(Invited) Solar-Driven Electrochemical Conversion of Carbon Dioxide to Hydrocarbons and Oxygenates

Abstract

Sustainable solar to fuel conversion could provide an alternative to mankind’s currently unsustainable use of fossil fuels [1]. Solar fuel generation by photoelectrochemical (PEC) and electrochemical methods is a potentially promising approach to address this fundamental challenge. To date, solar to fuel research efforts have been mostly focused on solar water splitting, which produces hydrogen [2]. In contrast, the conversion of CO 2 to hydrocarbons that could displace currently used fossil fuels is considerably less well developed [3]. Achieving a viable solar-driven EC CO 2 reduction energy conversion efficiency requires minimizing potential losses in all aspects of the device including the cathode, anode, electrolyte, and membrane. Achieving selective products requires management of multi-electron transfer reactions [4]. Strategies to optimize each component (anode, cathode, electrolyte, cell design) of our CO 2 electrolyzer cell to obtain high selectivity and energy conversion efficiency at low overpotential will be described. An overall cell design which has efficient gas to liquid mass transfer of CO 2 is employed [5]. Use of a CsHCO 3 buffered electrolyte increases selectivity to C 2+ products such as ethylene and ethanol [6]. A nanostructured anode is used which shows superior stability and high performance for oxygen evolution in the pH range of interest for CO 2 reduction. Finally, a cathode design has been developed which enables selectivity to hydrocarbons and oxygenates over a wide range of pH and cell voltage conditions. Solar-driven CO 2 reduction is accomplished by coupling the optimized electrolysis device to 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. Chu, S.; Cui, Y.; Liu, N. The Path towards Sustainable Energy. Nat. Mater. 2016 , 16 , 16–22. Ager, J. W.; Shaner, M. R.; Walczak, K. A.; Sharp, I. D.; Ardo, S. Experimental Demonstrations of Spontaneous, Solar-Driven Photoelectrochemical Water Splitting. Energy Environ. Sci . 2015 , 8, 2811–2824. Goeppert, A.; Czaun, M.; Jones, J.-P.; Surya Prakash, G. K.; Olah, G. A. Recycling of Carbon Dioxide to Methanol and Derived Products – Closing the Loop. Chem. Soc. Rev. 2014 , 43 , 7995–8048. Y. Hori, in Mod. Asp. Electrochem. , (Springer New York, New York, NY, 2008), pp. 89–189. Lobaccaro, P.; Singh, M. R.; Clark, E. L.; Kwon, Y.; Bell, A. T.; Ager, J. W. Effects of Temperature and Gas–liquid Mass Transfer on the Operation of Small Electrochemical Cells for the Quantitative Evaluation of CO 2 Reduction Electrocatalysts. Phys. Chem. Chem. Phys. 2016 , 18 , 26777–26785. Singh, M. R.; Kwon, Y.; Lum, Y.; Ager, J. W.; Bell, A. T. Hydrolysis of Electrolyte Cations Enhances the Electrochemical Reduction of CO 2 over Ag and Cu. J. Am. Chem. Soc. 2016 , 138 , 13006–13012.