urea, salicylic acid, methanol, and certain inorganic and organic carbonates.^[9] 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.^[10]

Electrocatalysis

The electrochemical reduction of carbon dioxide to various products is usually described as:


Reaction	Reduction potential E^o (V) at pH = 7 vs SHE ^[11]
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:

at ambient pressure and room temperature;
in connection with renewable energy sources (see also solar fuel)
competitive controllability, modularity and scale-up are relatively simple.^[12]

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^[13] have been evaluated.^[14]^[2]

Many such processes are assumed to operate via the intermediacy of metal carbon dioxide complexes.^[15] Many processes suffer from high overpotential, low current efficiency, low selectivity, slow kinetics, and/or poor catalyst stability.^[16]

The composition of the electrolyte can be decisive.^[17]^[18]^[19] Gas-diffusion electrodes are beneficial.^[20]^[21]^[22]

Catalysts

Catalysts can be grouped by their primary products.^[14]^[23]^[24] Several metal are unfit for CO₂RR because they promote to perform hydrogen evolution instead.^[25] 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.^[26]

Three common products are carbon monoxide, formate, or higher order carbon products (two or more carbons).^[27]

Carbon monoxide-producing

Carbon monoxide can be produced from CO₂RR over various precious metal catalysts.^[28] Steel has proven to be one such catalyst.,^[29] or hydrogen.^[30]

Mechanistically, carbon monoxide arises from the metal bonded to the carbon of CO₂ (see metallacarboxylic acid). Oxygen is lost as water.^[31]

Formate/formic acid-producing

Formic acid is produced as a primary product from CO₂RR over diverse catalysts.^[32]

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.^[31] Indium catalysts promote formate production because the Indium-Oxygen binding energy is stronger than the Indium-Carbon binding energy.^[33] 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.)^[34] These products are more valuable than C1 products, but the current efficiencies are low.^[35]