Mechanistic study on the interfacial variation of carbon electrode under electrochemical oxidation

Previously, we have reported that different procedures employed for electrochemical oxidation of glassy carbon (GC) can lead to remarkably different interfaces for the redox reactions of copper species in the presence of Cl⁻ anion. In this article, we further investigated the interfacial variation mechanism of other carbon electrodes employing both cyclic and potentiostatic oxidation methods in H₂SO₄ solution. The electrochemical studies were limited to the end-edge planes of vertically aligned multi-walled carbon nanotubes (eVAMWCNTs) and the edge planes of highly oriented pyrolytic graphite (eHOPG). Differences in the pristine surfaces and bulk structures of these two sp² carbon electrodes were merely resulted from the arrangements of graphene multilayers by rolling seamlessly or parallel alignment. Experimental results showed that cyclic oxidation favored the generation of oxygen-containing groups at eVAMWCNTs, due to that rigid MWCNT surface and bulk structures were unfavorable for the intercalation reaction of SO₄^{2 −} anion. On the other hand, potentiostatic oxidation was found to cause the oxidation of eVAMWCNTs via a different reaction route. By contrast, the oxidation of the soft eHOPG predominantly involved the intercalation of SO₄^{2 −} anion into the gaps between graphene layers, which might restrain the increase of oxygen-containing groups at eHOPG. As probed by Raman spectroscopy and the redox reactions of copper species, significant differences in the interfacial variation with two oxidation methods were observed for eVAMWCNTs. As indicated by an abnormal behavior of metallic copper deposition/dissolution, only the cyclic oxidation could quickly generate a very high ratio of oxygen-containing groups to other electron transfer sites (edge steps and defect sites) at the eVAMWCNT interfaces, similar to the previous observations for GC. Due to limitation in the increase of oxygen-containing groups at the eHOPG interfaces, two oxidation methods would not cause significant differences in the electrode responses for the redox reactions of copper species.