Black phosphorus nanosheets have emerged as a promising candidate for hydrogen storage due to their unique anisotropic structure and tunable electronic properties. Unlike isotropic materials such as graphene, black phosphorus exhibits a puckered lattice structure that creates distinct in-plane anisotropy. This structural characteristic allows for directional-dependent interactions with hydrogen molecules, potentially enhancing adsorption and storage capabilities. The tunable bandgap of black phosphorus, ranging from 0.3 eV in bulk to approximately 2 eV in monolayer form, further enables precise control over its electronic properties, which can be optimized for hydrogen storage applications.

The hydrogen storage mechanism in black phosphorus nanosheets primarily involves physisorption, where weak van der Waals forces facilitate the adsorption of hydrogen molecules onto the material's surface. Theoretical studies suggest that the puckered structure of black phosphorus provides a higher surface area-to-volume ratio compared to flat graphene sheets, leading to increased hydrogen adsorption sites. Additionally, the anisotropic nature of black phosphorus allows for selective functionalization along specific crystallographic directions, which can further enhance hydrogen binding energies. Experimental measurements indicate that black phosphorus nanosheets can achieve hydrogen storage capacities of up to 4.5 wt% under moderate pressure and temperature conditions, though practical applications require further optimization.

Graphene, a widely studied 2D material for hydrogen storage, offers high surface area and excellent electrical conductivity. However, its lack of a bandgap and isotropic structure limit its ability to selectively interact with hydrogen molecules. In contrast, black phosphorus provides a tunable electronic structure that can be modified via strain engineering, chemical doping, or layer thickness adjustment to optimize hydrogen adsorption. For instance, applying tensile strain along the armchair direction of black phosphorus can increase the interlayer distance, creating additional space for hydrogen intercalation. Similarly, doping with alkali metals such as lithium or potassium can enhance hydrogen binding energies through charge transfer mechanisms.

Despite these advantages, black phosphorus nanosheets face significant challenges related to environmental degradation. Exposure to ambient conditions leads to rapid oxidation and structural degradation, which severely impacts their hydrogen storage performance. Mitigation strategies include encapsulation with protective layers such as hexagonal boron nitride or graphene, which act as barriers against oxygen and moisture while preserving the material's electronic properties. Another approach involves chemical passivation using organic molecules or polymers that bind to the surface of black phosphorus, preventing oxidative degradation. Recent studies have demonstrated that passivated black phosphorus nanosheets retain their hydrogen storage capacity for extended periods under ambient conditions.

Comparing the hydrogen storage performance of black phosphorus and graphene reveals distinct trade-offs. Graphene exhibits superior stability and mechanical strength, making it more suitable for large-scale applications. However, its hydrogen storage capacity is limited by the weak physisorption of hydrogen molecules and the lack of tunable electronic properties. Black phosphorus, while offering higher theoretical storage capacities and tunability, requires careful handling and protection to prevent degradation. The choice between these materials depends on specific application requirements, including operating conditions, desired storage capacity, and long-term stability.

Degradation mitigation remains a critical area of research for black phosphorus nanosheets. In addition to encapsulation and passivation, advanced synthesis techniques such as chemical vapor deposition with in situ protection can produce high-quality, defect-free nanosheets with improved stability. Furthermore, alloying black phosphorus with other elements, such as arsenic or antimony, has shown promise in enhancing its environmental resilience without compromising hydrogen storage performance. These strategies aim to bridge the gap between laboratory-scale demonstrations and practical deployment in hydrogen storage systems.

The anisotropic properties of black phosphorus also enable novel hydrogen storage configurations. For example, vertically stacked black phosphorus nanosheets with controlled interlayer spacing can create hierarchical structures that maximize hydrogen adsorption while minimizing material volume. Such designs leverage the directional dependence of black phosphorus to optimize storage density and kinetics. Computational modeling suggests that these architectures could achieve storage capacities exceeding 6 wt% under optimized conditions, though experimental validation is ongoing.

In summary, black phosphorus nanosheets present a compelling alternative to graphene for hydrogen storage, leveraging their anisotropic structure and tunable electronic properties to achieve higher capacities and tailored performance. However, their susceptibility to degradation necessitates robust mitigation strategies to ensure long-term viability. Encapsulation, passivation, and advanced synthesis techniques offer pathways to overcome these challenges, while innovative structural designs further enhance storage potential. As research progresses, black phosphorus could play a pivotal role in advancing hydrogen storage technologies, complementing existing materials like graphene in the transition toward sustainable energy systems. The development of stable, high-performance black phosphorus-based storage solutions will require interdisciplinary efforts spanning materials science, chemistry, and engineering.