In recent decades, Cu has attracted considerable attention over other pure metals catalysts for its exceptional performance of the electrocatalytic reduction of CO2 (CO2RR) into valuable hydrocarbons and alcohols [1]. However, the low selectivity and stability of this catalyst remain significant challenges. Consequently, understanding and controlling the dynamics of electronic properties at the solid-liquid interface during CO2RR using operando techniques are essential.

By accessing transition-metal L-edges and light-element K-edges (O, N, C), soft X-ray Absorption Spectroscopy (sXAS) is the ideal tool to track catalytic redox mechanisms while simultaneously capturing local environmental details. At the BACH beamline within the ELETTRA Synchrotron facility (IT), a microfluidic electrochemical cell for operando XAS has been developed [2], enabling a detailed characterisation of oxidation states and electronic structure directly at the catalyst-electrolyte interface. 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.

Our ex situ and in situ sXAS study revealed that Nafion alters the chemical environment of the pristine Cu catalyst, leading to the formation of Cu²⁺ species, likely via partial dissolution induced by the ionomer's acidity, followed by coordination of dissolved copper species with the sulfonic groups of Nafion. These results were corroborated by ex situ Cu K-edge measurements, demonstrating the powerful synergy achieved by combining soft and hard XAS methodologies. Furthermore, to the best of our knowledge, this is the first report highlighting Nafion-induced dissolution effect on Cu-based catalytic materials.

Moreover, Faradaic efficiency (FE) measurements revealed differences in product selectivity when Nafion is added via spin-coating or drop-casting: while both approaches favour HCOOH as the primary product, spin-coating enhances CO formation and facilitates ethylene generation. These findings underscore the dual role of Nafion as both a structural binder and an active modifier of the first-stage catalytic behaviour, demonstrating the importance of in situ XAS for elucidating the catalyst-binder interactions during CO2RR.