Mesoporous carbon spheres produced by hydrothermal carbonization from rice husk: Optimization, characterization and hydrogen storage

In this study, carbon spheres were synthesized from rice husk with hydrothermal carbonization method at different temperatures and different reaction times. Surface areas and pore size distributions of carbon spheres were characterized by BET surface area device, sphere morphology by SEM, structural characterization by FTIR-ATR and XRD, and thermal properties and degradation mechanism by DTA/TG. In addition, hydrogen gas adsorption measurements of the samples were also carried out with the Hiden IMI PSI gas storage device. It can be said that the required temperature is 280 °C and the required reaction time is 6 h in order to obtain homogeneous and ideal sphere morphology carbon spheres from a lignocellulosic biomass in experiments carried out under different conditions in an acidic reaction medium. This is clearly seen from the SEM images. In addition, FTIR spectra and XRD patterns confirm that the sphere was obtained. According to the BET surface area and pore size distribution results, it can be said that the main significant difference is in the mesopore structure of the carbon spheres, even if the surface area values of the samples obtained at different temperatures and different reaction times increase linearly with temperature. In order to determine the usability of carbon spheres obtained in different conditions in the energy field, the gravimetrically measured H2 storage capacities at cryogenic temperature (77 K) and pressure range of 0–30 bar were determined as maximum 1.1% by weight. When the hydrogen storage capacities of the samples are evaluated together with the BET surface area values and pore size distributions, it shows that the hydrogen storage capacity in carbon spheres is directly proportional to the mesopore volumes. In addition, Langmuir and Freundlich isotherms related to adsorption equilibrium were investigated in order to better define the H2 adsorption that took place in these samples. Considering the regression coefficients of the obtained isotherms, it was determined that some of the carbon spheres were more compatible with the Langmuir isotherm and some with the Freundlich isotherm.