La contradicció entre la identitat viscuda i la identificació externa

In this thesis, screen printed graphite, graphene, reduced graphene oxide and

multiwalled carbon nanotube electrodes are used. These sp2 _{carbon based}

electrodes are amongst the most commonly used in electroanalysis today. Despite their popularity, and the fact the they have been in use for centuries (see timeline below), the fundamental electrochemical properties of these materials is still an ongoing research topic. In particular, the role played by the edge plane versus the basal plane has been explored extensively in recent years (illustrated in Fig. 1.22). Two opposing schools of thought have emerged.

The most prevalent theory is that the electrochemical activity occurs exclusively at edge plane sites and that the basal plane is practically inert [59, 66, 67]. This insight emerged from work on carbon nanotubes which brought into question the electrocatalytic properties of nanotubes as first reported by Britto and co-workers and further developed by Wang et. al in the early 2000s [57, 16]. Control experiments in which basal plane pyrolytic graphite (BPPG) electrodes were modified with graphite powder as well as carbon nanotubes showed that both modifications produced a similar electrocatalytic effect [68]. The same group followed this paper with a comparison of multiwalled carbon nanotube modified electrodes and edge plane pyrolytic graphite electrodes (EPPG) [69].

19th Century Humphry Davy uses graphite electrodes in the production of alkali metals

1960s First description of carbon nanotubes

1985 Sir Harry Kroto discovers C60

1991 Seminal publication by Iijima on carbon nanotubes

1996 Britto et. al describe first carbon nanotube modified

electrode for electroanalysis

2004 Geim and Novoselov produce graphene for the first

time from the mechanical exfoliation of graphite

2005/2006 Compton and Banks show that the edge sites of

graphite to be the sites of electrochemical activity

2014-2016 Unwin and co-workers contradict work of Compton

and Banks

T

1: Electrochemistry of sp

carbon electrodes

Cyclic voltammetry results showed that the electron transfer kinetics were very similar for both electrodes. It was therefore concluded that the reactive sites of carbon nanotubes occur at the ends of nanotubes. Furthermore, it was argued that any electrochemical activity observed from BPPG electrodes was due solely to defects in plane which exposed edge plane sites. Fix 1.20 shows the proposed microscopic structure of BPPG electrode comprising of discrete regions with edges which account for the electrodes electroactivity. This explanation for the electrocatalytic properties of carbon nanotubes was then extended to graphene electrodes in which its electrochemistry could be explained again through defect density[70, 71, 72]. This reasoning is now the textbook explanation for electrochemistry at carbon electrodes [51].

However, recent results by Unwin and co-workers may require a re-think of this accepted viewpoint. Voltammetry conducted at freshly cleaved BPPG electrodes suggests that there is in fact little difference between the electrochemical activity of the basal plane and edge plane [73]. This will no doubt be explored further in the coming years.

Figure 1.22: Schematic of the microstructure of graphite showing discrete regions which contain edge sites. [66]

1.9.2.1 Thin Layer Effects

Work on carbon nanotube modified electrodes has revealed that thin layer effects due to trapping of the electroactive species in the porous structure of the carbon nanotube network can have a considerable effect on its electrochemical characteristics (Fig. 1.23) [74, 75, 76] Differeniating between purely semi-infinite diffusion and a contribution of thin layers can be done by

graphing the log10 of peak current versus log10 of scan rate. A slope of around

0.5 indicates semi-infinite diffusion whereas a slope closer to 1 indicates thin layer effects. The justification for this is explained in the following paragraphs

by considering the Randles-Sevcik equation which applies to purely diffusional, electrochemically reversible behaviour and the contrasting thin layer effects.

Figure 1.23: Shematic of diffusion at a carbon nanotube modified electrode surface showing highlighting the contribution of both semi-infinite diffusion and thin layer diffusion of analyte within the carbon nanotube matrix. [75]

1.9.2.2 Determining the Influence of Thin Layer Effects

The Randles Sevcik equation is given by:

ip = (2.69 × 105)n 3 2AD 1 2Cv 1 2 (1.16) If we let (2.69 × 105)n32AD 1 2C = χ (1.17)

then the Randles Sevcik equation can be written as:

ip = χv 1

2 (1.18)

Taking the log10 of both sides of the equation:

log10ip = log10[χv

1

Then, if we note that log(AB) = log(A+B), the equation can be written as

log10ip = log10v

1 2 + log

10χ (1.20)

By using the fact that log(xy_{) = ylog(x) we find that}

log10ip =

1

2v + log10χ (1.21)

Hence, a graph of log10ip vs log10νshould have a slope of 1/2 for diffusion

limited processes obeying the Randles Sevcik equation.

Next we can consider the peak current for an n electron, reversible surface bound process:

Ip = n

2_F2

4RTνAΓ (1.22)

By taking the log10 of both sides and following a similar process as described

for the Randles-Sevcik equation the following expression is found:

log10ip = v + log10n2F2AΓ[(4RT )−1] (1.23)

Hence, in this case, graphing log10ip vs log10ν will give a slope of 1 indicating that thin layer effects dominate the voltammetric response.

1.9.2.3 Inner and Outer Sphere Redox Probes

Throughout this thesis, inner and outer sphere redox probes are used to characterise various electrode materials. In the case of the outer sphere process, the reaction centre is considered to occur in the outer Helmholtz plane and doesn’t penetrate the layer of solvation molecules that are adsorbed onto the electrode surface. In contrast, the inner sphere process occurs through

a common ligand1 _{and are strongly surface dependent [51]. In this thesis,}

ruthenium (III) chloride is the outer sphere redox probe that is used. The ferri/ferrocyanide redox couple is used to investigate surface effects. This redox

probe is described as a weakly interacting outer sphere probe according to McCreery in Fig 1.24 below.

Figure 1.24: Inner and Outer Sphere Redox probes at carbon electrodes[77]