Estimation of the electrochemical active site density of a metal-free carbon-based catalyst using phosphomolybdate (PMo12) as an adsorbate

Abstract

A method to estimate the electrochemical active site density (S_D) of carbon (C) and nitrogen-doped carbon (N/C-900) using phosphomolybdate (PMo₁₂) 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 (PMo₁₂) 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 × 10¹⁹ sites g⁻¹) is higher than that of C (∼0.17 × 10¹⁹ sites g⁻¹).