A model for the degradation of polyimides due to oxidation

Polyimides, due to their superior mechanical behavior at high temperatures, are used in a variety of applications that include aerospace, automobile and electronic packaging industries, as matrices for composites, as adhesives etc. In this paper, we extend our previous model in [S. Karra, K. R. Rajagopal, Modeling the non-linear viscoelastic response of high temperature polyimides, Mechanics of Materials, In press, doi:10.1016/j.mechmat.2010.09.006], to include oxidative degradation of these high temperature polyimides. Appropriate forms for the Helmholtz potential and the rate of dissipation are chosen to describe the degradation. The results for a specific boundary value problem, using our model compares well with the experimental creep data for PMR-15 resin that is aged in air.Comment: 13 pages, 2 figures, submitted to Mechanics of Time-dependent Material

An insight into Faradaic phenomena in activated carbon investigated by means of the microelectrode technique

5 pages, 4 figures, 1 table.-- Printed version published Sep 2007.Cyclic voltammetry was performed on activated carbon particles in a microelectrode setup to investigate the behaviour of an activated carbon with oxygen functionalities. Quinoid type redox peaks were clearly seen in the potential region around −0.5 V vs. Hg/HgO. After polarization below −0.4 V, an anodic peak confirms previous studies using a pristine carbon, but in the present work much higher in intensity. In addition, a corresponding cathodic peak, not previously reported, was also found. The appearance of this pair of peaks in a functionalized carbon may be connected to reversible hydrogen adsorption together with Faradaic reactions involving oxygenated functional groups.This work has been performed with financial support from MEC (project MAT2004-03480-C02) and FICYT (project IB05-086-C1). V. Ruiz acknowledges a predoctoral research grant from FICYT.Peer reviewe

A study of Faradaic phenomena in activated carbon by means of macroelectrodes and single particle electrodes

