Electrochemical hydrogen adsorption/storage in pure and functionalized single wall carbon nanotubes

Self-assembled sheets of single wall carbon nanotubes (SWNTs) were used as the working electrode for electrochemical hydrogen adsorption/storage in a three-electrode cell in 6M aqueous KOH or HNO3 solution. Hydrogen adsorption studies on pristine SWNTs, as well as SWNTs functionalized with electrodeposited nanoparticles of magnesium (Mg) and cobalt (Co), have been performed. The adsorbed hydrogen (uncorrected for possible water uptake via nanocapillarity) was measured to be 2.5 weight percent by thermogravimetric analysis (TGA) on a Mg-functionalized sample and 3.2 weight percent by Prompt-Gamma Activation Analysis (PGAA) for a pristine sample charged for 20 hrs. Weight loss occurs in the 105 degrees to 125 degrees C temperature range for both sample types. Hydrogen in Co-functionalized SWNTs and 6M HNO3 electrolyte appears to be strongly chemisorbed as indicated by the appearance of a C-H stretching line in the Fourier-transform infrared spectrum (FTIR) and absence of a desorption peak in the TGA data in the 25 degrees to 600 degrees C temperature range. Thermopower measurements scale with the TGA data and suggest that hydrogen uptake is associated with partial charge transfer. Ex-situ Raman spectroscopy shows a reproducible downshift of the SWNT tangential stretching mode consistent with charge transfer or chemisorption on electrochemical charging, and a substantial decrease under some conditions in resonance-enhanced intensity with increasing charging time. A SWNT sheet electrochemically coated with the conducting polymer polyaniline and then charged in 6M KOH shows possible hydrogen uptake of 1.5 weight % that desorbs at 70 degrees C.