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3STRUCTURE AND HYDROGEN SORPTION OF IRRADIATED NANOCARBON FILMS
E. M. Ibragimova, V. N. Sandalov, M. U. Kalanov, M. A. Mussaeva, M. I. Muminov
Institute of Nuclear Physics, Academy of Sciences of Uzbekistan pos. Ulugbek, Tashkent, 702132, Uzbekistan E-mail: ibragimova@inp.uz
The problem of safe storage/supply of hydrogen is quite urgent. The idea was to activate the | surface of nano-carbon films with ionizing irradiation so as to decrease the chemical potential of 3 hydrogen sorption and increase the number of dangling C- bonds, which can easily trap hydrogen and g then release it under ambient pressure. Irradiation of the carbon films in liquid nitrogen with 60Co ™ gamma-rays at 77 K resulted in structure modification of hydrocarbon nano-particles and formation of graphite precipitates, and also a two-times-growth of proton conductivity in a wide temperature interval 200-350 K. The estimated amount of hydrogen desorbed by the irradiated film at the normal pressure after 50 min was about 1.8 wt. %.
Introduction
The maximum possible saturation of carbon nano-tube (CNT) powder with hydrogen may be as high as 150 kg/m3 at 77 K under applied pressure. That is why CNT is considered as highly promising and safe container for hydrogen. According to the most publications [1-4] the single well CNT can be hydrogenised up to 1 wt. % at 300 K and 4 % at 77 K at ~gPa. The disadvantages of CNT: may burn in contact with air, hydrogen sorption/desorption cycle is limited by 500 times at ambient temperatures (the best samples have 5 % lost after 1000 cycles), after which it requires special recovery treatment.
Several direct methods of structure analysis are being developed for studying hydrogenised nano-carbon materials: neutron diffraction/scattering and X-ray diffraction technique can identify interstitial hydrogen sub-lattice, leading to increase of the lattice parameter, or impurity phases besides the carbon matrix. One should trace the reflection peak broadening and shift towards small angles [2] to determine local strains, a size and shape of nano-particles.
Two approaches exist for modification of nano-structures so as to improve hydrogen sorption ability: chemical treatment (NOx radicals or metal catalyst) or radiation treatment. The idea was to activate the highly developed total surface of CNT by ionizing irradiation so as to decrease the chemical potential of hydrogen sorption and increase surface defects of crystal lattice, i. e. dangling bonds C-, which can readily trap hydrogen and then release it. The attempt to irradiate CNT (single well nanotubes 01.25 nm I >2 |im) with high-energy electron beam in vacuum resulted in damage of the material [3]. The authors studied Langmuir isotherm q = kP/(1 + kP). At 1 bar of H2 at 77 K a
commensurate structure layer is formed, and at higher pressures it becomes a incommensurate one.
Experiment and discussion
The aim of this work was to study both structure and proton conductivity of wet carbon film coated paper under 60Co gamma-irradiation to different doses at the liquid nitrogen temperature and the normal atmosphere pressure. Since presence of water may block channels for hydrogen sorption, CNT powder samples are not recommended for a contact with water. Therefore gammairradiation of carbon film was performed in liquid nitrogen so as to avoid any contact with oxygen and carbon dioxide, and also to open nanotubes for better hydrogen absorption.
The direct X-ray diffraction method of structure analysis was developed at the Institute of Nuclear Physics (INP) for studying nano-struc-ture materials, including nano-carbon powder or C-film on paper. Special instrumental modifications of the experimental set-up DRON-3 (X-ray diffraction spectrometer) were done to achieve a better X-ray beam collimation and an improved sensitivity so as to reveal possible nano-phase inclusions. The weight fraction of an inclusion, F, was calculated by the empirical equation [4]:
F = 1 -(1 +1.26/, Hm(1)
where I. and I are integrated intensities of impurity phases and a matrix lattice, respectively. The size of the nanoparticles was calculated from the XRD data with the use of the Scherrer-Selyakov formula as follows:
L = 0.9/ / FWHM cos q, (2)
where FWHM is the full width of peak at half maximum and l = 0.154178 nm is the wavelength of CuK radiation.
a
E. M. Ibragimova, V. N. Sandalov, M. U. Kalanov, M. A. Mussaeva, M. I. Muminov Structure and hydrogen sorption of irradiated nanocarbon films
Fig. 1 shows X-ray diffraction pattern of the C-film on paper taken at 300 K.
One can see two wide diffusion reflections with the intensity ratio 2:1 and inter-plane distances d/n « 0.554 and 0.393 nm, and also a few narrow selective peaks over them for non-irradiated sample (Fig. 1a). Since the paper substrate also contributed into the X-diffraction patterns, despite the patterns were taken from the carbon-coated side, the uncoated pure paper was also examined and the pattern contained only these diffusion maxima at the same intensity ratio 2:1.
