Revealing Nafion-Induced Copper Modifications by Combining In Situ and Ex Situ X-Ray Absorption Spectroscopy

The electrochemical CO₂ reduction reaction (CO₂RR) on a Cu-based catalyst has garnered significant attention, primarily due to the ability of copper to produce multi-carbon products. Here, in situ soft X-ray absorption spectroscopy (XAS) characterization is employed to investigate the dynamics of a copper-based catalyst during CO₂RR, probing for the first time the impact of the Nafion commonly applied as a proton-conductive binder to ensure mechanical integrity and minimize nanoparticles detachment, on the initial oxidation state of the Cu-catalyst. The results show that Nafion alters the chemical environment of pristine Cu catalyst, leading to the formation of Cu²⁺ species, likely through partial dissolution induced by the acidic nature of the ionomer, followed by coordination of the dissolved copper species with the sulfonic groups of Nafion. These results are corroborated with ex situ Cu K-edge measurements. The Faradaic efficiency measurements reveal product selectivity differences when Nafion is added by spin-coating or drop-casting: while both approaches favor 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 first-stage catalytic behavior, demonstrating the importance of in situ XAS for elucidating the catalyst-binder interactions during CO₂RR.