Controlling hierarchical porous structures of rice-husk-derived carbons for improved capacitive deionization performance†
We report on hierarchical porous carbons derived from rice husk (RH) through the combination of carbonization and two post-processing methods that were used as the electrode material for capacitive deionization (CDI). The carbonized RHs, consisting of carbon/inorganic composite materials, were used to control the hierarchical porous structures, depending on the existence of silicon domains. Hierarchical porous carbons, denoted as RHC-A and comprising dominant micropores and small fraction mesopores, were synthesized by steam activation to react with the carbon fraction in the presence of silicon components. On the other hand, mesopore-dominant porous carbons, denoted as RHC-H, were obtained selectively by removing silicon components acting as a natural template from carbonized RH by hydrofluoric acid treatment. In order to understand the effect of the pore structure on CDI performance, two RH-derived porous carbons were compared in terms of their performances in water purification. In the batch-mode experiment, the microporous activated carbon (AC) exhibited the highest maximum electrosorption capacity of 17.7 mg g−1, which was mainly determined by the specific surface area. Under the same conditions, however, the RHC-A was verified to have enhanced kinetic performance according to the Langmuir isotherm and the pseudo-second-order kinetic model. Here, the hierarchical porous structures with continuous mesopores interconnected by micropores contributed to the enhanced kinetic performance of the RHC-A, facilitating effective ion transport and adsorption. In a continuous mode similar to the practical CDI, the kinetically improved RHC-A achieved a higher salt-removal capacity of 8.09 mg g−1 than the 5.40 mg g−1 and 1.63 mg g−1 of the commercial activated carbon and RHC-H, respectively, at 1.5 V and with a feed rate of 20 mL min−1 in 100 ppm of NaCl solution.