Mechanistic Insights into Hydrogen Storage Performance from the Substitution Doping Method: A Case Study on Alkali Metal-Doped Carbon Nanotubes

The development of suitable nanostructures for hydrogen storage applications is crucial to advancing clean and efficient energy technologies. This study explores the potential of substituted carbon nanotubes (CNTs) for hydrogen storage with a focus on the substitutional doping strategy. A four-step screening approach identifies alkali metals (Li, Na, and K) as highly effective dopants. All dopant atoms bind strongly to the CNT surface, with binding energies (EB ≥ |−1| eV), ensuring structural stability for long-term applications. Binding energy analysis indicates that hydrogen adsorption occurs within the range defined by the U.S. Department of Energy (|−0.15| ≤ EB ≤ |−0.6| eV). The best-performing structure, K-doped single-walled CNT, achieves a gravimetric capacity of 5.79 wt % and a desorption temperature of approximately 218 K. Moreover, mechanistic insights are provided through analyses of the density of states, PDOS, charge transfer, and band structure. These results highlight the promise of substitutionally doped CNTs for hydrogen storage and establish a broadly applicable strategy for dopant selection in low-dimensional nanomaterials.