Electrochemical CO2 reduction (CO2R) is a promising technology that could enable electricity generated from intermittent renewable sources to be stored in the form of carbon-neutral fuels and chemical precursors. Currently, metallic copper (Cu) is the only known electrocatalyst capable of reducing CO2 to hydrocarbons and alcohols. However, polycrystalline Cu produces up to 16 different reaction products at an applied potential of -1 V vs RHE, with hydrogen, methane, and ethene accounting for the majority of the charge passed. This lack of selectivity has motivated considerable interest in discovering ways to modify the Cu surface that enhance the reaction selectivity to a single desired product. Of particular interest are C2 products, such as ethanol and ethene, due to their high market value and fuel potential. Prior research efforts have identified several under coordinated Cu single crystal electrodes that exhibit an enhanced selectivity for C2+ products compared to polycrystalline Cu; however, such electrocatalysts are not scalable and are likely unstable. The selectivity of ethene relative to methane has also been enhanced by nanostructuring the Cu surface by electrochemical cycling and by the electrodeposition of Cu (I) oxide thin-films. However, these electrocatalysts are generally more selective for the formation of hydrogen than hydrocarbons. We note that all reports in the current literature observe a higher selectivity for hydrocarbons than oxygenates over metallic Cu and that there are currently no known methods of enhancing the oxygenate selectivity. Carbon monoxide (CO) reduction has been identified both experimentally and theoretically as the overpotential-determining step in the reduction of CO2 to hydrocarbons and alcohols over polycrystalline Cu. CO is known to cover a substantial portion of the Cu surface at steady state, resulting in the suppression of the hydrogen evolution reaction (HER) by competitive adsorption for active surface sites. Interestingly, the reaction selectivity observed during CO2R over polycrystalline Cu has been reported to change dramatically at potentials negative of -1 V vs RHE, with hydrogen and methane rapidly increasing at the expense of C2+ products. The onset potential of this selectivity shift agrees well with the calculated onset potential of CO2 depletion due to concentration polarization within the hydrodynamic boundary layer at the Cu surface. The local depletion of CO2 results in a lower steady state coverage of adsorbed CO on the Cu surface, reducing the rate of C-C coupling and enhancing the methanation of CO. This analysis suggests that higher CO coverages on Cu would result in an increase in the rate of C-C coupling between CO-derived intermediates, such as formyl, at the expense of C-H bond formation. Based on the aforementioned trends in the current literature, we hypothesized that a phase segregated bimetallic alloy of Cu with a CO-generating metal would enhance the selectivity to oxygenated multi-carbon products at the expense of hydrogen and hydrocarbons. We expect this selectivity shift to occur because the supply of additional CO by spillover will enhance the steady state coverage of CO adsorbed to the Cu surface. While several different metals have been identified as being CO selective, such as Au, Ag, and Zn, only Ag is completely immiscible with Cu and exclusively produces CO at applied potentials where Cu is capable of reducing >95% of the CO that it produces. The reaction selectivity observed during CO2R over the phase segregated CuAg bimetallic electrodes has led to the conclusion that they possess a reaction selectivity unlike either metallic constituent. The individual proficiencies of each metal were utilized synergistically to sequentially reduce CO2, first to CO over Ag and then to hydrocarbons and alcohols over Cu. The supply of additional CO to Cu by spillover from Ag increases the surface coverage of this key reaction intermediate, resulting in a selectivity shift that favors the formation of multi-carbon oxygenates at the expense of hydrogen and hydrocarbons. This effect is so pronounced that these CuAg bimetallic electrodes are currently the only electrocatalyst yet discovered that is more selective for the formation of multi-carbon oxygenates than hydrocarbons. The selectivity of oxygenates relative to hydrocarbons was found to scale with the relative distribution of Cu and Ag facet terminations present at the electrode surface, in agreement with the CO spillover hypothesis. By tuning the surface composition of the CuAg bimetallic electrode, the selectivity to multi-carbon oxygenates relative to hydrocarbons was increased by a factor of ~6 as compared to polycrystalline Cu.
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Dec 20, 2016
S E Jeena + 2
Open Access