High-energy quasi-solid-state supercapacitors enabled by carbon nanofoam from biowaste and high-voltage inorganic gel electrolyte

Abstract Focusing on major issues of carbon materials like insufficient capacitance and limited energy supply in supercapacitors, we propose the strategy of developing advanced carbon and high-voltage inorganic gel electrolyte to efficiently solve these challenges. Firstly, the architecture of self-doped carbon nanofoam (A-CS650) is fabricated utilizing naturally rich cotton stalk through a facile procedure, which demonstrates exceptional performance contributed by synergistic features of large surface area, hierarchical porosity and rich defects. A-CS650 presents gravimetric and volumetric capacitances up to 282 F g−1 and 234 F cm−3 at 0.5 A g−1, and a high-rate capacitance retention of 72.7% at a large rate of 100 A g−1. With increasing the mass loading to 20 mg cm−2, A-CS650 still retains good performance. Especially, by using unique CMC-Na/Na2SO4 gel electrolyte, 1.8 V A-CS650//A-CS650 quasi-solid-state supercapacitor, for the first time, is constructed, which displays an outstanding energy density of 22.6 Wh kg−1, greatly exceeding the value in PVA/KOH electrolyte (7.3 Wh kg−1). Besides, this device exhibits considerable stability over 10000 cycles (81.6% capacitance retention). The insight from this work verifies great adaptability of biowaste-derived carbons toward supercapacitors, and may open a new technical platform to develop portable energy systems.