(Energy Technology Division Supramaniam Srinivasan Young Investigator Award Address) Using Mechanistic Understanding of Electrocatalytic CO2 and CO Reduction to Guide Catalyst Discovery
Authors: Hahn Christopher 
Published: May 2020

Many technical challenges remain for the implementation of CO2 electrolysis as a practical means for CO2 utilization. A key challenge in heterogeneous catalysis has been developing technologies that can selectively convert CO2 or CO into short‐chain multi‐carbon oxygenates due to complex reaction networks.1 I will first discuss how CO reduction can be used as a means to understand how the electrochemical potential, electrolyte pH, and Cu electrode morphology impact product selectivity.2 Proper use of these design principles on high roughness factor Cu electrodes enabled 100% selectivity for CO reduction to multi-carbon oxygenates at low overpotentials.3 Next, I will discuss how engineering the surface structure of Cu electrocatalysts led to the discovery of structure-reactivity relationships.4 Using a combination of electrocatalysis experiments and in situ surface probe microscopy, we demonstrate that undercoordinated sites are selective motifs for oxygenates and C-C coupling. By comparing these results with state-of-the-art Cu electrocatalysts from the literature, we show that different morphologies have similar intrinsic activities for CO2 reduction. Afterwards, I will discuss a tandem catalysis approach for improving upon these normalized CO2 reduction activities, which is enabled by utilizing bimetallic electrodes consisting of Au nanoparticles on polycrystalline Cu (Au/Cu).5 At low overpotentials, the Au/Cu electrocatalyst has a synergistic catalytic activity superior to that of either Cu or Au, indicating that tandem catalysis mechanisms can be used to increase the energy efficiency for alcohol production. By comparing Au/Cu to Cu, I will highlight common potential-driven trends in the selectivity to oxygenated and multi-carbon products. Combined, all of these findings outline key principles for designing electrocatalytic systems that are able to produce valuable chemicals and fuels from CO2 or CO with high energy efficiency. Finally, I will conclude the presentation with our perspective on key research opportunities for the design and implementation of CO2 vapor-feed reactors, a necessary endeavor to achieve the performances required for practical implementation.6,7

1. Nitopi, S. et al. Progress and Perspectives of Electrochemical CO2 Reduction on Copper in Aqueous Electrolyte. Chemical Reviews
119, 7610-7672 (2019).

2. Wang, L. et al. Electrochemical Carbon Monoxide Reduction on Polycrystalline Copper: Effects of Potential, Pressure, and pH on Selectivity toward Multicarbon and Oxygenated Products. ACS Catalysis
8, 7445-7454 (2018).

3. Wang, L. et al. Electrochemically converting carbon monoxide to liquid fuels by directing selectivity with electrode surface area. Nature Catalysis
2, 702-708 (2019).

4. Hahn, C., Hatsukade, T., Kim, Y.-G., Vailionis, A., Baricuatro, J.H., Higgins, D.C., Nitopi, S.A., Soriaga, M.P. & Jaramillo, T.F. Engineering Cu surfaces for the electrocatalytic conversion of CO2: Controlling selectivity toward oxygenates and hydrocarbons. Proceedings of the National Academy of Sciences
114, 5918 (2017).

5. Morales-Guio, C.G. et al. Improved CO2 reduction activity towards C2+ alcohols on a tandem gold on copper electrocatalyst. Nature Catalysis
1, 764-771 (2018).

6. De Luna, P., Hahn, C., Higgins, D., Jaffer, S.A., Jaramillo, T.F. & Sargent, E.H. What would it take for renewably powered electrosynthesis to displace petrochemical processes? Science
364, eaav3506 (2019).

7. Higgins, D., Hahn, C., Xiang, C., Jaramillo, T.F. & Weber, A.Z. Gas-Diffusion Electrodes for Carbon Dioxide Reduction: A New Paradigm. ACS Energy Letters
4, 317-324 (2019).