This paper reviews recent progress made in identifying electrocatalysts for carbon dioxide (CO2) reduction to produce low-carbon fuels, including CO, HCOOH/HCOO−, CH2O, CH4, H2C2O4/HC2O4−, C2H4, CH3OH, CH3CH2OH and others. The electrocatalysts are classified into several categories, including metals, metal alloys, metal oxides, metal complexes, polymers/clusters, enzymes and organic molecules. The catalyts' activity, product selectivity, Faradaic efficiency, catalytic stability and reduction mechanisms during CO2 electroreduction have received detailed treatment. In particular, we review the effects of electrode potential, solution–electrolyte type and composition, temperature, pressure, and other conditions on these catalyst properties. The challenges in achieving highly active and stable CO2 reduction electrocatalysts are analyzed, and several research directions for practical applications are proposed, with the aim of mitigating performance degradation, overcoming additional challenges, and facilitating research and development in this area.

A review of catalysts for the electroreduction of carbon dioxide to produce low-carbon fuels

Two major energy-related problems confront the world in the
next 50 years. First, increased worldwide competition for
gradually depleting fossil fuel reserves (derived from past
photosynthesis) will lead to higher costs, both monetarily and politically. Second, atmospheric CO_2 levels are at their highest recorded level since records began. Further increases are predicted to produce large and uncontrollable impacts on the world climate. These projected impacts extend beyond climate to ocean acidification, because the ocean is a major sink for atmospheric CO2.1 Providing a future energy supply that is secure and CO_2-neutral will require switching to nonfossil energy sources such as wind, solar, nuclear, and geothermal energy and developing methods for transforming the energy produced by these new sources into forms that can be stored, transported, and used upon demand.

Frontiers, Opportunities, and Challenges in Biochemical and Chemical Catalysis of CO2 Fixation

Photoreduction of CO2 into sustainable and green solar fuels is generally believed to be an appealing solution to simultaneously overcome both environmental problems and energy crisis. The low selectivity of challenging multi-electron CO2 photoreduction reactions makes it one of the holy grails in heterogeneous photocatalysis. This Review highlights the important roles of cocatalysts in selective photocatalytic CO2 reduction into solar fuels using semiconductor catalysts. A special emphasis in this review is placed on the key role, design considerations and modification strategies of cocatalysts for CO2 photoreduction. Various cocatalysts, such as the biomimetic, metal-based, metal-free, and multifunctional ones, and their selectivity for CO2 photoreduction are summarized and discussed, along with the recent advances in this area. This Review provides useful information for the design of highly selective cocatalysts for photo(electro)reduction and electroreduction of CO2 and complements the existing reviews on various semiconductor photocatalysts.

Cocatalysts for Selective Photoreduction of CO2 into Solar Fuels.

The past several decades have seen a significant rise in atmospheric carbon dioxide levels resulting from the combustion of hydrocarbon fuels. A solar energy based technology to recycle carbon dioxide into readily transportable hydrocarbon fuel (i.e., a solar fuel) would help reduce atmospheric CO2 levels and partly fulfill energy demands within the present hydrocarbon based fuel infrastructure. We review the present status of carbon dioxide conversion techniques, with particular attention to a recently developed photocatalytic process to convert carbon dioxide and water vapor into hydrocarbon fuels using sunlight.

Toward Solar Fuels: Photocatalytic Conversion of Carbon Dioxide to Hydrocarbons

The direct and catalyzed electrochemistry of CO2 partake in the contemporary attempts to reduce this inert molecule to fuels by means of solar energy, either directly, after conversion of light to electricity, or indirectly in that all elements of comprehension derived from electrochemical experiments can be used in the design and interpretation of photochemical experiments. Following reviews of the activity in the field until 2007–2008, the present review reports more recent findings even if their interpretation remains uncertain. It also develops useful notions that allow analyzing and comparing more rigorously the performances of existing catalysts when the necessary data are available. Among the general trends that transpire presently and are likely to be the object of active future work emphasis is put on the favorable role of acid addition in homogeneous catalytic systems and on the crucial chemical role of the electrode material in heterogeneous catalysis.

