Hydrogen storage in platinum loaded single-walled carbon nanotubes

Hydrogen storage in platinum loaded single-walled carbon nanotubes

international journal of hydrogen energy xxx (xxxx) xxx Available online at www.sciencedirect.com ScienceDirect journal homepage: www.elsevier.com/l...

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international journal of hydrogen energy xxx (xxxx) xxx

Available online at www.sciencedirect.com

ScienceDirect journal homepage: www.elsevier.com/locate/he

Hydrogen storage in platinum loaded single-walled carbon nanotubes Anshu Sharma Department of Physics, Indian Institute of Technology Delhi, New Delhi 110016, India

highlights  A simple and effective method to load Pt on SWNT is presented.  Morphological changes in SWNT after loading of Pt is discussed.  Investigation on thermal stability of SWNT with and without Pt loading.  H2 storage capacities of SWNT and SWNT/Pt at 77 K and RT are discussed.

article info

abstract

Article history:

To study the hydrogen storage capacity, platinum (Pt) nanoparticles were deposited on

Received 21 March 2019

single-walled carbon nanotubes (SWNT) using hexachloroplatinic acid (H2PtCl6$6H2O) as a

Received in revised form

precursor. To verify Pt deposition on the surface of the SWNT, a Transmission Electron

30 April 2019

Microscope (TEM) was used to obtain surface morphology. The TEM images show that Pt

Accepted 4 September 2019

nanoparticles were homogeneously distributed on the surface of SWNT. Commercial

Available online xxx

SWNT were also used to compare the results. Thermal Gravimetric Analysis at heating rate of 5  C/min is measured for pure SWNT and Pt loaded SWNT. Before hydrogen storage

Keywords:

measurements these samples were reduced in 10% of H2 in Ar, flowing at 900  C in a

Single-walled carbon nanotubes

tubular furnace for 1 hour. Hydrogen storage capacity of these SWNT was investigated

Platinum nanoparticles

under 25 bar pressure and room temperature as well as liquid nitrogen temperature.

Hydrogen storage

© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Physisorption

Introduction Hydrogen has the highest energy density 120 MJ/kg over all possible fuels and this value is three times higher than gasoline. It is most abundant and an environmentally friendly energy carrier as water is its end combustion product. Regardless of all advantages, its low volumetric density (0.0899 kg/m3) at ambient conditions make it commercially limited [1,2]. Therefore hydrogen storage in a squeezed, reasonable and safer way is major technical barrier which prevents its widespread implementation as a substitute

energy carrier [3e5]. For hydrogen storage several methods including high-pressure hydrogen, liquid hydrogen, physisorption and chemisorption have been proposed. Hydrogen storage in the form of metal hydrides, compressed gas or hydrogen liquefaction possesses unadorned disadvantages [6e10]. These downsides have induced new storage conceptions as physical adsorption of hydrogen on nanomaterials. Porous materials such as metal organic frameworks has become the interest of scientific community as they shows a lot of advantages over above mentioned methods due to their fast adsorption/desorption kinetics [11e15]. Particularly

E-mail address: [email protected] https://doi.org/10.1016/j.ijhydene.2019.09.025 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Sharma A, Hydrogen storage in platinum loaded single-walled carbon nanotubes, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.025

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unique carbon materials, such as carbon nanotubes (CNT), has considered as an auspicious contender for gas adsorption. Carbon is among the very few materials which are light and solid at room temperature (RT). CNT has many advantages like low-cost availability, heat resistance, reversibility, large thermal and chemical stability. In bundles of CNT different adsorption sites are possible for hydrogen such as interstitial networks between the single-walled carbon nanotubes (SWNT) in a bundle, pore of the SWNT and on the external surface of the tube wall or in the grooves formed at the connection between adjacent tubes [16]. Physisorption which is responsible for the hydrogen adsorption in CNT is based on the vander Waals forces between adsorbate and adsorbent. Low hydrogen storage capacities of nanomaterials at ambient conditions [17,18] indicates that at RT interaction due to vander Waals forces is as small as gas molecules have thermal energy on surface of the adsorbent. Therefore reduction in the temperature of the system induces the interaction between adsorbent and gas molecule. Physisorption is considered as a very capable method for hydrogen storage as it is based on the vander Waals forces between adsorbent and adsorbate due to which it shows fast kinetics [19,20]. To increase the hydrogen storage capacities numerous methods such as metal doping and ligand exchange have been reported. It is reported that metal precursor loaded on CNT surface exhibit an upgraded catalytic activity. A robust interaction between nanomaterial and metal precursor is essential for preparation of highly dispersed and stable catalyst. Before loading of metal catalyst particle on the surface of CNT its pretreatments such as acid functionalization introduces active sites on its surface. These pretreatments are also helpful to remove fullerene-like caps or the metal clusters at the ends of the tubes without destroying the nanotubes which allow easy access of the hydrogen to the tube interior. Properties of the functional groups, their dispersion and concentration also impact the dispersion of noble metals. Several experimental methods are introduced in CNTs loaded with metal nanoparticles which include Fe, Co, Mg, Ag, Pt, Li and Pd [21e25]. In this paper, for dissociation of hydrogen molecules and its subsequent adsorption with the possibility of Pt as a catalyst loaded on SWNT at 77 K and RT has been studied.

