Nanoscale electrochemistry of sp2 carbon materials: from graphite and graphene to carbon nanotubes

Carbon materials have a long history of use as electrodes in electrochemistry, from (bio)electroanalysis to applications in energy technologies, such as batteries and fuel cells. With the advent of new forms of nanocarbon, particularly, carbon nanotubes and graphene, carbon electrode materials have taken on even greater significance for electrochemical studies, both in their own right and as components and supports in an array of functional composites. With the increasing prominence of carbon nanomaterials in electrochemistry comes a need to critically evaluate the experimental framework from which a microscopic understanding of electrochemical processes is best developed. This Account advocates the use of emerging electrochemical imaging techniques and confined electrochemical cell formats that have considerable potential to reveal major new perspectives on the intrinsic electrochemical activity of carbon materials, with unprecedented detail and spatial resolution. These techniques allow particular features on a surface to be targeted and models of structure–activity to be developed and tested on a wide range of length scales and time scales. When high resolution electrochemical imaging data are combined with information from other microscopy and spectroscopy techniques applied to the same area of an electrode surface, in a correlative-electrochemical microscopy approach, highly resolved and unambiguous pictures of electrode activity are revealed that provide new views of the electrochemical properties of carbon materials. With a focus on major sp2 carbon materials, graphite, graphene, and single walled carbon nanotubes (SWNTs), this Account summarizes recent advances that have changed understanding of interfacial electrochemistry at carbon electrodes including: (i) Unequivocal evidence for the high activity of the basal surface of highly oriented pyrolytic graphite (HOPG), which is at least as active as noble metal electrodes (e.g., platinum) for outer-sphere redox processes. (ii) Demonstration of the high activity of basal plane HOPG toward other reactions, with no requirement for catalysis by step edges or defects, as exemplified by studies of proton-coupled electron transfer, redox transformations of adsorbed molecules, surface functionalization via diazonium electrochemistry, and metal electrodeposition. (iii) Rationalization of the complex interplay of different factors that determine electrochemistry at graphene, including the source (mechanical exfoliation from graphite vs chemical vapor deposition), number of graphene layers, edges, electronic structure, redox couple, and electrode history effects. (iv) New methodologies that allow nanoscale electrochemistry of 1D materials (SWNTs) to be related to their electronic characteristics (metallic vs semiconductor SWNTs), size, and quality, with high resolution imaging revealing the high activity of SWNT sidewalls and the importance of defects for some electrocatalytic reactions (e.g., the oxygen reduction reaction). The experimental approaches highlighted for carbon electrodes are generally applicable to other electrode materials and set a new framework and course for the study of electrochemical and interfacial processes

Electrochemistry at highly oriented pyrolytic graphite (HOPG) : lower limit for the kinetics of outer-sphere redox processes and general implications for electron transfer models

The electron transfer (ET) kinetics of three redox couples in aqueous solution, IrCl62−/3−, Ru(NH3)63+/2+ and Fe(CN)64−/3−, on different grades of highly oriented pyrolytic graphite (HOPG) have been investigated in a droplet-cell setup. This simple configuration allows measurements to be made on a very short time scale after cleavage of HOPG, so as to minimise possible effects from (atmospheric) contamination, and with minimal, if any, change to the HOPG surface. However, the droplet-cell geometry differs from more conventional electrochemical setups and is more prone to ohmic drop effects. The magnitude of ohmic drop is elucidated by modelling the electric field in a typical droplet configuration. These simulations enable ohmic effects to be minimised practically by optimising the positions of the counter and reference electrodes in the droplet, and by using a concentration ratio of electrolyte to redox species that is higher than used conventionally. It is shown that the ET kinetics for all of the redox species studied herein is fast on all grades of HOPG and lower limits for ET rate constants are deduced. For IrCl62−/3− and Fe(CN)64−/3−, ET on HOPG is at least as fast as on Pt electrodes, and for Ru(NH3)63+/2+ ET kinetics on HOPG is comparable to Pt electrodes. Given the considerable difference in the density of electronic states (DOS) between graphite and metal electrodes, the results tend to suggest that the DOS of the electrode does not play an important role in the ET kinetics of these outer-sphere redox couples over the range of values encompassing HOPG and metals. This can be rationalised because the DOS of all of these different electrode materials is orders of magnitude larger than those of the redox species in solution, so that with strong electronic coupling between the redox couple and electrode (adiabatic electron transfer) the electronic structure of the electrode becomes a relatively unimportant factor in the ET kinetics

