Electrochemistry at Highly Oriented Pyrolytic Graphite (HOPG): Toward a New Perspective

Electrochemistry of Fe3+/2+ at highly oriented pyrolytic graphite (HOPG) electrodes: kinetics, identification of major electroactive sites and time effects on the response

“…17,18,29 In many of the above-mentioned studies, the reactions of the redox species were so fast (close to reversible) as to be effectively limited by diffusion, and so it was only possible to provide lower limits for k 0 . 2,3,18 Here, we present a study of the Fe 3+/2+ redox couple in aqueous solution, which is known to have much slower ET kinetics, 30 but is still regarded as an outer-sphere process, 29 although one that can be promoted (enhanced) by some adsorbed anions at metals such as platinum. 31,32 Thus, with careful experimental design, this is a very attractive redox system with which to make kinetic comparisons on different electrodes.…”

“…5,18 In fact, ET kinetics at freshly cleaved HOPG has been shown to be at least as fast as on platinum for IrCl 6 2-/3-and Fe(CN) 6 4-/3-, and the standard heterogeneous ET rate constant (k 0 ) for Ru(NH 3 ) 6 3+/2+ is comparable to that for platinum. 18 Further, the step edge density (which can affect the local density of electronic states 2,3,27,28 ) has been found to have little or no impact on the overall ET kinetics of macroscopic HOPG. 15,19 This various evidence suggests that these processes are adiabatic.…”

“…2,3,5,[15][16][17][18] This new finding has come about from a wide range of measurements 2, 3 -macroscopic/microscopic measurements of adsorbed redox species, 17,19 nanoscopic surface electroactivity imaging, 5,[20][21][22][23] and metal nanoparticle nucleation, 24 and for a wide range of redox reactions, including dopamine oxidation, 16,19 adsorbed anthraquinone reduction, 17 (ferrocenylmethyl)trimethylammonium (FcTMA + ) oxidation, 8,25,26 Ru(NH 3 ) 6 3+ reduction, 5,8,18,20,22 IrCl 6 2-reduction 18 and Fe(CN) 6 4-oxidation. 5,18 In fact, ET kinetics at freshly cleaved HOPG has been shown to be at least as fast as on platinum for IrCl 6 2-/3-and Fe(CN) 6 4-/3-, and the standard heterogeneous ET rate constant (k 0 ) for Ru(NH 3 ) 6 3+/2+ is comparable to that for platinum.…”

“…A consideration in HOPG electrochemistry is electrode history effects, 2,3,5,8,22,25,26 particularly the elapsed time after cleavage during which atmospheric contamination can occur. .…”