A new view of electrochemistry at highly oriented pyrolytic graphite

Major new insights on electrochemical processes at graphite electrodes are reported, following extensive investigations of two of the most studied redox couples, Fe(CN)64–/3– and Ru(NH3)63+/2+. Experiments have been carried out on five different grades of highly oriented pyrolytic graphite (HOPG) that vary in step-edge height and surface coverage. Significantly, the same electrochemical characteristic is observed on all surfaces, independent of surface quality: initial cyclic voltammetry (CV) is close to reversible on freshly cleaved surfaces (>400 measurements for Fe(CN)64–/3– and >100 for Ru(NH3)63+/2+), in marked contrast to previous studies that have found very slow electron transfer (ET) kinetics, with an interpretation that ET only occurs at step edges. Significantly, high spatial resolution electrochemical imaging with scanning electrochemical cell microscopy, on the highest quality mechanically cleaved HOPG, demonstrates definitively that the pristine basal surface supports fast ET, and that ET is not confined to step edges. However, the history of the HOPG surface strongly influences the electrochemical behavior. Thus, Fe(CN)64–/3– shows markedly diminished ET kinetics with either extended exposure of the HOPG surface to the ambient environment or repeated CV measurements. In situ atomic force microscopy (AFM) reveals that the deterioration in apparent ET kinetics is coupled with the deposition of material on the HOPG electrode, while conducting-AFM highlights that, after cleaving, the local surface conductivity of HOPG deteriorates significantly with time. These observations and new insights are not only important for graphite, but have significant implications for electrochemistry at related carbon materials such as graphene and carbon nanotubes

Electrochemical Properties of Carbon Surfaces

Pretreatment of glassy carbon (GC) electrodes with 2-propanol, acetonitrile, or cyclohexane had a significant effect on electrode kinetics, adsorption, and capacitance. Reagent grade solvents slowed electron transfer rates for dopamine, ascorbic acid, Fe 3+/2+ , and Fe(CN) 6 3-/4-and decreased adsorption of anthraquinone-2,6-disulfonate (AQDS) and methylene blue (MB). However, if activated carbon (AC) was present in the solvent during pretreatment, the result was increased electron transfer rates and adsorption for several commonly studied redox systems. The large surface area of AC acts as a "getter" for solvent impurities and for species desorbed from the GC surface, leading to a carbon electrode surface with higher capacitance, higher adsorption of AQDS and MB, and faster electron-transfer rates for Fe(CN) 6 3-/4-, dopamine, and ascorbic acid. In addition, the treated surfaces were more reproducible, and aged electrodes were reactivated by AC in 2-propanol. The results imply that large, polar organic impurities are present on the polished GC surface which are removed by the combination of an organic solvent and activated carbon. These impurities contain oxygen detectable by XPS and appear to be weakly catalytic toward the Fe 3+/2+ redox system. The goal of understanding the factors which control electron transfer (ET) kinetics at carbon electrodes has remained elusive, despite the wide use of carbon electrodes and the extensive investigations of their electrochemical behavior. 1-5 A prerequisite to determining ET mechanisms at carbon electrodes is the preparation of reproducible and hopefully well-defined surfaces for which surface structure and electrochemical behavior may be correlated. However, the propensity of most carbon surfaces to oxidize and/or adsorb impurities leads to generally variable surface structures and accompanying variability in properties. A wide variety of surface preparations for carbon electrodes has been described, particularly for glassy carbon. [5][6][7][8][9][10][11][12] Depending on the application, surface preparation can be critical to performance, with apparently minor changes in procedure leading to large effects. In addition to practical considerations of reproducibility and stability, surface preparations bear heavily on the larger question of the relationship of surface structure and ET reactivity. To further complicate matters, results from several laboratories including our own have established that carbon surface properties can affect ET kinetics for different redox systems in very different ways. [13][14][15][16][17] For example, Ru(NH 3 ) 6 3+/2+ is fairly insensitive to surface preparation, with the observed heterogeneous ET rate constant (k°) varying by less than a factor of 10 for a wide range of surface modifications. In contrast, k°for Fe 3+/2+ in 0.2 M HClO 4 can vary by factors of 100-1000, due to catalysis by surface carbonyl groups. 14 Pretreatment procedures which dramatically affect ET kinetics for dopamine and ascorbic acid may have little effect on "outer-sphere" systems such as Ru(NH 3 ) 6 3+/2+ , IrCl 6 3-/2-, or Co(en) 3 3+/2+ . Therefore, several redox systems with different ET mechanisms should be considered when the effects of surface preparation techniques are examined. Previous reports from our laboratory have proposed systematic procedures for assessing surface variables which affect ET kinetics for particular redox systems. 5,14,17 Polishing is the most common preparation procedure for carbon electrodes, 4,12 particularly for glassy carbon (GC) and microdisk electrodes made from carbon fibers. During the process of developing chemical modifications for polished GC surfaces, we often noted large effects of organic solvent exposure on ET kinetics. It became apparent that understanding these solven

Slow diffusion reveals the intrinsic electrochemical activity of basal plane highly oriented pyrolytic graphite electrodes

This paper reports a method for distinguishing the electroactivity of different types of sites on heterogeneous electrode surfaces, exemplified through studies of basal plane highly oriented pyrolytic graphite (HOPG) electrodes. By depositing a thin film of Nafion with incorporated redox species (i.e., tris(2-2'-bipyridyl)ruthenium(II), Ru(bpy)(3)(2+), and hexaaminoruthenium(III), [Ru(NH3)(6)](3+) onto HOPG, diffusion is greatly slowed down. On the time scale of cyclic voltammetry, one can then distinguish between different scenarios of electrode activity because sites on the electrode, with different activity, become diffusionally decoupled. In particular, we show that one can discriminate readily between limiting cases in which the basal plane of HOPG is considered to be either (i) completely active or (ii) inert (with only step edges active). Experimental measurements coupled to modeling show unequivocally that the basal plane of HOPG is electrochemically active. The methodology described and the results obtained have important implications for understanding the intrinsic activity of the basal plane and step edges of graphite electrodes and related carbon-based electrode materials