Density functional theory calculations explain copper's unique ability to convert CO2 into hydrocarbons, which may open up (photo-)electrochemical routes to fuels.

How copper catalyzes the electroreduction of carbon dioxide into hydrocarbon fuels

Rising atmospheric levels of carbon dioxide and the depletion of fossil fuel reserves raise serious concerns about the ensuing effects on the global climate and future energy supply. Utilizing the abundant solar energy to convert CO2 into fuels such as methane or methanol could address both problems simultaneously as well as provide a convenient means of energy storage. In this Review, current approaches for the heterogeneous photocatalytic reduction of CO2 on TiO2 and other metal oxide, oxynitride, sulfide, and phosphide semiconductors are presented. Research in this field is focused primarily on the development of novel nanostructured photocatalytic materials and on the investigation of the mechanism of the process, from light absorption through charge separation and transport to CO2 reduction pathways. The measures used to quantify the efficiency of the process are also discussed in detail.

Photocatalytic reduction of CO2 on TiO2 and other semiconductors.

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

We report new insights into the electrochemical reduction of CO2 on a metallic copper surface, enabled by the development of an experimental methodology with unprecedented sensitivity for the identification and quantification of CO2 electroreduction products. This involves a custom electrochemical cell designed to maximize product concentrations coupled to gas chromatography and nuclear magnetic resonance for the identification and quantification of gas and liquid products, respectively. We studied copper across a range of potentials and observed a total of 16 different CO2 reduction products, five of which are reported here for the first time, thus providing the most complete view of the reaction chemistry reported to date. Taking into account the chemical identities of the wide range of C1–C3 products generated and the potential-dependence of their turnover frequencies, mechanistic information is deduced. We discuss a scheme for the formation of multicarbon products involving enol-like surface intermediates as a possible pathway, accounting for the observed selectivity for eleven distinct C2+ oxygenated products including aldehydes, ketones, alcohols, and carboxylic acids.

New insights into the electrochemical reduction of carbon dioxide on metallic copper surfaces

To date, copper is the only heterogeneous catalyst that has shown a propensity to produce valuable hydrocarbons and alcohols, such as ethylene and ethanol, from electrochemical CO2 reduction (CO2R). There are variety of factors that impact CO2R activity and selectivity, including the catalyst surface structure, morphology, composition, the choice of electrolyte ions and pH, and the electrochemical cell design. Many of these factors are often intertwined, which can complicate catalyst discovery and design efforts. Here we take a broad and historical view of these different aspects and their complex interplay in CO2R catalysis on Cu, with the purpose of providing new insights, critical evaluations, and guidance to the field with regard to research directions and best practices. First, we describe the various experimental probes and complementary theoretical methods that have been used to discern the mechanisms by which products are formed, and next we present our current understanding of the complex reaction networks for CO2R on Cu. We then analyze two key methods that have been used in attempts to alter the activity and selectivity of Cu: nanostructuring and the formation of bimetallic electrodes. Finally, we offer some perspectives on the future outlook for electrochemical CO2R.

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Progress and Perspectives of Electrochemical CO2 Reduction on Copper in Aqueous Electrolyte

The total analysis of gaseous and soluble products was carried out in the electrochemical reduction of CO2 (current density 5 mA cm−2) on various electrode metals. HCOO− was predominantly produced at Cd, In, Sn, and Pb cathodes. Ag and Au principally gave CO. CH4 was formed in an appreciable amount at Cu cathode.

Production of CO and CH4 in electrochemical reduction of CO2 at metal electrodes in aqueous hydrogencarbonate solution.

The electrochemical reduction of CO2 was studied with galvanostatic (5 mA cm−2) electrolysis at a copper electrode between 0 and 40 °C. The production of CH4 and C2H4 was confirmed by GC with 4 different columns. The faradaic efficiency of CH4 formation was about 65% at 0 °C and dropped with an increase of temperature, whereas that of C2H4 formation rose to ca. 20% at 40 °C.

Production of methane and ethylene in electrochemical reduction of carbon dioxide at copper electrode in aqueous hydrogencarbonate solution

CO2 is electrochemically reduced to CO in 0.5 M aqueous KHCO3 solution at a gold electrode at 18°C, the reaction proceeding with markedly low overvoltage, starting at –0.8 V vs. normal hydrogen electrode (N.H.E.); the faradaic efficiency for CO formation is 91% at –1.10 V vs. N.H.E. with a partial current of 3.7 mA cm–2, and the reaction probably proceeds via adsorbed intermediates.

Katsuhei Kikuchi

Papers

Production of CO and CH4 in electrochemical reduction of CO2 at metal electrodes in aqueous hydrogencarbonate solution.

Production of methane and ethylene in electrochemical reduction of carbon dioxide at copper electrode in aqueous hydrogencarbonate solution

Electrochemical reduction of carbon dioxides to carbon monoxide at a gold electrode in aqueous potassium hydrogen carbonate

Production of Methane and Ethylene in Electrochemical Reduction of Carbon Dioxide at Copper Electrode in Aqueous Hydrogencarbonate Solution.