Controlling oxygen functional species of graphene oxide for an electro-oxidation of L-ascorbic acid
Research highlights
► Electrocatalysis of graphene oxide in Vitamin-C (L-AA) oxidation. ► Controlling of oxygen species onto graphene oxide. ► Clarification of the functionality of reactive graphene surface.
Introduction
Direct liquid fuel cells (DLFC) have been attracting much attention due to the increasing demand of green energy and highly efficient conversion power sources. There are several kinds of fuels used in DLFC such as methanol, ethanol, formic acid, propanol, ethylene glycol and dimethyl ether [1]. In particular, direct methanol fuel cells (DMFC) have been emerged as the best candidate for portable applications, but the problems of DMFC are the toxicity of fuel, the high overpotential at the anode and the methanol permeability [2]. Thus, in terms of biological compatibility, L-ascorbic acid (L-AA) fuel cells (DAAFC), also known as Vitamin-C, bring many points of interests [3], [4] such as non-toxic fuel, less fuel crossover and carbon based electrodes in the absence of precious metals. L-AA oxidation at the anode with two electron transfer process, is described below as [5]:L-ascorbic acid → dehydroascorbic acid + 2H+ + 2e−
Of late, graphene with the novel physicochemical properties has been studied in the field of electrochemical sensing and energy related applications such as hydrogen storage material, capacitor, rechargeable batteries and fuel cells [6], [7], [8], [9], [10], [11], [12], [13], [14]. Stergiou et al. [13] reported that the C/O atomic ratio at graphene oxide appears to be a critical parameter affecting their electrochemical characteristics. In addition, Wang et al. [14] showed that graphene synthesized by the chemical reduction of graphene oxide has less overpotential value for L-AA oxidation. We realize that the oxygen species on the carbon surface play an important role in the oxidation of L-AA, but the mechanistic origin is still unclear. Therefore, to clarify this issue, we try to study the effect of the reactive oxygen species on the L-AA oxidation by controlling the amount of oxygen species via an electroreduction method.
Section snippets
Experimental details
Graphene oxide sheet was synthesized by modified Hummer method [8], [9], [15], [16]. To control the amount of oxygen functional groups of graphene oxide, an electroreduction was applied. In the progress, the graphene oxide of 5 mg, deionized water of 5 ml and 5 wt.% Nafion solution of 15 mg were gently sonicated for 2 h and then, graphene oxide ink of 20 μL placed onto the ITO substrate was dried at 40 °C for 24 h.
The electro-reduction of graphene oxide was conducted in a conventional three electrode
Results and discussions
TEM and SEM images of as-prepared graphene oxide were shown in Fig. 1(a) and (b), respectively. TEM image clearly shows the formation of a couple of graphene oxide sheets and SEM image represents a typical wrinkle shape [17].
We performed LSV of as-prepared graphene oxide in 0.1 M KH2PO4 and as shown in Fig. 2(a), the cathodic current starts to rise at − 0.7 V and there is a current plateau around − 0.9 V. Fig. 2(b) is current-time profile applying the constant potential of − 0.8 V. During the
Conclusions
The graphene oxide was prepared by the chemical exfoliation and the electroreduction was successfully optimized to control oxygen functional groups for L-AA oxidation. Electrochemical measurements showed that the reduction time of 1000 s is the most appropriate condition leading to enhanced electro-oxidation activity of L-AA. The result is attributed to not only the existence of unpaired electron at a low oxidation level, but also edge and edge-like oxygen functional groups.
Acknowledgements
This work was supported by the National Research Foundation (NRF) grant funded by the Korea government (MEST) (2010-0022453).
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