Strategies to Hierarchical Porosity in Carbon Nanofiber Webs for Electrochemical Applications

Morphology and porosity are crucial aspects for designing electrodes with facile transport of electrons, ions and matter, which is a key parameter for electrochemical energy storage and conversion. Carbon nanofibers (CNFs) prepared by electrospinning are attractive for their high aspect ratio, inter-fiber macroporosity and their use as self-standing electrodes. The present work compares several strategies to induce intra-fiber micro-mesoporosity in self-standing CNF webs prepared by electrospinning polyacrylonitrile (PAN). Two main strategies were investigated, namely i) a templating method based on the addition of a porogen (polymethyl methacrylate, polyvinylpyrrolidone, Nafion® or ZnCl2) in the electrospinning solution of PAN, or ii) the activation in ammonia of previously formed CNF webs. The key result of this study is that open intra-fiber porosity could be achieved only when the strategies i) and ii) were combined. When each approach was applied separately, only closed intra-fiber porosity or no intra-fiber porosity was observed. In contrast, when both strategies were used in combination all CNF webs showed high mass-specific areas in the range of 325 to 1083 m2·g−1. Selected webs were also characterized for their carbon structure and electrical conductivity. The best compromise between high porosity and high electrical conductivity was identified as the fibrous web electrospun from PAN and polyvinylpyrrolidone.

Influence of Dye Size and Fiber Porosity on the Dyeing Kinetics and Time Lags in the Uptake of Cationic Dyes on Acrylic Fibers

The uptake curves of cationic thiazine dyes (based on thionine) and four other cationic dyes were determined for a series of acrylic fibers, consisting of the same polymer but varying porosities. The apparent diffusion coefficients ( Dapp) and the time lag of the dye uptake (Δ t) were calculated. Dapp and Δ t are functions of the size of the dye cations, the porosity of the fiber, and at least one other factor. We detected an influence of the glass transition temperature under dyeing conditions on the time lag, which was not found before for these or any other fibers.