Estimation of the electrochemical active site density of a metal-free carbon-based catalyst using phosphomolybdate (PMo 12 ) as an adsorbate.

A method to estimate the electrochemical active site density ( S D ) of carbon (C) and nitrogen-doped carbon (N/C-900) using phosphomolybdate (PMo 12 ) as a probe molecule is proposed. The complete coverage of the active sites by the probe molecules is established irrespective of the adsorbate concentration (1, 5, or 10 mM), potential cycling (1 or 10 cycles) and cleaning time (2, 5, or 10 min). A conversion factor derived from a smooth and polished glassy carbon disk of known geometrical area is used to estimate the electrochemical active surface area (ECSA) of the carbon catalyst from the S D . The relatively higher S D values estimated from DC voltammetry than from large-amplitude Fourier-transform alternating-current voltammetry (FTacV) is indicative of the contribution of capacitive charge in the former. Adsorbed probe molecules (PMo 12 ) can readily be desorbed from the catalyst surface by cycling the electrode to lower potentials. The active site density of N/C-900 (∼0.36 × 10 19 sites g -1 ) is higher than that of C (∼0.17 × 10 19 sites g -1 ).