Optimization of Water Hyacinth Stem-Based Oxygen-Functionalized Activated Carbon for Enhanced Supercapacitors

In the current world, storing and converting energy without affecting the natural ecosystem are considered a sustainable and efficient green energy source production technology. Especially, using low-cost, environmentally friendly, and high-cycle stability activated carbon (AC) from the water hyacinth (Eichhornia crassipes) waste material for charge storage application is the current attractive strategy for renewable energy generation. In this study, preparation of AC from water hyacinth using a mixed chemical activation agent followed by activation time was optimized by the I-optimal coordinate exchange design model based on a 3-factor/3-level strategy under nine experimental runs. The optimum conditions to prepare AC were found to be potassium hydroxide (≈17 g) and potassium carbonate (≈11 g), and the carbonization time was approximately 1 h. Under these augmented conditions, the maximum specific capacitance suggested by the designed model was found to be ≈75.2 F/g. The regression coefficient (R ² = 0.9979), adjusted (R ² = 0.9917), predicted (R ² = 0.8706), adequate precision (39.2795), and p-values (0.0062) proved the good correlation between actual and predicted values. The physicochemical and electrochemical properties of the final optimized AC were characterized by thermogravimetric/differential thermal analysis (TGA/DTA), X-ray diffractometry (XRD), Fourier transform infrared (FTIR), Brunauer-Emmett-Teller (BET), scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), selected area electron diffraction (SAED), and potentiostat (CV and EIS) instruments. Finally, the optimized AC electrode after 100 cycles at a current density of 2 A g^-1 retains an efficiency of 71.57%, indicating the good stability and sustainability of this material.