Introduction

The pursuit of sustainable and eco-friendly energy storage technologies has become a critical research priority, driven by escalating global energy demands and heightened environmental concerns1,2. Supercapacitors (SCs), known for their high power density, exceptional cycle stability, and rapid charge–discharge capabilities, hold significant potential for applications ranging from energy management systems and electric vehicles to smart grids and portable electronics3,4,5. However, conventional electrode materials, such as activated carbon6, metal oxides7, and conductive polymers8, face inherent limitations, including high production costs, environmental burdens, and sustainability challenges. Consequently, the development of novel, cost-effective, and environmentally friendly electrode materials is crucial for advancing supercapacitor technology9,10.

Biomass-derived porous carbon materials (BDPCs), a class of carbonaceous materials sourced from abundant renewable resources, have attracted considerable research interest in energy conversion and storage owing to their sustainability, cost-effectiveness, and environmental friendliness11,12. Extensive efforts are consequently devoted to exploring the energy storage applications of BDPCs derived from various precursors, including coconut shells13, corncobs14, water hyacinths15, and sugarcane bagasse16. A pivotal strategy involves the synergistic utilization of biomass’s innate three-dimensional architecture and artificial pore creation to construct hierarchical porous structure that optimize ion transport pathways and active site distribution. This ideal hierarchy integrates micropores (< 2 nm), mesopores (2–50 nm), and macropores (> 50 nm)17,18, each fulfilling a critical role: micropores provide abundant active sites for charge adsorption, mesopores facilitate efficient ion transport to enhance rate capability, and macropores function as ion reservoirs to shorten diffusion distances19,20. The efficacy of the strategy finds strong support in a growing corpus of empirical research. For instance, Harahap et al.21 synthesized hierarchical porous carbon from cassava peel via ZnCl₂ activation. The resulting material exhibited a high specific surface area of 634.7 m2 g-1, with a remarkable microporosity ratio of 94%. This microporous-dominated structure yielded an outstanding specific capacitance of 257 F g⁻1 at 1 A g⁻1 and excellent rate capability, retaining 92.43% of its capacitance at 10 A g⁻1. Similarly, Zhao et al.22 leveraged the intrinsic hierarchical network of loofah sponge to fabricate a 3D porous carbon electrode with a specific surface area of 772.87 m2 g-1, which delivered a high capacitance of 267 F g-1 at 0.5 A g-1 with 69.7% retention at 10 A g-1. In another example, Kwarteng et al.23 developed a novel porous carbon from onion flower seeds. The KOH-activated material featured a unique nanosheet morphology and an ultrahigh surface area of 2538.31 m2 g-1, achieving a specific capacitance of 200.37 F g-1 at 1 A g-1 along with excellent cycling stability (90.77% retention after 5000 cycles).

To date, the utilization of strawberry straw, a widespread agricultural waste whose disposal via burning or landfilling poses environmental threats24,25, for the synthesis of hierarchically porous carbon has remained unexplored. Transforming the waste into a valuable carbon material offers a dual-benefit solution to waste management and the development of sustainable supercapacitor electrodes26. It is noteworthy that strawberry straw exhibits the natural vascular bundle structure, which serves as critical channels for water and nutrient transport. These inherent aligned channels, when synergistically combined with a precisely regulated pore architecture through chemical activation, can collectively enhance electrolyte infiltration and facilitate rapid ion transport27. Moreover, biomass materials such as strawberry straw are rich in heteroatoms, which can be self-doped into the carbon framework during carbonization28. These inherent heteroatomic species are anticipated to not only modify the electronic structure and charge distribution of the resulting carbon but also improve its interfacial wettability, charge transfer kinetics, and overall electrical conductivity29,30.

In this study, strawberry straw biomass was firstly utilized as a sustainable precursor to synthesize O self-doped hierarchical porous carbon featuring naturally aligned channels through KOH activation, which not only offers abundant active sites and a diversified micro/mesoporous environment, but also ensures efficient ion transport. The as-prepared samples were comprehensively characterized to evaluate their morphological, structural, and electrochemical properties. The optimized electrode (SPC3) achieved a high specific capacitance of 277 F g-1 at 1 A g-1 in 6 M KOH, retaining 211 F g-1 at 10 A g-1 and showing 95.6% capacitance retention after 10,000 cycles, indicating excellent rate capability and cycling stability. Furthermore, a symmetrical supercapacitor based on SPC3 achieved an energy density of 20.9 Wh kg-1 and a power density of 6000 W kg-1.