Scanning electrochemical cell microscopy : a versatile technique for nanoscale electrochemistry and functional imaging

Scanning electrochemical cell microscopy (SECCM) is a new pipette-based imaging technique purposely designed to allow simultaneous electrochemical, conductance, and topographical visualization of surfaces and interfaces. SECCM uses a tiny meniscus or droplet, confined between the probe and the surface, for high-resolution functional imaging and nanoscale electrochemical measurements. Here we introduce this technique and provide an overview of its principles, instrumentation, and theory. We discuss the power of SECCM in resolving complex structure-activity problems and provide considerable new information on electrode processes by referring to key example systems, including graphene, graphite, carbon nanotubes, nanoparticles, and conducting diamond. The many longstanding questions that SECCM has been able to answer during its short existence demonstrate its potential to become a major technique in electrochemistry and interfacial science

Electrochemistry at carbon nanotube forests : sidewalls and closed ends allow fast electron transfer

The electrochemical properties of the closed ends and sidewalls of pristine carbon nanotube forests are investigated directly using a nanopipet electrochemical cell. Both are shown to promote fast electron transfer, without any activation or processing of the carbon nanotube material required, in contrast to the current model in the literature

Structural correlations in heterogeneous electron transfer at monolayer and multilayer graphene electrodes

As a new form of carbon, graphene is attracting intense interest as an electrode material with widespread applications. In the present study, the heterogeneous electron transfer (ET) activity of graphene is investigated using scanning electrochemical cell microscopy (SECCM), which allows electrochemical currents to be mapped at high spatial resolution across a surface for correlation with the corresponding structure and properties of the graphene surface. We establish that the rate of heterogeneous ET at graphene increases systematically with the number of graphene layers, and show that the stacking in multilayers also has a subtle influence on ET kinetics. © 2012 American Chemical Society

Nanoscale electrocatalysis : visualizing oxygen reduction at pristine, kinked, and oxidized sites on individual carbon nanotubes

There is a prevailing and widely adopted view that carbon nanotubes, which are finding considerable application in energy, healthcare, and electronics applications, are highly (electro)catalytically inert unless modified, doped, or defected. By visualizing the electrochemical reduction of oxygen (hydrogen peroxide generation) at high resolution along pristine (defect-free) regions of individual single-walled carbon nanotubes, we show that there is, in fact, significant activity comparable to that of standard gold electrocatalysts. Moreover, the activity is greatly enhanced at strained (kinked) sites and regions modified by oxidation. Single-walled carbon nanotubes are thus effective electrocatalysts in their own right and not just supports for other materials

Redox-dependent spatially resolved electrochemistry at graphene and graphite step edges

The electrochemical (EC) behavior of mechanically exfoliated graphene and highly oriented pyrolytic graphite (HOPG) is studied at high spatial resolution in aqueous solutions using Ru(NH3)63+/2+ as a redox probe whose standard potential sits close to the intrinsic Fermi level of graphene and graphite. When scanning electrochemical cell microscopy (SECCM) data are coupled with that from complementary techniques (AFM, micro-Raman) applied to the same sample area, different time-dependent EC activity between the basal planes and step edges is revealed. In contrast, other redox couples (ferrocene derivatives) whose potential is further removed from the intrinsic Fermi level of graphene and graphite show uniform and high activity (close to diffusion-control). Macroscopic voltammetric measurements in different environments reveal that the time-dependent behavior after HOPG cleavage, peculiar to Ru(NH3)63+/2+, is not associated particularly with any surface contaminants but is reasonably attributed to the spontaneous delamination of the HOPG with time to create partially coupled graphene layers, further supported by conductive AFM measurements. This process has a major impact on the density of states of graphene and graphite edges, particularly at the intrinsic Fermi level to which Ru(NH3)63+/2+ is most sensitive. Through the use of an improved voltammetric mode of SECCM, we produce movies of potential-resolved and spatially resolved HOPG activity, revealing how enhanced activity at step edges is a subtle effect for Ru(NH3)63+/2+. These latter studies allow us to propose a microscopic model to interpret the EC response of graphene (basal plane and edges) and aged HOPG considering the nontrivial electronic band structure