Utilization of intrinsic inorganic elements: A novel self-activation approach for hierarchical porous carbons from hydrochars

Abstract

Low surface area and the lack of hierarchical porous carbons (HPCs) structure limit the application areas of hydrochars. Therefore, in this study, self-activation of hydrochars by means of intrinsic inorganic elements was demonstrated as a novel approach for obtaining HPCs. Furthermore, the fate of intrinsic elements during the hydrothermal carbonization (HTC) process and the effects of intrinsic elements’ quantity on chemical modification of hydrochars were evaluated. Hydrochars were produced at 220, 230, 240 °C; 23-, 24- and 25-h and 1/3, 1/4 and 1/5 (w/w) ratios. Hydrochars with highest and lowest intrinsic inorganic element quantities were decided with Taguchi experimental design to use in chemical (with ZnCl₂, K₂CO₃ and H₃BO₃) and self-activation modification process conducted by heating hydrochars at 700 °C and 800 °C for 1 h. The worthy percentage of K in SSH passed to the process liquid, P completely remained in the hydrochars regardless of HTC condition. While the high inorganic element quantity in the unmodified hydrochars resulted in hydrochars with low surface area during the chemical modification, the opposite trend was observed for self-activation process. Surface area of unmodified hydrochars was increased from nearly 2 m²/g to the 400 m²/g with self-activation process. Additionally, all self-activated hydrochars had pore size lower than <2 nm in Density Functional Theory (DFT), which indicate the formation of micro and mesoporous structures addition to the macropores. Consequently, this study showed that inorganic elements have vital role for modification processes and HPCs can be successfully produced via intrinsic inorganic elements with shorter time and single stage.