Hydrogen Storage Performance of Sunflower Stalk-Derived Activated Carbons Produced via ZnCl2 and KOH Activation

In this study, the hydrogen storage capacities of activated carbons derived from sunflower stalk wastes were enhanced by initial chemical activation using different activating agents (ZnCl₂ or KOH) at biomass ratios of 1:1, 2:1, and 3:1 (w/w), followed by carbonization at varying temperatures (600°C, 700°C, 800°C, and 900°C) based on their surface area performance. The optimization and characterization of the prepared samples were systematically conducted using BET, FTIR, DTA/TG, and SEM/EDX techniques. SEM/EDX analysis revealed a marked increase in porosity and notable alterations in the elemental composition of the activated carbon surfaces as a function of the activating agent and carbonization temperature. Hydrogen storage capacities of the optimized samples were measured as a function of pressure at both room and cryogenic temperatures. As a result of the optimization process, the samples with the highest surface areas were identified as AC-Z2-700 and AC-K2-700, with AC-Z2-700 exhibiting the highest hydrogen storage performance. Storage capacities increased with rising pressure and decreasing temperature for both samples, while the isotherm profiles varied significantly between room and cryogenic conditions. The experimental data fitted well with the Henry and Freundlich isotherms at room temperature and with the Langmuir isotherm at cryogenic temperature. Furthermore, kinetic analyses indicated that the adsorption followed a pseudo-second-order model and that the dominant mechanism was intraparticle diffusion within the pores of the activated carbon. Overall, the findings demonstrate that sunflower stalk is a promising and sustainable precursor for producing high-surface area activated carbons with competitive hydrogen storage capabilities, contributing to both clean energy applications and environmental sustainability.