Boron nitride nanotubes (BNNTs) represent a promising class of nanomaterials for hydrogen storage, offering distinct advantages over conventional carbon-based materials such as carbon nanotubes (CNTs) and metal-organic frameworks (MOFs). Their unique structural and chemical properties make them particularly suitable for applications requiring high thermal stability, mechanical robustness, and efficient hydrogen adsorption. This article examines the potential of BNNTs as hydrogen storage media, comparing their performance with CNTs and MOFs while focusing on oxidation resistance and high-temperature stability.

BNNTs share a structural similarity with CNTs, consisting of rolled-up hexagonal lattices. However, while CNTs are composed of carbon atoms, BNNTs are formed by alternating boron and nitrogen atoms. This compositional difference imparts several superior properties to BNNTs, particularly in harsh environments. One of the most significant advantages of BNNTs is their exceptional thermal stability. Unlike CNTs, which begin to oxidize at temperatures around 400-500°C in air, BNNTs remain stable up to approximately 800-900°C. This high oxidation resistance is due to the strong covalent bonds between boron and nitrogen, as well as the inherent chemical inertness of BNNTs. For hydrogen storage applications, this property ensures structural integrity and performance stability even under elevated temperatures, making BNNTs suitable for systems where thermal management is critical.

Mechanical strength is another area where BNNTs outperform CNTs. The Young's modulus of BNNTs is comparable to that of CNTs, ranging between 1-1.2 TPa, but BNNTs exhibit superior resistance to mechanical deformation under high-stress conditions. This robustness is advantageous for hydrogen storage systems that undergo repeated charging and discharging cycles, as the material must maintain its structural integrity over time. Additionally, BNNTs demonstrate excellent resistance to radiation damage, further enhancing their durability in demanding environments.

Hydrogen adsorption in BNNTs occurs through physisorption, where hydrogen molecules are weakly bound to the nanotube surfaces via van der Waals interactions. The hydrogen storage capacity of BNNTs is influenced by factors such as tube diameter, chirality, and functionalization. Studies have reported hydrogen uptake values for BNNTs in the range of 1-3 wt% under moderate pressures and ambient temperatures. While this is comparable to the performance of CNTs, BNNTs exhibit more consistent adsorption behavior due to their uniform electronic structure. Unlike CNTs, which can display metallic or semiconducting properties depending on chirality, BNNTs are consistently wide-bandgap semiconductors, leading to more predictable hydrogen interactions.

Functionalization of BNNTs can further enhance their hydrogen storage capabilities. For example, doping BNNTs with transition metals or alkali metals has been shown to increase hydrogen uptake by introducing additional binding sites. Metal-doped BNNTs can achieve storage capacities of up to 4-5 wt%, approaching the targets set by the U.S. Department of Energy for practical hydrogen storage systems. The chemical stability of BNNTs also prevents undesirable side reactions during functionalization, a common issue with CNTs that can degrade their performance over time.

When compared to MOFs, BNNTs offer distinct advantages in terms of thermal and chemical stability. MOFs are highly porous materials with large surface areas, often exceeding 2000 m²/g, which enables high hydrogen adsorption capacities at cryogenic temperatures. However, MOFs typically suffer from poor thermal stability, with many frameworks decomposing at temperatures above 300°C. In contrast, BNNTs maintain their structure and functionality at much higher temperatures, making them more suitable for applications where thermal cycling is involved. Additionally, MOFs are prone to degradation in the presence of moisture or reactive gases, whereas BNNTs are inherently resistant to such environmental factors.

The high-temperature performance of BNNTs is particularly noteworthy. In systems where hydrogen is stored or released at elevated temperatures, BNNTs demonstrate minimal degradation compared to CNTs or MOFs. For instance, in thermochemical hydrogen storage cycles, BNNTs can withstand repeated exposure to temperatures exceeding 600°C without significant loss of capacity. This stability is attributed to the strong boron-nitrogen bonds and the absence of reactive carbon atoms that could participate in oxidation or reduction reactions.

Another critical factor is the reversibility of hydrogen adsorption in BNNTs. Unlike some hydride-based storage materials that require complex regeneration processes, BNNTs allow for straightforward release of hydrogen through pressure or temperature swings. The adsorption-desorption kinetics in BNNTs are also favorable, with rapid uptake and release rates that are comparable to or better than those of CNTs. This reversibility is essential for practical hydrogen storage systems, where quick refueling and discharging are necessary.

Despite these advantages, BNNTs are not without challenges. The primary limitation is their current production cost, which remains higher than that of CNTs due to the complexity of synthesis methods. However, advances in scalable fabrication techniques, such as plasma-enhanced chemical vapor deposition, are expected to reduce costs in the future. Additionally, while BNNTs exhibit good hydrogen adsorption properties, further research is needed to optimize their capacity to meet commercial targets. Strategies such as defect engineering, hybrid nanostructures, and advanced doping could potentially enhance performance.

In summary, BNNTs present a compelling alternative to CNTs and MOFs for hydrogen storage, particularly in applications demanding high thermal stability, mechanical strength, and oxidation resistance. Their ability to maintain performance under extreme conditions, coupled with favorable hydrogen adsorption characteristics, positions them as a viable candidate for next-generation storage materials. While challenges related to cost and capacity optimization remain, ongoing research and technological advancements are likely to address these barriers, paving the way for broader adoption of BNNTs in hydrogen energy systems.