Nanoscale Electrochemistry of sp² Carbon Materials: From Graphite and Graphene to Carbon Nanotubes

Abstract

2 CONSPECTUSCarbon 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 nano-carbon, 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 article 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 sp 2 carbon materials -graphite, graphene and single walled carbon nanotubes (SWNTs) -this article 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 outersphere redox processes. 3(ii) Demonstration of the high activity of basal plane HOPG towards 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. graphene grown by 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 ...

Nanoscale Electrocatalysis of Hydrazine Electro-Oxidation at Blistered Graphite Electrodes

“…62 The porous structure of blistered regions may aid the evolution of N2 nanobubbles, resulting in an enhancement of electrochemical activity. Finally, and significantly, the electronic properties of graphite change dramatically by the introduction of defects, 63 in particular, resulting in a high density of state near the Fermi level at defective graphite, 17,52,64 which would be expected to significantly enhance electrocatalysis. (g) ExampleTafel plots from the labelled regions in (f).…”

“…[13][14][15] This is particularly true of graphite, which comprises stacked graphene layers with weak van der Waals force present between the layers. 7 While outer sphere redox processes, and some more complex electron-proton coupled processes, occur readily at the basal structure of graphite, [16][17][18][19][20][21] electrocatalytic (bond-breaking) reactions often require additional efforts to promote the electrochemical activity. There are 3 broad, and somewhat interrelated, approaches and effects to consider: (i) selective doping of sp 2 materials by various heteroatoms (e.g.…”

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

“…basal plane (terraces) and step edges that are easily characterised by a variety of techniques. 5,6 Further, HOPG is generally regarded as a model to which other sp 2 carbon materials, such as carbon nanotubes 7 and graphene, 8,9 are often compared.…”

“…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.…”

“…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.…”