oil bath at 80  C for 18 h using constant stirring. Then, this acid treated product was cooled up to RT, filtered and washed with deionized water till the remained mixture grasped pH 7 then this filtrate was dried for 2 h at 100  C in an air oven. This acid treated SWNT powder used for further step. Then loading of Pt nanoparticles on the surface of SWNT were performed using a simple and efficient chemical root method. In this procedure, H2PtCl6 (10 mL) were added to the ethylene glycol (20 mL) under constant stirring to ensure the complete solubility of H2PtCl6 then acid treated SWNT (400 mg) were dispersed in to the above solution and ultrasonication was performed about 30 min. Further, this reaction mixture was stirred in an oil bath for 4 h at 120  C. Then, this mixture was filtered and washed with deionized water till filtrate reached pH7 and extracted by centrifugation using at 4000 rpm. This prepared soot was then dried in an air oven for 12 h at 80  C. In this way loading of Pt nanoparticles on the surface of SWNT was achieved [26].

Characterization techniques Transmission Electron Miicroscope (TEM) was used to investigate the morphology, size and shape of the samples. A high resolution TEM (JEOL JEM-2010F) at 200 kV was used to record the TEM images. A carbon-coated copper grid with 300 meshes was used to prepare a sample. Thermal Gravimetric Analysis (TGA) was carried out in a thermogravimetric analyzer Hitachi DSC7020. Measurements were performed in an alumina pans with heating rate of 5  C/min in air atmosphere. The final mass of the sample ranged from 5 to 12 mg. Hydrogen adsorption measurements are performed using an automated Sie-vert’s type apparatus (Setaram-HyEnergy PCT-Pro-2000) equipped with a microdoser from HyEnergy. To control the temperature, the sample was submerged in a thermal bath with liquid nitrogen (77 K) or water (300 K). The amounts of used sample are approximately 100 mg and 150 mg for SWNT and SWNT/Pt respectively.

Results and discussion TEM

Experimental details Chemicals Chloroplatinic Acid 8 wt% in water (Sigma Aldrich, Germany) and large surface area SWNT having length 5e35 mm, diameter ~1e2 nm with purity >95% (US Research Nanomaterials Inc., TX, USA) were used in this experiment. Chemical reagents such as carboxylic acid, nitric acid, ethanol etc were of commercial grade and were used as purchased.

Synthesis of SWNT/Pt nanocomposites First of all, as purchased SWNT was functionalized using carboxylic acid. In this process as purchased SWNT (500 mg) was dispersed in 25 ml of concentrated HNO3 and refluxed in

TEM measurements were carried out to measure the location, distribution and particle size of Pt nanoparticles. The as prepared SWNT/Pt nanocomposites were investigated by TEM. Fig. 1 shows the TEM image contrast of pure SWNT and SWNT/Pt composites i.e. after loading of Pt nanoparticles on SWNT. TEM images indicate that the Pt nanoparticles which are dispersed on the surface of the SWNT having an average size of 20e30 nm and do not agglomerate with each other to form a large cluster. TEM images also implies that Pt nanoparticles loaded directly on surface defect sites and pores and no single Pt nanoparticle is resolved in the TEM images which evident the strong bonding between Pt atoms and carbon atoms at the defects. Therefore above presented TEM images shows the efficiency of a simple method to load Pt nanoparticles on the surfaces of SWNT.

Please cite this article as: Sharma A, Hydrogen storage in platinum loaded single-walled carbon nanotubes, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.025

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Fig. 1 e TEM images of (a) SWNT (b) SWNT/Pt. It is cleared from Fig. 2 that as purchased SWNT and Pt loaded SWNT are considerably less stable at high temperature approximate 800  C and 600  C respectively. Here TGA measurements were performed to characterize the stability of SWNT and SWNT/Pt nanocomposite toward hightemperature.

