Thermal Analysis of Boron Nitride CNTs for Hydrogen Storage Applications

In an article recently published in the journal Materials Today Communications, researchers demonstrated the potential of carbon nanotubes (CNTs) and boron nitride nanotubes (BNNTs) as hydrogen (H2) storage materials by investigating their thermal conductivities using atomistic nonequilibrium molecular dynamics (NEMD) simulations. 

Thermal Analysis of Boron Nitride CNTs for Hydrogen Storage Applications

​​​​​​​Study: Thermal conductivities of hydrogen encapsulated boron nitride and hybrid boron nitride – carbon nanotubes using molecular dynamics simulations. Image Credit: Kateryna Kon/Shutterstock.com

BNNT, CNT, and boron nitride–carbon (BN-C) heteronanotubes were studied in pristine and defective states for their thermal conduction, with varying quantities of encapsulated H2. The performance of reactive forcefield (ReaxFF) was compared with non-reactive Tersoff-adaptive intermolecular reactive empirical bond order (AIREBO). Although ReaxFF consistently resulted in lower thermal conductivity (k) for all the analyzed nanotubes, it provided a better qualitative thermal transport prediction.

On the other hand, the combination of Tersoff + AIREBO + Lennard-Jones (TALJ) potentials provided a good quantitative prediction of k. However, the reason for the acoustic phonon remained unclear. Near-linear mixing rule for k in CNT-BNNT composition confirmed by both ReaxFF and TALJ potentials suggested that the interfacial heat transport at heterojunctions was smooth.

CNTs and BNNTs

CNTs and BNNTs have a structural similarity that allows their configuration into various geometries like hetero-nanotubular structures, nanometric films, nanotube arrays, and nanocomposites.

Based on the dimension, chirality, and defects, CNT is either semiconducting or semi-metallic with a band gap of 1 electronvolt. On the other hand, BNNTs are dielectric and have an approximate bandgap of 5.5 electronvolt irrespective of dimensions, chirality, or defects in their electrical behavior. 

Lightweight CNTs and BNNTs have acceptable thermal stabilities and exceptional thermal conductivities that help in the thermal management of miniaturized electronic devices. Chirality-pure CNTs have found their applications in high-density, high-performance electronic devices. However, the applications of BNNT remain unexplored due to the difficulty in synthesizing highly pure BNNT. Moreover, the synthesis of single-walled CNT-BNNT heteronanotubes with a heterojunction between CNT and BNNT layers was unsuccessful.

Molecular dynamic (MD) simulation is a robust tool that allows molecular level studies on nanoscale materials in terms of their thermal transport mechanisms. Moreover, MD simulations are often applied to study the thermal properties of BNNTs and CNTs. The computationally predicted value is used as a replacement for the experimental value due to the challenges involved inBNNT synthesis.

C and BN nanotubular materials come with endemic lattice and impurity defects introduced during their synthesis and other property manipulations. Although CNTs were previously explored to understand the impact of defects on their thermal conductivity, BNNTs and BN-C heteronanotubes remain unexplored in the same context.

Thermal Conductivities of H2 Encapsulated BNNT and BNNT-CNT using MD Simulations

In the present study, NEMD simulations were performed to calculate heat transport in pristine, defective, and hydrogen encapsulated, capped single-walled BNNT and BNNT-CNT hybrid structures. The entrapped weight % of H2 molecules inside the nanotubes were 0.00, 3.25, and 6.50.

Furthermore, the ReaxFF force field helped analyze the interatomic interactions of B, N, C, and H atoms, and the results were compared with those obtained from combined potentials of B-C-N interaction’s Tersoff potential, H-H interaction’s AIREBO potential, and C-B-N interaction’s LJ potential. The combination of Tersoff-AIREBO-LJ potential was termed as TALJ potential.

The studies were conducted on the effect of H2 encapsulation on the thermal conductivities of pristine BNNT, CNT, and the BN-C (50%) hybrid using two different potentials. Comparing TALJ and Tersoff potential for nanotubes with and without H2 content revealed that the thermal conductivities showed a decreasing trend with increasing H2 content in nanotubes.

The ReaxFF potential predicted a greater effect of H2 content in all three nanotubes. With increasing H2 content, maximum k reduction was observed in CNT, followed by BN-C hybrid and BNNT. This reduction in k was due to increased phonon scattering that arose due to collisions between nanotube atoms and H2 molecules and H2 molecules adsorption on the nanotube walls.

ReaxFF potential described C/B/N nanotube atom’s short-range bonded interactions and nanotube- H2 molecule’s long-range interactions that are non-bonded. The interactions between nanotube atoms-H2 molecules led to physisorption and chemisorption of H2 molecules on the inner side of the nanotube wall. The TALJ potentials accounted for the C-B-N atom’s short-range bonded interactions and physisorption-based, long-range non-bonded interactions with H2 molecules.

Conclusion

To summarize, NEMD simulations were performed along with ReaxFF forcefield and TALJ potentials to investigate the thermal conductivities of BNNT and BN-C heteronanotubes. The effect of a single atom (B, C, or N) and paired atom (B-N) vacancies and variable H2 content on thermal conductivities were explored at 300 kelvin. Compared to TALJ potentials, The ReaxFF potential predicted lower k values for all three nanotubes.

ReaxFF potential could determine the effect of composition on thermal conductivities across BNNT-CNT hybrids and capture the reactive interactions between the nanotube and H2 molecules. Moreover, parametrization by the inclusion of phonon dispersion quantitatively estimated k for BNC nanotubes.

ReaxFF described both long- and short-range interactions with its set of parameters. Furthermore, ReaxFF's expandable element coverage made this computational modeling useful for various applications like investigating the potential of H2 encapsulated nanotubes to store energy and sensor activity of molecule-encapsulated C-BN heteronanotubes.

Reference

Dethan, JFN., Yeo, J., Rhamdhani, MA ., Swamy, V. (2022) Thermal conductivities of hydrogen encapsulated boron nitride and hybrid boron nitride – carbon nanotubes using molecular dynamics simulations. Materials Today Communications. https://www.sciencedirect.com/science/article/pii/S2352492822008017?via%3Dihub

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Bhavna Kaveti

Written by

Bhavna Kaveti

Bhavna Kaveti is a science writer based in Hyderabad, India. She has a Masters in Pharmaceutical Chemistry from Vellore Institute of Technology, India, and a Ph.D. in Organic and Medicinal Chemistry from Universidad de Guanajuato, Mexico. Her research work involved designing and synthesizing heterocycle-based bioactive molecules, where she had exposure to both multistep and multicomponent synthesis. During her doctoral studies, she worked on synthesizing various linked and fused heterocycle-based peptidomimetic molecules that are anticipated to have a bioactive potential for further functionalization. While working on her thesis and research papers, she explored her passion for scientific writing and communications.

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