(229p) Synthesis and radio frequency oxygen-plasma treatment of graphene for electrodes of electrochemical capacitors

Chuen-Chang Lin^1,* and Sheng-Yen Fan¹

¹Department of Chemical & Materials Engineering, National Yunlin University of Science and Technology, 123 University Road Sec. 3, Douliu, Yunlin 64002, Taiwan

^*E-mail: linchuen@yuntech.edu.tw

1. Introduction

   The capacitive properties of graphene films depended on the number of graphene layers and the graphene films with three layers possessed higher specific capacitance than those of thicker graphene films [1]. Mono-layered graphene was synthesized with a very low precursor supply, but a higher flow resulted in few layer graphene [2]. Furthermore, the large enhancement in capacitance may be attributed to the possible pseudocapacitance because of the availability of oxygen-containing functional groups [3]. In order to add oxygenated functionalities to the surface of grapheme, it positioned ~30 cm downstream from the plasma source for minimizing surface damage was modified by microwave radical generator oxygen–plasma surface treatment at different times [4]. Thus, in this research, graphene was grown on the Nickel foam using thermal chemical vapor deposition (CVD) at different volume flow rates of Ar, then RF (radio frequency) oxygen-plasma was used to modify the surface properties of graphene, such as functional groups at different power levels/times, and finally maximum capacitance at better conditions was sought.

2. Experiment

The pretreated Nickel foam substrate was heated at 1000 ^oC in H₂ (100 sccm) and Ar (250 sccm) for 10 min to reduce the surface oxide layer. Next, graphene was grown on the annealed Nickel foam using CVD with a gas mixture of CH₄ (6 sccm), H₂ (100 sccm), as well as different volume flow rates (250, 400, and 550 sccm) of Ar for 10 min at 1000 ^oC, and then cooling to ambient temperatures at a rate of 10 ^oC /min in Ar with the same volume flow rate as graphene grown. Finally, the graphene electrode was modified by RF plasma with 30 sccm of oxygen at different power levels (50, 75, and 100 W) and times (3, 10, and 30 min). The cyclic voltammetry was undertaken with a 6 M aqueous electrolyte (KOH) for the prepared graphene electrodes.

3. Results and Discussion

    There is some contribution to the specific capacitance from foam Ni, it seems unimportant (See Fig. 1). The graphene grown with 400 sccm of Ar possessed the highest specific capacitance (See Fig. 1). The reason behind this behavior may be explained as follows. The ratio of I_2D/I_G (0.68) for the graphene grown with 400 sccm of Ar is about equal to the ratio of I_2D/I_G (0.704) for the graphene grown with 550 sccm of Ar, then they are almost the same layer numbers, which are fewer than that for the graphene grown with 250 sccm of Ar due to lower ratio of I_2D/I_G (0.442). The fewer the layer number, the lighter the weight, which leads to increasing the specific capacitance for the graphene grown with 400 sccm of Ar. The graphene grown with 400 sccm of Ar possessed more wrinkles than others. The wrinkles in the graphene enlarge the specific surface area, which also leads to increasing the specific capacitance for the graphene grown with 400 sccm of Ar. Furthermore, higher power leads to higher specific capacitance and longer time also leads to higher specific capacitance (See Fig. 2). The reason behind this behavior may be that the higher the power and time, the greater the percentage of the carbonyl functional group (C＝O) (See Fig. 3) at which fast faradic reactions take place and give rise to pseudo-capacitance, leading to higher specific capacitance.

References

1) W. Chen, Z. Fan, G. Zeng, and Z. Lai, Journal of Power Sources, 225 (2013) 251-256.

2) S. Kumar, N. McEvoy, T. Lutz, G. P. Keeley, V. Nicolosi, C. P. Murray, W. J. Blau, and G. S. Duesberg, Chem. Commun., 46 (2010) 1422-1424.

3) A. K. Mishra and S. Ramaprabhu, The Journal of Physical Chemistry C, 115 (2011) 14006-14013.

4) N. McEvoy, H. Nolan, N. A. Kumar, T. Hallam, and G. S. Duesberg, Carbon, 54 (2013) 283-290.