The effect of surface functional groups on the performance of graphite powders used as electrodes

Abstract

The use of commercial graphite powders (different sources and particle sizes), as electrode materials, was evaluated in an electrochemical cavity cell. The graphite powders were characterized by electron microscopy, XRD, vibrational spectroscopy, porosimetry, contact angle, Boehm titration and cyclic voltammetry. Despite similarities, the graphite powders differed significantly in terms of surface functional groups (SFGs), hydrophobicity and particle size. The surface chemistry of both graphites was modified by HCl treatment, followed by 200 °C heating, prior to electrode fabrication, which was found to play an important role on the electrode performance. Cyclic voltammetries were carried out for the two different graphites in acid medium and absence of reagent, and the results were correlated to Boehm titration, showing evidences of the presence of acidic and basic SFGs. After HCl + heat treatment, it was observed the diminishing of the SFGs for both commercial graphite powders tested. The electro-reduction of benzyl bromide was used as standard reaction, yielding 1,2-diphenylethane and toluene as products. A 23 factorial design was used to optimize some electrode parameters. The compaction of the graphite electrodes showed to be fundamental for obtaining good results. HCl + heat treatment favored the 1-electron process, giving 1,2‑diphenylethane as major product on both electrodes (GF, 61% and GA, 52%). The absence of graphite powder treatment favored the 2-electron process, furnishing toluene as major product (GF, 55% and GA, 43%).