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End date: 2019 × Start date: 2016 × DDC: 540 × Type: article × Publisher: Amsterdam : Elsevier ×

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    Performance evaluation of conductive additives for activated carbon supercapacitors in organic electrolyte
    (Amsterdam : Elsevier, 2016) Jäckel, Nicolas; Weingarth, Daniel; Schreiber, Anna; Krüner, Benjamin; Zeiger, Marco; Tolosa Rodriguez, Aura Monserrat; Aslan, Mesut; Presser, Volker
    In this study, we investigate two different activated carbons and four conductive additive materials, all produced in industrial scale from commercial suppliers. The two activated carbons differed in porosity: one with a narrow microporous pore size distribution, the other showed a broader micro-mesoporous pore structure. Electrochemical benchmarking was done in one molar tetraethylammonium tetrafluoroborate in acetonitrile. Comprehensive structural, chemical, and electrical characterization was carried out by varied techniques. This way, we correlate the electrochemical performance with composite electrode properties, such as surface area, pore volume, electrical conductivity, and mass loading for different admixtures of conductive additives to activated carbon. The electrochemical rate handling (from 0.1 A g−1 to 10 A g−1) and long-time stability testing via voltage floating (100 h at 2.7 V cell voltage) show the influence of functional surface groups on carbon materials and the role of percolation of additive particles.
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    Thermal conductivity and temperature profiles in carbon electrodes for supercapacitors
    (Amsterdam : Elsevier, 2014) Burheim, Odne S.; Aslan, Mesut; Atchison, Jennifer S.; Presser, Volker
    The thermal conductivity of supercapacitor film electrodes composed of activated carbon (AC), AC with 15 mass% multi-walled carbon nanotubes (MWCNTs), AC with 15 mass% onion-like carbon (OLC), and only OLC, all mixed with polymer binder (polytetrafluoroethylene), has been measured. This was done for dry electrodes and after the electrodes have been saturated with an organic electrolyte (1 M tetraethylammonium-tetrafluoroborate in acetonitrile, TEA-BF4). The thermal conductivity data was implemented in a simple model of generation and transport of heat in a cylindrical cell supercapacitor systems. Dry electrodes showed a thermal conductivity in the range of 0.09-0.19 W K-1 m-1 and the electrodes soaked with an organic electrolyte yielded values for the thermal conductivity between 0.42 and 0.47 W K-1 m-1. It was seen that the values related strongly to the porosity of the carbon electrode materials. Modeling of the internal temperature profiles of a supercapacitor under conditions corresponding to extreme cycling demonstrated that only a moderate temperature gradient of several degrees Celsius can be expected and which depends on the ohmic resistance of the cell as well as the wetting of the electrode materials.
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    Application of sulfonated nanoporous carbons as acid catalysts for Fischer esterification reactions
    (Amsterdam : Elsevier, 2015) Tamborini, L.H.; Militello, M.P.; Balach, J.; Moyano, J.M.; Barbero, C.A.; Acevedo, D.F.
    Heterogeneous acid catalysts were prepared by sulfonation of nanoporous carbons (NPCs). The NPCs were produced by pyrolysis of resorcinol–formaldehyde nanoporous resins (NPRs). The NPRs were synthesized as wet gels by condensation of resorcinol and formaldehyde in a sol–gel polycondensation using Na2CO3 as catalyst. A cationic polyelectrolyte (poly(diallyl dimethyl ammonium chloride)) was used as pore stabilizer, allowing to dry the gels in air without any special procedures. Five NPRs with different properties were synthesized by varying the monomer to catalyst ratio (Resorcinol/Na2CO3, R/C). The morphological and textural characterizations of the NPCs were performed by scanning electron microscopy and nitrogen adsorption–desorption isotherms. The results indicate that using a molar ratio of R/C = 200, a nanoporous carbon NPC with large surface area (695 m2/g) is obtained. This NPC was sulfonated by reaction with three different sulfonating agents: (i) concentrated sulfuric acid; (ii) fuming sulfuric acid and (iii) chlorosulfonic acid in order to obtain a novel Fischer esterification catalyst. The amount of acid groups attached to the NPC surface was determined by titration using a modified Boehm method. The catalytic activity, for Fischer esterification reaction of different sulfonated NPCs, was compared with sulfonated NPRs, sulfonated commercial porous carbons and polymeric acid catalyst (cationic ion exchange resins, sulfonated fluoropolymers). The modification of NPCs with concentrated sulfuric acid seems to render the materials with more catalytic activity. The best sulfonated material NPC (PC200-H2SO4) shows a high catalytic activity for the esterification of acetic acid (90.8%) and oleic acid (60.6%) with ethanol. The conversion and conversion rate values are better than commercial acid catalysts. The results suggest that sulfonated NPC catalysts are promising materials for the synthesis of biodiesel and related reactions.