Thursday, 2 June 2016: 15:20
Sapphire Ballroom E (Hilton San Diego Bayfront)
In recent years, capacitors and rechargeable batteries have attracted attention mainly due to the urgent need for efficient and low emission energy storage devices. Supercapacitors have been investigated due to their pulse power capability, long life cycle, and high kinetic of charge/discharge making them an alternative in the development of friendly energy storage devices. Among the materials which can be using in these devices, conducting polymer has been proposed as active material. In these materials, two different coupled charge transport occur: electron transport inside the polymer chain and an injection and ejection of counter ions to compensate the generated electronic charges. This last one is generally accepted as the slow step of the process. Besides, the same process could lead to the electrochemical degradation of the films. Considering the exposed above, this work aims at the investigation of the ageing process of electrochemical capacitors using LBL POMA/PTAA films and its effect on the electrochemical stability and compare them with poly(o-methoxyaniline) casting film. The films were preparation by both methods, LBL and casting, were followed by UV-VIS spectroscopy and by mass measurements. Their morphologies was measured by Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (FEG-SEM). The electrochemical performance was measured using galvanostatic, cyclic voltammetry and electrochemical impedance spectroscopy techniques. The results show that POMA/PTAA LBL films have a higher electrochemical stability than POMA casting films which is an important property to build electrochemical devices. POMA casting material has a specific capacitance of 98 F g-1 at the beginning of charge/discharge measurement, and, after 3000 cycles, this value falls down to 22 F g-1. On the other hand, LBL POMA/PTAA film presents an initial specific capacitance of 99 F g-1, and, after 3000 cycles of charge/discharge, this value is 98 F g-1. Which is about For LBL film, the initial capacitance value is close to the cast film, on the other hand, a value less than 1% degradation after 3000 cycles. Moreover, the cyclic voltammograms obtained for both films after 3000 cycles reveals that POMA casting film presents no more redox process while POMA/PTAA LBL film shows a small change in its VC fingerprint. Impedance data shows important changes upon electrochemical ageing. Among them, the most important variation occurs in the interface pore/polymer chains. Besides, it was not observed any significant change in the capacitance of the electrical double layer, meaning that the electroactive area of polymers is constant after the ageing which is in agreement with the morphological data. For LBL films, the charge transfer resistance and the charge transfer capacitance, which are related to charge compensation during the electrochemical process, do not change which corroborate the stability of the material. For casting POMA sample, otherwise, there are important increase in the charge transfer resistance and a decrease in the charge transfer capacitance which indicate severe degradation due to the ionic transport from the pores to the polymer chains. In this case, the decrease of charge transfer capacitance values could be related to a decrease in the amount of the ions to compensate the positive charges generated in the polymer chains. In the same direction, the increase of 1000 times in the charge transfer resistance for the casting film indicates also an important decrease in its electrochemical activity. Finally, casting POMA samples presents also a 10 times increase in the polymer resistance, indicating also a polymer chain degradation besides the pore/polymer chain interface. Then, to explain the results described in the previous Figures, we consider the fact that generally, in conducting polymers, the oxidation (reduction) processes leads to the intercalation (deintercalation) of counter ions to compensate the generated charge in the material. This explanation describes the typical behavior observed for POMA casting film. From a different point of view, in POMA/PTAA LBL films, the PTAA layer provides, at least partially, the counter ion balance to the positive charge generated in the POMA chains. As consequence, there is an important decrease in the ion transport from the solution to compensate the positive generated charges leading to an increase of the LBL material electrochemical stability.