Microfibrillar carbon from native cellulose

Use of pyrolytic carbon from cellulose has been limited in practice to activated adsorbent carbon, but cellulose-derived carbon retaining the nanoscale microfibrillar morphology offers rich possibilities as an advanced material. Here we developed novel methods to prepare such materials by an improved drying of wet cellulose prior to pyrolysis. This procedure is an adaptation from electron microscopy techniques, i.e. rapid freeze drying of suspension and solvent exchange drying, both being effective in preventing coagu­ lation of cellulose microfibrils/microcrystals. Pyrolytic carbon from such material has a large external surface area, with the graphitic carbon crystallites roughly aligned along the fiber axis. These features are potentially useful in developing novel carbon nanomaterials for electrodes, catalyst supports, or composite material elements. Introduction On the other hand, however, it is known that the structure of carbon material is strongly dependent Organic substances can be carbonized by heating on that of the original organic solid. Regarding the to 250-600 °c in an inert atmosphere, and this raw material, there is a distinction between process is utilized in the production of various 'graphitizable' and 'nongraphitizable' carbons. In carbon materials. While cellulosic materials are general, nonmelting solids such as cellulose belong used as carbon precursors on a large scale, their to the latter. While the common sources of cellu­ use is mostly limited to activated carbon as an losic carbon are those of higher plants, there are adsorbent. High-strength graphite fibers were special types of native cellulose with unusually produced from rayon in the early days, but it was high crystallinity. replaced by poly(acrylonitrile) (Bahl et al. 1998). Figure I shows schematically the shape of native The reason is twofold: (i) even though cellulose has microfibrils of higher plants, algaljtunicate, and a carbon content of 44.4%, its pyrolytic carbon bacterial celluloses (see e.g. Brown 1982). Car­ yield is as low as 10-20% at 500°C and even less bonization of these highly crystalline celluloses can than 10% at 1000 °C; (ii) cellulose does not soften give novel forms of pyrolytic carbon. In fact, we or melt by heating except under special conditions previously found strong correlations between (Nordin et al. 1973; Back et al. 1974), and there­ crystallinities of the starting cellulose and the de­ fore cannot be stretched during pyrolysis for rived high-temperature-treated carbon (Kim et al. improving crystallinity and orientation of graphite 200Ia). Here we report attempts to develop novel, crystallites. nanofibrillar forms of carbon from native cellulose