The conversion of biomass into value-added products has recently received much attention for a broad range of applications. In this work, water lettuce was converted into calcium carbonate (CaCO3)/carbon via hydrothermal carbonization (HTC) and pyrolysis at 900 °C. The HTC temperature and time varied in the range of 160–200 °C for 6–18 h, respectively. X-ray diffraction analysis indicated that the samples consisted of a mixture of calcite and vaterite phases of CaCO3 and amorphous carbon. The ratio of calcite and vaterite phases varied with HTC time. The Fourier transform infrared spectroscopy (FTIR) result showed the characteristic absorption bands confirming the presence of CaCO3. Scanning electron microscopy (SEM) images revealed the large crystal of CaCO3 and fine carbon particles. From the N2 sorption analysis, the sample prepared from the HTC at 200 °C for 6 h had the highest specific surface area of 95 m2/g due to the development of micropores. The results presented in this work demonstrated that both HTC temperature and time play critical roles in altering the surface area and phase structure of CaCO3/carbon. The CaCO3/carbon derived from water lettuce can potentially be used and adapted for many applications.

Activated carbon fibers (ACFs) were successfully synthesized from kapok via a two-step process: (i) pre-carbonization and (ii) chemical activation. The pre-carbonization temperature was varied at 300℃, 400℃, and 500℃. The mixing ratio of the pre-carbonized product and potassium hydroxide (KOH) was 3:1, while the activation temperature was 800℃. The effect of pre-carbonization temperature on the morphology, surface area and porosity, chemical functional group, and phase structure of ACFs was investigated and discussed. The characterization results showed that ACFs exhibited an amorphous carbon structure with a hollow fiber shape resembling the kapok. The specific surface area decreased from 487 m2×g-1 to 326 m2×g-1 as the pre-carbonization increased. The pore structure of ACFs possessed a major contribution of micropores, and mesopores became more dominant at a high pre-carbonization temperature. The potential use of ACFs as electrode materials in supercapacitors was electrochemically tested by cyclic voltammetry and galvanostatic charge-discharge measurements. The ACFs obtained from pre-carbonization at 500℃ had the highest specific capacitance of 31.9 F×g-1 at a current density of 1 A×g-1. The results in this work will be a helpful guideline for the further design and development of ACFs from kapok for supercapacitor applications.