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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