Abstract
It is becoming increasingly important to find “green” solvents for chemical and electrochemical purposes. Poly(ethylene glycol) has been identified as an environmentally benign, non-volatile, viscous solvent with interesting properties that could be beneficial in many applications. For example, poly(ethylene glycol) is known to be a strong absorber of carbon dioxide, which could lead to applications in carbon capture and utilisation technologies. The electrochemical properties of poly(ethylene glycol) are therefore of interest.Voltammetry in poly(ethylene glycol) was performed. Diffusion coefficients for both oxidised and reduced forms of several redox species were determined and verified using a combination of double potential step experiments, cyclic voltammetry and computer simulations using the commercial electrochemical software DigiElch™. The optimised diffusion coefficients were approximately two orders of magnitudes smaller compared with aqueous solution, which was attributed to the higher viscosity of the polymer solvent.
The slow diffusion in viscous poly(ethylene glycol) gives low measured currents in voltammetry. To overcome the slow mass transport, a hydrodynamic technique based upon a laminar Couette flow in a microgap was developed. An increase in current of over two orders of magnitude was observed. A Levich-type equation was derived and was consistent with experimental data for the reduction of Fe(CN)63-(aq) under Couette flow conditions. Studies in poly(ethylene glycol) could indicate complex interactions between the polymer solvent and the platinum electrode. A proposed mechanism involves the reversible adsorption of poly(ethylene glycol) to the platinum electrode above +0.4 V (vs. SCE).
The grafting ability of poly(ethylene glycol) was investigated by anodic coupling to glassy carbon and boron-doped diamond surfaces. Modification of carbon substrates
with poly(ethylene glycol) caused a suppression in the apparent rate of electron transfer for the Fe(CN)63-/4- redox couple. The apparent rate of electron transfer after poly(ethylene glycol) attachment correlated with grafting potential, time and polymer chain length. It is proposed that the hydrophilic redox species cannot diffuse far into the poly(ethylene glycol) layer, which slows the apparent rate of electron transfer. However, the apparent rate of electron transfer for a more hydrophobic redox species, 1,1’ ferrocene dimethanol (Fc(CH2OH)2, was not affected by grafting potential, time or chain length. The apparent selectivity of the PEG-modified electrode enabled the mediated oxidation of potassium ferrocyanide (K4Fe(CN)6) using a Fc(CH2OH)2 mediator. This mediated oxidation approach enhanced the voltammetric response for micromolar concentrations of the mediator, which could have potential uses in sensing applications.
The electrochemical grafting of a PEGylated castor oil, Kolliphor® EL, is also demonstrated. A similar suppression in the voltammetric response for the Fe(CN)63-/4- redox couple was observed and five ferrocene mediators were assessed in their ability to mediate electron transfer to Fe(CN)63-/4-. The electron shuttle ability for the five ferrocene mediators decreased in the order: dimethylaminomethyl ferrocene > n-butyl ferrocene > ferrocene dimethanol > ferrocene acetonitrile > ferrocene acetic acid. It is proposed that the electron shuttle ability is influenced by (i) the reversible potential for the mediator molecule, (ii) the heterogeneous electron transfer kinetics for the mediator molecule, (iii) electrostatic interactions, and (iv) mediator aggregation at the modified surface.
Finally, a one-step electroless deposition method is developed, which utilises the reducing ability of poly(ethylene glycol). A platinum salt precursor solution is prepared in PEG200 and heated at 500 °C for 30 min in atmospheric conditions. PEG200 acts as both the solvent and as the reducing agent. The high temperature causes PEG200 to evaporate, which results in the formation of metallic Pt nano-fibrous deposits. The electrochemically active surface area and the electro-catalytic activity of the Pt nano fibrous deposits was determined. Only a slight increase in catalytic activity is observed for the nano-fibrous deposits compared with a bulk polycrystalline Pt disc.
| Date of Award | 11 Nov 2015 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Frank Marken (Supervisor) |