Anodische Oberflächen- und Volumenoxidation graphitischer Materialien in neutralen und alkalischen wäßrigen Lösungen / Anodic Surface and Bulk Oxidation of Graphitic Materials in Neutral and Basic Aqueous Solutions

Abstract

Anodic oxidation of graphitic materials in aqueous electrolytes is a complex process which may yield e.g. higher stage intercalation compounds and lamellar graphite oxides, acidic surface or defect oxides and gaseous or colloidal decomposition products. Kind and concentration of the electrolyte solution are the most important parameters to control this wide-spread distribution of products. Based on electrochemical and microscopic studies on HOPG and carbon fibres, a mechanistic concept of electrolyte effects is proposed: In dilute aqueous solutions, attack of OH• radicals formed in the primary electrochemical step is the dominant reaction pathway leading to formation of covalent carbon-oxygen bonds. Due to their reactivity, OH• radicals are able to attack basal plane carbon sites and also to create 3-dimensional defect oxides. Dissociation or capture of OH• radicals - which is favoured in basic and in most concentrated neutral solutions, respectively, - results in less active radical species and limits the attack to thermodynamically less stable carbon atoms at edge sites which may be either oxidized or removed. Due to acidification of the diffusion layer of anodes even in neutral aqueous electrolytes, direct current oxidation always yields higher stage acid intercalation compounds and lamellar graphite oxide as by-products. This can be almost prevented by alternating current oxidation.