The development of an energy efficient carbon dioxide (CO₂) electroreduction process could simultaneously curb anthropogenic CO₂ emissions and provide a sustainable pathway for fuel generation. If these electroreactors employed CO₂ feedstocks from major emission sources (e.g., thermal power plants) and excess electricity from intermittent locally available renewable sources (e.g., solar, wind), then chemicals of economic value could be generated in a carbon neutral fashion. To this end, the research community has devoted significant effort to developing catalysts that exhibit energy efficient and selective reduction of CO₂^1,2. Carbon monoxide (CO) electrosynthesis is perhaps the simplest and, to date the most extensively studied, carbon dioxide transformation. Gold (Au) is a particularly attractive catalyst for this reaction due to its reasonable selectivity, > 80%, and activity, > 3mA/cm², at moderate applied potentials (-0.4 V to -0.7 V vs RHE)^3,4. To further improve performance, Au has been extensively studied in many forms including oxide-derived Au⁵, nanoparticles of various sizes⁶, and ultrathin nanowires⁷. However, there has been less focus on role of the substrate material on the performance of the electrocatalysts, an interaction that has been shown to be relevant in several other electrochemical reactions (i.e., oxygen reduction reaction⁸, hydrogen evolution reaction⁹).

Substrates improve electrochemical performance by aiding in the dispersion, stabilization, and utilization of the active electrocatalyst sites, especially for smaller nanoparticles that lack stability and are susceptible to aggregation. Prior work by Ma et al. demonstrated that silver nanoparticles supported on titania produced a two-fold increase in CO partial current density over its carbon substrate counterpart, resulting in comparable results to unsupported silver nanoparticles with only 40% of the metal loading¹⁰. We compare metal oxides (i.e. TiO₂, Ag₂O, CuO) and carbonaceous materials as substrates due to their differing physical and electrical properties.  Most metal oxide substrates appear highly susceptible to the competing hydrogen evolution reaction resulting in low faradaic efficiency for CO and other CO₂ reduction products.  By comparison, carbonaceous supporting materials produced large yields of CO at medium overpotentials but primarily H₂at both very high and very low overpotentials (Figure 1), resulting in a parabolic profile of faradaic efficiency versus working electrode potential.  Supported electrocatalysts were evaluated in both a 3-electrode cell for the purpose of quantitative analytical measurements and in a flow reactor for an engineering assessment of performance and durability.

Acknowledgments

            We gratefully acknowledge the financial support of the Massachusetts Institute of Technology Energy Initiative and the Kuwait – MIT Center for Natural Resources and the Environment. The assistance of Dr. Kyler Carroll, Mr. Jarrod Milshtein, and Mr. John Barton is much appreciated.

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