Microwave exfoliation of a biochar obtained from updraft retort carbonization for supercapacitor fabrication

Biochar can offer a sustainable and renewable solution to energy storage problems. In this work, a biochar (which we refer to as an exfoliated biochar) with well-formed edge and basal planes was developed and characterized, and its electrochemical properties were tested. Using a three-electrode configuration for study purposes, the original specific capacitance (C_sp) of 283 Fg⁻¹ increased to 517 Fg⁻¹ after exfoliation, with an energy density (E_d) of 258.5 WhKg⁻¹, while retaining 95% of its capacity after 5000 cycles with an average charge time of 120 s. The CPE fitting of the EIS showed an ‘n’ value of 0.773, confirming diffusion-based charge storage and a very low ESR. In a two-electrode real world test using a PAT El-cell, a retention capacity of 96.5% was obtained, with a charge time of 210 s. In addition, C_sp and E_d values of 493 Fg⁻¹ and 246 WhKg⁻¹ were obtained. The self-discharge profile showed a two-stage self discharge before exfoliation (0.75–0.2–0.08 A), and a one-stage stable self-discharge profile from 0.9 to 0.18 A after exfoliation, demonstrating the high stability of the biochar nanosheets after exfoliation. This work shows that nanosheet forms of biochars can generate a high energy density and offer a long cycle life with minimum self-discharge, making them suitable for supercapacitor applications.