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.

References

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