CO2 + H2O + 2e– → HCO2– + OH– (1)
Additionally, conversion of CO2 to formate is often considered the mechanistic gateway for the generation of other organic products including multicarbon alcohols. However, this process is hindered by an excessive activation energy barrier that typically leads to electrochemical overpotentials on the order of one volt. Even processes that employ solar energy in place of traditional electricity cannot overcome this limitation, since the photochemical process requires ≥ 3eV photons to overcome the reaction barrier, a portion of the solar spectrum that is not readily available at the earth’s surface. The concept is further complicated by the fact that the 2-electron reduction of CO2 to C1 products (formic acid and carbon monoxide) has been known for over 100 years, and during this time little progress has been made in reducing the excessive activation energy needed to carryout this transformation. Add to this the challenge that reduction of aqueous CO2 always competes with formation of H2from the reduction of water lowering the faradaic efficiency of the process.
Our investigation of electrocatalyzed systems has led to the finding that the nature of the electrode interface is a critical parameter when heavy post-transition metal electrodes are employed. It has been known for some time that tin (and to a lesser extent) indium electrodes are intrinsically catalytic for the reduction of CO2 to formate producing modest overpotentials (~500mV) and good faradaic efficiency. There is some debate about the stability of such systems, however. To further understand these systems, we have carried out detailed FTIR spectroelectrochemical investigations of them along with the related bismuth and lead electrode systems. For both the tin and indium systems, our studies points to a meta-stable surface oxide as a key catalytic component. Improved electrocatalytic performance and time stability are obtained when a thick oxide layer is placed on the electrode surface by anodization prior to CO2 electrolysis. Upon the initiation of a CO2 electrolysis, this layer is rapidly reduced to a thin oxide at the and thereafter remains stable and electrocatalytic for an extended time period. This finding stands in contrast to the Pourbaix diagram for these materials, which indicates that the pure metallic state is the stable state at potentials where CO2 reduction is observed to occur. For both of these systems, a surface metal hydroxide is identified as a key species. This moiety interacts with dissolved CO2 to form a surface confined metal bicarbonate, and it is the redox chemistry of this species that leads to formate production.
Interestingly, the bismuth system appears to sustain the catalytic reduction of CO2 directly at the metal surface. Surface oxides, though present appear to act as an inert interface. Lead electrodes are also electrocatalytic for CO2 to formate, and like bismuth the charge transfer event appears to occur directly at the metal surface. However, the surface oxide in this case serves to buffer the interfacial pH. Thus, islands of oxide act as a sort of “proton bottle” providing protons to the electrode surface at a concentration that facilitates the rate of CO2 reduction, but not to such a large extent that H2 formation competes with the processes of interest. This buffer effect is also critical in controlling the generation of base at the electrode interface as indicated in equation (1) The unanticipated kinetic benefit of the lead surface oxide is derived through the accidental similarity of the CO2/bicarbonate and lead oxide pKas. The various aspects of the interfacial oxide interplay with the reduction of CO2, noted here suggests strategies for the design of interfaces with enhanced ability to produce formate from CO2.