Electrochemical CO₂ reduction (CO₂R) is a promising strategy towards developing commodity chemicals as an alternative from fossil fuels towards a net zero carbon economy. Cu electrodes allow for conversion to multi-carbon (C_≥2) products including ethylene, ethanol, and 1-propanol. While numerous strategies and electrocatalyst designs have facilitated high selectivity towards C_≥2 products in neutral and basic aqueous electrolytes, CO₂ underutilization is an inherent problem owing to the deleterious reaction of CO₂ and hydroxide to generate carbonate. Employing an electrolyte with low pH is a strategy in enhancing CO₂ utilization. However, the hydrogen evolution reaction (HER) is more facile than in higher pH electrolyte. Improvement in selectivity towards C_≥2 products via HER suppression can be obtained with organic coatings. We have previously demonstrated at neutral pH the in-situ electrodeposition of pyridinium-based molecular additives allowing enhancement in CO₂R and high suppression of HER on Cu and Ag electrodes. Extending this strategy to pH ≤ 2 electrolyte, we report several novel molecular additive frameworks bearing polyaromatic motifs, and we observe enhanced CO₂R selectivity to C_≥2products. Notably, an increase in the partial current density towards CO₂R is observed. Complete selectivity towards hydrogen is obtained on unmodified polycrystalline Cu foil. Mechanistic investigations suggest that different molecular film structures, even while enhanced CO₂R is observed, have varying effects on the electrode microenvironment regarding proton and CO₂ mass transport. Higher current density CO₂R towards C_≥2 products is demonstrated in a gas diffusion electrode (GDE) assembly. Our work illuminates the ability to tune heterogenous CO₂R electrocatalyst performance via a systematic structure-function study of molecular architectures, leading to improved selectivity, durability, and activity towards the production of value-added chemicals.

Gavin Heim

Graduate Student, California Institute of Technology

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