Using tanned leather waste to derive biochars for supercapacitor electrodes in various electrolytes

Chromium-tanned pigskin leather was carbonized in three ways: (1) pyrolysis in an inert atmosphere at 800 °C, (2) pre-carbonization at 500 °C followed by the activation with KOH at 800 °C, and (3) direct activation with excess KOH at 800 °C. The yield of biochars was comparable, 24–28 wt.%. The elemental composition of derived biochars and the content of inorganic elements in ash were determined, and the predominance of chromium and iron was discussed. The changes in the molecular structure after carbonization were assessed with the help of FTIR and Raman spectra. The KOH activation increased specific surface area and porosity but had minimal impact on conductivity. The electric conductivity of powders was determined as a function of applied pressure, achieving a value of approximately 1 S cm⁻¹ at 10 MPa. The electrochemical performance was evaluated using aqueous electrolytes (1 M Na₂SO₄ and 6 M KOH), as well as an organic electrolyte (1 M tetraethylammonium tetrafluoroborate in acetonitrile, TEABF₄/ACN). The highest specific capacitance achieved was 187.7 F g⁻¹ in 6 M KOH using the three-electrode system. Biochar was also tested in a symmetrical supercapacitor, achieving a specific capacitance of 150.9 F g⁻¹ in 6 M KOH, and maintaining 95% capacitance after 5000 charge/discharge cycles. The supercapacitor in 6 M KOH showed the highest power density of 10.5 kW kg⁻¹, whereas the one in 1 M TEABF₄/ACN demonstrated the highest energy density of 21.3 Wh kg⁻¹.