# Carbon Nanotubes as Future Energy Storage System

V Vasu, D Silambarasan

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V Vasu, D Silambarasan. Carbon Nanotubes as Future Energy Storage System. Mechanics, Materials Science & Engineering Journal, 2017, 9 (1), 10.2412/mmse.52.18.599. hal-01496308

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# Carbon Nanotubes as Future Energy Storage System

V. Vasu¹, D. Silambarasan¹

1 – School of Physics, Madurai Kamaraj University, Madurai, India

<figure><img src="image_2.png" /></figure>

DOI 10.2412/mmse.52.18.599 provided by [Seo4U.link](https://linkAGE04U.seo)

**Keywords:** carbon nanotubes, metal oxides, hydrogen, storage.

**ABSTRACT.** Hydrogen is considered to be a clean energy carrier. At present the main drawback in using hydrogen as the fuel is the lack of proper hydrogen storage vehicle, thus on-going research is focused on the development of advance hydrogen storage materials. Many alloys are able to store hydrogen reversibly, but the gravimetric storage density is too low for any practical applications. Theoretical studies have predicted that interaction of hydrogen with carbon nanotubes is by physisorption of hydrogen on the exterior and in the interior surfaces. Hence the CNTs appear to be the ultimate solution due to their chemical stability, large surface area, low density and hollowness. Recent studies indicate that the physisorption on pure CNTs may not be a feasible method of storing hydrogen. Hence, the functionalization of CNTs with metal hydrides is a subject of increasing scientific interest, to improve the hydrogen storage capacities. Lithium borohydride is a complex hydride that is received considerable attention due to its high gravimetric and volumetric hydrogen storage capacities. Our experimental investigation deals with the hydrogenation of SWCNTs functionalized with borane and also we have studied SWCNTs with different metal oxides composite like TiO₂, SnO₂ and WO₃. SWCNTs functionalization with borane was carried out by drop casting method. SWCNTs-metal oxide composite was prepared by both drop casting method and electron beam evaporation method. These results were discussed in detail in the present work. Studies were carried out with the aim to achieve higher storage capacity of hydrogen. It is found that the maximum storage capacity of 4.77 wt.% was observed for the SWCNTs functionalized with borane. The achieved hydrogen storage capacity in this investigation is close to the U.S. DOE target.

**Introduction.** Being an efficient energy carrier, hydrogen is believed as the appropriate candidate to meet the energy requirements with the increase in population. It is abundant, environmentally friendly fuel that has the potential to reduce our dependence on fossil fuels, but several significant challenges must be overcome before it can be widely used. The issues are namely, production, storage, transportation, conversion and applications. Hydrogen production and conversion are already technologically viable in the present scenario, but its storage and transportation encounter challenges. This work focuses on the investigations of hydrogen storage. Solid state storage form of hydrogen is considered to be the most appropriate and promising way than other forms such as gaseous and liquid. A nano-technological approach to solve the problem of storing hydrogen on materials is one of the main motivations behind this experimental research drive.

Among the nano materials, carbon nano materials are the most and widely investigated candidate for
hydrogen storage. CNTs play a major role in the curriculum of hydrogen storage than other forms of
carbon nanostructures. Carbon nanotubes (CNTs) are one of the allotropes of carbon with a
cylindrical nanostructure. These cylindrical sp² bonded carbon atoms possess unusual properties,
which are valuable for nanotechnology, electronics, optics and other fields of materials science and
technology. Generally, SWCNTs offer better adsorption and desorption properties than other type of
CNTs because of their maximum surface area. Hence, we have chosen SWCNTs for our
investigations. The initial investigations carried out (by various group) in bare CNTs for hydrogen
storage suggested that CNTs are not an efficient material to store hydrogen for practical applications
[1]. However, it has been shown that the addition of functional molecules, atoms and ions with CNTs

© 2017 The Authors. Published by Magnolithe GmbH. This is an open access article under the CC BY-NC-ND license
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enhances the binding energy of hydrogen as well as the storage capacity via two processes, namely, (i) providing multiple sites for adsorption, (ii) electron charge transfer between metal and carbon atoms. Hence, this research work is focused on functionalization of SWCNTs, addition of metal, metal hydrides and metal oxides to SWCNTs for hydrogen storage.

