Electrochemical impedance characteristics and electroreduction of oxygen at tungsten carbide derived micromesoporous carbon electrodes

Abstract

The electrical double layer characteristics and oxygen electroreduction kinetics in 0.5M H2SO4 aqueous solution has been studied at micromesoporous tungsten carbide derived carbon C(WC) electrodes. Carbon powders with various specific surface areas (1280–2116m2g−1) have been prepared from WC at chlorination temperatures 900°C, 1000°C and 1100°C. The porous structure of carbon substrate was characterised using nitrogen sorption, X-ray diffraction, high resolution TEM, electron energy loss spectroscopy, selected area electron diffraction and scanning electron microscopy with energy-dispersive X-ray spectroscopy methods. Cyclic voltammograms at various potential scan rates from 2 to 70mVs−1, and rotating disc electrode data at rotation velocities from 0 to 3000revmin−1, were measured within the region of potentials from +0.4V to −0.6V vs. Hg|Hg2SO4|sat.K2SO4 in H2O (MSE). At E>−0.2V, the electroreduction of oxygen is mainly limited by the charge transfer step, and at −0.6V<E<−0.2V, by the mixed kinetics. The oxygen electroreduction mainly proceeds through the peroxide formation intermediate step on all electrodes studied. Despite of that the electrodes tested were very stable during the electrochemical experiment, indicating that the C(WC) is a suitable catalyst support material for polymer electrolyte membrane fuel cell. The electroreduction rate of oxygen depends strongly on the structure (graphitisation level) of carbide derived carbon used for preparation of an electrode and the oxygen reduction overvoltage decreases in the order C(WC) 1100°C>C(WC) 1000°C>C(WC) 900°C. Very high low-frequency capacitance values, independent of alternative current (ac) frequency at f<0.1Hz, have been established for C(WC) 1100°C, demonstrating that at ac f→0, mainly pseudocapacitive behaviour with adsorption limited step of reaction intermediates has been observed.