In the last decades, copper has attracted considerable attention over other pure metal catalysts for its exceptional performance in the electrocatalytic reduction of CO₂ into valuable hydrocarbons and alcohols [1]. However, the selectivity of this reaction remains a key challenge for industrial applications. It is well-established that the selectivity is influenced by the oxidation state of the catalytic material [2], highlighting the importance of understanding and controlling the dynamics of electronic properties at the solid/liquid interface during CO₂ reduction reaction (CO₂RR). In-situ / operando spectroscopic techniques, such as X-ray absorption (XAS), are essential as they can provide important information about the chemical state of the elements of interest, though it is technically challenging in the soft X-ray regime.

To overcome these limitations, a strategy increasingly adopted to study solid/liquid interfaces under ultra-high vacuum conditions involves using electrochemical cells equipped with X-ray transparent Si₃N₄ membranes. These membranes allow the separation of the liquid phase from the vacuum while enabling X-ray spectroscopy. Such cells facilitate the investigation of changes in the oxidation state of catalytic material during electrochemical reaction by measuring XAS spectra at the core level edges, typically using total fluoresce yield (TFY) mode.

This strategy has been successfully implemented at BACH beamline, in collaboration with the technical service group of IOM-CNR [3]. The microfluidic electrochemical cell (ME-cell) features inlet and outlet channels, which allow for the renewal of the electrolyte, and a three-electrode system, comprising an Ag/AgCl leakless as reference electrode (RE), a Pt wire as counter electrode (CE) and a working electrode (WE) made of an Au-coated Si₃N₄ membrane onto which the catalytic material is deposited.

Stability studies of electrodeposited copper nanoparticles (CuNPs) were carried out in alkaline media, where dissolution and re-deposition phenomena were observed. Our results revealed that electrodeposited CuNPs on graphene substrates present higher stability after re-deposition, showing to be a more promising catalyst for CO2RR. Additionally, a significant effect on the chemical state of pristine CuNPs was identified after incorporating the well-known Nafion polymer as a binder. In-situ XAS measurements demonstrated that Nafion induces a persistent copper (II) oxidation state under near-catalytic conditions during CO₂RR, which disappears after the first cycle of activation, suggesting a dynamic transformation of the catalytic surface. Furthermore, in collaboration with the University of Trieste, we observed that the Nafion deposition methodology (e.g., spin coating vs. drop-casting) impacts the product selectivity during the reduction mechanism of CO2.

Our results emphasize the importance of in situ XAS measurements to understand the stability of copper-based catalysts in alkaline media. Moreover our results highlight the critical role of polymer incorporation in tailoring the performance of Cu-based catalysts for CO2 reduction reaction.