**Experimental.** The preparation of materials involved the methods of drop casting, electron beam evaporation (e-beam) and spin coating. In the drop casting method, the appropriate amount of materials is mixed by grinding and ultrasonicated in 2-proponal for fixed durations. The same procedure is followed for spin coating technique. In the e-beam technique, the appropriate amount of materials is mixed by grinding and made into pellet, which is then evaporated with the e-beam current of 20-30 mA under high vacuum conditions for a fixed duration of time. We have designed a Seiverts' like hydrogen storage setup for the hydrogenation process of the samples prepared by drop casting and spin coating techniques. In the e-beam technique, samples are evaporated in hydrogen atmosphere, i.e. hydrogen is stored in the composite samples during the preparation of hydrogen storage medium (HSM) itself as one-step process. Various characterization techniques such as AFM, SEM, FTIR, RAMAN, CHNS-elemental are employed to analyze the samples.

**Results and discussions.** The SWCNTs functionalized with borane system show a maximum hydrogen storage capacity of 4.77 wt.% at 50°C [2-4]. Here, borane helps to anchor hydrogen molecules onto SWCNTs in the ideal binding energy limits. Fig. 1 (a), 1 (b), 1 (c) and 1 (d) shows TEM image, Raman spectrum of SWCNTs, IR spectrum of SWCNTs functionalized with $BH_3$ and Raman spectra for SWCNTs (C), SWCNTs functionalized with $BH_3$ (CB), hydrogenated SWCNTs functionalized with $BH_3$ (CBH) and dehydrogenated SWCNTs (DCB), respectively. Raman spectrum provides valuable information about the purity and defects in the CNT samples. The D/G intensity ratio of SWCNTs (C) is 0.08, which informs the high purity of SWCNTs and for functionalized sample (CB), it is 0.21 and for the hydrogenated samples, the ratio is increased with the degree of hydrogenation. For the dehydrogenated sample (DCB), the D/G ratio is 0.225. This dehydrogenated sample is again hydrogenated and this step is continued. The change in D/G ratio, from 0.21 (CB) to 0.225 (DCB) is 0.015 (7.1%) after the first cycle. For the second cycle, the difference is 0.02 (~9.5%). In the third cycle, it changes to 11.6%. There is an increase of 2.1-2.4% in the D/G ratio between two successive cycles. If we take the average of change in D/G ratio of the dehydrogenated SWCNTs after each cycle, it is about ~2.3%. The entire hydrogenation and dehydrogenation cycles are independent events and this effect is not cumulative. The quality of CNTs deteriorates due to dehydrogenation is only about 2.3%. This indicates that the functionalized SWCNTs are restored to the original level after dehydrogenation and this is confirmed by Raman study. Hence, at the end of any number of cycles the change in D/G ratio value and the deterioration in the sample are around ~2.3%. This is the limitation in our method. The expected deviation in storage capacity is within 5% about the mean value. Zhang *et al.* [1] observed the percentage of change in D/G ratio and it was about 3%. The whole hydrogenation and dehydrogenation experiments are stabilized and repeatable. The achieved hydrogen storage capacity in this investigation is close to the U.S. DOE target. The same way is applied for all the types of samples and their deterioration level of the samples is estimated.

The composites such as, SWCNTs-SnO$_2$, SWCNTs-WO$_3$ and SWCNTs-TiO$_2$ are prepared by both e-beam and drop casting techniques. The surface morphology of SWCNTs dispersed in SnO$_2$ thin film prepared by e-beam technique is shown in Fig. 2 (a). The 3D AFM image reveals the inclusion of SWCNTs in SnO$_2$ thin film, which resulted in the formation of circular cone protrusions with the SnO$_2$ background. The SWCNTs in the composite may be aggregated due to van der Waals forces to form circular cone protrusions. It is noted from the AFM image that, the SWCNTs are arranged perpendicularly to the plane of the substrate rather than a random arrangement on SnO$_2$ thin film surface. Similar kind of occurrence of circular cone protrusion of CNTs on SnO$_2$ thin film background was obtained by Wisitsoraat *et al.* [5] and the corresponding 3D AFM image is presented in Fig. 2 (b). They pointed out the possible reason for this effect is that, the CNTs self-organized themselves
while they are moving towards the substrates in line with the material evaporation trajectory, which is almost perpendicular to the substrate. Similar morphologies for SWCNTs-WO₃ and SWCNTs-TiO₂ composites are obtained.

