Electrochemical CO2 Reduction over Cu-Based Gas Diffusion Electrodes: A Complementary Spectroscopic Study
Authors: Matam Santhosh Kumar , Sharma Preetam , Yu Eileen , Drivas Charalampos , Wilding Martin , Ramanan Nitya , Gianolio D , Isaacs M , Guan S , Davies P R , Catlow C. Richard A 
Published: December 2024

Electrochemical CO2 reduction (eCO2R) over copper gas diffusion electrode (Cu-GDE) has the potential to generate multi carbon products (C2+ such as ethylene and ethanol) with renewable electricity at a commercial scale [1-3]. To unlock the potential, selectivity towards a desired product with acceptable current densities are a must [1-3]. Efforts are being made to understand catalyst structure, activity and selectivity relationships by different techniques, including a few in situ and operando X-ray absorption spectroscopy (XAS) studies [3-7]. However, there is no consensus on the active and selective site/phase composition of copper for ethylene production. On the one hand, the occurrence of only metallic Cu during the reaction is reported [3-7] while on the other hand, some of these studies find no correlation between the metallic Cu and eCO2R products distribution [3,4]. Differently, the occurrence of Cu(OH)2 and Cu2O (along with metallic Cu) during the reaction is also reported [5,6]. The discrepancies could be a result of varied experimental conditions and in situ / operando cell designs [3-7]. The latter could play a key role in obtaining reliable structure-activity data [8]. The observations emphasise the need for targeted and cell specific operando spectroscopic studies to unravel structure, activity and selectivity relationships [3].
In the present study we report a novel operando XAS GDE flow cell with a three-phase system (i.e., gaseous CO2, liquid electrolyte and solid catalyst) to conduct experiments in a transmission mode that probes the Cu-GDE from surface to bulk and provides information on the copper oxidation and coordination states during the reaction. It would, however, not be able to distinguish between the surface and bulk composition [3] and hence complementary surface sensitive quasi in situ X-ray photoelectron spectroscopy (XPS) is employed to probe copper dynamics on the Cu-GDE surface [9]. The data are then complemented by Raman spectroscopy that monitors not only the surface adsorbed species but also the evolution of copper speciation (even very amorphous species) during the reaction [10].
The dynamic behaviour of copper during the reaction is captured by X-ray absorption near edge structure (XANES) spectra. Linear combination analyses (LCA) show that within the first 20 min of the reaction, copper is only in oxidised state (mainly CuO and minority Cu2O), which is corroborated by the quasi in situ Raman spectroscopy and X-ray photoelectron spectroscopy(XPS). The majority of CuO is reduced to Cu2O (44%) and metallic Cu (44%) at 30 min of the reaction. The metallic Cu content increased to 77% at 60 min of the reaction, however remarkably the remainder (23%) of the copper is in the Cu2+ oxidation state. The latter is not detected by quasi in situ XPS that instead shows copper only in < 2+ oxidation state, potentially as metallic Cu. This implies that the bulk composition is different from the surface composition which is further confirmed by the quasi in situ Raman that shows the presence of copper oxides, Cu(OH)2 and CO adsorbed metallic Cu at 60 min. Based on the complementary spectroscopic data, eCO2R data will be correlated with the catalyst structure dynamics and structure-activity relations will be discussed.

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