Operando X-ray absorption spectroscopic flow cell for electrochemical CO2 reduction: new insight into the role of copper species

Abstract

We present a novel operando X-ray absorption spectroscopic (XAS) flow cell, consisting of a gas chamber for CO₂ and a liquid chamber for the electrolyte, to monitor electrochemical CO₂ reduction (eCO₂R) over a gas diffusion electrode (GDE). The feasibility of the flow cell is demonstrated by collecting XAS data (during eCO₂R over Cu-GDE) in a transmission mode at the Cu K-edge. The dynamic behaviour of copper during eCO₂R is captured by XAS, which is complemented by quasi in situ Raman and X-ray photoelectron spectroscopy (XPS). The linear combination analyses (LCA) of the X-ray absorption near edge structure (XANES) indicate that copper oxides are the only species present during the first 20 min of eCO₂R, which was corroborated by complementary Raman and XPS. Significantly, the complementary spectroscopic data suggest that the copper composition in the bulk and on the surface Cu-GDE evolve differently at and above 30 min of eCO₂R. LCA indicates that at 60 min, 77% of copper occurs as metallic Cu and the remaining 23% in the Cu(II) oxidation state, which is not evident from the XPS results that show 100% of the copper is in <2+ oxidation state. Thus, Cu(II) is probably in the bulk of Cu-GDE, as is also evident from Raman spectroscopic result. The ethylene formation correlates very well with the occurrence of copper oxides and hydroxide species in Cu-GDE. The results not only demonstrate the applicability and versatility of the operando XAS GDE flow cell, but also illustrate the unique advantages of combining XAS with complementary Raman and XPS that enables the monitoring of the catalyst structural evolution from the bulk to surface and surface-adsorbed species.