The effect of metal codoping on hydrogen storage has been meticulously studied in small cubic C8 nanocluster within the framework of density functional theory (DFT). Initially, a C8 nanocluster was doped with two Li atoms [ C8(Li)2], achieving a hydrogen uptake of 15.5 wt% with an adsorption energy of 0.16 eV. Although this configuration demonstrates a high hydrogen storage capacity, its thermodynamic stability under ambient conditions is limited due to weak binding interactions between Li and H2 molecules. By introducing metal atoms that have stronger binding with the C8 framework, it is expected to enhance the overall structural stability. For that, we have chosen Na, K, Be, Mg, Ca, Sc, Ti, V, and Cr metal atoms along with Li to investigate the influence of codoping on hydrogen storage characteristics. The Ti- and V-codoped structures exhibited significant distortion of the C8 nanocluster during optimization primarily due to strong charge transfer, steric repulsion arising from the larger atomic radii of Ti and V, and partial bond breaking within the nanocluster framework and were, therefore, excluded from further calculations. The resulting codoped structures—C8LiNa, C8LiK, C8LiBe, C8LiMg, C8LiCa, C8LiSc, and C8LiCr— yielded hydrogen uptake of 16.1 wt%, 14.6 wt%, 11.2 wt%, 13.7 wt%, 12.4 wt%, 9.8 wt%, and 11.5 wt%, respectively, all surpassing the U.S. Department of Energy 2025 target of 5.5 wt%. Among these, the LiCr codoped C8 nanocluster exhibited significantly improved adsorption energies of 0.31 eV, which is within the ideal range of 0.2–0.6 eV for faster adsorption–desorption kinetics. Furthermore, Gibbs free energy corrections to H2 adsorption energy at various temperatures and pressures revealed superior thermodynamic stability of the C8LiCr structure, suggesting its promising potential for practical hydrogen storage applications. These results highlight the significant impact of metal codoping as a powerful strategy for enhancing hydrogen uptake, stability, and overall H2 storage performance in nanostructured materials.

Unlocking hydrogen storage potential of carbon nanocluster: a computational study of metal codoping effects
    Abstract
        Graphical abstract
    1 Introduction
    2 Computational details
    3 Results
        3.1 Geometric optimization
        3.2 Geometric parameters
        3.3 Thermodynamic stability
        3.4 H2 adsorption
            3.4.1 Adsorption energy
        3.5 Charge analysis
            3.5.1 Natural bond orbital (NBO) charge analysis: a deeper look into electronic interactions
        3.6 ESP: visualizing charge transfer
        3.7 Electronic properties
        3.8 Global reactivity descriptors
            3.8.1 HOMO–LUMO gap (EHL)
            3.8.2 Chemical hardness (η)
            3.8.3 Electrophilicity index (ω)
        3.9 QTAIM analysis
        3.10 RDG analysis
        3.11 AIMD analysis
        3.12 Discussion
    4 Conclusion
    Acknowledgements 
    References

• Poonam Parkar(Department of Physics, The Institute of Science, Dr. Homi Bhabha State University, Mumbai 400032, India)
• Ajay Chaudhari(Department of Physics, The Institute of Science, Dr. Homi Bhabha State University, Mumbai 400032, India) Corresponding author