Molecular tuning of CO₂-to-ethylene conversion

Abstract

The electrocatalytic carbon dioxide (CO₂) reduction reaction (CO₂RR) to value-added fuels and feedstocks provides a sustainable and carbon-neutral approach to the storage of intermittent renewable electricity. The highly selective generation of economically desirable C₂ products such as ethylene from CO₂RR remains a challenge. Tuning the stabilities of intermediates to favour a desired reaction pathway offers the opportunity to enhance selectivity, and this has recently been explored on copper (Cu) via control over morphology, grain boundaries7, facets, oxidation state and dopants. Unfortunately, the Faradaic efficiency for ethylene is still low in neutral media (60 per cent at a partial current density of 7 mA cm⁻² in the best catalyst reported so far), resulting in a low energy efficiency. Here we present a molecular tuning strategy—the functionalization of the surface of electrocatalysts with organic molecules—that stabilizes intermediates for enhanced CO₂RR to ethylene. Using electrochemical, operando/in situ spectroscopic and computational studies, we investigate the influence of a library of molecules, derived via electro-dimerization of arylpyridiniums, on Cu. We find that the adhered molecules improve the stabilization of an atop-bound CO intermediate, thereby favouring further reduction to ethylene. As a result of this strategy, we report the CO₂RR to ethylene with a Faradaic efficiency of 72 per cent at a partial current density of 230 mA cm⁻² in a liquid-electrolyte flow cell in neutral medium. We report stable ethylene electrosynthesis for 190 hours in a membrane-electrode-assembly-based system that provides a full-cell energy efficiency of 20 per cent. These findings indicate how molecular strategies can complement heterogeneous catalysts by stabilizing intermediates via local molecular tuning.