(17b) CO2 Electroreduction Pathway to Multi-Carbon Products

The development of a cost effective process for the electrochemical reduction of CO₂ to fuels and chemicals could enable a shift to a sustainable energy economy.  Coupled to a renewable energy source such as wind or solar, such a process could generate carbon neutral fuels or fine chemicals that are conventionally produced from petroleum.  To control which product is formed, it is important to understand the pathway to multi-carbon products.  We began by studying the reaction on metallic copper.  It is well known that copper is capable of catalyzing the electroreduction of CO₂ into both single carbon and multi-carbon hydrocarbons with high current efficiency.  We employed a custom electrochemical cell coupled to gas chromatography and NMR to achieve accurate current and voltage measurement coupled with sensitive product detection.  The products we observed are in good agreement with past studies which have reported methane, ethylene, CO, formate, ethanol, 1-propanol, allyl alcohol, acetaldehyde, propionaldehyde, acetate, and methanol as products.  In addition to the reported products we also detected several minor products that have not been reported before to our knowledge: ethylene glycol, glycolaldehyde, hydroxyacetone, acetone, and glyoxal.  This expanded knowledge of the products of CO₂ electroreduction on copper and their voltage dependence leads us to consider a possible mechanistic pathway based on the enol tautomers of the multicarbon products being the active species on the electrode surface.  To gain further insight into the mechanism, we also looked at the activity of other metals and the electroreduction of CO, which is believed to be an important intermediate, and compared the results to CO₂ reduction on Cu.