Experiment

Materials

Strawberry straw was purchased from Taobao Co., Ltd. N-Methyl-2-pyrrolidone (NMP) was sourced from Macklin Biochemical Technology Co., Ltd. Potassium hydroxide (KOH) was supplied by Taicang Hu Test Reagent Co., Ltd. Polyvinylidene fluoride (PVDF) and acetylene black (AB) were provided by Tianjin Chemical Technology Co., Ltd. Hydrochloric acid (HCl, 36 wt%) and ethanol (C2H5OH) were obtained from Sinopharm Chemical Reagent Co., Ltd. All chemicals were used without further purification.

Synthesis of SPC

The carbon precursor was prepared from strawberry straw through a carbonization process. The experimental procedure was conducted as follows: First, the strawberry straw was cut into 1 cm segments and soaked in a 1 M HCl solution for 1 h. Subsequently, the samples were alternately washed with deionized water and anhydrous ethanol to remove surface impurities thoroughly, followed by drying in an oven at 80 °C for 3 h. The dried samples were placed in an alumina crucible and loaded into a tube furnace. Under a nitrogen (N₂) atmosphere, the temperature was increased to 600 °C at a heating rate of 10 °C/min and held for 2 h to ensure complete thermal decomposition. After the system cooled to room temperature, the black carbonized material, denoted as SPC, was collected for analysis.

The carbonized product (SPC) was finely ground and mixed with KOH at mass ratios of 1:2, 1:3, and 1:4, respectively. The mixtures were then transferred to a tube furnace and activated under a nitrogen atmosphere by heating to 800 °C at a rate of 8 °C/min, with a dwelling time of 2 h at the target temperature. After the system cooled naturally to room temperature, the products were repeatedly washed with 1 M HCl until the filtrate reached a neutral pH. Finally, the purified products were dehydrated in a vacuum oven at 100 °C for 8 h, yielding a series of porous carbon materials labeled as SPCx (x = 2,3,4). The detailed synthesis process is illustrated in Fig. 1.

Fig. 1
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Diagram of synthesis of SPCx from Strawberry straw.

Characterization

The microstructure and elemental composition of the samples were characterized using a field emission scanning electron microscope (SEM, SU-8100, Hitachi, Japan). The crystal structure parameters were determined using an X-ray diffractometer (XRD, Rigaku ULTIMAIV, Japan) with Cu-Kα radiation over a 2θ range of 5° to 80° Raman spectroscopy (Xplora PLUS, Horiba, Japan) was utilized to analyze the graphitization degree and defects in the carbon materials. The chemical composition and bonding states of the elements were characterized by X-ray photoelectron spectroscopy (XPS, ESCALAB 250XI, USA). The specific surface area and pore size distribution were determined using an Automated Surface Area and Porosimetry Analyzer (AUTOSORB iQ, Quantachrome, USA) via N₂ adsorption–desorption isotherms.

Electrode preparation and electrochemical characterization

The working electrode was prepared by thoroughly mixing the synthesized SPCx, polyvinylidene fluoride (PVDF) binder, and acetylene black conductive agent in an agate mortar at a mass ratio of 8:1:1. The mixture was ground until a homogeneous black slurry was obtained. Subsequently, the slurry was uniformly coated onto a pre-cleaned nickel foam substrate (1 × 1 cm2). The coated substrate was transferred to a vacuum drying oven and dried at 100 °C for 24 h. Finally, the dried electrode was pressed at 20 bar to form the working electrode for electrochemical tests.

The electrochemical performance of the SPCx material was evaluated using an electrochemical workstation (CHI660E, China) in a 6 M KOH electrolyte. A standard three-electrode configuration was employed, with the SPCx electrode serving as the working electrode, a platinum plate as the counter electrode, and a Hg/HgO electrode as the reference electrode. Cyclic voltammetry (CV) was conducted within a potential window of -1.0 to 0 V (vs. Hg/HgO). Galvanostatic charge–discharge (GCD) tests were performed in the voltage range of -1.0 to 0 V at current densities ranging from 1 to 10 A g⁻1. Additionally, electrochemical impedance spectroscopy (EIS) was carried out over a frequency range from 100 kHz to 0.1 Hz to further investigate the electrode kinetics.

