Hierarchical porous activated carbon prepared from biowaste of lemon peel for electrochemical double layer capacitors

Highlights: • Carbon electrodes are derived from the biowaste of lemon peel by chemical activation employing KOH. • Activation temperature and the mass ratio of precursor and KOH are optimized to achieve hierarchical porosity. • Carbon obtained at the activation condition of 1:1 mass ratio and 600 °C (ALP-600) exhibits excellent capacitive performance. • ALP-600 shows high specific capacitance of 152.14 Fg^-1 in 0.5 M H₂SO₄. • The symmetric capacitor employing ALP-600 retains 99.5% of coulombic efficiency over 10,000 cycles. Conversion of bio-waste into activated carbon is an environmentally friendly approach that reduces the pressure on biowaste disposal. Here we describe the preparation of activated carbon from the biowaste of the lemon peel by carbonization at 400 °C followed by chemical activation employing KOH. The activation parameters like temperature and mass ratio of precursor and KOH are optimized to obtain carbon material of better capacitive performance. The mass ratio and temperature of activation are optimized as 1:1 and 600 °C (ALP-600) respectively by systematic analysis of the capacitive performance of carbon materials in acidic (0.5 M H₂SO₄) and alkaline (2 M KOH) electrolytes. The superior performance of ALP-600 despite its lower surface area (744.78 m² g^-1) is attributed to its hierarchical porosity as the mesopores improve the capacitive performance by providing connectivity between the pores. ALP-600 delivers a higher specific capacitance of 152.14 Fg^-1 and better power rate capability evident from the quasi-rectangular shape of cyclic voltammogram even at a higher scan rate of 100 mVs⁻¹ in the acidic electrolyte. The high energy density of 4.67 Whkg⁻¹ and a high-power density of 8113 Wkg^-1 is achieved with the symmetric capacitor. The symmetric capacitor of configuration ALP-600 || 0.5 M H₂SO₄ || ALP-600 exhibits excellent cycling stability over 10,000 cycles with 99.5% coulombic efficiency and insignificant capacitance loss.