Abstract.- The electrochemical behaviour of a chemically activated carbon with oxygencontaining surface groups was studied using a conventional macroelectrode configuration with disc electrodes and the single particle microelectrode technique. The results of both experimental set-ups were compare taking into account the visible peaks of the surface groups, capacitance and Faradaic currents. Galvanostatic cycling and cyclic voltammetry performed at different potential windows clearly indicated that the microelectrode configuration was more sensitive to Faradic phenomena (i.e. oxygenated functional groups). The incorporation of mainly CO 2 -evolving groups after positive polarization may cause the degradation of the carbon material, leading to a distortion in its capacitive behaviour as a result of a restriction of the available surface area. In a three-electrode cell, the formation of an active material into a manageable working electrode often involves a polymer. The elaboration of disc-type electrodes (macroelectrodes) is generally used for the characterization of the performance of an active material. The addition of polymer to the activated carbon may cause a reduction in accessible surface area for the electrolyte (an effect that is strongly influenced by the way in which the polymer and the carbon are mixed) [3] investigated a chemically activated carbon noting the Faradaic behaviour of the material at the potentials where quinone/hydroquinone reactions are believed to take place. This carbon had up to 3.5% oxygen and 1.2 mmol CO/g as determined by temperature-programmed desorption (TPD). They found no distinct peaks in the interval 0.2 to -0.8 V vs. Hg|HgO. Smeared out humps and variations of both anodic and cathodic currents with surface functionality concentrations were attributed to the oxidation/reduction of these functionalities. Béguin et al. [4] studied activated carbons with 10 % and 6 % oxygen after activation with KOH. The activated carbons were mixed with polymeric binder and graphite before the preparation of the electrodes. They did not find any distinct peaks but attributed anodic and cathodic humps (smeared out in the scans up to -1.7 V) found at -0.4 V and -1 V vs. Hg|HgO to the oxidation and reduction of surface functionalities. Jurewicz et al. In the single particle microelectrode technique [6], a single carbon particle is used as working electrode. Thus, the information obtained corresponds exclusively to the activated carbon. Two key aspects in the cyclic voltammetry experiments performed in the microscopic configuration are the low current intensities registered, with the subsequent reduction in the ohmic drop (IR), and the increased sensitivity to both the Faradaic and non-Faradaic currents generated within the pore system. It has previously been shown that the microelectrode technique may yield more distinct peaks when Faradaic phenomena are studied due to smaller potential gradients in the particles themselves compared to those of the macroscopic electrodes Hg|HgO. The nature of the active material studied in this paper differed in structure and in chemical composition from the carbide-derived activated carbon used in previous studies These results raise the question whether differences in peak observations between the two methods are due to the higher sensitivity of the microparticle method or simply due to circumstantial variations in the activated carbon chemistry. In order to clarify this 4 point, experiments were carried out, for the first time to our knowledge, so that both configurations (disc-type electrode and single-particle electrode) with the same active material and under similar experimental conditions (electrolyte solution, current density and potential window) could be compared. 2.-EXPERIMENTAL 2.1.-Material A chemically activated carbon was produced from mesophase pitch (AR24). This pitch, derived from synthetic naphthalene and thus low in ash content, was chemically activated with KOH (3:1 KOH to carbon ratio) at 700 °C for 1 h. The activation procedure and the characteristics of the activated carbon obtained have been previously described 5 The electro-oxidation of the activated carbon was carried out in a conventional macroelectrode system, without using any binder. Around 100 mg of material was subjected to electro-oxidation at +0.3 V in a N 2 bubbling solution of 6M KOH, the same electrolyte as for the other experiments. After the electro-oxidation process, the sample was washed using distilled water and then dried in a vacuum oven at 110 °C overnight. The amount of oxygen functional groups incorporated after this electrochemical treatment was evaluated by TPD under inert atmosphere (He). Changes is surface area were negligible as the BET surface area decreased from 2000 to 1989 m 2 /g. 2.2.-Conventional three-electrode cell configuration Coin-type electrodes were prepared by mixing the activated carbon (90 wt. %) with polyvinylidene fluoride, PVDF (10 wt. %). The working electrodes were fabricated from this mixture by pressing ~30 mg of mixture into 8 mm (diameter) discs with a thickness of 200 μm. Platinum and Hg|HgO were used as the counter and reference electrodes (0.098 V versus NHE), respectively. The electrochemical measurements were performed in a 6M KOH aqueous solution. 2.3.-Single particle microelectrode The cell for the single particle microelectrode technique is mounted on a microscope, and a single particle of the activated carbon is brought into contact with a highly conductive carbon fibre attached to a gold probe surrounded by de-aired electrolyte. This acts as the working electrode. A Ni felt with a larger surface area than the working electrode was used as counter electrode. Hg|HgO was used as the reference electrode. 6 Particles of the activated carbon (~100 µm) were used for the single particle experiments. For a detailed description of the single particle microelectrode technique and further details regarding the absence of electrochemical activity of the carbon fibre, see references 2.4.