Comment on electrochemical kinetics at ordered graphite electrodes.

A carbon electrodes have been used in electrochemistry for more than a century, there has been renewed recent interest in electron transfer at carbon surfaces due to the development of graphene and carbon nanotube (CNT) materials. This Comment regards reports of heterogeneous electron transfer (ET) rates on highly ordered pyrolytic graphite (HOPG), which is often used as a model for single crystal graphite. The atomically smooth, hexagonal “basal” plane is exposed by cleaving HOPG, and then voltammetry and related techniques are used to investigate ET rates to various redox systems, often ferrocene (Fc), Fe(CN)6 , or Ru(NH3)6 . The resulting heterogeneous electron transfer rate constant (k, cm/s) is used to investigate the factors which affect the reactivity of the electrode, including any differences between graphene, CNTs, basal plane HOPG, and other carbon materials. Since HOPG basal plane is the most ordered and best characterized graphite surface, it is appropriate to compare ET kinetics on the basal plane to more recently developed carbon electrodes. In the early 1990s, we concluded that HOPG basal plane exhibits k values for 18 redox systems which are 1−3 orders of magnitude slower than those on glassy carbon, which has mixed basal and edge plane. For Fe(CN)6 3‐/4‐ in 1 M KCl, the observed k on low-defect basal plane was 10−5 to 10−6 cm/s, while that on glassy carbon with its many exposed graphitic edges was >0.1 cm/s, depending on pretreatment. We attributed the low k, low capacitance, and low electrochemically observed adsorption of anthraquinone 2,6 disulfonate (AQDS) to the low density of electronic states on basal plane originally reported by Gerischer and Yeager. However, more recent publications from several authors have reported a wide range of k values for Fe(CN)6 3‐/4‐ and Fc on basal plane HOPG, CNTs, and graphene. For example, reports regarding kinetics of Fe(CN)6 3‐/4‐ on basal plane HOPG have concluded that “basal plane HOPG is highly active” or “the basal plane was effectively inert”. In order to fully understand the dependence of ET kinetics on the nature of the graphite surface, two discrepancies need to be resolved. First, why do reported k values for supposedly simple outer-sphere redox reactions vary by orders of magnitude on basal plane HOPG? Second, should we expect k on basal plane HOPG for such redox systems to be similar to that on the sides of CNTs or the basal surface of a single-layer graphene sheet? When comparing ET rates for different carbon electrode surfaces, there are at least three significant phenomena which can dramatically affect the observations: redox mechanism, surface density of electronic states, and the presence of edge plane sites on the electrode surface. Regarding mechanism, a recent review described how “electrocatalytic” redox systems such as Fe and dopamine oxidation in water have observed ET rates which are strongly dependent on the presence of specific sites on the carbon surface, such as oxygen-containing functional groups or hydrogen bonding sites. Metal deposition and surface modification by diazonium-derived radicals are both much faster at edge plane defects than on low-defect HOPG basal plane, thus permitting “decoration” of the edges with metals or organic molecules. In contrast, the “outer sphere” redox reactions such as Fc and Ru(NH3)6 3+/2+ do not require specific surface sites but are still affected by the electronic structure of the electrode material. Fc and Fe(CN)6 3‐/4‐ are often used as “simple” outer sphere redox systems but deserve special note. Fc and its derivatives have a high k (>5 cm/s) on Pt electrodes and appear “reversible” at commonly used scan rates. Since determination of such high rates is difficult, kinetic variations due to the carbon surface may be masked by the upper limit of the kinetic measurement technique. Fe(CN)6 3‐/4‐ is notorious for various surface interactions and nonideality, notably degradation with time and exposure to light. Fe(CN)6 3‐/4‐ can be a useful indicator for the nature of the surface, but it is definitely not “simple” or well behaved. Regarding the second point of the electronic structure of the electrode, it has long been recognized that k for outer-sphere ET should depend on the density of electronic states (DOS) on the surface of the electrode and that most metals have a high DOS with no gaps or large variations with potential. The much slower ET for outer-sphere redox systems on silicon surfaces at potentials within the band gap is a prominent example of the effect of low (or zero) DOS on electrode kinetics. This and related phenomena are the basis of the large body of research on semiconductor electrochemistry. Figure 1 shows several examples of the calculated density of electronic states (DOS) for graphitic materials. Single crystal graphite (Figure 1A) has a small overlap of valence and conduction bands at the Fermi level, with a factor of >100× lower DOS than Au. Yeager and Gerischer first noted that the low capacitance of an HOPG basal plane electrode is a consequence of its low DOS near the Fermi level. The DOS of CNTs depends on their diameter, and they occur as both “metallic” and “semiconductor” tubes. Figure 1B shows that the DOS for a CNT can vary significantly relative to the orbital energies of redox systems, with significant effects on the observed electron transfer rates. Most preparation methods for CNTs lead to a mixture of metallic and semiconducting tubes, with a range of DOS distributions. Although the “sidewall” surface of CNTs and the basal surface of graphene are similar to basal HOPG in terms of structure and the lack of functional groups, they differ substantially in electronic structure, notably the