High yield conversion of biowaste coffee grounds into hierarchical porous carbon for superior capacitive energy storage

Abstract

Recently great efforts have been focused on converting biowastes into high-valued carbon materials.However, it is still a great challenge to achieve high carbon yield and controllable porous distribution in both industrial and academic research. Inspired by the multi-void structure of waste coffee grounds, herein we fabricated hierarchical porous carbon via the combination of catalytic carbonization and alkali activation. The catalytic carbonization process was applied to obtain well-defined mesoporous carbon with carbon yield as high as 42.5 wt%, and subsequent alkali activation process produced hierarchical porous carbon with ultrahigh specific surface area (3549 m 2 g −1 ) and large meso-/macropores volume (1.64 cm 3 g −1 ). In three-electrode system, the electrode exhibited a high capacitance of 440 F g −1 at 0.5 A g −1 in 6 M KOH aqueous electrolyte, superior to that of many reported biomass-derived porous carbons. In two-electrode system, its energy density reached to 101 Wh kg −1 at the power density of 900 W kg −1 in 1-Ethyl-3-Methylimidazolium Tetrafluoroborate (EMIMBF 4 ). This work provided a cost-effective strategy to recycle biowastes into hierarchical porous carbon with high yield for highperformance energy storage application.In recent years, with the purpose of turning waste into treasure and reducing environmental hazard, waste management for the production of high-valuable carbon materials has received considerable attention 1 . Specially, biowastes are viewed as the most outstanding natural carbon precursors by virtue of their rich source, low-cost and sustainability 2-4 . So far, many biowastes have been recycled to synthesize carbon materials, but their applications are still limited by low carbon yield and unsatisfied pore structure 5-7 . Thus, effective methods are necessary to control carbonization process and optimize pore distribution for meeting the demands of various intended applications.Until now, several strategies have been developed for the fabrication of porous carbons, including hydrothermal carbonization 8 , hard template 9 and molten-salt route 10 , etc. Generally the widely used carbon-rich precursors are polymers 11 , metal-organic frameworks 12 , organic complex 13 and carbohydrates 14 . Meanwhile, natural biomass is also widely used as carbon precursors, such as salvia splendens 15 , clover stems 16 , moringa oleifera branches 17 , ginkgo leaves 18 , and pollens 19 . Moreover, with considering environmental protection and resource recycle, biowastes are viewed as more promising natural carbon precursors 20 . Currently, various biowastes from plants or animals have been used for porous carbons production, such as kraft lignin 21 , soybean residue 22 , dairy manure 23 , bagasse wastes 24 and peanut shell 1 . In many cases, the added catalysts (or activators) are difficult to penetrate into the inside of carbon precursors, resulting in low carbon yield and unmanageable pore distribution 25 . Besides, many kinds of biowastes are difficult to collect or have a low production. I...

Supercapacitors in the Light of Solid Waste and Energy Management: A Review

“…The device achieved a high energy density of 101 W h kg −1 and a maximum power density of 18 000 W kg −1 in EMIMBF 4 besides retaining 92% of capacitance after 10 000 cycles. [64] Agricultural xylose acquired from hemicellulose has also been utilized to fabricate electrodes for supercapacitors. Waribam et al reported a two-step process of hydrothermal carbonization and KOH-H 3 PO 4 activation to convert xylose into porous carbon microspheres with surface area as high as 692 m 2 g −1 .…”

“…[ 63 ] Alternatively, waste coffee grounds obtained from leftovers of coffee machines have also been used to develop carbon for supercapacitor electrodes. [ 64 ] The carbon showed an exceptional specific surface area of 3549 m 2 g −1 and delivered a capacitance of 440 F g −1 at 0.5 A g −1 with 6 m KOH. The device achieved a high energy density of 101 W h kg −1 and a maximum power density of 18 000 W kg −1 in EMIMBF 4 besides retaining 92% of capacitance after 10 000 cycles.…”

“…The device achieved a high energy density of 101 W h kg −1 and a maximum power density of 18 000 W kg −1 in EMIMBF 4 besides retaining 92% of capacitance after 10 000 cycles. [ 64 ]…”

“…Therefore, of late, due emphasis has also been laid on the evolution of eco‐friendly supercapacitors. Benign materials retrieved from carbon‐rich agricultural and food biowaste, [ 9,52–75,105,115,116 ] industrial waste, [ 76–79 ] paper, [ 74,80,81 ] and cotton [ 82 ] have been used in their fabrication. Additionally, carbon acquired from discarded polymers [ 83–95 ] as well as valuable metals and metal composites recuperated from electronic waste including spent batteries [ 96–102 ] have been utilized in the preparation of supercapacitor electrodes.…”

Comparison of acid exfoliators in carbon nanosheets synthesis from stinging nettle (Urtica dioica) for electrochemical applications

“…The preparation of nanostructured materials from waste materials has drawn tremendous interest in recent years 16 , 17 . Many researchers have demonstrated that carbon materials for use as adsorbent or electronic materials can be synthesized by potassium hydroxide from low-cost waste materials such as rice husk 18 , Acai stone 19 , corncob 20 , pine cone flower 21 , banana peel 22 , water hyacinth 23 , waste coffee grounds 24 , pineapple leaf fibre 25 , bamboo 26 , wood sawdust 27 , peanut shell 28 – 30 , jute 31 , gulfweed 32 and pomelo peel 33 . Nettle ( Urtica dioica ) is a plant widely distributed in many areas of Asia, Europe, America and Africa 34 .…”