Experimental advances in electrode kinetics

At the present, numerous kinetic theories are available to electrochemists in describing chemical reactions at the electrode. These exist in varying degree of complexity, ranging from simplistic empirical formulae to more rigorous models that account for microscopic physical phenomena pertaining to the reactive species and their environment. However, experimental work documented in support of the validity and comparison between these theories is scarce. This is largely due to difficulty in attaining and controlling the experimental conditions where applicability of the theories break down. As such, the current work utilizes state-of-the-art electrochemical experimental techniques in order to test some of the most prominent electrode kinetics theories via comparison to simulations. This is done with the aim of identifying their limitations and providing new insight into fundamental mechanistic processes that occur during an electrochemical reaction.
Part 1 of this thesis describes the preliminary topics of electrode reactions and electrochemical techniques. Fundamentals of electrode processes (Chapter 1) and an overview of kinetics theories involved (Chapter 2) are presented along with the detail of instrumentation used (Chapter 3).
Part 2 reports the comparison between the use of Butler-Volmer (BV) and Marcus-Hush-Chidsey (MH) formalisms in the simulation of voltammetric responses of model processes, namely: the oxidation of 9,10-Diphynylanthracene and reduction of 2-Nitropropane at the High-Speed Channel Microband Electrode (Chapter 4), and the reduction of Cyclooctatetraene at the hemispherical mercury electrode (Chapter 5). It was found that both formalisms yields excellent fits for the 9,10-Diphenylanthracene system which involves relatively fast kinetics. However, for the 2-Nitropropane and Cyclooctatetraene systems which involve much slower kinetics, the MH formalism yields inferior quality of fittings as a result of its inherent assumption of equal reorganization energies for oxidation and reduction.
Part 3 demonstrates the ability of the asymmetric form of the Marcus-Hush-Chidsey (aMH) model, a formalism developed in-house which extends on the Marcus-Hush-Chidsey formalism, allowing it to account for asymmetry between the oxidative and reductive processes. When tetraphenylethylene was used as the model experimental system (Chapter 6), the aMH model was found to yield excellent quality of fittings comparable to that of the BV formalism while the MH model displays an inferior quality of fittings. Further comparisons between the MH and aMH models in fitting experimental systems of 2-Methyl-2-Nitropropane, Cyclooctatetraene and Europium (III) (Chapter 7) reveals the correlation between the transfer coefficient used within the BV formalism and the asymmetric parameter of the aMH model. This gives physical insight to the BV transfer coefficient as the indicator of the differences between the force constants of the oxidized and reduced species.
Part 4 concerns further developments of physical insights into microscopic processes of heterogeneous electron transfer based on the aMH model. In particular, cyclic voltammetry carried out at variable temperature has been applied to 1-Nitropentane (Chapter 8) and 3-Nitrophenolate (Chapter 9) as a method for the appraisal of the value of reorganization energy of the reactive species. Furthermore, the ability of the model to correlate the variation of electron transfer rate to the degree of adiabaticity was also evaluated using 3-Nitrophenolate as the reagent (Chapter 10). In this experiment, a varying degree of adiabaticity was achieved by changing the distance of approach between the reagent and the electrode through the use of supporting electrolyte cations of differing size. Chapter 11 extends the study on adiabaticity to a system with a lesser degree of adiabaticity, namely 2-Nitropropane. Here, the potential energy curve model developed by Waldeck and Mishra was also used to rationalize the adiabaticity behavior of the experimental system.

Advisor: Compton, R