Fabrication of biomass-derived activated carbon with interconnected hierarchical architecture via H3PO4-assisted KOH activation for high-performance symmetrical supercapacitors

Biomass-based activated carbon materials provide a novel approach for the development of high-performance electrode materials for supercapacitors without the need for fossil fuel energy sources. Herein, activated carbon with an interconnected hierarchical architecture was designed and fabricated from hazelnut shells via H3PO4-assisted KOH activation. The results indicated that the activated carbon material with an interconnected pore framework was successfully fabricated and exhibited excellent electrochemical performance. The pore size distribution of the activated carbon could be tuned by the quantity of KOH that was added. A hierarchical architecture containing a gradient distribution from nanopores to micropores had the optimal connectivity and improved the electric double-layer capacitance and capacitance retention rate, while abundant heteroatoms on the surface or edges of the carbon matrix enhanced the pseudocapacitance. The highest specific capacitance was achieved 338 F·g−1 at a current density of 0.2 A·g−1 and the highest capacitance retention was 86.3% at 10 A·g−1 in a three-electrode system. The symmetrical capacitor (SC) device had an energy density of 16.42 W·h·kg−1 at a power density of 160 W·kg−1 in Na2SO4 electrolyte. In Na2SO4 gel electrolyte, the SC device exhibited a high energy density of 22.46 W·h·kg−1 at a power supply of 450 W·kg−1 and outstanding cycle stability with 133% of the initial capacity after 10,000 cycles in an operating potential of 1.8 V. This work provides an efficient strategy for the preparation of high-performance activated carbon electrodes from hazelnut shells.