The preparation of carbon nanotube/metallic particle hybrids using pressed porous stainless steel pellets as a substrate is described. The catalytic growth of carbon nanotubes was carried out by CVD on a nickel catalyst obtained by impregnation of pellets with a highly dispersive colloidal solution of nickel acetate tetrahydrate in ethanol. Granular polyethylene was used as the carbon source. Metallic particles were deposited by thermal evaporation of Pt and Ag using pellets with grown carbon nanotubes as a base. The use of such composites as fuel cell electrodes is discussed.
Fuel cells are electrochemical sources of current in which the chemical energy of the fuel is directly transformed to the electrical energy through the redox chemical reactions proceeding at catalytic electrodes [
In this paper, experiments on carbon nanotube growth on porous pellets prepared from stainless steel powders and direct deposition of catalytic metallic particles of Pt and Ag on carbon nanotubes obtained at given substrates are presented.
The advantage of this approach implies that after CNTs’ growth on porous pellets followed by carbon nanotubes decoration by catalytic nanoparticles, this hybrid structure represents the fuel cell material in which three functions are combined: catalytic electrode, gas diffusion layer, and current collector.
The pellets were pressed from commercially available stainless steel (russian type X23H18) powders. The pellets obtained were disks with a diameter of 10 mm and thickness of 2 mm. One of the pellet’s sides was impregnated with highly dispersive colloidal solution of nickel acetate in ethanol. For this purpose, 0.236 g of chemically pure Ni acetate tetrahydrate (Ni(ac)2·4H2O) was dissolved in 5 mL of ethanol on heating up to boiling temperature. Then the solution was allowed to air cool to room temperature. The porous stainless steel pellets were impregnated with the transparent solution by deposition of 3–5 drops on one side of the pellets. After air drying, gel-like material was formed in pores and on the pellet surface. Such pellets were used as substrates with deposited catalyst for carbon nanotube growth in a CVD process of 15 min duration. The gel was decomposed and reduced to nickel nanoparticles under heating at an early stage of CVD process providing fast nanotube growth. The synthesis of carbon nanotubes was carried out at 800°C using granular polyethylene as the source of carbon [
Morphology of the samples obtained was investigated by optical and transmission electron microscopy (Tesla BS-500). The metallic Pt and Ag nanoparticles were deposited by means of thermal evaporation in a vacuum.
Figure
TEM image of carbon nanotube/metallic particles hybrids obtained on the pressed stainless steel pellets. Typical metallic particle agglomerations (black flakes) were observed for both Pt and Ag.
From the analysis of optical and electron microscopy measurements, it is found that the formation of carbon nanotubes takes place both on the surface of the pellet and inside of pores of the subsurface layer (Figure
TEM image of carbon nanotube/metallic particles hybrids inside of pressed stainless steel pellets. As an example, metallic particles agglomerations of Pt (black flakes) with a few nanoparticles (black circles) are presented.
Carbon nanotubes had an arbitrary orientation and were intertwisted to one another forming thin “felt” mat on the surface of the pellet.
As it was already mentioned, catalytic particles of Pt and Ag were produced by thermal evaporation in a vacuum. As one can see from Figure
TEM image of unsupported carbon nanotubes with (a) deposited Ni nanoparticles and (b) through holes of nanometer scale on walls of carbon nanotubes.
The formation of the holes can be understood by the following way. Under electron irradiation, both the effect of heating of Ni nanoparticles and dissolving of carbon in the particles take place. At high temperature, the metallic particles evaporate together with dissolved carbon creating the hole in carbon nanotube wall.
A model of the hydrogen-oxygen fuel cell has been developed in order to measure in situ electrical characteristics of porous nanocomposite carbon-metal electrodes (Figure
Laboratory model of hydrogen-oxygen fuel cell.
It was found that both hydrogen and oxygen penetrated through porous electrodes when applying an excess pressure (0.15 bar).
The results of electrical characteristics measuring of the experimental fuel cell on the basis of the electrodes developed and are shown in Figure
Electrical characteristics of the fuel cell based on carbon nanotube/metallic particle hybrids formed on porous stainless steel pellets as electrodes. 1
Open circuit voltage of the fuel cell was 0.94 V, the shape of the current-voltage characteristic was almost linear (Figure
Preparation of CNT/metallic particles hybrids on pressed porous stainless steel pellets was carried out. Carbon nanotubes were grown by CVD on a nickel catalyst obtained by impregnation of pellets with highly dispersive colloidal solution of nickel acetate in ethanol followed by heat treatment. The effect of redeposition of metallic agglomerations and burning out of holes in carbon nanotube walls under microscope electron beam was observed.
The use of such composites in fuel cell technology can simplify the fuel cell design since, in this hybrid structure catalytic electrode, gas diffusion layer and current collector are combined.
This work was partially supported by the Russian Foundation for Basic Research (Grant no 12-08-00755-a).