Development and application of pipet-based electrochemical imaging techniques

This thesis describes the development of an electrochemical scanned probe microscope, SECCM, outlining the need for such a development, by highlighting previous techniques and their limitations. SECCM consists of a double barrel capillary pulled to small dimensions, filled with electrolyte solution and a redox mediator of choice, with a QRCE is inserted into each channel. A potential is applied between the QRCEs, whilst modulating the pipet normal to the surface. The probe is translated towards the surface and once contact is established, a modulation in the ion current arises due to the physical oscillation of the probe, which is then used as a feedback parameter for imaging. The potential at the working electrode substrate is also controlled. SECCM is introduced using a model test substrate, gold bands on glass, showing that the probe is able to track topographical features, making simultaneous electrochemical measurements. Ion conductance measurements between the two QRCEs, are shown to be sensitive to the nature of the substrate investigated. The fundamental electrochemical behaviour of CVD graphene and SWNT is investigated. A multimicroscopy approach is used for CVD graphene studies, correlating surface structure and activity, deducing heterogeneous electron transfer kinetics through simulation. The SWNT samples are studied in two different morphologies: as 3D forests; and, as a 2D network. In the forests, the probe is positioned at the ends and sidewalls, making spot measurements. The voltammetric behaviour shows very similar responses, whilst in the network, a nanosized probe is scanned across the surface, showing relatively uniform activity across an entire tube. These new insights indicate that SWNTs are highly active electrode materials. The fabrication and characterisation of SECM-SICM probes, in a straightforward manner is also presented. These types of probes were found to be ideal for the investigation of biological samples, being extremely easy and quick to fabricate

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

Mass transport enhancement in redox flow batteries with corrugated fluidic networks

We propose a facile, novel concept of mass transfer enhancement in flow batteries based on electrolyte guidance in rationally designed corrugated channel systems. The proposed fluidic networks employ periodic throttling of the flow to optimally deflect the electrolytes into the porous electrode, targeting enhancement of the electrolyte-electrode interaction. Theoretical analysis is conducted with channels in the form of trapezoidal waves, confirming and detailing the mass transport enhancement mechanism. In dilute concentration experiments with an alkaline quinone redox chemistry, a scaling of the limiting current with Re0.74 is identified, which compares favourably against the Re0.33 scaling typical of diffusionlimited laminar processes. Experimental IR-corrected polarization curves are presented for high concentration conditions, and a significant performance improvement is observed with the narrowing of the nozzles. The adverse effects of periodic throttling on the pumping power are compared with the benefits in terms of power density, and an improvement of up to 102% in net power density is obtained in comparison with the flow-by case employing straight parallel channels. The proposed novel concept of corrugated fluidic networks comes with facile fabrication and contributes to the improvement of the transport characteristics and overall performance of redox flow battery systems.Indisponível

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

3D-printed fluidic networks for high-power-density heat-managing miniaturized redox flow batteries

The miniaturization of redox flow cells (RFCs) paves the way to novel energy conversion concepts combining power delivery and heat regulation. Envisioning the integration of high-power-density RFCs into electronic devices such as microprocessors, lasers, or light-emitting diodes for the purpose of providing power and heat management simultaneously, we introduce and investigate interdigitated, tapered multiple-pass microfluidic networks in miniaturized flow cells. Employing 3D-printing for the facile and inexpensive fabrication of these networks, we demonstrate RFCs with maximum power densities of up to 1.4 W cm−2 at room temperature and net power densities of up to 0.99 W cm−2 after subtracting pumping power losses. The electrolytes employed modest concentrations of 0.4 M K4Fe(CN)6 and 0.2 M 2,6-dihydroxyanthraquinone in alkaline electrolyte. We thereby show that rational tailoring of fluidic networks in RFCs is key for the development of devices effectively combining power delivery and thermal management.ISSN:1754-5692ISSN:1754-570

Mixed Anionic and Cationic Redox Chemistry in a Tetrathiomolybdate Amorphous Coordination Framework

We report the electrochemistry of a hitherto unexplored Na2MoS4 phase as conversion electrode material for Na‐ and Li‐cation reversible storage. The material adopts an amorphous coordination polymer structure, with mixed Mo and S valences. Ex‐situ XPS and in‐situ XRD analysis reveal a complex interplay between Mo and S redox, while excluding the formation of free sulfur, lithium sulfide or other crystalline phases. Na2MoS4 behaves as a mixed ionic‐electronic conductor, with electronic conductivity of 6.1 × 10 ‐4 S cm ‐1 , that allowed the use of this material without any conductive carbon in an electrochemical cell. A reversible capacity of 500 mAh g ‐1 is attained corresponding to 5‐electron redox exchange, with the end‐member reaching from Na5MoS4 . This study not only emphasizes on the excellent charge storage performances of Na2MoS4 for Li or Na batteries, but also enriches the emerging library and knowledge of sulfide phases with mixed anionic and cationic redox

Localized high resolution electrochemistry and multifunctional imaging : scanning electrochemical cell microscopy

We describe highly localized electrochemical measurements and imaging using a simple, mobile theta pipet cell. Each channel (diameter <500 nm) of a tapered theta pipet is filled with electrolyte solution and a Ag/AgCl electrode, between which a bias is applied, resulting in a conductance current across a thin meniscus of solution at the end of the pipet, which is typically deployed in air or a controlled gaseous environment. When the position of the pipet normal to a surface of interest is oscillated, an oscillating component in the conductance current is generated when the meniscus at the end of the probe comes into contact with the surface and undergoes periodic (reversible) deformation, so as to modulate the solution resistance. This oscillating current component can be used to maintain gentle contact of the solution from the pipet cell with the surface and as a set point for high resolution topographical imaging with the pipet. Simultaneously, the mean conductance current that flows between the pipet channels can be measured and is sensitive to the local nature of the interface, informing one, for example, on wettability and ion flow into or out of the surface investigated. Furthermore, conductor or semiconductor surfaces can be connected as a working electrode, with one of the electrodes in the pipet serving as a quasi-reference electrode. This pipet cell then constitutes part of a dynamic electrochemical cell, with which direct voltammetric-amperometric imaging can be carried out simultaneously with conductance and topographical imaging. This provides multifunctional electrochemical maps of surfaces and interfaces at high spatial resolution:Me prospects for the use of this new methodology widely are highlighted through exemplar studies and a brief discussion of future applications

Mapping nanoscale electrochemistry of individual single-walled carbon nanotubes

We introduce a multiprobe platform for the investigation of single-walled carbon nanotubes (SWNTs) that allows the electrochemical response of an individual SWNT to be mapped at high spatial resolution and correlated directly with the intrinsic electronic and structural properties. With this approach, we develop a detailed picture of the factors controlling electrochemistry at SWNTs and propose a definitive model that has major implications for future architectures of SWNT electrode devices

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