Hydrogen Adsorption on Boron Nitride Nanotubes

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A P A T H T O R O O M T E M P E R A T U R E

H Y D R O G E N S T O R A G E

B Y

A S W I N . R A V I N D R A N

& A R A V I N D H A W K . J . P

Hydrogen Adsorption on Boron

Nitride Nanotubes



Introduction

Hydrogen is an ideal material as an alternative energy source. A serious bottleneck for the full usage of hydrogen is the lack of appropriate storage materials.Metal hydrides, high-pressurized tanks, and liquid

hydrogen have been tested and investigated as possiblestorage forms or materials, which, however, have severaldrawbacks such as low capacity, safety problems, orimpractical release temperatures. We have focused ondeveloping hydrogen storage facility by using

physisorption property of nanostructurematerials.



Contd..

Materials at nanoscale have advantages over their bulk structures such as large surface area andpotentially high binding energy. Single-wall carbonnanotubes, for example, have all their atoms at the

surface and have been tested as a hydrogen storagefor the past several years. However, their reporteddesorption temperature and capacity have not beenconfirmed. The goal in hydrogen storage is a capacity

of about 6% by weight at ambient temperature andpressure, which is gauged by the surface area andhydrogen binding energy



. The key is therefore to find materials that have thehydrogen binding energy of 0.2–0.3 eV as estimatedfrom the van’t Hoff equation9 and a sufficientsurface area of a few thousands m2/g

In this paper, we report our study on boron nitride(BN) as a potential hydrogen storage throughpseudopotential density functional calculations done

by Seung-hoon jhi & Young-Kyun Kwon of California



The adsorption of molecular hydrogen on surfaces isin nature a physisorption due to strong H-H bondand closed-shell electronic configuration. The

binding energy of H2 on solid surface is thus usually

very small. For example, the binding energy of hydrogen on activated carbons is about 60 meV

which inevitably leads to very high pressure (severalhundred bars) and/or very low temperature s, Kd to

store meaningful amounts of hydrogen in activatedcarbons despite their high surface area of up to 2600m2/g.



We focused on BN because it is light and similar instructure to carbon so that high surface area BN can

be synthesized. An interesting feature of BN is thatthe B-N bond has ionic character, which may induce

an extra dipole moment and hence a strongeradsorption of hydrogen. This feature may in fact betrue for all ionic materials because the (induced)dipole moment of hydrogen is sensitive to local

electric fields. Hexagonal boron-nitride shBNdconsists of heteropolar sp2-like bonding, and Rubioet al.12 showed that BN could form tubularstructures through ab initio calculations.



We focused on BN because it is light and similar instructure to carbon so that high surface area BN can

be synthesized. An interesting feature of BN is thatthe B-N bond has ionic character, which may induce

an extra dipole moment and hence a strongeradsorption of hydrogen. This feature may in fact betrue for all ionic materials because the (induced)dipole moment of hydrogen is sensitive to local

electric fields. Hexagonal boron-nitride shBNdconsists of heteropolar sp2-like bonding, and Rubioet al.12 showed that BN could form tubularstructures through ab initio calculations.



Figure 2 shows the binding curves of hydrogenmolecule on BN graphitic sheet for various bindingsites: top of boron (B), nitrogen atoms (N), andhexagon center (HC). The binding energy of H2 onthe BN sheet is calculated to be about 90 meV. Wenote that the difference in binding energy fordifferent sites is quite small that the diffusion of

hydrogen can be fast in BN at moderatetemperatures.



. Schematic drawing of various BN structure. (a) A graphiticsheet of hexagonal boron nitride with carbon (dark filledcircle) doping, (b) hBN with 537 defects denoted by gray filledcircles, and (c) (10,0) boron nitride nanotube with hydrogen (smallgray circles) on top of it. Boron and nitrogen atoms are depicted aslarger and smaller empty circles in all three figures.



The binding energy in BN nanotubes is even larger by about 10% than in planar BN sheet, which ispresumably due to the buckling of BN bonds.

Another interesting observation is that the bindingenergy in BN nanotube has a more variance fordifferent sites than in planar BN, which is also due tothe corrugation of B-N bonds in BN nanotube.

The diffusion of hydrogen is therefore slower insmall diameter BN nanotubes than in largerdiameter ones.



The nanotube in that study has bamboo likestructure and its surface area is not significant(about 200 m2/g) compared to that of high surface-area activated carbon s,2600 m2/gd.11 Since thecapacity of hydrogen storage depends on the surfacearea, the study demonstrated that BN can be ahydrogen storage material with a strong binding

energy and hence is in support of our calculations.Synthesis of high surface-area BN at large quantitiesis required for further studies and applications.





Applications in Automobile sector

Our study demonstrates a pathway to the finding of proper media that can hold hydrogen at ambientconditions through physisorption which would be agreat use in the future to store huge amountof hydrogen at room tempertature for usingas a alternative energy source for cars, planes& rockets .



Advantages

We can store hydrogen fuel for vehicles occupying

very less space thus being compact as being

weightless and can be stored in room

temperature without need of external components .



Q U E R I E S ? ? ?



THANK YOU

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Hydrogen Adsorption on Boron Nitride Nanotubes

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