Electrolyte ions-matching hierarchically porous biochar electrodes with an extended potential window for next-generation supercapacitors

Abstract

Engineering high-performance carbonaceous electrode materials from earth-abundant biomass has attracted substantial attention for its applicability in next-generation supercapacitors (SCs). However, these materials exhibit low specific energy due to the dominance of mesopores and a limited potential window. To overcome these shortcomings, herein, we synthesize Miscanthus sinensis (silver grass)-derived hierarchically-porous activated carbons (SHACs) via pyrolysis, carbonization, and KOH activation. We test the SHAC electrodes with different electrolytes, showing how an electrolyte–electrode pair can be tuned to boost energy and power densities. Owing to the synergetic effect of the size-balanced proportion of micropores matched with the size of electrolyte ions, in KOH electrolyte, the SHAC electrode produces a high specific capacitance (592 F g⁻¹) while, simultaneously, providing faster charging compared to Na₂SO₄ electrolyte. We rationalize these findings with molecular dynamics simulations, demonstrating the avoidance of power-density trade-off, typical for microporous SCs. Upon adding K₃Fe(CN)₆ redox species to KOH electrolyte (hybrid electrolyte), capacitance increases 2.53 fold (380 to 963 F g⁻¹ at 5 A g⁻¹) due to the synergy of capacitive and faradaic energy storage mechanisms. In the hybrid electrolyte, a SHAC electrode-embedded symmetric SC (SSC) offers a high cycling stability (97%) with 1.6 V wide operational voltage and permits energy storage and power density higher than those reported so far for aqueous electrolyte-based SSCs and asymmetric SCs. In addition, these SSCs provide long-lasting operational capabilities that are useful for driving various portable electronic devices. The obtained results demonstrate a feasible methodology to utilize the maximum available surface area of carbonaceous materials for electrochemical energy storage applications.