Role of Surface Oxide Layer during CO 2 Reduction at Copper Electrodes
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Role of Surface Oxide Layer during CO2 Reduction at Copper Electrodes Cheng-Chun Tsai, Joel Bugayong, and Gregory L. Griffin Department of Chemical Engineering, Louisiana State University, Baton Rouge, LA 70803, U.S.A.
ABSTRACT We have compared the rates of CO formation on Cu and Cu oxide surfaces during the electrochemical reduction of CO2 in aqueous media. On metallic Cu surfaces, H2 formation is the main reaction at potentials less cathodic than –1.16 V(NHE). At this potential the formation of CO becomes significant, while CH4 appears at potentials more cathodic than –1.36 V(NHE). On electrodeposited Cu oxide surfaces there is a complex transient response. During reduction at constant potential ( –1.1 V(NHE) ), there is a large, transient cathodic current that corresponds to reduction of the oxide layer. After this initial oxide reduction, the current density stabilizes and the formation rates of H2 and CO show a more slowly varying transient behavior. The H2 formation rate is roughly 3x higher than on freshly cleaned Cu foil, but is largely independent of the thickness of the initial oxide layer. In contrast, the CO formation rate is at least one order of magnitude higher on the (reduced) Cu oxide samples than on Cu foil at the same potential. These results are interpreted as evidence that CO formation is enhanced at low-coordination number Cu sites present on freshly nucleated Cu clusters following oxide reduction. INTRODUCTION The electrochemical reduction of CO2 to produce chemicals and liquid fuels has been identified as a priority research direction and a basic research need by the U.S. Department of Energy [1]. Global consumption of liquid fossil fuels is currently around 80 million barrels (oil equivalent)/day, and is projected to rise to 100 million barrels/day by 2030 [2]. The current value corresponds to a CO2 release rate into the atmosphere of 3.6 Gt C/year [3]. For comparison, the net consumption of CO2 by terrestrial biological sinks (i.e., plant life, including land use changes) is estimated to be 1.1 GtC/year [4]. Thus it is apparent that an alternative, sustainable method of producing hydrocarbon fuels will be necessary if these fuels are to remain a viable long-term option for society. Copper is widely recognized for its unique selectivity for producing hydrocarbon products during electroreduction of CO2 in aqueous media [5, 6]. Low H2 overpotential metals (e.g., Pt, Ni, Fe) primarily form H2; these metals are also considered to bind CO too strongly to form any other products. High overpotential metals (Hg, Cd, Pb, Tl, In, Sn) cannot bind CO2 strongly enough for dissociative reactions, and instead produce HCOOH as the major CO2 reduction product. Medium overpotential metals (Ag, Au, Zn) can dissociate CO2, but they do not bind CO strongly and release it as their primary product (although this may be of interest for the electrochemical production of H2/CO syngas mixtures). Copper alone has the ability to form and retain adsorbed CO for subsequent electrochemical hydrogenation.
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