Understanding the Impact of N‑Arylpyridinium Ions on the Selectivity of CO2 Reduction at the Cu/Electrolyte Interface

Copper is the only pure metal electrocatalyst capable of converting carbon dioxide to hydrocarbons at significant reaction rates. However, the poor product selectivity of this process on copper remains a critical challenge. Modification of the aqueous electrolyte/copper interface with organic thin films has emerged as a promising means for tuning the selectivity toward valuable C≥2 hydrocarbons. Recently, it was demonstrated that the addition of N-substituted arylpyridinium derivatives to the electrolyte substantially alters the reaction selectivity (Han et al. ACS Cent. Sci. 2017, 3, 853–859). The changes in selectivity were shown to sensitively depend on the chemical structure of the added N-substituted arylpyridinium. For example, 1-(4-tolyl)­pyridinium (T-Pyr) increases the Faradaic efficiency of C≥2 products to ≈80% (compared to ≈25% observed for the unmodified Cu/electrolyte interface), whereas 1-(4-pyridyl)­pyridinium (P-Pyr) blocks the pathways to C≥2 products. It has been demonstrated that the reduction of N-substituted arylpyridinium derivatives leads to the formation of organic thin films on Cu electrodes. However, the mechanisms by which these thin films regulate the reaction selectivity are poorly understood. Herein, using surface-enhanced infrared absorption spectroscopy, we elucidate how films formed from T-Pyr and P-Pyr give rise to distinct interfacial properties that result in the observed differences in catalytic selectivity. We find that the two films alter the reactivity of the surface in two distinct ways: (1) in the presence of T-Pyr, the interfacial pH increases at moderate current densities (