(a) SWNT (b) SWNT/Pt

100

95

Weight (%)

(a) 90

Hydrogen adsorption 85

80

(b) 75 0

200

400

600

800

1000

Temperature (oC)

Fig. 2 e TGA of (a) SWNT and (b) SWNT/Pt.

TGA Determination of the amount of loaded Pt nanoparticles catalyst on the surfaces of SWNT was analyzed by TGA in an air atmosphere. It is a simple analytical technique that is frequently used to determine the decomposition temperature of the CNT.

0.60

(a)SWNT at 77K

Adsorption Desorption

0.55

Figs. 3 and 4 shows the hydrogen adsorption/desorption analyses of as purchased SWNT and SWNT/Pt at 77 K and RT respectively. The measured isotherm shows no saturation at RT. At low temperature a slight increase in adsorption can be observed. At low temperature no saturation of the pressure isotherms can be determined at pressure up to 25 bar. The adsorption reached 0.55e0.60 wt% at 77 K and 25 bar in the case of pure SWNT but as Pt is loaded in SWNT adsorption isotherm reached 0.65e0.70 wt% at 77 K and 25 bar which is greater than pure SWNT. This indicates that the high pressure adsorption is confirmed in SWNT but very high values could not be reproduced. The adsorption occurs due to attractive potential of the pore walls [27,28]. After loading Pt catalyst hydrogen could be physisorbed on SWNT exposed surfaces. Experimental results shown here indicated that at cryogenic temperatures the hydrogen adsorption in the interstitial sites of SWNT bundles

0.020

(b)SWNT at RT

Adsorption Desorption

0.45

Absolute H2 uptake (wt%)

Absolute H2 uptake (wt%)

0.50

0.015

0.40 0.35

0.010

0.30 0.25

0.005

0.20 0.15 0.10

0.000

0.05 0.00 0

5

10

15

Pressure (bar)

20

25

-0.005

0

10

20

30

Pressure (bar)

Fig. 3 e H2 adsorption/desorption isotherms of (a) SWNT at 77 K (b) SWNT at RT. Please cite this article as: Sharma A, Hydrogen storage in platinum loaded single-walled carbon nanotubes, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.025

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0.70

Adsorption Desorption

0.65 0.60

(a)SWNT/Pt at 77K

0.020

(b)SWNT/Pt at RT

Adsorption Desorption

0.018

Absolute H2uptake (wt%)

AbsoluteH2 uptake (wt%)

0.55 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 -0.05

0.016 0.014 0.012 0.010 0.008 0.006 0.004

0

5

10

Pressure (bar)

15

20

25

0

10

Pressure (bar)

20

30

Fig. 4 e H2 adsorption/desorption isotherms of (a) SWNT/Pt at 77 K (b) SWNT/Pt at RT.

possessing a very wide tube diameter and extreme separation could not sufficient to achieve the gravimetric hydrogen storage for DOE target [29].

Conclusion A facile method to load Pt catalysis on SWNT is devised successfully. This study highlights a very simple but efficient synthesis approach for high loading of Pt on SWNT with uniform dispersion. Through the acid treatment large quantities of homogeneously distributed active sites can be generated for anchoring Pt nanoparticles. Physical characterizations revealed that Pt nanoparticles were successfully deposited on the surface of SWNT with a very good density. CNT samples, including SWNT and Pt loaded SWNT were used for hydrogen storage purpose by a gravimetric measurement method. Hydrogen storage capacity of these samples were found less than 1 wt% in both of the cases having conditions of hydrogen gas pressure 25 bar at 77 K and 25 bar at RT. These results suggest that high pressure hydrogen storage is confirmed in SWNT but very high values could not be reproduced. Therefore it is concluded that hydrogen storage capacity of SWNT is far below the DOE targets and it is unfeasible for on-board hydrogen uptake systems. In the other side, for significantly improving their hydrogen storage capacity and kinetics an effective additive can be used in SWNT.

Acknowledgements The author gratefully acknowledges the financial support of the Department of Science and Technology, Ministry of Science and Technology, Govt. of India under women scientists scheme (WOS-A) grant SR/WOS-A/PS-62/2013. Author is highly thankful to Max Planck Institute for the Structure and Dynamics of Matter, Germany for providing experimental facilities for sample preparation and Max Planck Institute for Intelligent System, Stuttgart, Germany for providing hydrogen storage measurements under Max Planck-India mobility grant supported by Max Planck Society, Germany and Indo-German Science and Technology Centre, Department of Science and Technology, Ministry of Science and Technology, Govt. of India.

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Please cite this article as: Sharma A, Hydrogen storage in platinum loaded single-walled carbon nanotubes, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.025