Enhanced hydrogen storage in lithium-doped defective fullerenes: Experimental optimization, adsorption mechanisms, and kinetic-isotherm modeling

Abstract

This study investigates the enhancement of hydrogen storage capacity in fullerene (C60) through lithium doping and defect formation. Defective fullerenes (DC60) were synthesized via high-energy ball milling, and Li-doped variants were prepared using hydrothermal methods. FTIR revealed the disappearance of C60's characteristic bands and the emergence of new bands in doped and defective samples. Thermal analysis showed reduced stability and altered degradation mechanisms due to defect formation and lithium incorporation. SEM indicated significant morphological changes, with increased agglomeration in Li-doped particles. Particle size variations and symmetry loss were also observed post-milling.Hydrogen storage performance depended on lithium concentration, temperature, and doping time. Among tested samples, the sample doped with 0.1 M LiNO₃ at 200 °C for 12 h (Li-D-C60–01 M-200C-12 h) showed the highest hydrogen uptake, attributed to its large surface area and micropore volume under high-pressure adsorption conditions and cryogenic temperatures, where excess adsorption behavior was observed.Isotherm models fitted well with Freundlich and Langmuir equations, while kinetic data followed the pseudo-second-order model, indicating intra-particle diffusion as the rate-limiting step. EIS analysis demonstrated improved conductivity and reduced impedance in Li-doped samples due to enhanced diffusion-based charge transport. Pearson correlation analysis revealed strong positive relationships between hydrogen storage capacity and both BET surface area (r = 0.9033) and micropore volume (r = 0.8867), with the dominant influence arising from BET surface area; this indicates that Li doping affects performance primarily through modifications in pore accessibility and surface electronic structure.Moreover, Li centers act as controllable hydrogen release valves, enabling safer and more reversible hydrogen desorption compared to pristine fullerene systems. These results demonstrate the promising potential of Li-doped defective fullerenes in hydrogen storage applications.