The purpose of this work was to investigate the electrocatalytic activity of a molecular clathrochelate, containing cobalt as active centre, for the hydrogen evolution reaction (HER) in aqueous acidic media. The electrocatalytic activity of this cobalt complex has been measured using a glassy carbon working electrode, with the complex either dissolved in the electrolyte (homogeneous phase) or electro-grafted at the surface (heterogeneous phase). The complex was electrografted via diazonium derivatives reduction, in an attempt to form a surface monolayer of cobalt clathrochelate, covalently bonded to the glassy carbon substrate. The chemical composition and morphology of the modified electrode have been characterized by electron microscopy, atomic force microscopy and X-ray photoelectron spectroscopy. The HER kinetics on the two electrodes has been analyzed by cyclic voltammetry and electrochemical impedance spectroscopy in two different media: in acetonitrile (by adding increasing amounts of equivalent protons) and in 0.1 M H2SO4 aqueous solution. Tafel slopes and exchange current density values have been determined on both electrodes, in both media, and compared. It was found that the onset of the HER requires a significantly lower overpotential (approximate to 800 mV less) when the complex is electrografted at the surface of the working electrode. The hydrogen production rate, determined by gas chromatography, is reported.