Electrocatalytic Reduction of Carbon Dioxide: Let the Molecules Do the Work

Abstract

This review summarizes the development of electrochemical CO 2 reduction catalysts in the UNC Energy Frontier Research Center for Solar Fuels. Two strategies for converting CO 2 to CO or formate have been explored. In one, polypyridyl complexes of Ru(II) have been used to reduce CO 2 to CO in acetonitrile and in acetonitrile/water mixtures. In the absence of CO 2 water is reduced to H 2 by these complexes. With added weak acids in acetonitrile with added water and CO 2 , reduction to syngas mixtures of CO and H 2 is observed. A single polypyridyl complex of Ru(II) has been shown to be both a catalyst for water oxidation and CO 2 reduction in an electrochemical cell for CO 2 splitting into CO and O 2 . In parallel, Ir pincer catalysts have been shown to act as selective electrocatalysts for reducing CO 2 to formate in acetonitrile with added water and in pure water without competition from electrocatalytic H 2 production. Details of the catalytic mechanisms of each have also been investigated.The world's energy future is being driven by increased demand which is currently met by increasing use of fossil fuels with their associated environmental impact through the greenhouse gas effect [1]. With technologies that evolve to become cost effective, renewable energy is the appealing energy source of the future with the sun the ultimate energy source. However, large-scale use of solar energy requires large land areas and a vast energy storage capability for nighttime and on-demand utilization. Natural photosynthesis is appealing with energy conversion through stored biomass but, with typical solar efficiencies of B1 %, is impractical as a source for the high energy densities required to power urban centers and industrial complexes. A long-term solution is potentially available based on ''artificial photosynthesis'' with solar fuels the product, water splitting into H 2 and O 2 or reduction of CO 2 to a carbon fuel or carbon fuel precursor [2]. Solar-driven reduction of CO 2 by water occurs in green plants and photosynthetic bacteria and is an inspiration for the development of an artificial photosynthesis-based equivalent. Given the impressive efficiencies reached with current semiconductor technology for photovotaic devices, far higher efficiencies should be reachable in artificial photosynthesis compared to natural photosynthesis with a target of [10 % being a reasonable goal. The products of artificial photosynthesis are so-called solar fuels, highenergy density molecules with solar energy stored in chemical bonds. As noted above, target reactions are water splitting into hydrogen and oxygen, and reduction of CO 2 to CO, formic acid, methanol or other reduced forms of carbon, with hydrocarbons especially attractive because

Progress and Perspective of Electrocatalytic CO2 Reduction for Renewable Carbonaceous Fuels and Chemicals

“…Since the 1970s, some metal–organic complexes have been applied as a class of typical homogeneous electrocatalysts, because their special coordinative structures and active centers can tightly bind with CO 2 molecules . The electrocatalysts based on metal–organic complexes have attracted significant attention for decades due to the remarkable selectivity, but also have some unpopular disadvantages, such as complicate synthesis processes, low reduction activity, and toxic effects . Heterogeneous metal electrocatalysts have been developed later, accompanying with some advantageous characteristics, such as low toxicity, facile synthesis processes and superior electrocatalytic activity .…”

Selective electrochemical reduction of CO2 to CO with a cobalt chlorin complex adsorbed on multi-walled carbon nanotubes in water

“…[1][2][3] There have been extensive studies on electrocatalytic reduction of CO 2 with cobalt and nickel macrocycles. [4][5][6] The selective electrocatalytic reduction of CO 2 to CO has been achieved using nickel macrocycles in water without formation of H 2 .…”

Sustainion Imidazolium‐Functionalized Polymers for Carbon Dioxide Electrolysis

“…Interest has increased in the conversion of CO 2 into fuels and chemicals as a way to mitigate climate change, as a source of renewable feedstocks for fuels and industrial chemicals, and for energy storage . The electrolysis of CO 2 to form CO, formic acid, and other compounds has been observed at industrially relevant rates, and the processes have high selectivity .…”