Synthesis, characterization and electro-catalytic applications of metal Nanoparticles-decorated Carcon Nanotubes for hydrogen storage

Abstract

Since their discovery in 1991, CNTs have shown extraordinary properties and as result, these materials are being investigated for several different applications. Synthesis and electrochemical application of CNTs for hydrogen storage provide new possibilities for replacement of gasoline use in vehicles due to its cost and negative environmental impact.

The study investigated the metal nanoparticles modified multi-walled carbon nanotubes as possible storage material for hydrogen. Herein, carbon nanotubes were successfully synthesized by pyrolysis of iron (II) phthalocyanine under Ar/H2 reducing atmosphere at 900 oC for 30 min. The micro-structural information of the as-prepared carbon nanotubes was examined by Transmission electron microscopy (TEM). It was found that the prepared CNTs were multi-walled with iron particles impurities present on the surface. Synthesized MWCNTs were found to have open tips as shown by TEM images. These materials were purified and functionalized with acid groups as confirmed by Fourier transform infra-red spectroscopy (FTIR). A successful decoration of MWCNTs by Cu, CuO, Fe, Fe2O3, Ni and NiO nanoparticles was confirmed by Scanning electron miscroscopy (SEM) and Transmission electron microscopy (TEM). TEM images showed that metal nanoparticles and metal oxides were well dispersed on the surface of the MWCNTs. The chemical composition of the as-prepared MWCNTs was confirmed by XRD (showing the presence of metal impurities and amorphous carbon).

Synthesized materials were applied in electrochemical techniques such as cyclic voltammetry, chronopotentiometry and controlled potential electrolysis. These techniques have shown that modification of glassy carbon bare electrode (GCE) with carbon nanotubes decorated with metal nanoparticles (Cu, Ni and Fe), improves the current density, charge-discharge voltages and discharge capacity for hydrogen storage (in a 6 M KOH aqueous electrolyte). It was shown that MWCNTs exhibit high conductivity, porosity and high surface area for hydrogen

storage. The increase in discharge capacity was as follows: GCE < GCE-MWCNT < GCE-MWCNT-M (M = Cu, Ni, Fe and/or metal oxides). This confirmed a successful modification of GCE with MWCNTs and MWCNT-M (M = Cu, Ni, Fe and/or metal oxides). The maximum discharge capacity of 8 nAh/g was obtained by GCE-MWCNTs-Ni electrode, corresponding to an H/C value of 28.32 x 10

It was confirmed that both Ni loading and MWCNTs loading have an impact on the current response, charge-discharge voltages and discharge capacity. A maximum current density and discharge current was reached when a 4wt% nickel was loaded. A decrease in current density and discharge current was observed for nickel loading of higher than 4wt%. Thus suggests a possible decrease in surface area of the adsorbed material on the surface of the electrode for hydrogen storage. As more MWCNTs were added, a decrease in current density was observed. A 2wt% MWCNTs gave higher discharge current and this was possibly due to less hindrance on the surface of the electrode for hydrogen to diffuse.

It was shown that calcining the metal nanoparticles result in particles agglomeration, as confirmed by Transmision electron microscopy (TEM). This resulted in a decrease in surface area of the working electrode. A low current response was observed compared to the uncalcined Ni nanoparticles. The highest exchange current density was obtained while using a GCE-MWCNT-Nical as compared to the GCE-MWCNT-Niuncal electrode. The applied discharge current in CPE was also shown to have influence on the discharge capacity. An increase in discharge capacity for the GCE-MWCNT-Ni (2wt% MWCNTs and 4wt% Ni) electrode was observed as more discharge current was applied. A decrease in discharge capacity for hydrogen was observed as more content of the MWCNT-Niuncal nanocomposite are added on the active surface area of the glassy carbon electrode.