-Experimental procedures The electrochemical measurements were performed in both cases with an Autolab potentionstat supplied by Eco Chemie. Cyclic voltammetry experiments were carried out at a sweep rate of 1 mV/s and galvanostatic cycling was performed at a current density of 230 mA/g. The potential window used was kept inside the limits of the thermodynamic stability of the electrolytic medium, 6M KOH. All the potentials refer to Hg|HgO reference electrode. Several discs and particles were analyzed in order to obtain reliable data. Reproducible results were obtained in all of the cases. Using a digital microscope it was possible to obtain several images of the particles studied. With the help of the carbon fibre, controlled with a micromanipulator, each particle was moved around at different angles and its volume was estimated. Additionally, the apparent density of the active material was evaluated according to the method described elsewhere 8 The cut-off value increased towards more negative values (starting at 0, -0.2 V and progressively increasing to 0, -0.9 V). The voltammetry characteristics obtained show significant differences for each configuration Previous studies have suggested that smaller electrodes may have a more uniform potential From This behaviour is more pronounced when the potential range shifts to more negative values. The voltammograms also show a slight increase in the anodic current as the potential window moves towards the cut-off value, which points to reversible hydrogen adsorption Galvanostatic cycling in the potential interval 0 to -0.7 V at a current density of 230 mA/g also reveasls significant differences between both devices In the cycle corresponding to the macroscopic device, only a slight curvature in the charging branch can be observed. This non-capacitive behaviour is due to Faradaic currents occurring at the same time as the double layer charging current Faradaic currents are usually attributed to both reversible (e.g. reversible redox reactions of surface functional groups of the activated carbon or hydrogen adsorption) and irreversible processes (e.g. hydrogen evolution or irreversible reduction of functional groups in the carbon surface). The linearity of the discharge branch seems to indicate that there are irreversible processes involved in the charge. This may be correlated with the asymmetry of the humps observed in the voltammograms at around -0.5 V (see The electrochemical processes that take place during positive polarization in carbon materials have been studied in literature In addition to the functionalities already present, other oxygen containing groups introduced into the carbon matrix by positive polarization are believed to be present. For this reason, characterization of the raw material after electro-oxidation was performed. The type and amount of functionalities introduced into the activated carbon after cycling at +0.3 V was investigated by electro-oxidation of the activated material without adding any binder. The overall increase in oxygen determined by direct elemental analysis is also noticeable, as it changes from 3.5 wt. % in the starting material to 15 wt. % in EO-AC+0.3. The decrease in capacitance observed after aggressive oxidation of the material is in agreement with previous results obtained by Zuleta et al. [7]. The suggested pathway for such reactions involves a loosely bound CO 2 group that dissociates from the carbon surface and reacts further in the electrolyte with OH -to form CO − 2 3 . Thus, excessive oxidation leads to chemical changes in the structure of the carbonaceous material, affecting subsequent electrochemical behaviour, including the mobility of the ions. 12 Furthermore, the humps indicative of Faradaic processes that are the result of the functional groups present in the material (due to the activation process, illustrated in 4.-CONCLUSIONS The specific capacitance values obtained by both techniques are similar. However, the single particle microelectrode technique revealed the occurrence of several important phenomena not clearly visible when using macroscopic disc electrodes to characterize the activated carbons. The microelectrode technique also disclosed the pseudocapacitance caused by Faradaic phenomena in basic media, which are be ascribed to the redox reactions of oxygenated groups and to the irreversible evolution of hydrogen. The electro-oxidation of the carbon surface at positive potentials (+0.3 V) changes the capacitive behaviour of the material due to pore blockage, which hinders access of the electrolyte to the porous network. However, electro-oxidation did not significantly affect the surface functionality of the single bonded oxygen groups. 13 ACKNOWLEDGEMENTS.-This work has been carried out with financial suppor

A study of Faradaic phenomena in activated carbon by means of macroelectrodes and single particle electrodes

6 pages, 7 figures.-- Printed version published Jul 1, 2008.The electrochemical behaviour of a chemically activated carbon with oxygen-containing surface groups was studied using a conventional macroelectrode configuration with disc electrodes and the single particle microelectrode technique. The results of both experimental set-ups were compare taking into account the visible peaks of the surface groups, capacitance and Faradaic currents. Galvanostatic cycling and cyclic voltammetry performed at different potential windows clearly indicated that the microelectrode configuration was more sensitive to Faradic phenomena (i.e. oxygenated functional groups). The incorporation of mainly CO2-evolving groups after positive polarization may cause the degradation of the carbon material, leading to a distortion in its capacitive behaviour as a result of a restriction of the available surface area.This work has been carried out with financial support from MEC (Project MAT2004-03480-C02) and FICYT (Project IB05-086-C1). V. Ruiz acknowledges a predoctoral research grant awarded by FICYT.Peer reviewe