Electron transfer kinetics on natural crystals of MoS2 and graphite

Here, we evaluate the electrochemical performance of sparsely studied natural crystals of molybdenite and graphite, which have increasingly been used for fabrication of next generation monolayer molybdenum disulphide and graphene energy storage devices. Heterogeneous electron transfer kinetics of several redox mediators, including Fe(CN)63−/4−, Ru(NH3)63+/2+ and IrCl62−/3− are determined using voltammetry in a micro-droplet cell. The kinetics on both materials are studied as a function of surface defectiveness, surface ageing, applied potential and illumination. We find that the basal planes of both natural MoS2 and graphite show significant electroactivity, but a large decrease in electron transfer kinetics is observed on atmosphere-aged surfaces in comparison to in situ freshly cleaved surfaces of both materials. This is attributed to surface oxidation and adsorption of airborne contaminants at the surface exposed to an ambient environment. In contrast to semimetallic graphite, the electrode kinetics on semiconducting MoS2 are strongly dependent on the surface illumination and applied potential. Furthermore, while visibly present defects/cracks do not significantly affect the response of graphite, the kinetics on MoS2 systematically accelerate with small increase in disorder. These findings have direct implications for use of MoS2 and graphene/graphite as electrode materials in electrochemistry-related applications

Towards the nanoscale : electrocatalysts and their supports

Electrocatalysts and their support materials used for fuel cell (FC) technology remain at the forefront of research in this field. Typically FC electrocatalysts are comprised of Pt nanoparticles (NPs) supported by carbon. Improvement of both the efficiency and the durability of the materials is required to increase the overall FC performance. To achieve these goals requires a fundamental understanding of electrocatalysis at composite materials and the exploration of alternative materials. These aspects are explored in this thesis. Highly oriented pyrolytic graphite (HOPG) and poly-(3,4-ethylenedioxythiophene) PEDOT-coated HOPG were used as support materials for the electrodeposition of Pt NPs. The NPs were characterised using atomic force microscopy (AFM) which showed that by applying an ultra-thin (ca. 2 nm) of PEDOT, a conducting polymer (CP), onto HOPG, there was less tendency for NP aggregation, with no preferential deposition, i.e. at step edges, and also smaller particles were formed. PEDOT-coated HOPG as the support material for Pt NPs showed a significant enhancement of electroactivity for methanol oxidation, by an order of magnitude, compared with similarly prepared NPs on native HOPG. An alternative support material; explored in this thesis, was polycrystalline boron doped diamond (pBDD), owing to its stability in harsh environments, analogous to FCs. During growth, boron uptake varies across the exposed surface of pBDD, leading to a heterogeneous substrate with typical grain sizes of 5-40 μm. Two new scanned probed techniques; intermittent contact - scanning electrochemical microscopy (IC-SECM) and scanning electrochemical cell microscopy (SECCM) were employed to investigate the impact of this heterogeneity on the local electrochemical properties. Maps using IC-SECM revelaed that the entire surface was active, but that areas with higher boron concentration were more electroactive. Grain boundaries showed no enhanced activity. The maps were sucessfully correlated to the boron dopant density using micro-Raman mapping and field emission scanning electron microscopy (FE-SEM). Similarly, SECCM maps also proved that the entire surface is electrochemically active with the heterogeneities relating to boron content. For data obtained by both techniques finite element simulations (FEM) were employed to extract values for the standard rate constant, k0. With knowledge of the fundamental properties of pBDD, the successful fabrication of a pBDD rotating disk electrode (RDE) is reported which is fully characterised. By functionalisation of pBDD with Pt NPs, the oxygen reduction reaction (ORR) has been studied and compared with a bulk Pt RDE. These preliminary studies show potential for gaining insight into the kinetics of the ORR

Electrochemistry of well-defined graphene samples: role of contaminants

We report the electrochemical characterisation of well-defined graphene samples, prepared by mechanical exfoliation. Mechanical exfoliation is the method of choice for high purity graphene samples, despite the inherent complexity of the approach and the small scale of the resultant flakes. However, one important, yet presently unclear area, is the role of adsorbates such as processing residue, on the properties of the graphene layer. We report high resolution microscopic and electrochemical characterisation of a variety of poly(methyl methacrylate) (PMMA) transferred graphene samples, with the explicit aim of investigating the relationship between electrochemical activity and sample purity.</p

Electrochemical “read–write” microscale patterning of boron doped diamond electrodes

