From sunflower shells to hybrid-power cells: Boron-enhanced carbon electrodes for next-generation Zn-ion supercapacitors

Zinc-ion hybrid supercapacitors have emerged as a promising technology, combining the high energy density of batteries with the high-power density of supercapacitors. This study investigates the performance of zinc-ion hybrid supercapacitors utilizing boron-doped (B-doped) and undoped activated carbon (AC) as electrode materials. Recognizing the importance of sustainability, we utilized activated carbon derived from locally abundant sunflower seed shells through a controlled pyrolysis process. The synthesized B-doped and undoped AC materials were comprehensively characterized using advanced techniques, including X-ray Diffraction (XRD) to confirm the amorphous carbon structure, Fourier-Transform Infrared (FTIR) spectroscopy to identify functional groups, and Thermogravimetric Analysis (TGA) to assess the thermochemical properties and volatile matter content. Raman spectroscopy revealed that the intensity ratio of the D-band to G-band (I_D/I_G) was 0.938 for the B-doped AC and 0.832 for the undoped AC, indicating an increased level of disorder in the carbon lattice due to boron incorporation. This was further supported by X-ray Photoelectron Spectroscopy (XPS), which confirmed the presence of boron in the B-doped AC, validating the successful doping process. BET analysis revealed a significant increase in surface area for the B-doped AC (600 m²/g) compared to the undoped AC (200 m²/g), which contributed to the enhanced electrochemical performance of the B-doped material. Electrochemical performance was evaluated through methods such as Cyclic Voltammetry (CV), constant-current charge-discharge tests, and Electrochemical Impedance Spectroscopy (EIS). The study examined the influence of ZnSO₄ electrolyte concentration (ranging from 0.5 to 2 M) on the performance of the Zn-ion hybrid supercapacitor. Notably, the B-doped AC material exhibited superior performance, delivering a gravimetric capacitance of approximately 105 F/cm² in 1.5 M ZnSO₄ electrolyte at a current density of 0.1 mA/cm², with 100 % coulombic efficiency retained over 100 cycles. This performance was significantly enhanced compared to the undoped AC material, which delivered around 45 F/cm² under the same conditions. The findings underscore the potential of B-doping in improving the electrochemical properties of sustainable carbonaceous materials, offering an effective pathway toward high-performance zinc-ion hybrid supercapacitors using locally available resources.