Boron nitride nanotubes (BNNTs) and nanosheets have emerged as promising candidates for hydrogen storage due to their unique structural and chemical properties. These materials exhibit high thermal stability, exceptional mechanical strength, and tunable hydrogen adsorption characteristics, making them attractive alternatives to conventional carbon-based nanomaterials. This article explores the potential of BNNTs and nanosheets for hydrogen storage, comparing their performance with carbon nanotubes (CNTs) and detailing synthesis methods, doping strategies, and experimental results.

BNNTs share a structural similarity with CNTs, consisting of rolled hexagonal lattices. However, while CNTs are composed of carbon atoms, BNNTs feature alternating boron and nitrogen atoms. This difference in composition grants BNNTs several advantages, particularly in high-temperature and oxidative environments. BNNTs demonstrate remarkable thermal stability, withstanding temperatures up to 900°C in air, whereas CNTs oxidize at much lower temperatures, around 400-500°C. This oxidation resistance makes BNNTs suitable for applications where thermal management is critical, such as in fuel cells or high-temperature hydrogen storage systems.

Mechanical strength is another area where BNNTs excel. Studies have shown that BNNTs possess a Young’s modulus of approximately 1.2 TPa, comparable to CNTs, but with superior resistance to mechanical degradation under harsh conditions. This robustness ensures long-term structural integrity in hydrogen storage applications, where repeated adsorption-desorption cycles can strain the material.

Hydrogen adsorption in BNNTs occurs through physisorption, where weak van der Waals forces bind hydrogen molecules to the nanotube surface. The adsorption capacity depends on factors such as surface area, pore volume, and functionalization. Pristine BNNTs typically exhibit moderate hydrogen uptake, with experimental values ranging from 1.5 to 3.0 wt% at cryogenic temperatures (77 K) and moderate pressures (1-10 bar). However, doping and functionalization strategies can significantly enhance this capacity. For instance, introducing transition metals like palladium or platinum onto BNNT surfaces can facilitate spillover effects, where hydrogen molecules dissociate and migrate to additional binding sites, increasing storage density.

BNNT nanosheets, or hexagonal boron nitride (h-BN) sheets, also show potential for hydrogen storage. Their two-dimensional structure provides a large surface area for hydrogen adsorption, and their inherent defects and edges can serve as additional binding sites. Experimental studies report hydrogen uptake values of 2.0 to 4.5 wt% for functionalized h-BN nanosheets under similar conditions. The advantage of nanosheets lies in their ease of processing and scalability compared to nanotubes.

Synthesis techniques for BNNTs include chemical vapor deposition (CVD), ball milling, and high-temperature methods. CVD is the most widely used, producing high-purity BNNTs with controlled diameters and lengths. Ball milling, followed by annealing, offers a cost-effective alternative but may introduce more defects. High-temperature synthesis, such as plasma or laser ablation, yields high-quality BNNTs but at higher energy costs. Each method influences the final hydrogen storage performance by altering the material’s crystallinity, defect density, and surface chemistry.

Doping strategies further optimize BNNTs for hydrogen storage. Incorporating elements like lithium or magnesium can enhance hydrogen binding energies through charge polarization. For example, lithium-doped BNNTs have demonstrated hydrogen uptake capacities of up to 5.0 wt% at room temperature, attributed to the strong electrostatic interaction between hydrogen molecules and the doped sites. Similarly, covalent functionalization with organic groups can increase surface hydrophobicity, improving hydrogen adsorption under humid conditions.

In contrast, carbon nanotubes have been extensively studied for hydrogen storage but face limitations. While CNTs can achieve high theoretical capacities, their practical performance often falls short due to challenges in achieving uniform pore sizes and preventing aggregation. Additionally, CNTs are prone to oxidation, limiting their use in high-temperature environments. BNNTs, with their inherent oxidation resistance and chemical stability, offer a more durable alternative.

Experimental results highlight the volumetric and gravimetric capacities of BNNT-based systems. Gravimetric capacity, expressed as weight percentage, is crucial for mobile applications where weight is a constraint. Volumetric capacity, measured in g/L, determines the space efficiency of the storage system. BNNTs have shown balanced performance in both metrics, with gravimetric capacities reaching 4-5 wt% and volumetric capacities around 40-60 g/L under optimized conditions. These values are competitive with other advanced storage materials like metal-organic frameworks (MOFs) but with added benefits of thermal and mechanical stability.

Challenges remain in scaling up BNNT production and reducing costs to make them commercially viable for hydrogen storage. Current synthesis methods are energy-intensive, and achieving uniform doping or functionalization at large scales is difficult. However, ongoing research into scalable fabrication techniques and novel doping approaches continues to improve the feasibility of BNNT-based hydrogen storage systems.

In summary, boron nitride nanotubes and nanosheets present a compelling option for hydrogen storage, combining thermal stability, mechanical strength, and tunable adsorption properties. Their advantages over carbon nanotubes, particularly in oxidative environments, make them suitable for demanding applications. While further development is needed to optimize synthesis and doping processes, the progress so far underscores the potential of BNNTs in advancing hydrogen storage technologies.