Electrochemical study of anthraquinone groups, grafted by the diazonium chemistry, in different aqueous media-relevance for the development of aqueous hybrid electrochemical capacitor

Abstract

The cyclic voltammetry behavior of anthraquinone (AQ) modified-glassy carbon and carbon powder-based composite electrodes was investigated as a function of the solution pH. The modification of glassy carbon electrode and carbon powder was performed by electrochemical and chemical reduction of the diazonium ions, respectively. The modification was performed by using 1-diazoanthraquinone and 2-diazoanthraquinone as anthraquinone precursor and yielded materials that differ in the point of attachment of the anthraquinone moieties. 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 shape of the cyclic voltammogram is affected by the solution pH. A well-defined cyclic voltammogram is observed for the AQ-modified glassy carbon based composite electrode in alkaline electrolyte whereas the electrochemical response for the carbon powder-based composite electrodes is more complex. On the other hand, the opposite is true in acidic media as a well-defined cyclic voltammogram is seen for the composite electrode. The amount of electrochemically addressable AQ molecules was found to depend on the solution pH. The relevance of the results for the use of anthraquinone-modified carbon in electrochemical capacitor is discussed.

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.