(a) CV curves of the PSAC//PSAC symmetrical supercapacitor at different scan rates in the voltage window of 0-1.2 V. (b) Nyquist plots of the PSAC//PSAC symmetrical supercapacitor. (c) Galvanostatic charge/discharge curves of the PSAC//PSAC symmetrical supercapacitor at different current densities. (d) Specific capacitances of the as-assembled PSAC symmetrical supercapacitor based on the total mass of the active materials of the two electrodes at different current densities. (e) Ragone plots of the PSAC symmetrical supercapacitor and other previously reported carbon-based symmetric supercapacitors. (f) Cycling performance of the PSAC symmetrical supercapacitor at a current density of 10 A g À1 (inset shows the photograph of the GCD curves for recycling and lighting LED bulbs).

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... planes of graphitic carbon layers, respectively. 24 The broad diffraction peak of carbon is attributed to the abundant micropores in the samples. From the spectra of PSAC-3 and PSC, it could be deduced that the activation of KOH could contribute to increasing porosity as the spectra of PASC-3 show a broader peak. 25 The Raman spectra of PSAC-3 in Fig. S3 † indicate two peaks located at about $1342 cm À1 and $1591 cm À1 , corresponding to D (disordered carbon) and G (graphitic carbon). 26 The G band originates from graphitic inplane stretching from sp 2 carbon and the D band corresponds to defects and disorders in the carbon structure by virtue of disordered carbon atoms with dangling ...

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... electrochemical performance of PSAC-3 based supercapacitors is evaluated in a fully assembled two-electrode cell with 1 M KOH electrolyte. As shown in Fig. 3a, the CV curves of the supercapacitors at different scan rates varying from 5 to 100 mV s À1 present quasi-rectangular-like shapes with no obvious distortion, which conrms the EDLC capacitive performance. 51 However, the curves are deformed with an increase of the scan rate by virtue of the slow charge-discharge kinetics of PSAC-3 ...

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... the EDLC capacitive performance. 51 However, the curves are deformed with an increase of the scan rate by virtue of the slow charge-discharge kinetics of PSAC-3 materials in the KOH electrolyte. And further ion diffusion could not adequately access the surface of PSAC-3 in such a short time at high scan rates. 52 The Nyquist plots of the device (Fig. 3b) indicate a small impedance of 2.39 U (6.17 kHz) with a vertical line in the lowfrequency region, demonstrating the fast and efficient ionic transport in the KOH electrolyte. 53 Moreover, the GCD curves exhibit regularly triangular shapes with no distinct voltage drop in the whole voltage window of 0-1.2 V (Fig. 3c), suggesting a low ...

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... The Nyquist plots of the device (Fig. 3b) indicate a small impedance of 2.39 U (6.17 kHz) with a vertical line in the lowfrequency region, demonstrating the fast and efficient ionic transport in the KOH electrolyte. 53 Moreover, the GCD curves exhibit regularly triangular shapes with no distinct voltage drop in the whole voltage window of 0-1.2 V (Fig. 3c), suggesting a low internal resistance and a good electrochemical reversibility. Low equivalent series resistance (ESR) of the assembled devices has also been proved by their low IR drops (voltage drops). 54 Fig. 3d shows the comparison of the capacitance of the device in the current density range from 1 to 10 A g À1 , which gives rise ...

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... Moreover, the GCD curves exhibit regularly triangular shapes with no distinct voltage drop in the whole voltage window of 0-1.2 V (Fig. 3c), suggesting a low internal resistance and a good electrochemical reversibility. Low equivalent series resistance (ESR) of the assembled devices has also been proved by their low IR drops (voltage drops). 54 Fig. 3d shows the comparison of the capacitance of the device in the current density range from 1 to 10 A g À1 , which gives rise to the change of capacitance from $426 to $343 F g À1 , demonstrating a good rate performance. The specic capacitance is still as high as 310 F g À1 with a retention of 72.8% at a high current density of 20 A g À1 ...

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... high as 310 F g À1 with a retention of 72.8% at a high current density of 20 A g À1 , which could be attributed to the interconnected structure with developed hierarchical porosity, contributing to rapid electrolyte transfer and enhanced degree of graphitization, which imparts good electrical conductivity. 55 With the increase of current density (Fig. 3d), the capacitance values show a slightly decreased trend due to the steric limitations of materials over which ions can only partially penetrate into the micropores. 56 The Ragone plot of the device in KOH electrolyte calculated from discharge curves at different current densities is displayed in Fig. 3e. The maximum energy density of ...

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... 55 With the increase of current density (Fig. 3d), the capacitance values show a slightly decreased trend due to the steric limitations of materials over which ions can only partially penetrate into the micropores. 56 The Ragone plot of the device in KOH electrolyte calculated from discharge curves at different current densities is displayed in Fig. 3e. The maximum energy density of the device is up to $85 W h kg À1 at a power density of 1.2 kW kg À1 . As is known, the energy density is suppressed with the increase of current density since the limited pores on the surface are accessed by electrolyte ions for fast discharging at high current density, whereas almost all pores could be ...

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... available supercapacitors (3-5 W h kg À1 ) and other previously reported biomass-derived heteroatom-doped carbonaceous symmetric supercapacitors. [58][59][60][61][62] Notably, the PSAC-based symmetric supercapacitor shows about 95.04% retention of its initial capacitance aer 10 000 charge and discharge cycles at a current density of 10 A g À1 (Fig. 3f), verifying an outstanding durability and long-term cycling stability. The hierarchical porous structure, excellent pore stability, enhanced wettability and good conductivity are responsible for the outstanding cycling performance. 63 Meanwhile, the GCD curves in the 1st and 10 000th cycle at 1 A g À1 are almost fully overlapped except ...

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... applications. 65,66 To further meet the practical application requirements for modern wearable and portable energy storage devices, a tandem device is assembled by connecting two prototype units in series. And then, the device is used to light up a red light-emitting-diode (LED, the lowest working potential is 2.0 V), as depicted in the inset of Fig. 3f. This tentative test conrms that the facile and costefficient synthesis strategy for designing analogous biomassderived materials such as PSAC-3 paves the way for a new direction for expanding their practical applications in energy storage and ...