Preparation and Characterization of Cobalt Nanoparticle-Decorated Multi-Walled Carbon Nanotubes

In order to increase gas storage capacity such as hydrogen and methane, various wt% of cobalt were decorated on the surface of MWCNTs which was already treated nitric and sulfuric acids to provide functional groups on the surface of MWCNTs. The characteristic peak of SO4 in FT-IR spectrum was at 1220 cm, which was resulting from sulfuric acid treatment. This peak implies that acid treatment containing sulfuric acid can give rise to the incorporation of sulfur onto the surface of MWCNTs. According to the results of XRD, cobalt were thoroughly reduced under hydrogen around 673 K. This indicates that cobalt oxides need the temperature of 673 K to be reduced under hydrogen. In case of 15 wt% cobalt decoration on the surface of MWCNTs, cobalt particle size was around 10 nm. Keywords; Carbon nanotubes, Chemically modified carbons, Gas storage Instruction Hydrogen fuel is one of the cleanest and idealized energy sources. In order to use hydrogen fuel efficiently, hydrogen storage is an essential condition for widespread utilization all over the world. In recent years, there has been considerable experimental and theoretical interest in the use of nanostructured carbon materials, especially in the form of tubes [1-4] and fibers [5,6] as potential hydrogen sorbents [7]. Unfortunately, these materials do not meet the DOE targets. The DOE has asked for achievable hydrogen storage capacity to reach 4.5 wt% by the FY 2005, 6.0 wt% by the FY 2010, and 9.0 wt% by the FY 2015 [8]. Carbon nanotubes (CNTs), having high surface-to-volume ratios, are ideal for fast kinetics because of their reversible characteristics during hydrogenation and dehydrogenation. Dillon [9] et al. first reported the reversible hydrogen storage properties of carbon soot containing 0.1-0.2 wt% single-walled carbon nanotubes (SWCNTs) at 300 Torr. The hydrogen storage capacity of pure SWCNTs could be extrapolated to reach 5-10 wt% after calculating their purity. An overview of experimental data on hydrogen adsorption in carbon nanotubes (CNTs) was given in the literature by Ding et al. [10]. In additional, it was found that multi-walled carbon nanotubes can have a hydrogen storage capacity at room temperature of 1.97 wt% H2 at 4 MPa [11], 3.7 wt% H2 at 6.9 MPa [12], 4 wt% H2 at 10 MPa [13], and 6.3 wt% H2 at 14.8 MPa [14]. These high values, however, have not so far been verified independently. The storage values are dependent on many parameters of the carbon nanotubes, including their structure, structure defects, pretreatment, purification, geometry (surface area, tube diameter, and length), arrangement of tubes in bundles and/or ‘ropes’, storage pressure, temperature, etc. The nature of the factors that influence the level of hydrogen storage is still disputed [7]. However, recent studies [15, 16] have shown that the hydrogen storage capacity on a pristine CNTs is less than 0.01 wt% at room temperature. Also, Lawrence and Xu [17] also reported that hydrogen only 0.6 wt% was adsorbed on the CNT bundle at 294 K and 10 MPa. Ni supported activated carbon reported by Zieliński et al. [7] stored up to 0.53 % at 3 MPa against 0.1 % for as-received activated carbon. More recently, it was reported by Kim et al. that 6 wt% Ni nanoparticle-dispersed multi-walled carbon nanotubes released hydrogen of 2.8 wt% in the range of 340 – 520 K [18]. As described above, the important point for hydrogen storage is the use of transition metals such as Fe, Co, Ni, Cu, etc. In this study, prior to hydrogen storage study using cobalt-decorated MWCNTs, Co-MWCNTs were investigated by various analysis tools. Also, the optimum condition of cobalt dispersion on the surface of MWCNTs is provided.