In this article, we cover how boron nitride has been explored as a potentially game-changing material for hydrogen energy storage technologies.
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Hydrogen Storage: Focus on Boron Nitride
Despite years of research, materials like metal hydrides and coordination hydrides face limitations such as low storage density, inappropriate thermodynamic temperature, slow kinetic rates, and poor cycling stability. Recent attention has been directed towards two-dimensional (2D) nanostructured materials, particularly those composed of lighter-weight elements, for their potential in hydrogen storage.
A newly emerging “white graphene”, hexagonal boron nitride (h-BN), has gained significant attention due to its unique electronic and thermodynamic properties. Unlike graphene, which consists of C atoms, BN is composed of boron (B) and nitrogen (N) atoms in a 1:1 ratio.
Many works have focused on various strategies such as specific surface area, functionalization, substitutional doping and porosity to enhance the outcome of BN as a hydrogen storage material.
Theoretical Insights
The van der Waals correction factor in DFT is one of the important parameters affecting the potentiality of h-BN as a hydrogen storage material. The average adsorption energy decreases with the increase in H2 molecules. The contribution of 1s orbital of the H atom in the conduction band leads to a weak tuning of bandgap with the increasing H2 adsorption. 1 To tackle this issue of decreased adsorption energy, doping host materials with metals, especially transition metals ,or alkali is the adopted solution.
The H2 adsorption on the undoped h-BN, is found to be a weak physisorption. Doping with transition metals like V, Cr, Mn, Nb, Mo, Te, Ta, W or Re, successfully enhances the adsorption ability. Among all the transition metals doped on the h-BN, W atom shows the highest binding energy with BN. 2
Various studies are also performed theoretically to study the effect of doping of BN with Ca, Li, and C on the absorption ability. Ca atoms are preferentially bound to vacancy sites, with binding energies of 6.75 eV and 6.37 eV for Ca adsorption on B vacancy and B–N divacancy sheets, respectively. The Ca-decorated BNNSs with B vacancy and B–N divacancy defects demonstrated promising hydrogen storage capabilities, achieving theoretical gravimetric densities of 7.6 wt% and 8.0 wt%, respectively. 3
Prof. Mounkachi and his team from Mohammed V University in Rabat, Morocco, also studied hydrogen storage applications using Li, Na, K, Ca nd Sc decorated two-dimensional t-graphene like BN (t-B4N4) monolayer. Li-decorated t-B4N4 shows excellent thermal stability with higher monolayer adsorption energy than Li@t-graphene with a maximum hydrogen absorption capacity of 12.47 wt%. 4
The C-doped h-BN, specifically 2Ti/2C-BN, is also found as a potential carrier for hydrogen storage. The double carbon-doped h-BN exhibited enhanced hydrogen storage capacity, with a gravimetric capacity of 6.33 wt% under mild pressure conditions at ambient temperature. 5
Parallelly, Prof. Sattar and his team from Linnaeus University, Sweden, theoretically predicted the possibility of bilayer h-BN as a candidate for Hydrogen storage using first principle calculations. Negative binding energies suggest stability, but high desorption temperatures for up to six H2 molecules may hinder reversible adsorption. Effective carrier masses are calculated to understand the impact of H2 adsorption on electron or hole transport. Diffusion barriers indicate a small energy barrier for H2 molecular propagation across hexagonal minimum-energy sites. 6
Experimental investigations
The theoretical calculations suggest optimizing the morphology, introducing heteroatoms, and modifying h-BNNS with metal nanoparticles can significantly enhance its hydrogen storage capabilities. However, there is a need for experimental validation to confirm these theoretical findings and address practical challenges in the synthesis and stability of modified h-BN materials for hydrogen storage applications.
Chemical synthesis is the prevailing method for sample preparation, offering advantages in adjusting the elemental composition and creating lattice substitution defects for increased hydrogen adsorption sites. The synthesis of h-BN nanosheets using various technologies leads to materials with diverse lateral size, vertical thickness distributions, and chemical compositions.
Porous h-BN microsponges and microbelts, created through chemical reactions, exhibited high and reversible H2 sorption capacities. The hydrogen storage potential of the original h-BN is attributed to physisorption, with larger specific surface area and porosity contributing to greater absorption under low-temperature and high-pressure conditions. 7
The functionalization of h-BN also plays a crucial role in adsorption ability due to increased BET surface area. The ability is found to be higher at cryogenic temperature when compared to room temperature and increased with increasing pressure.
Prof. Chen from Oak Ridge National Laboratory, successfully synthesized defect-free h-BN doped Ni and h-BN grafted PLLA to improve the hydrogen storage performance of pristine h-BN. This strategy is adopted to separate the h-BN nanosheets and increase the interlayer separation. PLLA-h-BN showed a hydrogen storage capacity of around 6.1 wt%, whereas the capacity increased to 7.3 wt% for Ni-h-BN. 8
A lightweight and compact hydrogen storage medium utilizing acid-treated halloysite clay nanotubes (A-HNTs) in combination with hexagonal boron nitride nanoparticles (h-BN) can be a material of choice. The resulting 5 wt% h-BN decorated acid-treated halloysite clay nanotubes exhibited a significantly increased storage capacity of 2.88 wt%, compared to the mere 0.22 wt% observed in pristine HNTs.
Future Challenges and Commercialization
The above findings highlight the potential of BN as a promising material for hydrogen storage applications, though it is crucial to acknowledge that commercialization remains a distant goal.
However, an optimistic outlook suggests that this challenge could be overcome through the implementation of efficient regeneration. By addressing this concern, h-BN emerges as a compelling economic and promising candidate for hydrogen storage in the foreseeable future.
References and Further Reading
Jhi, S.-H., et al. (2004). Hydrogen adsorption on boron nitride nanotubes: a path to room-temperature hydrogen storage. Physical Review, B 69(24), p. 245407. doi.org/10.1103/PhysRevB.69.245407
Thupsuri, S., et al. (2021) A study of the transition metal doped boron nitride nanosheets as promising candidates for hydrogen and formaldehyde adsorptions. Physica E: Low-dimensional Systems and Nanostructures, 134, p. 114859.
Ma, L., et al. (2021) First-principles study of hydrogen storage on Ca-decorated defective boron nitride nanosheets. Physica E: Low-dimensional Systems and Nanostructures, 128, p. 114588.
Kassaoui, M. E, L., et al. (2022) Improvement of the hydrogen storage performance of t-graphene-like two-dimensional boron nitride upon selected lithium decoration. Physical Chemistry Chemical Physics, 24(24), pp. 15048-15059.
Wang, T., et al. (2022) Hydrogen storage in C-doped h-BN decorated with titanium atom: A computational study. Chemical Physics Letters, 803, p. 139853.
Rai, D. P., et al. (2021) Hydrogen storage in bilayer hexagonal boron nitride: a first-principles study. ACS omega, 6(45), pp. 30362-30370.
Weng, Q., et al. (2014) One‐step template‐free synthesis of highly porous boron nitride microsponges for hydrogen storage. Advanced Energy Materials, 4(7), p. 1301525.
Liang, H., et al. Intercalation optimized hexagonal boron nitride nanosheets for high efficiency hydrogen storage. Applied Surface Science, 604, p. 154118.
Muthu, R. et al. (2016). Synthesis, characterization of hexagonal boron nitride nanoparticles decorated halloysite nanoclay composite and its application as hydrogen storage medium. Renewable Energy, 90, pp. 554-564. doi.org/10.1016/j.renene.2016.01.026
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