<figure><img src="image_3.png" /><figcaption>Fig. 1. TEM image (a) and Raman spectrum of SWCNTs (b), IR spectrum of SWCNTs functionalized BH₃ (c) and Raman spectra for all the functionalized, hydrogenated and dehydrogenated samples (d).</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Fig. 2. 3D AFM image of SWCNTs-SnO₂ composite prepared by e-beam technique of our sample (a) and (b) 3D AFM image of CNTs-SnO₂ composite prepared in e-beam technique by Wisitsoraat et al. [5].</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 3. SEM images of SnO₂ (a), WO₃ (b), TiO₂ (c) and SWCNTs-SnO₂ (d), SWCNTs-WO₃ (e) and SWCNTs-TiO₂ (f) composites prepared by drop casting method.</figcaption></figure>

The composite, SWCNTs-SnO₂ prepared by e-beam technique exhibit a hydrogen storage capacity of 2.4 wt.% [6]. SnO₂ alone shows a hydrogen uptake of 0.6 wt.%. The same composite prepared by
drop casting technique show a storage capacity of 1.1 wt.% at 100°C. Hydrogen uptake of SWCNTs-WO₃ composite prepared by e-beam technique is found to be 2.7 wt.%. Here, WO₃ shows a hydrogen uptake of 0.5 wt.%. The composite, SWCNTs-WO₃ prepared by drop casting method show a storage capacity of 0.9 wt.% at 100°C [7]. The amount of hydrogen uptake by the composite, SWCNTs-TiO₂ prepared by e-beam technique is found to be 3.2 wt.%. TiO₂ alone shows a hydrogen storage capacity of 1.4 wt.%. The same composite, made by drop casting method shows a storage capacity of 1.3 wt.% [8]. Fig. 3 presents the SEM images of SnO₂ (a), WO₃ (b), TiO₂ (c) nanoparticles and SWCNTs-SnO₂ (d), SWCNTs-WO₃ (e) and SWCNTs-TiO₂ (f) composites prepared by drop casting method. The results are summarized in the table 1. These results indicate that the deposition of SWCNTs with metal oxide materials (SnO₂, WO₃ and TiO₂) is possible using e-beam technique without any significant structural decomposition of SWCNTs. The tubular nature of SWCNTs after coating is examined using Raman spectrum. The hydrogen uptake exhibited by these composite materials confirm the synergistic effect exist between CNTs and metal oxides. The hydrogen adsorption in composite samples (SWCNTs-SnO₂, SWCNTs-WO₃ and SWCNTs-TiO₂) prepared by e-beam technique possess weak chemical binding. During the evaporation of material in hydrogen ambient, one can expect the incorporation of hydrogen in atomic form on the composite, which will then result in the strong binding of hydrogen with the composite. But, the obtained range of the binding energy belongs to weak chemical bonding on the composite samples. One of the possible reasons for this kind of adsorption is that, during the process of incorporation of hydrogen, initially the H atoms have lower coverage on the substrate (along with the composite). As the time of evaporation increases, the coverage of H atoms on the substrate also increases (i.e. the available H atoms start to incorporate on the neighboring sites on composite and this process continues upto higher coverage). This may lead to the formation of hydrogen molecules upon higher coverage. During the course of desorption, the adsorbed H₂ molecules get desorbed molecularly.

