Use this URL to cite or link to this record in EThOS: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.793900
Title: Electrochemical fundamentals and application of new carbon materials
Author: Liu, Danqing
ISNI:       0000 0004 8497 7184
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2018
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Abstract:
The work presented in this thesis focuses on studying the electrochemistry of nanomaterials using an innovative combination of state of the art small-scale electrochemical methods and complementary microscopy techniques. Three classes of material are considered: (i) the functionalization and electrocatalytic applications of sp2 carbon materials, with a focus on single-walled carbon nanotubes (SWNTs), with nickel hydroxide nanoparticles (Ni(OH)2 NPs) electrodeposited onto them through two different approaches; (ii) a fundamental electrochemical study of boron doped diamond (BDD) (sp3 carbon), in which the microscopic factors that determine the aqueous solvent window in chloride-containing salt solutions were examined by using scanning electrochemical cell microscopy (SECCM); (iii) a study of the electrochemical behaviour of sp2 carbon materials at the nanoscale by employing SECCM, focusing on the electron transfer (ET) kinetics of graphene on a Cu foil and the hydrogen evolution reaction (HER) electrocatalytic activities of graphene-like hexagonal boron nitride (h-BN) on different metal substrates (Cu and Au). Using a macroscale droplet method, functionalized SWNTs with nickel hydroxide, Ni(OH)2, nanoparticles (NPs) of different crystallographic orientations are formed by two approaches. The first involves the electrodeposition of Ni NPs that are converted to Ni(OH)2 through potential cycling in alkaline media to form ordered -phase Ni(OH)2 NPs (indirect approach). The second uses the electrochemical generation of a relative high concentration of OH-, in the presence of Ni2+, to lead to the formation of disordered -phase Ni(OH)2 NPs (direct approach). Ni(OH)2 NPs prepared by the direct approach show a remarkable electrocatalytic activity towards alcohol oxidation, with specific activities (SA) of ~2.8 kA g-1 for 0.5 M methanol and ~3.7 kA g-1 for 0.5 M ethanol. In contrast, SWNT-Ni(OH)2 produced by the indirect approach shows SA values about an order of magnitude lower. This work reveals the capability of electrochemistry for synthesizing different nanostructures and highlights the correlation of particle crystallography with electrocatalytic properties. Electrochemical studies can be extended to the microscale and nanoscale by using voltammetric SECCM. The solvent window of BDD, a sp3 carbon material, in aqueous potassium chloride solutions is investigated both for oxygen- and hydrogen-terminated surfaces. The solvent window is shown to be strongly linked to the local dopant concentrations, with lower dopant concentrations leading to a wider window. The surface termination is a particularly important factor, with the solvent window of the H-terminated surface being wider than for the O-terminated surface for similar boron dopant levels. Furthermore, the anodic potential window of the O-terminated surface is greatly diminished due to chloride electro-oxidation. This study provides new perspectives on the factors that control the solvent window of BDD at the level of a single facet for a complex heterogeneous electrode. Finally, the electrochemistry of graphene and graphene-like h-BN is explored, at nanoscale, as examples of very important 2D materials. High-resolution SECCM is employed to assess the ET kinetics for the Ru(NH3)63+/2+ couple across as-grown CVD graphene produced by chemical vapour deposition. A correlation between the ET kinetics and the number of graphene layers is found, indicating that monolayer graphene exhibits faster ET kinetics than that observed for the bi- and multilayer graphene. These differences in ET kinetics provide insight into how the electrochemical properties of graphene-based devices can be manipulated with wide-reaching applications including in sensors and energy storage. Furthermore, the electrocatalytic performances of as-grown CVD h-BN on a Cu foil (h-BN/Cu) and h-BN transferred to a Au substrate (h-BN/Au) toward the HER are studied by using single-barrel SECCM. The Tafel slopes are estimated to be 136 mV per decade and 108 mV per decade for h-BN/Cu and h-BN/Au, respectively and the exchange current densities are 4 × 10-8 A cm-2 and 1 × 10-6 A cm-2. Importantly, the results suggest that the enhanced HER activity is associated with the interaction between the h-BN and the underlying metal substrate which promotes hydrogen adsorption on the h-BN surface. This work indicates that heterostructures formed by h-BN when supported on different metal substrates could be to modulate the electrocatalytic activity towards the HER.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.793900  DOI: Not available
Keywords: QD Chemistry
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