Hierarchically porous carbonaceous materials derived from waste citrus biomass for high-performance capacitive deionization

Capacitive deionization (CDI) is a cost-effective and environmentally friendly water desalination method that removes salt ions by applying a low voltage. Preparation of highly porous CDI electrode materials from high-abundance sources with low-cost, eco-friendly, and simple processing methods is still challenging. This study used lime peel (LP) biomass to prepare carbon, followed by KOH activation to increase the amorphous morphology and porosity at the elevated temperature range of 400–1000 °C. Thermogravimetry, Fourier-transform infrared spectroscopy, X-ray diffraction analysis, Raman spectroscopy, field-emission scanning electron microscopy, energy dispersive X-ray, and contact angle study was adopted to evaluate the thermal stability, elemental composition, microstates, morphology, and hydrophilicity of the prepared materials. The prepared material was cast on graphite electrodes and subjected to cyclic voltammetric, chronopotentiometric, and electrochemical impedance spectroscopic measurements to analyze their capacitive performance. The ion removal efficiency from the water of the materials was tested in a homemade CDI cell. Owing to the amorphous morphology, hierarchical porous structure, and superior wettability, these materials generally showed better capacitive performance and excellent salt adsorption capacity (SAC) in NaCl solution. The effects of the use of an activating agent and carbonization temperature employed for preparing materials, and the flow rate, salt concentrations, and applied voltage of the CDI experiment on the SAC were revealed. At optimized conditions, the SAC was found to be 33.57 mg g⁻¹ along with a pseudo-first-order rate constant of ca. 0.11 min⁻¹. Thus, LP is a potential, cost-free carbon precursor for producing cheap, efficient, and non-toxic electrode materials for CDI applications.