<table><caption>Table 1. Hydrogenation and dehydrogenation parameters of different materials.</caption><thead><tr><th>System</th><th>Preparation method</th><th>Hydrogen storage capacity (wt.%)</th><th>Desorption temp. range (°C)</th><th>Bind. energy range (eV)</th></tr></thead><tbody><tr><td>SWCNT-BH<sub>3</sub></td><td>Drop casting</td><td>4.77</td><td>90-125</td><td>0.28-0.31</td></tr><tr><td rowspan="2">SWCNT-SnO<sub>2</sub></td><td>e-beam</td><td>2.40</td><td>200-350</td><td>0.36-0.49</td></tr><tr><td>Drop casting</td><td>1.10</td><td>170-210</td><td>0.35-0.38</td></tr><tr><td rowspan="2">SWCNT-WO<sub>3</sub></td><td>e-beam</td><td>2.70</td><td>175-305</td><td>0.35-0.45</td></tr><tr><td>Drop casting</td><td>0.90</td><td>175-215</td><td>0.35-0.38</td></tr><tr><td rowspan="2">SWCNT-TiO<sub>2</sub></td><td>e-beam</td><td>3.20</td><td>120-215</td><td>0.31-0.38</td></tr><tr><td>Drop casting</td><td>1.30</td><td>160-205</td><td>0.34-0.37</td></tr></tbody></table>

On the other hand, the hydrogenation of composite samples (SWCNTs-SnO₂, SWCNTs-WO₃ and SWCNTs-TiO₂) prepared by drop casting technique is attributed to the mechanism of spillover. During hydrogenation, the metal oxide nanoparticles may dissociate the hydrogen molecule and migrate the hydrogen atoms to the nearest vacant sites on CNTs. More amount of hydrogen can occupy on the adsorption sites offered by CNTs in this way. During the course of desorption, the adsorbed hydrogen atoms were supposed to recombine into molecular hydrogen (reverse spillover) and get desorbed from the samples [9]. The average binding energy of hydrogen released from most of the samples lie in the recommended range of 0.2-0.4 eV. Thus, the attached hydrogen has weak chemical binding on the samples and importantly all the hydrogenated systems maintain their stability at room temperature. In this binding energy limits, the interaction between the host
material and hydrogen molecules are primarily due to the combination of electrostatic, inductive and covalent charge transfer mechanisms [10]. Nikitin *et al.* [11] hydrogenated the SWCNT films using atomic hydrogen. They observed the hydrogen desorption in the temperature range between 200 and 300°C and noted that the chemisorbed hydrogen (C-H) desorbs from the surface of SWCNT. Here, the desorption temperature ranges of our systems lie around the desorption temperature range reported by Nikitin *et al.* [11]. Hence, one can emphasize that the stored hydrogen associated with CNTs may have the attachment on its surface.

**Summary.** Among the hydrogen storage materials investigated in this thesis work, SWCNTs functionalized with BH₃ turn out to be a good HSM with the maximum hydrogen storage capacity and the lower desorption temperature. The binding energy of hydrogen also exists in the ideal limits. The material exhibits excellent reproducibility and lesser deterioration level of only 2.4%. The HSM based on SWCNTs-metal oxide composites shows considerable (but not poor) performance. The preliminary investigation of hydrogen storage in SWCNTs-water soluble polymer composite shows interesting results. Hydrogen uptake of the materials investigated here depends on the nature of interaction between materials and hydrogen, preparation method as well as the method of hydrogenation. Hence, enhanced performance towards the hydrogen adsorption and desorption of these materials can be achieved by modifying the above stated factors.

**Acknowledgements.** Madurai Kamaraj University is gratefully acknowledged for academic and financial support through University Stipendiary Research Fellowship (USRF) and UGC for the award of BSR fellowship.

**References**

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[2] D. Silambarasan, V.J. Surya, V. Vasu and K. Iyakutti, *Int. J. Hydrogen Energy* **36**, 3574-3579 (2011).

[3] D. Silambarasan, V. Vasu, V.J. Surya and K. Iyakutti, *IEEE Trans. Nanotechnol.* **11**, 1047-1053 (2012).

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Cite the paper

V. Vasu, D. Silambarasan (2017). *Carbon Nanotubes as Future Energy Storage System*. *Mechanics, Materials Science & Engineering*, Vol 9. doi:10.2412/mmse.52.18.599