'Single entity' electrochemistry at carbon nanotubes

In this thesis the electrocatalytic properties of carbon nanotubes was studied at both ensemble and single entity levels. For the former CNTs modified electrodes are characterised using cyclic voltammetry and for the latter the ‘nano-impacts’ method is used. This thesis develops a better understanding of diffusion at modified electrodes. Further, the electro-catalytic behaviour of CNTs was evaluated for the Br2/Brand VO₂⁺/VO²⁺ redox couples using the aforementioned techniques. These redox couples are used as catholytes in redox flow battery technology and are identified to undergo sluggish kinetics at carbon surfaces.
Chapter 1 introduces the fundamental principles of electrochemistry and the electrochemical techniques used in this thesis. Chapter 2 gives general information regarding the chemicals and instrumentation. Chapter 3 briefly summarises the use of CNTs in various electrochemical fields as a catalyst and introduces the role of diffusion under different electrode geometries - semi-infinite at planar electrodes and thin-layer at modified electrodes in evaluating the electrocatalytic property at modified electrodes. Further in Chapter 4 the effect of the heterogeneous distribution of CNTs on a substrate electrode as a result of drop-casting was studied. It was concluded that imaging techniques should be used in parallel with voltammetry techniques to interpret voltammetric data.
Next, the possible electrocatalysis of the Br2/Brredox couple was studied at CNTs. As a pre-requisite, Chapter 5 determines the formal potential of the redox couple using the correct form of Randles-Ševčík equation which reflects on the non-unity stoichiometry of the reaction. This chapter also evaluates the follow-up chemistry leading to the formation of tribromide ions during the reaction. This was conducted using simulations by Mr. Haotian Chen and it was confirmed that the formation of tribromide ion occurs when the concentration of reactant (Br–) is more than 0.1 M. Using the determined formal potential in Chapter 6 the electrochemical rate constant of Br2/Brredox couple was obtained at bare glassy carbon (GC) electrode and at single b-MWCNTs (using ‘nanoimpacts’) by combining experimental data with simulations. It was observed that bMWCNTs were only moderately electrocatalytic compared to a bare GC electrode.
In Chapter 7 the redox kinetics of VO₂⁺/VO²⁺ redox couple was studied at b-MWCNTs and was compared to the kinetics at bare GC. Interestingly we found that b-MWCNTs were positively catalytic towards the oxidative reaction and ‘negatively catalytic’ towards the reduction reaction as concluded from ‘nano-impacts’ experiment. The asymmetry is explained in terms of the multistep nature of the electrode reaction and the relative sequence of electron transfer and chemical steps.
The high surface area of CNTs can facilitate the adsorption of the electroactive species which makes it difficult to unambiguously distinguish between thin-layer effects and the adsorption using cyclic voltammetry alone since the peak current scales linearly with scan rate in both cases. Therefore, in chapter 8 we distinguish these phenomena using complementary ‘nano-impact’ studies where the oxidation of 4-hexyltresorcinol was used as a proof of concept. It was concluded in that case that the oxidation reaction was diffusion and not adsorption controlled.

Advisor: Compton, R