Hierarchically structured Co3O4/graphene-like carbon hybrid on carbon felt for high-performance negative electrodes in aqueous symmetric supercapacitors

Pseudocapacitive metal oxides such as cobalt oxide (Co 3 O 4 ) offer high theoretical capacitance but suffer from sluggish kinetics and poor stability as negative-electrode materials for supercapacitors. Here, a hierarchically structured Co 3 O 4 /graphene-like carbon hybrid on carbon felt (Co 3 O 4 /GLC/CF) is developed via a sustainable, two-step hydrothermal–pyrolysis route using basil-seed biomass. The design exploits a three-fold synergy between the two components: kinetically, a distinct diffusion-controlled contribution (~24.2% of the total charge storage), reasonably associated with the Co 3 O 4 phase, that complements the EDLC-dominated response of the GLC framework (b-values of 0.85 and 0.74 for the anodic and cathodic peaks, respectively; 75.8% capacitive contribution at 60 mV s −1 ); structurally, the nanoparticles act as interlayer spacers that suppress restacking of the GLC sheets, yielding a defect-rich, hierarchical mesoporous architecture (BET surface area 47.64 m 2 g −1 , mean pore diameter ~20.1 nm) that promotes efficient electrolyte access; and texturally, FTIR and XPS results confirm that the Co 3 O 4 phase enhances surface hydrophilicity, improving electrolyte wettability and lowering ion-diffusion resistance, consistent with the low charge-transfer resistance obtained from impedance spectroscopy (R p = 2.4 Ω). Operating within a negative potential window of −1.15 to 0 V (vs. Ag/AgCl) in 3 M KOH, the electrode delivers a specific capacitance of 231.3 F g −1 at 1 A g −1 . A symmetric supercapacitor (SSC) (Co 3 O 4 /GLC/NF//Co 3 O 4 /GLC/CF) built on this hybrid negative electrode achieves a wide voltage window of 1.6 V, an energy density of 105.5 Wh kg −1 at 800 W kg −1 (25.1 Wh kg −1 at 8000 W kg −1 ), and retains 86.4% of its initial capacitance after 8000 cycles at 20 A g −1 . These results establish the biomass-derived Co 3 O 4 /GLC hybrid as a scalable, high-performance negative-electrode strategy for aqueous symmetric supercapacitors.