Scanning electrochemical cell microscopy is utilised as a read–write pipette-based probe to both electrochemically modify the local surface chemistry of boron doped diamond and “read” the resulting modification, at the micron scale. In this specific application, localised electrochemical oxidation results in conversion of the H-terminated surface to –O, electrochemically visualised by monitoring the current change for reduction of Ru(NH3)63+. This methodology, in general, provides a platform for read–write analysis of electrodes, opening up new analytical avenues, particularly as the pipette can be viewed as a microfluidic device

Influence of ultrathin poly-(3,4-ethylenedioxythiophene) (PEDOT) film supports on the electrodeposition and electrocatalytic activity of discrete platinum nanoparticles

Coating a carbon electrode surface, specifically highly oriented pyrolytic graphite (HOPG) with an ultrathin film of poly-(3,4-ethylenedioxythiophene), PEDOT, provides a support on which a high density of uniformly dispersed Pt nanoparticles (NPs) can readily be formed by electrodeposition. The NPs tend to be much smaller, have a higher surface coverage, better dispersion and show a much lower tendency to aggregate, than Pt NPs produced under identical electrochemical conditions on HOPG alone. The electrocatalytic activity of the NPs was investigated for methanol (MeOH) and formic acid (HCOOH) oxidation. Significantly, for similarly prepared particles, Pt NP-PEDOT arrays exhibited higher catalytic activity (in terms of current density, based on the Pt area), towards MeOH oxidation, by an order of magnitude, and towards HCOOH oxidation at high potentials, than Pt NPs supported on native HOPG. These findings can be rationalised in terms of the enhanced oxidation of adsorbed CO, a key reaction intermediate and a catalyst poison. This research provides strong evidence that employing conducting polymers, such as PEDOT, as a support substrate, can greatly improve particular catalytic reactions, allowing for better catalyst utilisation in fuel cell technology

Intermittent-contact scanning electrochemical microscopy (IC-SECM) as a quantitative probe of defects in single crystal boron doped diamond electrodes

We demonstrate, for the first time, the use of electrochemical imaging to identify defect and defect free areas in single crystal boron doped diamond (BDD) electrodes. Specifically, we show that defects contain different boron dopant concentrations than the surrounding single crystal matrix and that these variations can be visualized using intermittent contact−scanning electrochemical microscopy (IC‐SECM). The measured IC‐SECM tip currents provide quantitative information on the rates of electron transfer across the single crystal BDD electrodes, which correlate with variations in boron doping levels. In some instances, IC‐SECM outperforms alternative methods such as Raman microscopy and cathodoluminescence imaging, due to its intrinsic surface‐sensitivity, expanding the application and impact of electrochemical microscopy for materials characterization. The results obtained, and procedure outlined, are particularly valuable for the assessment and screening of single crystal BDD electrodes

Active sites for outer-sphere, inner-sphere, and complex multistage electrochemical reactions at polycrystalline boron-doped diamond electrodes (pBDD) revealed with scanning electrochemical cell microscopy (SECCM)

The local rate of heterogeneous electron transfer (HET) at polycrystalline boron-doped diamond (pBDD) electrodes has been visualized at high spatial resolution for various aqueous electrochemical reactions, using scanning electrochemical cell microscopy (SECCM), which is a technique that uses a mobile pipet-based electrochemical cell as an imaging probe. As exemplar systems, three important classes of electrode reactions have been investigated: outer-sphere (one-electron oxidation of ferrocenylmethyltrimethylammonium (FcTMA+)), inner-sphere (one-electron oxidation of Fe2+), and complex processes with coupled electron transfer and chemical reactions (oxidation of serotonin). In all cases, the pattern of reactivity is similar: the entire pBDD surface is electroactive, but there are variations in activity between different crystal facets which correlate directly with differences in the local dopant level, as visualized qualitatively by field-emission scanning electron microscopy (FE-SEM). No evidence was found for enhanced activity at grain boundaries for any of the reactions. The case of serotonin oxidation is particularly interesting, as this process is known to lead to deterioration of the electrodes, because of blocking by reaction products, and therefore cannot be studied with conventional scanning electrochemical probe microscopy (SEPM) techniques. Yet, we have found this system nonproblematic to study, because the meniscus of the scanning pipet is only in contact with the surface investigated for a brief time and any blocking product is left behind as the pipet moves to a new location. Thus, SECCM opens up the possibility of investigating and visualizing much more complex heterogeneous electrode reactions than possible presently with other SEPM techniques