"Enhancing the Selectivity of Electrochemical CO₂ Reduction to Multi-Carbon Oxygenates"

Atmospheric carbon dioxide (CO₂) is a potential source of renewable carbon for the production of fuels and chemicals. However, the hydrogen required to reduce CO₂ must be derived from water and the necessary energy must be supplied by a renewable source for this process to be sustainable. One approach to this goal is the utilization of electrical energy generated by photovoltaics to drive the electrochemical CO₂ reduction reaction (CO₂RR). Previous research has shown that the overall rate of CO₂RR and the distribution of products formed depend primarily on the electrocatalyst used as the cathode. Copper (Cu) is the only monometallic electrocatalyst capable of reducing CO₂ into potential fuels or multi-carbon chemicals with a total Faradaic efficiency (FE) greater than 1%. Unfortunately, the Faradaic efficiency of CO₂RR over Cu is limited by parasitic losses associated with the relatively facile hydrogen evolution reaction (HER). Thus, there is a contemporary interest in developing methods of suppressing HER and modifying the multi-carbon product selectivity typically observed over Cu. The latter is a daunting challenge because little is known about the mechanism by which CO₂ is reduced to multi-carbon products over Cu and alloying Cu with other metals generally inhibits its CO₂ reduction activity.

In the first half of this presentation we will report the insights gained into the mechanism of CO₂ reduction over Cu obtained by interfacing an electrochemical cell with a mass spectrometer such that volatile species are sampled directly at the electrode-electrolyte interface. Using this approach, the composition of the local reaction environment is quantified over Cu, revealing an abundant quantity of carbonyl-containing species near the cathode surface despite an absence in the bulk electrolyte. Thus, aldehydes are identified as transient products in the reduction of CO₂ to primary alcohols over Cu. Furthermore, evidence that these aldehydes are reactants in the mechanism of carbon chain growth over Cu will also be presented along with a research plan to validate this hypothesis.  

In the second half of this presentation we will report the results of our investigation of CO₂RR over CuAg surface alloys, which exhibit a multi-carbon oxygenate selectivity ~50% higher than pure Cu. The incorporation of relatively large Ag atoms into the Cu surface induces compressive strain in the surrounding Cu surface atoms, which results in an observable shift in the valence band structure of Cu to deeper levels. This electronic structure modification weakens the adsorption energy of H, resulting in a 60-75% reduction in the HER activity of Cu during CO₂RR. Interestingly, the inhibition of HER does not impact the ability of Cu to produce products derived from CO₂, leading to a 10-15% boost in the total Faradaic efficiency for multi-carbon products. Furthermore, the carbonyl-containing product selectivity increases at the expense of ethene due to the suppression of HER, which reduces the rate of C-O bond scission, and the reduced oxophilicity of the compressively strained Cu, which presumably reduces the adsorption energy of these carbonyl-containing products.