Preparation and modification of polyacrylonitrile microcellular foam films for use as electrodes in supercapacitors

Results obtained for polyacrylonitrile microcellular foam thin films deposited on carbon fibers are presented. It appears that surface modifications by CO2 flow or water steam ar high temperature lead to an increase of the film surface area, as well as a proportional increase of capacitance. The films were characterized by Fourier transform infrared spectroscopy, scanning electron microscopy. and the Brunauer-Emmett-Teller technique. Extensive electrochemical studies (cyclic voltnmmetry, ae impedance, galvanic cycles, and cycle life tests) were performed both in aqueous and organic media in order to evaluate their use ax electrodes in a supercapacitor. The importance of the nature of the electrolyte was demonstrated using lithium perchlorate and tetraalkyl-based salts. The high performances of these activated materials were correlated both to their high surface area and their microporous distribution. (C) 2000 The Electrochemical Society. S0013-4651(00)04-096-9. All rights reserved.open1130sciescopu

Oxygen reduction in acid media catalysed by heat treated cobalt tetraazaannulene supported on an active charcoal: Correlations between the performances after longevity tests and the active site configuration as seen by XPS and ToF-SIMS

The influence of heat treatment (HT) on CoTAA supported on SX Ultra samples before and after electrochemical tests has been investigated using the rotating disk electrode (RDE) technique and cyclic voltammetry, The heat treated sample configuration was also scrutinized by two different surface spectroscopic techniques: X-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectrometry (ToF-SIMS). After a 100 h polarization period at 500 mV (RHE) (diffusion activation domain), optimal performances were found to occur with the 600 degrees C heat treated samples (HT 600). However, the most salient feature was the observation of a fair activity and stability for the 800 degrees C heat treated samples (HT 800). Similar behavior was exhibited both with H2SO4 and Nafion(R) electrolytes. Collected data could be interpreted on the basis of RDE and cyclic voltammetry theories, suggesting a fast dioxygen mass transfer within the catalytic material in comparison with interfacial process kinetics. After these electrochemical tests a complete demetallation was taking place, as seen by XPS within the HT 800 samples. A protonation process was also clearly identified on the nitrogen ions. An attempt has been made to propose a catalytic cycle in which variations of the different nitrogen oxidation states are occuring. The major obstacle encountered lay in the lack of XPS evidence of the high nitrogen oxidation states. From ToF-SIMS data the existence of bonding between C, N and O can be established with NO3- formation. Under the same treatment conditions this process occurs differently with H(2)TAA precursor. From the collected data it turned out that catalytic cycles involving nitrogen ions possess a lower turnover than those with cobalt ion participation