Operando proton-transfer-reaction time-of-flight mass spectrometry of carbon dioxide reduction electrocatalysis

Electrochemical carbon dioxide reduction is a potential pathway for sustainable production of fuels and chemicals. However, the detailed catalytic mechanism in cells using high-current gas diffusion electrodes remains uncertain. Here we use proton-transfer-reaction time-of-flight mass spectrometry (PTR–TOF–MS) to perform operando analysis of intermediates and products generated by electrochemical carbon dioxide reduction in gas-diffusion-electrode-based flow cells with copper-based electrocatalysts. PTR–TOF–MS allows for sensitive detection of C₁–C₄ minor and major intermediates and products, measurement of their ¹³C isotope composition and precise identification of onset potentials. We find that formaldehyde and acetaldehyde are not the major intermediates for formation of methanol and ethanol/ethylene, respectively, and that propionaldehyde reduction is on the major pathway for 1-propanol formation. Interestingly, the discrimination against ¹³C in the reaction products is substantially larger than for biological CO₂ fixation in photosynthesis and Fischer–Tropsch synthesis of hydrocarbons. [Figure not available: see fulltext.].