Higher cation concentrations in the liquid electrolyte lead to higher cation concentrations in the EDL, which in turn favor the stabilization of -[𝑆𝑂 - . ] 678 via better electrostatic interactions and lead to additional energetic costs for the adsorption of sulfonates on Pt(111). This is consistent with the observation that higher cation concentrations deter the adsorption of sulfonate, i.e., anodic shifts in the adsorption peak on the SHE scale (Figure ). Although C + appears in Eq. 7, it is neither a reactant nor a product in the reaction, as C + forms the coordinating environment for -[𝑆𝑂 - . ] 678 . Therefore, the electrode potential should be referenced to the SHE scale even if the cation is proton. To evaluate the validity of this hypothesis, we increased the concentration of Na + from 0 to 100 mM in 2 mM HClO4, and observed that the adsorption peak of sulfonates in Nafion shifted positively at a rate of ~27 mV/dec(Na + ) (Figure ,c), suggesting that higher concentrations of cations cause anodic shifts in the sulfonate adsorption peak. In addition, the adsorption peak potential remains largely the same on the SHE scale when the composition of the cations is varied while keeping the total concentration of cations constant, i.e., 0.1 M HClO4 and 0.003 M NaClO4+0.097 M HClO4 (Figure ), supporting that it is the concentration, rather than the identity, of cation impacts the adsorption adsorption behavior of sulfonates in Nafion. Further, the lack of the adsorption peak shift when the solvent was switched from H2O to D2O suggests that no O-H(D) scission or formation is involved in the sulfonate adsorption on Pt(111) (Figure ). This is consistent with the proposed mechanism (Eq. 7), in which no covalent bond between cations, including protons, and sulfonate is present. The mechanisms proprosed above are generally in line with the potential dependent infrared spectra (Supplementary Note 3).