Ionomers based on protic ionic liquid for proton-coupled electrochemical reactions

• A protic ionic liquid, [DBU][H 2 SO 4 ], was incorporated into catalyst layers as a proton-conductive additive to partially replace the perfluorinated ionomers Nafion. • Electrodes with a Nafion:[DBU][H 2 SO 4 ] ratio of 0.5:0.5 showed superior electrochemical performance in both toluene hydrogenation and aqueous hydrogen evolution reaction. • The high ionic conductivity of [DBU][H 2 SO 4 ], combined with Nafion’s mechanical integrity and interfacial robustness, produced a clear synergistic effect. • Thermogravimetric analysis confirmed that [DBU][H 2 SO 4 ] exhibits superior thermal stability compared to Nafion. • This strategy offers a sustainable approach to ionomer design, reducing reliance on perfluorinated compounds while improving overall performance and thermal stability. This study explores the use of a [DBU][H 2 SO 4 ], protic ionic liquid (PIL), as a proton-conductive additive in catalyst layers for proton-coupled electrochemical reactions, aiming to complement or partially replace conventional polymeric ionomers such as Nafion, which has raised increasing concern regarding per- and polyfluoroalkyl substances (PFAS). Electrochemical performance was evaluated using a Pt/C catalyst for two representative electrochemical reactions—the toluene hydrogenation and the hydrogen evolution reaction (HER). Three proton-conducting compositions were compared: Nafion only (1:0), a mixture of Nafion and [DBU][H 2 SO 4 ] (0.5:0.5), and [DBU][H 2 SO 4 ] only (0:1). In both reaction systems, electrodes incorporating the mixed ionomer demonstrated reduced overpotentials, attributed to the higher ionic conductivity of [DBU][H 2 SO 4 ] compared to Nafion. Beyond overpotential reduction, the Nafion–[DBU][H 2 SO 4 ] blend also improved faradaic efficiency and operational stability, outperforming the Nafion-only and [DBU][H 2 SO 4 ]-only configurations. These synergistic improvements arise from the combination of the [DBU][H 2 SO 4 ]’s efficient proton transport with Nafion’s mechanical integrity and interfacial adhesion. Importantly, the partial substitution of fluorinated ionomers with non-fluorinated PILs offers a more sustainable approach to ionomer design. The findings highlight the potential of PILs as co-functional additives that simultaneously enhance electrochemical performance and reduce environmental impact in both organic and aqueous systems.