(Invited) Elucidation of Activity of Copper and Copper Oxide Nanomaterials for Electrocatalytic and Photoelectrochemical Reduction of Carbon Dioxide

Abstract

Copper and copper oxide based materials are, in principle, promising components (supports, reactive sites, and visible-light absorbing semiconductors) of electrocatalysts and photocathodes for reduction of carbon dioxide. Electrochemical and photoelectrochemical approaches are generally suitable for the low-temperature CO2-conversion to carbon-based simple organic fuels or utility chemicals.Different concepts of utilization, including nanostructuring, doping, admixing, preconditioning, modification or functionalization of various copper and copper oxide based systems for catalytic electroreduction and photoelectrochemical reduction of CO2 will be elucidated, as well as important strategies to enhance the systems’ overall activity and stability will be discussed. Our recent developments in the area of electrochemical and photoelectrochemical CO2R in aqueous electrolytes at copper-based catalysts and copper oxide-based p-type semiconductors (e.g., Cu2O and related mixed oxides) are focused on the preparation of stable efficient systems exhibiting possibly high activity and selectivity toward a single specific product. Obviously the binding strength (chemisorption) of CO2 and the CO-type reaction intermediates would be different at Cu and Cu-oxo species what translates to differences in activity and selectivity. We have also concentrated on intentional modification of copper and copper oxide electrodes by the addition of other metals, metal oxides, coordination compounds or even organic species to alter reaction pathways, stabilize certain reaction intermediates, and to enhance reactivity. For effective heterogeneous photoelectrochemical CO2R at copper oxide semiconductors, the improvement in stability through formation of the p-n junction and/or the interfacial modifications, e.g. by fabrication of robust, not-inhibiting over-layers having also the capability of controlling the adsorption energy and possible activation steps, is crucial. The potential gain coming from the photoelectrochemical approach concerns the ability of driving the CO2R at less negative potentials relative to what can be achieved in conventional electrocatalysis. For example, careful comparison of the data obtained during electrocatalytic and photoelectrochemical reduction of carbon dioxide at hierarchical bilayered films of copper(I) oxide decorated with tungsten(VI) oxide nanowires clearly implies that upon illumination with visible light the electroreduction of CO2 starts at ca. 0.3 V (vs. RHE) whereas under conventional electrochemical conditions, the reaction is driven below -0.2 V. Although methanol have been postulated as dominating product, the mechanisms of operation of both systems are different: while the reduced, mostly Cu0 sites, are catalytically active during electrochemical CO2R, the photoelectrochemical approach utilizes Cu2O semiconductor and long-lived charge carriers (electrons) photogenerated at the interface. The role of the WO3 over-layer can be summarized as follows: the heterojunction formed by p-type (Cu2O) and n-type (WO3) semiconductors improves the system’s overall stability, suppresses competitive hydrogen evolution, and it seems to minimize charge recombination effects. The advantage of driving CO2R at less negative potentials could be fundamentally useful for construction of water-splitting type electrolyzers for conversion of carbon dioxide. In such electrochemical reactor, the oxygen evolution (water oxidation) reaction proceeds at anode, whereas CO2 is supplied in the cathode compartment where it is electrochemically (rarely photoelectrochemically) reduced.