The electrochemical reduction of carbon dioxide is the conversion of carbon dioxide to more reduced chemical species using electrical energy. It is one possible step in the broad scheme of carbon capture and utilization. The first examples of electrochemical reduction of carbon dioxide are from the 19th century, when carbon dioxide was reduced to carbon monoxide using a zinccathode. Research in this field intensified in the 1980s following the oil embargoes of the 1970s. Electrochemical reduction of carbon dioxide represents a possible means of producing chemicals or fuels, converting carbon dioxide to organic feedstocks such as formic acid, carbon monoxide, ethylene, ethanol and methane. Among the more selective metallic catalysts in this field are tin for formic acid, gold for carbon monoxide and copper for ethylene, ethanol or methane.
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 on earth. Some anaerobic organisms employ enzymes to convert CO2 to carbon monoxide, from which fatty acids can be made. In industry, a few products are made from CO2, including urea, salicylic acid, methanol, and certain inorganic and organic carbonates. In the laboratory, carbon dioxide is sometimes used to prepare carboxylic acids. No electrochemical process involving CO2 has been commercialized.
Electrocatalysis
The electrochemical reduction of carbon dioxide to CO is usually described as: The redox potential for this reaction is similar to that for hydrogen evolution in aqueous electrolytes, thus electrochemical reduction of CO2 is usually competitive with hydrogen evolution reaction. Electrochemical methods have gained significant attention: 1) at ambient pressure and room temperature; 2) in connection with renewable energy sources 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, ethane, etc., and the products are mainly dependent on the selected catalysts and operating potentials. A variety of homogeneous and heterogeneous catalysts 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, slowkinetics, and/or poor catalyst stability. The composition of the electrolyte can be decisive. Gas-diffusion electrodes are beneficial.
