Impact of Side Chains in 1-n-Alkylimidazolium Ionomers on Cu-Catalyzed Electrochemical CO2 Reduction

Abstract

This study presents the impact of the side chains in 1-n-alkylimidazolium ionomers with varying side chain lengths (C_nH_2n+1 where n = 1, 4, 10, 16) on Cu-catalyzed electrochemical CO₂ reduction reaction (CO₂RR). Longer side chains suppress the H₂ and CH₄ formation, with the n-hexadecyl ionomer (n = 16) showing the greatest reduction in kinetics by up to 56.5% and 60.0%, respectively. On the other hand, C₂H₄ production demonstrates optimal Faradaic efficiency with the n-decyl ionomer (n = 10), a substantial increase of 59.9% compared to its methyl analog (n = 1). Through a combination of density functional theory calculations and material characterization, it is revealed that the engineering of the side chains effectively modulates the thermodynamic stability of key intermediates, thus influencing the selectivity of both CO₂RR and hydrogen evolution reaction. Moreover, ionomer engineering enables industrially relevant partial current density of -209.5 mA cm^-2 and a Faradaic efficiency of 52.4% for C₂H₄ production at 3.95 V, even with a moderately active Cu catalyst, outperforming previous benchmarks and allowing for further improvement through catalyst engineering. This study underscores the critical role of ionomers in CO₂RR, providing insights into their optimal design for sustainable chemical synthesis.

Keywords: CO2 reduction; Cu catalyst; DFT; binder; ionomer.

Figure 1

a) Chemical structures of 1‐n‐alkylimidazolium‐containing ionomers with different alkyl side‐chain lengths. b) Schematic representation of the glassy carbon electrode coated with the Cu nanoparticles (NPs) and 1‐n‐alkylimidazolium ionomers. c) Schematic illustration of electrochemical CO₂ reduction process on the ionomer‐coated Cu NPs electrode.

Figure 2

Characterization of the electrodes coated with the Cu NPs and as‐synthesized 1‐n‐alkylimidazolium ionomers. a) SEM image of Cu–PSImC10 and corresponding elemental mapping for b) Cu, c) N, and d) C (scale bar = 1 µm). e,f) X‐ray photoelectron spectroscopy of all electrodes in the (e) Cu 2p _{3 /2} and (f) N 1s regions.

Figure 3

Catalytic activity and selectivity of the electrodes coated with Cu NPs and 1‐n‐alkylimidazolium ionomers. a–c) Partial current densities for each major product (H₂, CH₄, and C₂H₄) during electrochemical CO₂ reduction at different applied potentials in a CO₂‐saturated 0.1 M KHCO₃. d) Faradaic efficiency for C₂H₄ measured after CO₂ reduction for 1 h. The data reported are an average of at least four different measurements on freshly prepared samples, and the error bar represents the standard deviation.

Figure 4

Free energy diagram (in eV) for the formation reactions of a) H₂, b) CH₄, and c) C₂H₄, with various alkyl chain lengths of ionomers (PSImC1, PSImC4, PSImC10, and PSImC16) on the Cu(111) surface, computed from DFT calculations. The same calculations without the ionomers (Pure Cu(111)) are also compared with those from the references.^{[ 25} , ^{26 ]} The rate‐determining steps (RDS) and the corresponding intermediate states for each case are chosen from the references.^{[ 25} , ^{26 ]} For the H₂ and CH₄ formations, the reaction coordinate describes a proton‐coupled electron transfer at each step (i.e., n(H⁺ + e⁻) with n = 1). For the C₂H₄ formation, the reaction coordinate follows the RDS identified from the reference.^{[ 27 ]}

Figure 5

Water contact angle measurements on the as‐prepared electrodes coated with the Cu NPs and 1‐n‐alkylimidazolium ionomers. The contact angle increases with the alkyl side chain length.

Figure 6

a) Partial current densities (j) for major products (H₂, CO, and C₂H₄) with the PSImC10 ionomer in a membrane electrode assembly at various cell voltages. b) Faradaic efficiencies (FE) for the major products with the PSImC10 at 3.95 V compared to those with the commercial Sustainion ionomer. c,d) Comparison of PSImC10, Sustainion, and literature data on maximum (c) and (d) $F{E}_{C_2{H}_4}$ versus cell voltage. The results from this study are indicated as PSImC10 and Sustainion. For comparative analysis, literature data were extracted from studies using Cu catalysts of moderate catalytic activity (i.e., commercial Cu nanoparticles or sputtered Cu). Symbols: square (PSImC10), pentagon (Sustainion), inverted triangle (Nafion), and circle (PTFE).