Figure 2. Electrochemical double layer probing from the spectral broadening of pyrazine and water core levels.

(a,b) Representative intensity-normalized N 1s and O 1s core-level peaks, respectively, acquired at different applied potentials to the working electrode (WE) in a KOH 0.4 mM aqueous solution (d_EDL=15.2 nm) containing 1.0 M pyrazine (OCP, open circuit potential, APXPS, ambient pressure X-ray photoelectron spectroscopy, LPPy, liquid phase pyrazine, GPW, gas phase water, LPW, liquid phase water, and BG, background) and (c) double-layer capacitance (obtained from electrochemical characterization) as a function of the applied potential. The double layer capacitance trend has been fitted within a range of 400 mV centred on the PZC, by using both Gouy–Chapman (GC) and Gouy–Chapman–Stern (GCS) models⁶,¹⁸; (d) LPPy N 1s and LPW O 1s full-width at half-maximum (FWHM) trends as a function of the applied potential within the EDL region (APXPS, ambient pressure X-ray photoelectron spectroscopy, CV, cyclic voltammetry, and EC, electrochemistry). The error bars were determined via repeated measurements of the FWHM of a given core level, propagated with the experimental spectral resolution (see Methods section).