Inner- or outer-sphere. We should still learn about heterogeneous electronic transfer

There are a lot of carbon-based electrodic materials and among those, pyrolytic and glassy carbon are extensively applied in several electrochemical studies. In order to figure out the advantages of carbonbased materials, researchers have used redox couples: compounds connected through reduction-oxidation reactions, where the couple can interact strongly or not with the material. In this way, electrochemical processes are often missed or left behind when characterizing electrodic materials. It is reminded how one can make use of basic electrochemical theory in order to analyze and take into account what the data truly shows. Figure 1 (a – d) shows the electrochemical behavior of ferro/ferricyanide (a) and melatonin (c) on pyrolytic carbon electrode. It can be observed that scan rate may change the apparent kinetics of the electrochemical process. At low scan rates the electronic transfer seems evidently nonlinear (cf. See Fig. 1(b)); however, in higher scan rate values such behavior changes. It is worth mentioning that such range of scan rates are widely used to characterize eletrodic materials. On the other hand, the electrochemical profile of melatonin shows an oxidation peak at 0.65 V, while the relationship between the peak potential (at different scan rates) and log v reveals α = 0.12, according to Laviron’s equation. Such slope value suggests a quasi-reversible process (cf. Fig. 1(d)), contrary to some works describing the process as irreversible. Cyclic voltammetry and further treatment of the data have shown that full comprehension is not easy to reach. There are several factors that should be considered prior to determine the electronic transfer or mass transport properties of a redox couple, and therefore the properties of an electrodic material.