Supercapacitors based on carbons with tuned porosity derived from paper pulp mill sludge biowaste

Hydrothermal carbonization followed by chemical activation is utilized to convert paper pulp mill sludge biowaste into high surface area (up to 2980 m2 g−1) carbons. This synthesis process employs an otherwise unusable byproduct of paper manufacturing that is generated in thousands of tons per year. The textural properties of the carbons are tunable by the activation process, yielding controlled levels of micro and mesoporosity. The electrochemical results for the optimized carbon are very promising. An organic electrolyte yields a maximum capacitance of 166 F g−1, and a Ragone curve with 30 W h kg−1 at 57 W kg−1 and 20 W h kg−1 at 5450 W kg−1. Two ionic liquid electrolytes result in maximum capacitances of 180-190 F g−1 with up to 62% retention between 2 and 200 mV s−1. The ionic liquids yielded energy density-power density combinations of 51 W h kg−1 at 375 W kg−1 and 26-31 W h kg−1 at 6760-7000 W kg−1. After 5000 plus charge-discharge cycles the capacitance retention is as high at 91%. The scan rate dependence of the surface area normalized capacitance highlights the rich interplay of the electrolyte ions with pores of various sizes.