(Keynote) New Classes of Copper-Free Electrocatalysts for CO2 Reduction Based on Transition Metal/Post Transition Metal Alloys and Intermetallic Compounds

Abstract

Binary alloys and intermetallic compounds composed of a first row transition metal and a post transition metal such as aluminum or gallium provide a new source of heterogeneous electrocatalysts for the reduction of CO2. Unlike the related Group 13 and 14 single metal systems (tin and indium which only generate formate and CO, the mixed metal systems form a variety of C1 products, along with carbon-carbon bonded products. The alloy systems resemble the indium and tin systems, in that they show a catalytic dependence on the surface oxides present.We have reported that Ni3Al reduces CO2 to a variety of C3 oxygenates,1-2 while Lewis et. al. has noted that various NixGay alloys generate C2 products from CO2.3 In both cases, the higher order products are observed in low yield. However, these examples are important since they demonstrate that metallic systems that do not contain copper can form carbon-carbon bonds from CO2. Recently we discovered that chromium-gallium bimetallic combinations can produce oxalate in high faradaic yields in aqueous electrolytes.4 Here again an oxide plays a key role in the observed electrocatalysis.The Cr-Ga system, which may best be thought of as an alloy of Cr2O3 and Ga2O3 is particularly intriguing since it produces oxalate at modest potentials (~-1.5 V vs. Ag/AgCl) in aqueous electrolyte. Prior reports on oxalate electrogeneration from CO2 required the use of nonaqueous electrolytes and significantly more negative potentials (>-2.0 V vs. Ag/AgCl). In the current system, we find that CO2 reduction can be carried out at potentials where oxalate formation competes favorably with H2 production yielding faradaic efficiencies in excess of 50%. Our new findings argue for a previously unexplored mechanism for the formation of carbon-carbon bonds that circumvents the generation of a [CO2 –• ] intermediate. Preliminary data hints at a mechanism that goes through a CO intermediate. Interestingly this system is strongly cation dependent, showing reactivity patterns that suggest two distinct active sites are present at the electrode interface.For all of the alloy and intermetallic systems we have investigated, there appears to be strong correlation between the morphology of the mixed metal electrocatalyst and the product distribution. The chemical source of this effect remains under study. It is also observed that the composition of the underlying electrode strongly influences the product composition. For example, while Ni3Al on glassy carbon produces a variety of C3 products, the same catalyst when placed on a p-type CIGS photocathode primarily produces methanol with no indication of carbon-carbon coupling processes.A detailed mechanistic understanding of these systems is likely to offer new opportunities for the electrosynthesis of organic products from CO2 with high faradaic yield and modest overpotential.