| Summary: | <p>This thesis presents five analytical electrochemistry projects which are written as separate chapters. The research presented herein covers a wide aspect of electrochemistry, ranging, experimentally, from a 'forensic-style' electrochemical analysis of the mercury/water biphasic system post-sonication, to studying the interaction of marine phytoplankton using electrochemistry, and theoretically, simulations providing new insights to non-ideal interactions in voltammetry leading to phase-transition like behaviour. The scientific scope of the chapters is broad and multi-disciplinary, some may even seem 'wacky'; but, they are all at-heart fundamentally based in electrochemistry and are adventurous projects pushing beyond the traditional boundaries of the current field. The novelty and scientific advances of each Chapter are summarised below.</p>
<p>Chapter 1 serves to introduce the fundamental concepts of electrochemistry, which are the building blocks for the projects discussed subsequently in this thesis. Here the discussion includes equilibrium electrochemistry, electrode kinetics and mass transport. Moreover, different aspect of electrochemistry in practice are discussed, such as the use of a three-electrode system, the effects of electrode geometry, and the voltammetric techniques – chronoamperometry and cyclic voltammetry. </p>
<p>For electrons to transfer across an electrode interface, the analyte must come within an electron tunnelling distance of the electrode. What if the analyte is insoluble in solution and the unavoidable sedimentation of solids voids contact with the electrode? Chapter 2 tackles this problem by facilitating the electrochemistry of insoluble solids with a bespoke carbon-black composite. The latter is used as a modification to the electrode so as to bind the insoluble analyte, provide electrical contact and at the same time it is sufficiently porous allowing the composite to 'see' the electrolyte solution. The electrochemical behaviour of this composite material is fully characterised with three well-known aqueous insoluble redox couples, and are used to obtain the first cyclic voltammogram of solid red-cinnabar (α-HgS). Further highlighted in this Chapter is an in situ electrochemical X-ray diffraction experiment, facilitated by this carbon-black composite, demonstrating the possibility to electrochemically navigate across the narrow phase-width of the superconductor β-Fe<sub>1+x</sub>Se post-synthesis.</p>
<p>What happens when ultra-sound sonication is applied to a mercury/water biphasic system? Chapter 3 presents a 'forensic-style' investigation, where the 'clouds' of grey particles seen emanating from the mercury/water interface were investigated and identified as Hg@HgO core-shell particles, characterised for the first time using a combination of electrochemical methods and X-ray diffraction experiments. Furthermore, as discovered electrochemically, sonication results in the simultaneous formation of molecular Hg(OH)<sub>2</sub> in the aqueous phase at a concentration close to a saturation limit of 0.24 mM; which is likely formed from the sonochemical reaction of liquid mercury with hydroxyl radicals; the latter is generated from acoustic cavitation.</p>
<p>Chapter 4 investigates the electrochemical interactions possible with marine phytoplankton. These are photosynthetic, chlorophyll-containing, single-cellular organisms which are globally significant as the nutritional source of all life forms in the ocean and has a dominant role in the ocean's carbon cycle. This Chapter showcases a pioneering in situ fluoro-electrochemical set up to quantify the rate of phytoplankton 'killed' by electro-oxidation. The electrochemical oxidation occurs via the propagation of hydroxyl radicals, formed at the electrode interface, reacting with the individual phytoplankton distant from the electrode. Moreover, a total of six species of phytoplankton are screened to develop a proof-of-concept 'susceptibility library mapping' of phytoplankton species towards electro-oxidation.</p>
<p>Beyond the experimental-dominant work discussed thus far, theoretical simulations with a ground-up approach can often provide new insights from an entirely different perspective, which can synergistically augment or provoke experimental findings. In Chapter 5, the mathematical formulation, and numerical methods allowing electrochemical simulations are discussed. This Chapter builds upon the fundamental electrochemistry concepts as established in Chapter 1, enabling one to have full control over the simulation model – a contrast to a 'blackbox' approach.</p>
<p>Chapter 6 develops a general simulation model for voltammetry of species adsorbed from solution, involving coupled mass transport and adsorption which distinguishes the relative contributions from the adsorption kinetics, the mass transport and electron transfer kinetics of the reactants and products. The model adopts Langmuirian adsorption kinetics to allow the effects of adsorption on voltammetry to be generally accounted for. The new insights include: a steady-state voltage-current response for a macro-electrode when the rate-determining step becomes 'bottlenecked' by the slow desorption of the product. Furthermore, when the rate of adsorption and desorption of the reaction and product are fast compared to the mass transport and are not rate-determining, the simulated current for the redox process via the adsorption pathway can be up to several times higher to that when electron transfer occurs via the solution phase.</p>
<p>Continuing from the general adsorption model discussed in Chapter 6, Chapter 7 turns to investigate the effects of non-ideal surface interactions on voltammetry involving one adsorbed species. In particular, the simulation focuses on non-ideal interactions leading to phase transition like behaviour, which is not uncommon in voltammetry reflecting strong surface interactions. By coupling interfacial electrode kinetics with the Frumkin adsorption isotherm, the underlining physicochemical processes occurring in voltammetry induced phase transitions are studied via simulation. Phase-transition-like behaviour is only observable in voltammetry under reversible electrode kinetics, which if not present would otherwise lead to a 'disconnection' from the adsorption isotherm. In the case when the phase transition occurs on the voltammetric timescale, the simulated interfacial conditions reveal intrusions into a 'forbidden' zone of the Frumkin adsorption isotherm – even in the presence of a significant excess in the concentration of the analyte – as a result of local dis-equilibrium.</p>
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