A DEMS approach for the direct detection of CO formed during electrochemical CO2 reduction

Observation of CO formed during electrochemical CO2 reduction using Differential Electrochemical Mass Spectrometry (DEMS) is complicated by the fragmentation of CO2 in the course of the ionization process. Since much more CO2 than CO enters the vacuum of the mass spectrometer, the ion current for mass 28 is dominated by the CO+-fragment of CO2. By reducing the cathode potential of the ion source of the mass spectrometer from-70 V to-27.5 V, fragmentation of CO2 is reduced to a negligible degree. This allows direct observation of electrochemically formed CO by measuring the ionic current for mass 28. We show that this method is superior to matrix calibration in which the ionic current for mass 44 corrected by the CO+/CO2+-intensity ratio is subtracted from the ionic current for mass 28. Using this method, we compare DEMS results for the electrochemical reduction of CO2 at gold electrodes obtained in two different cells, a conventional DEMS cell with the working electrode sputtered onto the membrane in contact with the vacuum and a flow cell where the interface to the vacuum is separated from the working electrode. We show that in the conventional cell at the interface between electrolyte and vacuum, the local CO2 concentration is reduced as the nearby vacuum interferes with the equilibria of reactions involving gases, and the local pH is increased. Therefore, in DEMS cells where the working electrode is positioned in the vicinity of the interface, the onset potential for CO2 reduction and hydrogen evolution are shifted and the observed faradaic efficiency for CO2 reduction are considerably reduced compared to literature values. This can be rectified by using flow cells that allow a spatial separation between vacuum/electrolyte interface and working electrode. We describe how the Dual Thin Layer Cell can be calibrated for detecting CO, thus allowing quantification of evolved amounts of CO from the ionic current for mass 28. (C) 2020 The Authors. Published by Elsevier B.V.