Catalysis of the electrochemical reduction of carbon dioxide

Global warming concern has dramatically increased interest in using CO2 as a feedstock for preparation of value-added compounds, thereby helping to reduce its atmospheric concentration. Here, we describe a dinuclear copper(I) complex that is oxidized in air by CO2 rather than O2; the product is a tetranuclear copper(II) complex containing two bridging CO2-derived oxalate groups. Treatment of the copper(II) oxalate complex in acetonitrile with a soluble lithium salt results in quantitative precipitation of lithium oxalate. The copper(II) complex can then be nearly quantitatively electrochemically reduced at a relatively accessible potential, regenerating the initial dinuclear copper(I) compound. Preliminary results demonstrate six turnovers (producing 12 equivalents of oxalate) during 7 hours of catalysis at an applied potential of –0.03 volts versus the normal hydrogen electrode.

http://science.sciencemag.org/content/sci/327/5963/313.full.pdf

Electrocatalytic CO2 conversion to oxalate by a copper complex.

https://www.rsc.org/suppdata/cc/b9/b900423h/b900423h.pdf

A molecular cage of nickel(II) and copper(I): a [{Ni(L)2}2(CuI)6] cluster resembling the active site of nickel-containing enzymes.

The hexanuclear metallacrown [Ni6L12] (L is μ–S–CH2–CH2–S–C6H4–Cl) has been demonstrated to functionally resemble the [NiFe] hydrogenases. Protonation of the [Ni6L12] cluster was studied employing 1H NMR spectroscopy by the sequential additions of dichloroacetic acid or p-toluenesulfonic acid monohydrate into solutions of [Ni6L12] in CD2Cl2 and DMF-d7, respectively; protonation takes place on the thioether sulfurs available in the metallacrown. Electrochemical properties of both the parent and protonated [Ni6L12] species have been studied using cyclic voltammetry. Protonated [Ni6L12] shows an interesting electrocatalytic property as it catalyzes the reduction of protons to molecular hydrogen in the presence of protic acids, such as dichloroacetic acid and chloroacetic acid at −1.5 and −1.6 V vs. Ag/AgCl in DMF, respectively. A catalytic cycle has been proposed based on the observations from the NMR spectroscopic and electrochemical studies of the metallacrown. The behavior of this electrocatalyst was further studied by its immobilization on the surface of a pyrolytic graphite electrode; reduction of a dichloroacetic acid solution in acetonitrile on the surface of the modified electrode occurs at 220 mV more positive potential compared to the unmodified electrode.

Reduction of protons assisted by a hexanuclear nickel thiolate metallacrown: protonation and electrocatalytic dihydrogen evolution

Organo Ruthenium–Nickel Dithiolates with Redox-Responsive Nickel Sites

The hexanuclear [Ni6L12] (2) wheel-type cluster adopts an unusual structural motif whereby four NiS4 square-planar and two NiS5 square-pyramidal units are conjoined by edge sharing; the NiS5 units resemble the Ni centre of the inactive state in the [NiFe] hydrogenase.

Raja Angamuthu

Papers

Electrocatalytic CO2 conversion to oxalate by a copper complex.

A molecular cage of nickel(II) and copper(I): a [{Ni(L)2}2(CuI)6] cluster resembling the active site of nickel-containing enzymes.

Reduction of protons assisted by a hexanuclear nickel thiolate metallacrown: protonation and electrocatalytic dihydrogen evolution

Organo Ruthenium–Nickel Dithiolates with Redox-Responsive Nickel Sites

Hexanuclear [Ni6L12] metallacrown framework consisting of NiS4 square-planar and NiS5 square-pyramidal building blocks