Carbonaceous Nanotubes Research Articles

Structural and Electrochemical Properties of Carbon Nanotubes and Nanofibers

We studied electrochemical and structural properties of electrodes prepared from new carbonaceous nanomaterials (CNM), specifically, one-walled nanotubes (ONT) and graphite nanofibers (GNF). The ONT were synthesized by an electric arc method using 3Co/Ni as a catalyst [1]. The ONT yield after purification was more than 90%. The ONT diameters, determined by Raman spectroscopy, were 1.2‐1.5 nm. The GNF were synthesized from a 4 : 1 mixture of CO and H 2 at 600ie with Fe : Cu = 7 : 3 as a catalyst. GNF diameters were 100‐300 nm. According to standard porosimetry [2], the specific volume of ONT micropores with radii r < 1.5 nm was 0.29 cm 3 /g, and for GNF, 0.065 cm 3 /g. The hydrogen-sorption capacity of ONT was 3.5 wt % at a hydrogen pressure of 10 MPa. Assuming that the micropore volume is equal to the inner volume of ONT and that the adsorbed hydrogen stays mainly inside ONT, it was found that the hydrogen density was 0.12 g/cm 3 (this value is very close to that obtained theoretically in [3]), i.e. was 1.7 times the liquid hydrogen density, which is caused by a strong interaction between hydrogen molecules and the inner surface of ONT. The electrochemical studies of CNM were performed in a 0.5 M H 2 SO 4 aqueous solution. The hydrogen evolution was observed at E < ‐0.4 V (RHE). Chronovoltammograms for ONT feature a sharp reversible current maximum at E = ‐0.3 V, caused probably by the generation, adsorption/desorption, and ionization of hydrogen, which confirms a strong interaction between hydrogen molecules and the inner surface of ONT. Unlike ONT, GNF possess no reversible mechanism of hydrogen oxidation‐reduction, which is probably explained by different hydrogen sorption mechanisms. On the CNM-based electrodes, the EDL charging occurs much faster and is more reversible as compared to activated carbons (AC). For example, at the EDL charging time τ of 20 s, for AC TSA capacitance Q = 100 F/g, and for GNF it is 20 F/g, while at τ = 2 s for AC TSA Q = 6 F/g, and for GNF it is 12 F/g. This effect is probably due to the absence of pore tortuosity in CNM.