Mesoporous activated carbon for supercapacitors derived from coconut fiber by combining H3PO4-assisted hydrothermal pretreatment with KOH activation

The utilization of biomass resources as carbon precursors exhibits a promising application prospect for supercapacitor electrode material preparation. Here, we present mesoporous activated carbon from coconut fiber by H3PO4-assisted hydrothermal pretreatment combined with a low KOH-to-hydrochar ratio activation. The results show that the hydrothermal pretreatment increases the micropores of the activated carbons by hydrolyzing the beta-glycosidic linkage of hemicelluloses and the aryl ether bond of lignin, causing depolymerization of lignin. H3PO4 promotes hydrolysis and the subsequent KOH activation process develops a substantial number of mesopores and surface oxygen groups. The activated carbon prepared under 10 wt% H3PO4-assisted hydrothermal pretreatment combined with a 1:1 KOH-to-hydrochar ratio activation (PHC-1), which is much lower than normal alkali consumption for activation, obtains a large specific surface area (1217.10 m2 g-1), substantial mesoporous ratio (46.84%), and noteworthy surface oxygen content (13.49%). These properties bring about an impressive electrochemical performance (315.5 F g-1 at 1 A g-1, 76.83% retention at 20 A g-1). By comparison, activated carbons derived from unpretreated and water hydrothermal pretreatment methods display significantly lower mesoporous ratios and specific capacitances. When integrated into a symmetric supercapacitor, the PHC-1 carbon electrodes maintain remarkable cycling stability (remains 94.8% after 10000 cycles). The high proportion of mesopores in conjunction with adequate micropores endows the activated carbon with significantly higher electrochemical performances compared with the carbons mainly composed of micropores. All these electrochemical data indicate that mesoporous activated carbon PHC-1, with outstanding properties prepared using coconut fiber, has potential applications in energy storage.