Electrochemical study of anthraquinone groups, grafted by the diazonium chemistry, in different aqueous media-relevance for the development of aqueous hybrid electrochemical capacitor
Introduction
Electrochemical capacitors are energy-storage devices that are ideally suited for rapid storage and release of energy. Compared with conventional capacitors, the specific energy density of these devices is several orders of magnitude higher and also they have a higher specific power density than most conventional batteries [1], [2]. Among the different type of electrochemical capacitors, electrochemical double layer capacitors (EDLCs) are very promising for use in high power electronic devices and electric vehicles [3]. These systems are based on the double layer structure at the electrode/electrolyte interface where charges accumulated by the electrode are compensated by electrolyte ionic species of opposite charge [1], [4]. High surface area carbons are generally used as the capacitive material because of their low cost, excellent cycle life and high specific surface area due to their microporous structure [4], [5]. Several studies have focused on the modification of carbon materials in order to increase the power and energy densities of EDLCs [6], [7], [8], [9], [10], [11], [12], [13]. As an example, surface modification with electroactive molecules such as anthraquinone can increase the performance of carbon electrodes by adding a faradaic contribution to the double layer capacitance [12], [13]. The modification can be carried out by using the diazonium cations electrochemical reduction method developed by Pinson and coworkers [14], [15]. This method involves the formation of aryl radicals, which subsequently react with the carbon surface by forming grafted layers according to Scheme 1 which describes the grafting of anthraquinone groups [14], [15]. This electrochemical modification of carbon surface has been extensively studied by Tammeveski and coworkers during the last decade [16], [17], [18]. Moreover, carbon modification can be also performed by chemical reduction of the diazonium ions [19], [20], which is very attractive and convenient for carbon black modification.
The resulting improvements of the electrode performances can be used for the development of hybrid EDLCs [3]. Anthraquinone-modified carbons are well suited for the negative electrode when combined in an hybrid electrochemical capacitor with a positive electrode consisting of MnO2, PbO2 or Ni(OH)2 [3]. The electrolyte used in hybrid EDLC should be optimized for both the negative and positive electrode. For instance, a MnO2 electrode requires the utilization of neutral electrolyte and PbO2 electrode can only be used in acidic electrolyte. As anthraquinone molecules are pH sensitive, the nature of the electrolyte will have a great influence on their redox reactions [21], [22], [23], [24], [25]. Physical and electrochemical behavior of grafted molecules can also be influenced by various factors such as the reaction conditions used for the formation of the diazonium ions [12], [20], [26] and the substrate [15].
The purpose of this article is to study the influence of the electrolyte pH on the electrochemical activity of grafted AQ molecules. Grafted layers were formed electrochemically and chemically on glassy carbon electrode and carbon powders (carbon black), respectively (Scheme 1). 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 relevance of the results for the use of anthraquinone-modified carbon in electrochemical capacitor is discussed.
Section snippets
Reagents
Black Pearls 2000 was obtained from the Cabot Corporation (named BP in the following sections), anthraquinone, 1-aminoanthraquinone, 2-aminoanthraquinone and tert-butylnitrite (90% solution in acetonitrile) were purchased from Aldrich. Unless otherwise stated, all reagents were obtained from Aldrich and were used without further purification. All solutions and subsequent dilutions were carried out using deionized water (Barnstead Nanopure II). Three sets of electrolytes were used, sulfuric acid
Electrochemical grafting of AQ on glassy carbon electrode
The electrochemical grafting of anthraquinone moieties on a glassy carbon electrode was carried out by using two different amine precursors; 1-amino-anthraquinone (1-AAQ) and 2-amino-anthraquinone (2-AAQ). These two amines allow the grafting of the same molecule, the only difference lies in the position of the bond between the grafted molecule and the carbon substrate (Scheme 2).
Cyclic voltammograms presented in Fig. 1a and b show the presence of well-defined cathodic waves located at 0.5 and
Cyclic voltammetry behavior of AQ-modified glassy carbon electrode and carbon powder
A close examination of the cyclic voltammograms of the AQ-modified glassy carbon and composite electrodes revealed some interesting features. Firstly, the shape of the CV for one type of electrode is affected by the electrolyte pH. Secondly, a well-defined CV is observed for the AQ-modified glassy carbon electrode in alkaline electrolyte whereas the electrochemical response for the modified carbon powder based composite electrode is more complex. On the other hand, the opposite is true in
Conclusions
The electrochemical behavior of anthraquinone-modified glassy carbon and carbon powder based composite electrode was investigated as a function of the solution pH. In agreement with recent reports [25], [29], the cyclic voltammograms can be quite complex and are influenced by the pH of the electrolyte as well as the nature of the substrate onto which the anthraquinone molecules are grafted. Most of the previous studies were related to application of AQ modified surfaces as pH sensors [36] and
Acknowledgments
This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) for a Discovery Grant to DB and by “Le Ministère du Développement économique, de l’Innovation et de l’Exportation” (MDEIE) from the Québec government. NanoQAM is also acknowledged.
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