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. Operando investigation of the CO2 reduction mechanism at gas diffusion electrodes is a challenging task. Here proton-transfer-reaction time-of-flight mass spectrometry has been deployed to analyse the intermediates and products at copper electrodes in real time.