Electrochemical Oxidation and Reduction of Carbonaceous Materials and Their Electrochemical Properties
Authors: Mikata Koji , Miyazaki Kohei , Fukutsuka Tomokazu , Abe Takeshi 
Published: February 2020

Introduction

Electric and hybrid electric vehicles have attracted much attention since there is serious concern about environmental problems and the depletion of fossil fuels. To improve the cruising ranges of the environmentally benign vehicles, rechargeable batteries have been used and world-widely investigated. One of the major problems of rechargeable batteries for EVs is that rare metals such as Co and Ni must be used for active materials. We focused on the rare-metal free electrodes. Carbonaceous materials are very attractive candidates alternative to the rare metal-based materials. This is principally due to their high heat resistances, low electric resistances, etc. In addition, they can be synthesized from renewable natural resources. Therefore, it will be interesting to study the carbonaceous materials as environmental-friendly active materials for electrochemical energy devices.
Chemically treated carbons have some unique physicochemical properties. For example, oxidized carbon materials show larger surface areas, resulting in the high capacitances for electric double layer capacitor and also high catalytic activities for fuel cells. Various methods for the oxidation of carbonaceous materials have been known, and recently, Hummers[1] and Staudenmaier methods[2] have been often used. However, it is difficult to control the reaction progress of oxidation by these methods.
In this work, we focused on the electrochemical method for the oxidation and reduction of carbonaceous materials and we showed the ir electrochemical properties.

Experiment

A three-electrode cell was used for electrochemical measurements. Glassy carbon plate, platinum wire, Ag/AgCl electrodes were used as working, counter, and reference electrodes, respectively. At first, glassy carbon oxide (GCO) was prepared by the oxidation at the potential of 1.5, 2.0, and 2.5 V (vs. Ag/AgCl) in 0.1 mol dm–3 Na2SO4 for 30 min. After that reduced-GCO (rGCO) was prepared by the reduction of GCO at –1.1 V for 30 min. Cyclic voltammetry was carried out between –0.5 and 0.7 V at a scan rate of 2.0 mV s–1 to measure the capacitance.
Glassy carbon was oxidized/reduced (2.5 / –1.1 V) repeatedly to progress the surface reactions. X-ray photoelectron spectroscopy was conducted for the characterization of the surface functional groups.

Results and discussion

The capacitances of rGCO prepared at a high potential (2.5 V) were much higher than those of GCO. When the oxidation potentials were set at 1.5 or 2.0 V, the capacitances remained unchanged before and after reduction. Capacitances of rGCO increased as the oxidation/reduction cycle number increased, and the capacitances became almost constant after the 10th cycle (Figure 1). In the oxidation process, some oxygen functional groups should be formed. XPS spectra showed that C-O groups mainly existed on the surface of GCO oxidized at 2.5 V. And it has been reported that epoxy groups were formed at the higher potentials [3]. Therefore, it was suggested that epoxy groups should be responsible for the increase of the capacitances. We also studied the correlation between the sweep range and the capacitances. Figure 2 shows cyclic voltammograms of GCO in the potential range of –2.5 and 2.1 V. Large oxidative and reductive currents were observed in this potential range. The details will be reported in the conference.

Reference

W.S. Hummers, J. Am. Chem. Soc.,
80, 1339 (1958).

L. Staudenmaier, Ber. Dtsch. Chem. Ges., 31, 1481 (1898).

H. Tateishi, et al, Carbon, 76, 40 (2014)

Figure 1