Ternary magnetic biochar composite from Citrullus Lanatus rind for supercapacitor’s electrode

Abstract

Metal oxides and conducting polymers are renowned as some of the promising materials for electrochemical energy storage (EES) devices, which include batteries, supercapacitors, and hybrid EES devices. This is due to their unique redox properties, significant theoretical capacitance, and environmentally benign nature. However, unsupported metal oxides and conducting polymer nanostructures suffer from particle aggregation, which decreases their electrochemical surface area. In recent years, the preparation of EES from renewable biomass has been developed taking into consideration the economic and environmental feasibility. Biochar is one of the major products of the thermochemical conversion of biomass. Applications of biochar for agricultural and environmental areas have been studied and reviewed extensively but biochar for energy storage materials has not been widely explored and examined. Therefore, the aim of this study is to convert the watermelon rind (WR) into magnetic biochar through a single-route self-purging pyrolysis method. Binary metal oxides (BMOs), such as nickel ferrite (NiFe2O4) and cobalt ferrite (CoFe2O4), were impregnated in dried watermelon rind to incorporate metal ions into the magnetic biochar. Response surface methodology (RSM) was employed to determine the magnetic watermelon rind biochar (MWRB) synthesis conditions (pyrolysis temperature, pyrolysis time, and WR: BMO ratio). The optimised magnetic biochar was combined with polyaniline (PANI) to produce a ternary magnetic biochar composite with PANI (TC-MWRB/PANI) via in-situ polymerisation to further enhance its electrochemical performance. RSM was also implemented to determine the TC-MWRB/PANI synthesis conditions (PANI concentration, sonication time, and sonication amplitude). Characterisations were done through field emission scanning electron microscopy (FESEM), energy dispersive Xray spectroscopy (EDX), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, Brunauer-Emmett-Teller (BET) surface area, and vibrating sample magnetometer (VSM). Electrochemical evaluations were performed through cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS). The XRD and FTIR results confirmed the successful formation of MWRB and TC-MWRB/PANI. The FESEM images revealed the porous structure of MWRB and fibrous-look PANI embedded on the surface of TC-MWRB/PANI, while the EDX results showed their associated elemental composition. The electrochemical investigations revealed excellent electrochemical performance of the MWRB and TC-MWRB/PANI for energy storage applications. Based on the electrode specific capacity, the regressed model and experimental results for the fabricated MWRBNiFe2O4 and MWRBCoFe2O4 were determined to be 191 C g-1 and 187 C g-1 and 200.05 C g-1 and 200.96 C g-1, at 5 mV s-1, respectively. In addition, the electrode specific capacity based on the regressed model and experimental results for the fabricated TC-MWRBCoFe2O4/PANI were determined to be 488.22 C g-1 and 491.29 C g-1. A two-electrode configuration with TC-MWRBCoFe2O4/PANI as a positrode and watermelon rind biochar (WRB) as a negatrode was fabricated to form a hybrid device (supercapattery) that operated in a stable potential window of 1.5 V. The energy density and power density of the device measured at a current density of 4 A g-1 were estimated to be 22.45 Wh kg-1 and 833.19 W kg-1, respectively. The fabricated supercapattery showed excellent cyclability with 97.46% specific capacity retention after 5,000 cycles.