Confident methods for the evaluation of the hydrogen content in nanoporous carbon microfibers
© Culebras et al.; licensee Springer. 2012
Received: 25 August 2012
Accepted: 5 September 2012
Published: 24 October 2012
Nanoporous carbon microfibers were grown by chemical vapor deposition in the vapor-liquid solid mode using different fluid hydrocarbons as precursors in different proportions. The as-grown samples were further treated in argon and hydrogen atmospheres at different pressure conditions and annealed at several temperatures in order to deduce the best conditions for the incorporation and re-incorporation of hydrogen into the microfibers through the nanopores. Since there are some discrepancies in the results on the hydrogen content obtained under vacuum conditions, in this work, we have measured the hydrogen content in the microfibers using several analytical methods in ambient conditions: surface tension, mass density, and Raman measurements. A discussion on the validity of the results obtained through the correlation between them is the purpose of the present work.
Hydrogen is known to be the most common element in the Milky Way, and it represents 74% in content, followed by helium (24%), oxygen (1%), and carbon (0.4%). It is found in a large amount of chemical compounds, particularly in carbon-rich and organic materials. Atomic hydrogen is unstable, and it is usually found in combination with other elements (hydrocarbons, polymers, water, etc.) or as a diatomic molecule. Hydrogen is used, as least in prototypes, in fuel cells, which is a very important issue in energy storage. Another interesting application is thermoelectricity; the electrical conductivity and the Seebeck coefficient can be engineered by changing the hydrogen content.
On average, the storage capacity of hydrogen in carbon nanostructures is of the order of 1.5 wt.%, although the storage capacity can significantly change with the desorption temperature or hydrostatic pressure. For instance, single-walled carbon nanotubes (CNTs) show a hydrogen uptake of 5 to 10 wt.% at 133 K and 40 kPa. It has also been shown that single- or multi-walled CNTs adsorbed a hydrogen amount of 3 to 4 wt.% at room temperature but at 10 MPa[7, 8].
An important problem in this research field is to have a confident measurement of the hydrogen content. This is not an easy matter because of the depletion of hydrogen when the fibers are in a vacuum environment, and many of the used techniques need vacuum conditions. Techniques such as elastic recoil detection analysis show unsatisfactory sensitivity since it works with the samples placed into a high vacuum chamber. It was also difficult to obtain confident results in the measurement of the hydrogen content by reflection electron energy loss spectroscopy; the measurement error was not lower than 20%.
In this work, we study carbon fibers grown by chemical vapor deposition, a method which allows obtaining a good-quality material under a reasonable cost. The vapor growth produces filaments of some centimeters of length and microfibers with a length smaller than 100 μ m. The manufacturing process of vapor-grown carbon fibers (VGCFs) has been previously described in the literature. They have been prepared incorporating metallic particles of group VII to the gas flow entering into the reactor. Although VGCFs synthesized with a metallic catalyst have received special attention in many fields because of their controllable structure and attractive mechanical and electrical properties[7, 13], one of the most important applications of VGCFs in the near future will be as hydrogen storage materials, mainly in the form of nanowires because of its large surface for hydrogen incorporation. In this work, we have studied the hydrogen content in VGCFs, both filaments and microfibers, restricting the analysis to techniques which do not need vacuum, and try to select the more confident technique for measuring the hydrogen content in porous materials, particularly in carbonaceous specimens.
Crystal growth and sample preparation
Set of samples grown as explained in the text
70% H2 + 30% CH4
H2 bubbling in C6H6
70% H2 + 15% CH4 + 15% C2H4
70% H2 +30% CH4
H2 bubbling in C6H6
Samples obtained after further treatment on samples A and B (FGS)
In order to carry out a rough comparison between the samples, thermogravimetric measurements (TGA) were performed using a TGA system. The samples were kept in Ar atmosphere, and the heating velocity was 2 K/min between 323 and 473 K and 20 K/min between 473 and 1,023 K.
Surface energy measurements
where the upper index p and d are used to distinguish the polar and dispersive components (;) of the surface energy. By using two liquids, a polar liquid (for instance, glycerol: mN/m and mN/m) and a non-polar liquid (as vaseline oil: mN/m), we were able to obtain and from the measurements of the contact angles with the VGCFs.
where ρ is the density in g/cm3 and x is the hydrogen content in wt.%. As the first step, the mass of each sample was determined. Then, the volume was established using a gas pycnometer with helium. The hydrogen content of each sample has been calculated from Equation 2.
A Raman confocal microscope (Renishaw 2000, Renishaw, Gloucestershire, UK) has been used in the analysis. It was provided with a Leica microscope (Leica, Solms, Germany), a nitrogen-cooled charge couple device, and an air-cooled Ar ion laser (514.5 nm) as excitation source. A ×50 objective has been utilized. The spectra of all the samples have been evaluated by the own software of the system.
Results and discussion
The Raman spectra of carbonaceous materials consist of two main broad peaks known as-band (stands for ‘graphite’) and-band (proportional to the level of ‘disorder’ or defects). The-band is located around 1,575/cm, and it is assigned to a doubly degenerated deformation vibration of the hexagonal ring corresponding to the E2g mode of graphite with crystal symmetry. The-band is located around 1,355/cm, and it is an indication of the crystal size. The existence of the-band points out the existence of disorder-induced scattering. During the process of carbonization of polymers through thermal treatment at increasing temperatures, the intensity of the-band decreases (indicating recrystallization). When the-band disappears completely, the material has turned into a well-ordered graphite material with no defects (obviously, a neglected defect concentration).
Therefore, we may suggest that the variation of the ratio between the intensity of the andpeaks indicates an alteration in the graphitic character of the material. For this reason, the relationship between the intensity of the peakshas been generally used to predict the elastic modulus of carbon thin films and CNTs. This relationship points out the alteration of crystalline perfection.
From the analysis and comparison of the different techniques used to measure the hydrogen content in the carbonaceous materials, we can conclude that the density and Raman measurements are the most confident techniques since there is a clear linear correlation between the hydrogen content extracted from the density measurements and theratio of the Raman peaks. In any of these techniques, vacuum is needed for the measurements, and we can ignore the discussion or the evaluation on the effect of the vacuum conditions on the final results. From the results, we can also conclude that the samples with more hydrogen content are those grown bubbling hydrogen in benzene: B compared with C and A (FGS), and E compared to D (FS).
We thank the financial support given by the project CSD2010-0044, which belongs to the ‘Consolider Ingenio’ Programme of the Ministry of Finances and Competitiveness.
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