Confident methods for the evaluation of the hydrogen content in nanoporous carbon microfibers
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Confident methods for the evaluation of the hydrogen content in nanoporous carbon microfibers ˜ 2 , Andres Cantarero1 , Jos´e Maria Amo2 , Concepcion ´ Domingo3 Mario Culebras1* , Antonio Madronero 4 ´ and Antonio Lopez
Abstract 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. Keywords: Raman dispersion, Chemical vapor deposition, Nanoporous materials
Background 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%) [1]. 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 [2]. Another interesting application is thermoelectricity; the electrical conductivity and the Seebeck coefficient can be engineered by changing the hydrogen content [3]. 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 [4] or hydrostatic pressure [5]. For instance, single-walled carbon nanotubes (CNTs) show a *Correspondence: [email protected] 1 Materials Science Institute, University of Valencia, PO Box 22085, 46071 Valencia, Spain Full list of author information is available at the end of the article
hydrogen uptake of 5 to 10 wt.% at 133 K and 40 kPa [6]. 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 [9], 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 [10]. It was also difficult to obtain confident results in th
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