Effects of the loading and polymerization temperature on the capacitive performance of polyaniline in NaNO3
Introduction
Due to the demand of power sources delivering high power or pulse power for limited time intervals with an acceptable capacity, the development of supercapacitors has been the interesting subject of many researches [1], [2], [3]. These devices have been recognized as a potential device improving the performance and service life of fuel cell and batteries [4], which are applicable in several systems, such as hybrid electric vehicles, fuel cells, cellular phones, PDAs etc. In addition, supercapacitors are considered to be a potential device in several future applications (e.g. an energy manager and tuner in electric systems).
Due to the Faradaic reactions, the energy density of a supercapacitor consisting of electroactive materials with several oxidation states or structures (e.g. transition metal oxides and conducting polymers) [1], [2], [5], [6], [7] is expected to be higher than that of a double-layer capacitors. Hence, conducting polymers, such as polyaniline (PANI), polypyrrole (PPy), polythiophene (PTh), and their derivatives with a large degree of π-orbital conjugation and various oxidation structures have been considered as the electroactive materials for the application of supercapacitors [1], [2], [3], [8], [9], [10], [11], [12], [13], [14]. Moreover, the mechanisms of redox transitions and stability of conducting polymers have been widely investigated [2], [3], [8], [9], [10], [11], [12], [13], [14], [15], [16] since these materials have been employed as cathodes in rechargeable batteries. From these studies, the electronic/ionic conductivity, electrochemical reversibility, doping/dedoping processes and degradation are strongly dependent upon the polymer structure. Since the performance of electrochemical supercapacitors is mainly controlled by the electrochemical kinetics of the redox couples within the electroactive materials [1], [2], [5], [7], [12], [13], [14], the electrochemical characteristics and stability of these electroactive materials have to be systematically investigated before practical usage. Based on the above viewpoints, effects of the preparation methods and conditions, the media of electrochemical tests and the heterocyclic structure of the copolymers on the capacitive performance (e.g. electrochemical reversibility and stability) of conducting polymers are worthy being studied [10], [17].
PANI and its derivatives are one of the most important conducting polymers because they have lots of advantages, such as ease of polymerization in aqueous media [18], good stability in air [19], simplicity in doping/dedoping [20], etc. In addition, these polymers have been employed as electrode materials in electrochromic devices due to the variable and controllable electrochromic property [21], [22]. Moreover, these polymers are also proposed as potential candidates for the application of electrochemical supercapacitors from the electrochemical and AC-impedance studies [1], [2], [3], [8], [9], [11], [12], [14], [23] although they are usually employed as the cathode materials for rechargeable batteries [24], [25]. The employment of PANI as electrode materials for the electrochemical supercapacitors offers several advantages in comparison with some other similar systems (e.g. RuO2, IrO2) [2]. First, PANI and its derivatives generally have a good conductivity. Hence, current collectors in a dispersed-matrix structure are not necessarily required [2]. Second, these polymers are relatively cheap and the preparation costs are competitive. Third, the specific capacitance (capacitance per gram of polymers) and redox reversibility of these polymers are high [11], [12], [14]. Thus, these PANI-like polymers are considered as potential candidates for the electrode material of electrochemical supercapacitors.
PANI has the general formula [(BNHBNH)y(BNQN)1−y]x, in which B and Q, respectively indicate the C6H4 ring in the benzenoid and quinonoid forms [26]. These structures can be identified by X-ray photoelectron spectroscopy (XPS) since it has been widely recognized as a powerful surface analysis tool for identifying the influences of environmental and fabricating variables on the dc conductivity and the doping/dedoping mechanisms of PANI and the physicochemical interactions of PANI interchains [27], [28]. In addition, the amount of side chains and interchain links of conducting polymers could be quantitatively measured by this unique technique [13], [29]. The present work aims to correlate the capacitive behavior of PANI with its textural structure examined by XPS, which includes the oxidation structure (the imine-like nitrogen/amine-like nitrogen ratio), the doping level (N+/N), and the defect density (CO and CO groups). In addition, the ideally capacitive characteristics of PANI-coated electrodes (denoted as PANI/C) with an acceptable capability in an approximately neutral NaNO3 solution are systematically investigated to show its applicability in the electrochemical supercapacitors. Effects of the polymerization temperature on the capacitive and degrading behavior of PANI films during the charge–discharge processes were systematically compared using cyclic voltammetry and chronopotentiometry. The effect of polymer loading on the capacitive characteristics was also investigated. Finally, the morphology of PANI films with different charge densities and different temperatures of polymerization was examined by a scanning electron microscope (SEM).
Section snippets
Experimental
The 10×10×3 mm graphite supports (Nippon Carbon EG-NPL, N.C.K., Japan) were first abraded with ultrafine SiC paper, rinsed in an ultrasonic bath of water for 10 min, then, etched in a 0.5 M H2SO4 solution at room temperature (r.t.) for 10 min, and finally rinsed with the ultrasonic bath of water for 40 min. The exposed geometric area of these pretreated graphite supports is equal to 1 cm2 and the other surface areas were coated with PTFE (polytetrafluorene ethylene) films. These supports were
Capacitive behavior of PANI with different charge densities of polymerization
Typical cyclic voltammograms measured in 1 M HNO3 and 1 M NaNO3 at 25 mV s−1 for a PANI/C electrode polymerized in a 1 M HNO3 solution containing 0.2 M aniline at 25 °C with the polymerization charge density of 0.45 C cm−2 are shown as curves 1 and 2 in Fig. 1, respectively. Curve 1 shows the typical voltammetric responses of PANI in strongly acidic media, e.g. the passive responses in the hydrogen adsorption/desorption region, a pair of redox peaks (labeled as C1/A1) between 0 and 0.3 V, the
Conclusions
PANI polymerized at temperatures between 4 and 50 °C with different polymerization charge densities showed the ideally capacitive behavior (i.e. highly electrochemical reversibility and high-power property) between −0.2 and 0.6 V in 1 M NaNO3. The PANI film prepared at a lower temperature exhibits a better electronic conductivity due to the higher degree of doping and the lower density of defects within the polymer, which were confirmed by the XPS study. These PANI films have the specific
Acknowledgements
The financial support of this work, by the National Science Council of the Republic of China under contract no. NSC 90-2214-E-194-012, is gratefully acknowledged.
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