Electrochemical study of anthraquinone groups, grafted by the diazonium chemistry, in different aqueous media-relevance for the development of aqueous hybrid electrochemical capacitor

The cyclic voltammetry behavior of anthraquinone (AQ) modified-glassy carbon and carbon powder-based composite electrodes was investigated as a function of the solution pH. The modification of glassy carbon electrode and carbon powder was performed by electrochemical and chemical reduction of the diazonium ions, respectively. The modification was performed by using 1-diazoanthraquinone and 2-diazoanthraquinone as anthraquinone precursor and yielded materials that differ in the point of attachment of the anthraquinone moieties. Cyclic voltammetry measurements were performed in several aqueous solutions in a pH range from 0.5 to 14 and the apparent redox potential and the anthraquinone surface concentration were determined. The shape of the cyclic voltammogram is affected by the solution pH. A well-defined cyclic voltammogram is observed for the AQ-modified glassy carbon based composite electrode in alkaline electrolyte whereas the electrochemical response for the carbon powder-based composite electrodes is more complex. On the other hand, the opposite is true in acidic media as a well-defined cyclic voltammogram is seen for the composite electrode. The amount of electrochemically addressable AQ molecules was found to depend on the solution pH. The relevance of the results for the use of anthraquinone-modified carbon in electrochemical capacitor is discussed.