Thus, all the observed selective peaks are due to the carbon film structure. Among them (Fig.la) two peaks with d/n « 0.4152 and 0.3591 nm belong to one type of hydrocarbon nano-particles and another structure line with d/n « 0.5125 nm is attributed to a different kind of hydrocarbon nano-particles [5, 6]. X-diffraction peaks related to graphite nano-precipitates were not observed here. The sizes of the hydrocarbon nano-particles were estimated about 27-28 nm.
After the first gamma-irradiation to the dose of 105 R (Fig. 1b) there appeared a very intensive narrow peak with d/n « 0.7131 nm. The analysis showed, that this reflection is a forbidden one (001) and belongs to (002) with d/n « 0.3591 nm from graphite nano-precipitates. The observed forbidden reflection is indicative of radiation-induced microstrains. The intensities of the selective reflections existing in non-irradiated sample decreased within 4-9 %. Besides, the sizes of the hydrocarbon nano-particles reduced down 23-24 nm. However, the following irradiation to a higher dose of 106 R (Fig. 1c) resulted in disappearance of the induced peak with d/n « 0.7131 nm, recovery of the initial intensities of the rest peaks, and thereby to relaxation of the radiation induced local strains.
Thus, gamma-irradiation of the carbon film at 77 K caused some nano-phase transformations. Fortunately, we did not observe any noticeable change in the diffusion maximums belonging to the paper substrate, which could overlap the obvious transformations of the superimposed narrow peaks from the carbon film.
The occurred broadening of the selective reflections and local micro-strains after the irradiation may have resulted from hydrogen adsorption by the non-saturated hydrocarbon nano-particles. Indeed, hydrogen can induce phase decomposition in carbon materials [5] and several alloys [7]. The hydrogen desorption of the carbon-coated paper samples was also studied by means of thermal-induced surface electric conductivity measurements by applying DC voltage of ~30 V within the temperature interval 200-360 K in the course of heating samples. The studied sample was put into the hermetic cell, pressed by the concentric stainless steel electrodes, then cooled in N2 vapor down 80 K. Gamma-irradiation was done at 300 K to a dose of ~105 R at the gamma-flux of 500 R/s in the
29, deg.
Fig. 1. X-ray diffraction pattern of carbon film on paper taken at 300 K: a — non-irradiated; b, c — gamma-irradiated at 77 K in liquid nitrogen to doses 105 and 106 R
ambient atmosphere (humidity in the building around 50 %, however in the channel 5 meters underwater the humidity must be higher).
The temperature dependences of the electric conductivity of non-irradiated or irradiated samples are shown in Fig. 2. The conductivity was almost temperature independent within the measurement interval except the sharp peak about 300 K. Gamma-irradiation resulted in the peak position decrease by 10 K and almost 2 times enhancement of the conductivity over the measured temperature interval. No pronounced peak was detected in dry samples therefore we ascribed the observed behavior to the proton conductivity, which was enhanced by the irradiation due to increasing the number of dangling C-bonds, which trapped hydrogen atoms under the irradiation and released them at heating and applied DC-voltage.
The peak proton conductivity of 100 nA and the temperature independent value of ~50 nA were measured after the irradiation. If to take into account the measurement time ~3000 seconds, the mass of the carbon film between the measuring electrodes in the cell m ~ 0.1 mg and the passed charge, one can estimate the mass % of hydrogenation of the sample. According to Faraday's law, the passed charge of 270 mAh/g is enough for
100
o
Fig. 2. Thermal stimulated conductivity of non-irradiated (open symbols) and gamma-irradiated (closed symbols) wet carbon films
Водородная энергетика и транспорт Хранение водорода
enriching CNT to 1 wt. % H. In our experiments the passed charge was 500 mAh/g, that corresponded to ~1.8 mass % of H released by the irradiated carbon film during 50 minutes.
Conclusions
Analyzing the results of gamma-irradiation of the carbon films, obtained with the help of both X-ray diffraction and surface electric conductivity, one can suggest, that the low-temperature shift of the conductivity peak at 300 K for non-irradiated sample after the gamma-irradiation may be caused by the induced change of the sizes and disposition of the existing hydrocarbon nano-particles (so called quantum dots).
Thus, the idea to activate the highly developed total surface of nano-carbon film with the ionizing gamma-irradiation so as to decrease the chemical potential of hydrogen sorption at the normal pressure seems to be right.
A two-times-growth of proton conductivity in a wide temperature interval 200-350 K was gained due to the gamma-irradiation, that corresponds to ~1.8 mass % of H released by the irradiated carbon film during 50 minutes.
Such experimental combination (radiation induced hydrogen sorption, structure analysis and proton conductivity during heating) allow us to monitor structure changes in carbon films, find a proper dose-dose rate-temperature-atmosphere conditions for improving the hydrogen sorption/desorption capability at the normal pressure and suggest the carbon films as possible materials for hydrogen storage or electrodes for fuel elements.
Acknowledgement
The researches are supported by STCU grant Uz23j and contract F2.1.2 with the Center for Science and Technology of Uzbekistan.
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Справочник-каталог
»Оборудование нетрадиционной и малой энергетики»
Изд-во АО ВИЭН
167 стр., 2000 г.
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