To rationalize the non-Nernstian shift behavior of the sulfonate adsorption peak (~30 mV per pH unit) (Figure ), we propose that the asorption of sulfonate in Nafion occurs in two steps as the potential scans anodically. When the electrode potential becomes more positive than the potential of zero charge (PZC), a fraction of sulfonate groups in the Nafion phase close to the electrode would be electrostatically drawn out of the Nafion phase to balance the positive charges on the electrode surface (Scheme 1), i.e., forming the electric double layer (EDL), as in: