A Ni-Al Thin Film Electrocatalyst for the Reduction of CO2 to Multi-Carbon Oxygenated Organics
Authors: Paris Aubrey R. , Bocarsly Andrew B. 
Published: February 2020

The amount of CO2 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 CO2 levels, coupled with an impending fossil fuel shortage, have inspired a search for electrocatalysts that transform CO2 into useful products, such as chemical feedstocks or fuels.

While several classes of CO2-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 CO2, but the majority favor the generation of hydrogen gas from protons in solution as opposed to CO2 reduction. Of the handful of single-metal electrodes capable of reducing CO2, 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 CO2 reduction catalysts, despite the fact that their abilities to transform CO2 often contrast greatly with the CO2 reduction capacity of their component metals. Most alloy catalysts studied generate either CO or HCOOH, and a handful can generate two-carbon products from CO2.6–10 Inspired by a desire to expand the scope of bimetallic alloy CO2 reduction catalysts, we have shown that nickel-aluminum thin films electrochemically reduce CO2 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 CO2, 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 CO2 reduction will be presented as determined using cyclic voltammetric analysis, bulk electrolysis, and electrochemical condition optimization. 1H-NMR, 13C-NMR, 2D-NMR, and mass spectrometry experiments are used to confirm the assertion that all multiple-carbon products are derived entirely from CO2 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 CO2 reduction catalysts, and our ongoing work in this area will be summarized.

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