Effective Strategies for Reducing Carbon Monoxide into Liquid Fuels By Copper Catalysts

Thursday, 17 May 2018: 10:30
Room 617 (Washington State Convention Center)
L. Wang (Stanford University), S. A. Nitopi (Stanford University Department of Chemical Engineering), M. Orazov, C. Morales-Guio (Stanford University), C. Hahn (SLAC National Accelerator Laboratory), and T. F. Jaramillo (Stanford University)
Understanding the surface reactivity of CO, a key intermediate during electrochemical CO2 reduction, is crucial for the development of catalysts that selectively target desired products for this reaction. In this study, a custom-designed electrochemical flow cell is utilized to investigate planar polycrystalline copper as an electrocatalyst for CO reduction under alkaline conditions. We observed a large positive shift in the overpotential for C-C coupled products under CO reduction conditions compared to CO2 reduction by using the same cell. By investigating the effects of CO partial pressure and electrolyte pH, the aforementioned large potential shift was identified as a pH effect, demonstrating that alkaline conditions can be used to increase the energy efficiency of CO and CO2 reduction to C-C coupled products. Common trends in selectivity were uncovered which indicate both the production of oxygenates and the growth of longer carbon chains are favored at lower overpotentials. These selectivity trends are generalized by comparing to current state of the art rough Cu catalysts, that are able to achieve high oxygenate selectivity by operating at the same geometric current density at lower overpotentials. Combined, these findings outline key design principles for CO and CO2 electrolyzers that are able to produce valuable liquid-phase products with high energy efficiency.

To evaluate these strategies, we carried out electrochemical CO reduction under alkaline conditions on a new high surface area Cu-based catalyst to operate at low overpotentials. With the Cu nanomaterials that were developed in our work, CO is reduced into multi-carbon oxygenates with almost 100% Faraday efficiency at a potential of only –0.23 V versus the reversible hydrogen electrode. Therefore, we conclude that the competing hydrogen evolution reaction can be supressed by successfully engineering the catalyst and electrochemical conditions. A maximum Faraday efficiency of 60% is obtained for ethanol at only –0.33V vs. RHE, demonstrating the high selectivity of this catalyst for liquid fuels at low overpotentials. Thus, this work successfully implements the design principles for pursuing liquid products that were learned from the work on polycrystalline Cu, demonstrating that there are energy-efficient sustainable pathways for the sequential conversion of CO2 to liquid fuels.