Electroactive polymer-based electrochemical capacitors using poly(benzimidazo-benzophenanthroline) and its pyridine derivative poly(4-aza-benzimidazo-benzophenanthroline) as cathode materials with ionic liquid electrolyte
Highlights
▸ A new processing technique gives polymer films with superior transport properties. ▸ Using this technique, Type IV capacitors were constructed. ▸ BBL-based capacitors store slightly more energy than a comparable Py-BBL device at low rates. ▸ The Py-BBL devices deliver much higher energy at higher rates. ▸ The Py-BBL devices were found to last least five times as long as the BBL devices.
Introduction
Conjugated polymers have a wide range of uses due to the property changes of the materials in their different states of oxidation. From neutral light-harvesting semiconductors to doped transparent electrode materials, the uses are as wide and varied as the number of polymer systems. In the area of charge storage, as portable electronic devices become more complex, and as renewable energy continues to gain interest, there will be interest and strong demand for any charge storage device that can provide both higher power and energy density. Specifically, properties that are unique to capacitors (high power (high C-rates)) are increasingly gaining interest. Traditional reasons for using polymer-based systems include the potential for lighter weight, lower cost, more damage resistance and more flexible packaging.
Capacitor (NEC) and Ultracapacitor (Pinnacle) were trademarks of early electrochemical capacitor companies in the 1970s [1]. These terms have since been broadened to describe any double layer or redox capacitor with specific energy and specific power intermediate to batteries and electrostatic capacitors respectively. Typically, an ultracapacitor is a device comprised of two carbonaceous electrodes, and a supercapacitor is a similar device in which two carbonaceous electrodes are catalyzed with metal oxides such as RuO2. The term electrochemical capacitor is used more generally to describe any capacitor that uses electrochemical reduction/oxidation processes to store charge. Electroactive polymer-based electrochemical capacitors (EPECs) were popularized by Rudge et al. in 1994 [1]. While there are many earlier papers that fall under the guidelines set forth by Rudge, the devices were initially described as batteries by the authors [1] and the terms ultracapacitor, supercapacitor and electrochemical capacitors are sometimes used interchangeably. Since then, a wide range of materials and architectures have been explored in electroactive polymer based capacitors.
EPECs are classified by the materials used on the anode and cathode [2], [3], [4], [5]. Types I and II utilize p-doping polymers on both electrodes. In the charged state, one electrode is fully charged and the other electrode is completely neutral. In the discharged state, each electrode is at a partially charged state. This not only limits the capacity of the cells but also the voltage and power, since only modest average voltages are possible. In a Type III device, the same polymer is used on both electrodes, but the polymer must both n- and p-dope, and both processes must be stable. This allows a higher voltage and power since voltage of the cell is raised from 0.5 V to over 2 V in most cases [6]. We have previously introduced a Type IV EPEC category, in which different polymers are used on the anode and cathode [6], allowing for separate optimization of both the n-doping and p-doping layers. Electroactive polymer-based electrochemical capacitors (EPECs), also offer the potential for increased charge storage capacity, because the entire volume of the polymer should be available for charge storage, whereas in traditional inorganic metal oxides, only the surfaces of the particles participate in the charge storage process [7].
Poly(benzimidazo benzophenanthroline) (BBL) [8] was first introduced as a temperature resistant insulating polymer. The material is difficult to process due to its poor solubility. Jenekhe [9] later found that BBL can be dissolved at higher concentrations in nitromethane with Lewis acids such as aluminum and gallium chloride, with solution concentration as high as 20 weight percent allowed for more facile processing of thin films. The first electrochemical analyses performed on BBL were in aqueous acid [2], [3], [4], [5]. Because the electron affinity of BBL is around 4.4 eV, reductive electrochemical doping is performed at a more positive potential than most electroactive polymers. Babel and Jenekhe [4] demonstrated BBL’s utility in field effect transistors.
The ability of n-doping materials to undergo many reduction cycles is important in EPECs. In fact most Type III and Type IV EPECs are limited by the stability of the n-doping material. While BBL may be a promising material n-doping, tuning the HOMO and LUMO of BBL by adding nitrogen to the backbone—making a pyridine derivative of BBL (Py-BBL)—may further improve the electrochemical stability of the reduction process. While it is likely that incorporating nitrogen may bring the reduction potential closer to 0, this might be a worthwhile trade-off.
Ionic liquid electrolytes such as 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIBTI) have been used by our group in the past and have demonstrated to be quite effective supporting electrolytes [6]. Reasons for using ionic liquid electrolytes include their low volatility, wide temperature use window, and good electrochemical and thermal stability [10].
In this work, we present the synthesis and characterization of poly(4-aza-benzimidazo benzophenanthroline) (Py-BBL), a new pyridine derivative of the cathode material BBL. In addition, we present an analysis and comparison of thin film Type IV EPECs with a working voltages of >2.0 V using BBL and Py-BBL as cathode materials.
Section snippets
Materials
1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIBTI) was synthesized from lithium bis(trifluoromethylsulfonyl)imide and 1-ethyl-3-methylimidazolium chloride and purified via column chromatography as reported previously [11], [12]. 3,4-Propylenedioxythiophene (ProDOT) was prepared and purified according to a literature procedure [13]. 2,3,5,6-Tetraamino pyridine trihydrogen chloride monohydrate was obtained by the reduction of 2,6-diamino-3,5-dinitro pyridine with Sn0 and
Results and discussion
The synthesis of Py-BBL (Fig. 2) parallels the synthetic scheme that is used in the synthesis of BBL. The extra nitrogen lowers the energy level of the highest occupied molecular orbital (HOMO) by 0.3 eV.
In an effort to increase the initial electro-activity of BBL and Py-BBL films, EMIBTI was added to the polymer solution during the film casting process. The resultant films exhibit significantly higher electroactivity [18]. Fig. 4 shows SEMs of BBL cast without and with EMIBTI. The films cast
Conclusions
A higher nitrogen analog of the ladder polymer BBL (Py-BBL) was prepared. Films of BBL and Py-BBL were co-cast from solution with a room temperature ionic liquid, EMIBTI. This process produced polymer films with superior performance at high charging rates; did not require a break-in period, and resulted in films with higher charge storage capability than the standard films. These respective films were then incorporated into Type IV electrochemical capacitors. Py-BBL-based electrochemical
Acknowledgments
The authors thank the Office of Naval Research (Drs. M. Anderson and P. Armistead) and the China Lake ILIR Program for the financial support of this project. The authors also thank the reviewers of our original manuscript for their suggestions and Paul Goodman for editing the revised manuscript.
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