Talk: Toward Sustainable Conversion of Carbon Dioxide to Fuels and Chemicals

Talk by Joel W. Ager

Adjunct Professor in the Department of Materials Science & Engineering, 

University of California Berkeley and Staff Scientist in the Materials Sciences Division, 
Lawrence Berkeley National Laboratory

Energy conversion processes underlie the progress of human civilization. Since the industrial revolution, energy conversion via the combustion of fossil fuels has become dominant, causing the continuing rise in Earth’s atmospheric CO2 levels which may be mankind’s most enduring legacy on the planet. Sustainable energy conversion technologies could mitigate or reverse this trend. Electrochemical conversion of CO2 (EC-CO2R) to chemical precursors and/or fuels is a promising approach; however, challenges remain both in terms of controlling the selectivity and in the overall energy efficiency of the process.

Currently employed electrocatalysts for EC-CO2R produce either two-electron products such as CO or formate (Au, Ag) or a mix of C1, C2, and C3 products with little deterministic control over their distribution (Cu). An alternative to the use of single metal electrocatalysts is a sequential catalysis approach in which CO2 is reduced to CO by either Au or Ag, followed by transport of the CO intermediate to Cu, where is can be further reduced. Both simulations and experiments using micropatterned bimetallic structures will be used to show that intermediate transport can occur over 10’s of microns, representing a significant fraction of the boundary layer thickness. Micropatterned catalysts with controlled ratios of the CO-producing metal (Au or Ag) and Cu allow for tuning of the CO activity at the Cu surface, including achieving concentrations above the equilibrium solubility. This approach allows tuning of the C2 product distribution with increasing CO activity favoring oxygenate formation (e.g. ethanol, acetaldehyde) over hydrocarbons such as ethylene.

These insights are used in the design of a solar-driven EC-CO2R device. Use of bimetallic CuAg “nanocoral” cathodes enables selectivity to hydrocarbons and oxygenates over a wide range of pH and cell voltage conditions. A nanostructured IrOx anode has multiday stability and high performance for oxygen evolution in the pH range of interest for CO2 reduction. Use of a CsHCO3 buffered electrolyte increases selectivity to C2+ products such as ethylene and ethanol Solar-driven CO2 reduction is accomplished by coupling the optimized electrolyzer to cell to Si solar cells. 1 sun efficiencies of over 4% for the production of hydrocarbons and oxygenates are achieved. Notably, the overall system also functions at >1% conversion efficiency at illumination intensities down to 0.3 suns. Use of a 4-terminal III-V/Si tandem cell leads to a conversion efficiency to hydrocarbons and oxygenates of over 5%.


man 23 apr 18


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