A Ni-Al Thin Film Electrocatalyst for the Reduction of CO2 to Multi-Carbon Oxygenated Organics (2017)

The amount of CO 2 in the atmosphere has increased dramatically since industrialized processes (e.g., manufacturing, agriculture) became commonplace. Current concentrations of this greenhouse gas exceed 400 ppm, far surpassing the pre-Industrial Revolution value of 280 ppm, 1 and it is unlikely that the worldwide average will dip below this massive value. The many environmental, human health, and societal challenges associated with elevated CO 2 levels, coupled with an impending fossil fuel shortage, have inspired a search for electrocatalysts that transform CO 2 into useful products, such as chemical feedstocks or fuels. While several classes of CO 2 -reducing electrocatalysts exist (e.g., molecular catalysts, framework complexes, nanoparticles), practical advantages favor the use of heterogeneous catalyst varieties. Single-metal electrodes have been well-characterized for their reactivity toward aqueous CO 2 , but the majority favor the generation of hydrogen gas from protons in solution as opposed to CO 2 reduction. Of the handful of single-metal electrodes capable of reducing CO 2 , only copper has been shown to generate multiple-carbon products from the single-carbon starting material. 2–4 Even so, copper electrodes have suffered from selectivity and stability challenges, 5 and much effort is now being focused on achieving highly reduced products that address these practical problems. Metal alloys constitute a relatively understudied class of heterogeneous CO 2 reduction catalysts, despite the fact that their abilities to transform CO 2 often contrast greatly with the CO 2 reduction capacity of their component metals. Most alloy catalysts studied generate either CO or HCOOH, and a handful can generate two-carbon products from CO 2 . 6–10 Inspired by a desire to expand the scope of bimetallic alloy CO 2 reduction catalysts, we have shown that nickel-aluminum thin films electrochemically reduce CO 2 to an array of oxygenated organic species containing one to three carbon atoms. The electrochemical transformation is performed in aqueous solution at –1.38 V vs. Ag/AgCl with good stability over time. This species is the first non-copper-containing electrocatalyst capable of generating three-carbon products from CO 2 , thereby addressing many selectivity and stability challenges posed by copper-based electrodes. We will describe the synthesis and materials characterization of nickel-aluminum alloy catalysts, including powder X-ray diffraction, energy-dispersive X-ray spectroscopy, and scanning electron microscopy. The material’s behavior toward CO 2 reduction will be presented as determined using cyclic voltammetric analysis, bulk electrolysis, and electrochemical condition optimization. 1 H-NMR, 13 C-NMR, 2D-NMR, and mass spectrometry experiments are used to confirm the assertion that all multiple-carbon products are derived entirely from CO 2 starting material, and mechanistic insights will be presented. This information will be used to suggest design principles for the further study of bimetallic species as heterogeneous CO 2 reduction catalysts, and our ongoing work in this area will be summarized. References 1. R. Monastersky, Nature , 497 , 13 (2013). 2. Y. Hori, A. Murata, R. Takahashi, and S. Suzuki, J. Chem. Soc. Chem. Commun. , 17–19 (1988). 3. K. P. Kuhl, E. R. Cave, D. N. Abram, and T. F. Jaramillo, Energy Environ. Sci. , 5 , 7050 (2012). 4. D. Ren, N. T. Wong, A. D. Handoko, Y. Huang, and B. S. Yeo, J. Phys. Chem. Lett. , 7 , 20–24 (2016). 5. Y. Hori, in Modern Aspects of Electrochemistry , Modern Aspects of Electrochemistry. M. E. Gamboa-Aldeco, Editor, vol. 42, p. 89–189, Springer New York (2008). 6. S. Ma et al., J. Am. Chem. Soc. , 139 , 47–50 (2017). 7. S. Ishimaru, R. Shiratsuchi, and G. Nogami, J. Electrochem. Soc. , 147 , 1864–1867 (2000). 8. A. S. Varela et al., J. Phys. Chem. C , 117 , 20500–20508 (2013). 9. D. Kim, J. Resasco, Y. Yu, A. M. Asiri, and P. Yang, Nat. Commun. , 5 , 4948 (2014). 10. D. A. Torelli et al., ACS Catal. , 6 , 2100–2104 (2016).