Electrochemical CO₂ reduction (CO₂R) can be integrated with renewable energy sources and water can be utilized as a direct proton source which is promising to provide a sustainable net-zero carbon cycle. However, using water as the proton source causes undesired competitive hydrogen evolution reaction (HER), and thus it is crucial to control selectivity for CO₂R. Various metal-based electrocatalysts have been investigated to convert CO₂ to CO, formate, ethylene, ethanol, or other C₂₊ chemicals. Recent efforts to understand the electrocatalytic activity have found that the cations in the electrolyte play an important role in modulating the activity even for a given electrocatalyst. In addition, the influences of the cations vary depending on the types of reactions and catalysts when the cations interact with the intermediate species, and CO₂R is a sensitive reaction to the presence of the cations.   Therefore, the catalyst and electrolyte interface must be carefully investigated. The relationship between the product distribution of CO₂R and its intermediate species can be monitored by operando spectroscopy studies that have attracted attention. We demonstrate that *CO, one of the crucial intermediates, can be observed by Raman or Infrared spectroscopy, and their vibrational wavenumbers depend on the electrolyte condition. Meanwhile, we also investigate selectivity for electrochemical CO₂ conversion to CO production between Ag nanoparticle and Ni-N-C single atom catalyst. The Ni-N-C was found to have low sensitivity to the type of alkali metal cation and universally high selectivity of CO₂R to CO over HER compared to the Ag nanoparticle, which showed the importance of the catalyst-electrolyte interface on the CO₂R activity. This contributes to increased CO₂ conversion efficiency even under low concentrations of available CO₂ gas. Modulation of the catalyst-electrolyte interface can provide new opportunities to promote challenging catalytic reactions.