Results and discussion

As shown in Fig. 2a, the X-ray diffraction (XRD) patterns of all samples exhibited a broad diffraction peak at approximately 23°, corresponding to the (002) crystal plane of graphite, confirming the presence of abundant amorphous carbon structures. A relatively weak diffraction peak appeared at around 43°, which was attributed to the (100) crystal plane of graphite, indicating the existence of graphitic microcrystalline structures31. Compared to the carbonized sample (SPC), the activated samples (SPC2-4) exhibit a significantly reduced intensity of the (002) diffraction peak. This is primarily attributed to the vigorous etching of the carbon framework during the KOH activation process, which generates a substantial volume of pores and defects. Furthermore, the enhanced intensity observed at low angles (2θ < 20°) for the activated samples indicates the formation of abundant micropores32,33. Further defect analysis was performed on all samples using Raman spectroscopy (Fig. 2b). The spectra exhibited two characteristic peaks: the D band at 1350 cm-1 and the G band at 1585 cm-1. The D band corresponds to structural defects and disorder in the carbon materials, while the G band originates from the in-plane vibration of sp2-hybridized carbon atoms, and representss an ordered graphitic structure20. The degree of structural disorder or defects in the materials was typically evaluated by the intensity ratio (ID/IG). After the introduction of KOH as an activating agent, the ID/IG values of SPCx increased significantly, indicating that more defects and pore structures were introduced during the material activation process. These defects and disordered structures provided supplementary active sites for electrochemical reactions and enhanced the material’s wettability, thereby contributing to improved specific capacitance34. Notably, SPC3 exhibited the highest ID/IG value, suggesting that it possessed the most pronounced defect density among the samples.

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(a) XRD patterns and (b) Raman spectra of SPC, SPC2 SPC3 and SPC4.

The microstructures of the supercapacitor electrode materials, including pristine SPC and KOH-activated SPCx, were systematically investigated using scanning electron microscopy (SEM), as shown in Fig. 3. Figure 3a-d showed that the unactivated SPC sample retained the characteristic fibrous bundle structure of its biomass-derived carbon precursor, with only a few sparse pores distributed on its surface. The ordered fibrous channel structure, originally for water and nutrient transport, was completely retained after carbonization and provided directed diffusion pathways for electrolyte ions in the electrode, thereby significantly enhancing the ion migration rate. When the KOH/C mass ratio was increased to 2:1, the SEM images of SPC2 (Fig. 3e-h) revealed the formation of abundant pores on the material surface, with pore sizes ranging from 2 to 5 μm. However, the pore structure exhibited poor uniformity, indicating that the KOH etching effect on the carbon skeleton remained incomplete at this activation ratio. When the KOH/C ratio was further increased to 3:1, the pore structure of SPC3 was significantly optimized. As shown in Fig. 3i-l, the pore size uniformly expanded to 5–6 μm, forming a well-developed porous system, while the fibrous channel remained intact. These ordered channels provided directional pathways for ion transport, and the highly interconnected porous structure synergistically enhanced electrolyte infiltration and ion diffusion, significantly improving ion transport efficiency. However, excessive activation in SPC4 introduced detrimental effects, as observed in Fig. 3m-p. The strong etching effect of KOH caused localized structural damage to the carbon framework, resulting in partial pore wall collapse (Fig. 3o) and a further reduction in pore connectivity, ultimately leading to reduced ion transport efficiency. The HRTEM characterization of the SPC3 sample was showed in Fig. 4. Figure 4a-b clearly showed a carbon framework composed of numerous worm-like micropores and mesopores, evidencing the hierarchical porous morphology. This structure is critical for achieving a high specific surface area and abundant active sites for electrolyte ions. Additionally, the amorphous nature of the carbon is unequivocally confirmed by the wide and diffused halo ring in the corresponding Selected Area Electron Diffraction (SAED) pattern (Fig. 4c).

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(a-d) SEM images SPC; (e–h) SEM images of SPC2; (i-l) SEM images of SPC3; (m-p) SEM images SPC4.

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TEM images of SPC3.

As shown in Fig. 5a, the elemental mapping from the energy-dispersive X-ray spectroscopy (EDS) of SPC3 revealed that C and O elements were uniformly distributed on the carbon framework, presenting a microscopic morphology where fibrous channel structures and porous characteristics coexist. To further investigate the chemical bonding states of SPC3, X-ray photoelectron spectroscopy (XPS) analysis was conducted. The XPS survey spectrum (Fig. 5b) confirmed that SPC3 was primarily composed of C and O elements, consistent with the EDS results, with atomic percentages of 94.8% for C and 5.2% for O. The high-resolution C 1 s spectrum (Fig. 5c) was fitted into three characteristic peaks, corresponding to C–C/C = C (284.8 eV), C–O (286.1 eV), and C = O (288.7 eV). Similarly, the O 1 s spectrum (Fig. 5d) can be divided into three characteristic peaks assigned to C = O (531.8 eV), C–OH/C–O–C (533.3 eV), and O–H (534.8 eV)35. These results demonstrated the presence of abundant oxygen-containing functional groups in SPC3 sample. The presence of these oxygen functional groups plays a positive role in enhancing the hydrophilicity of carbon materials, thereby improving electrolyte wettability and shortening ion diffusion pathways. Furthermore, specific oxygen functional groups can participate in redox reactions, contributing to pseudocapacitance36.

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(a) EDS and element mapping of SPC3; (e–h) XPS survey spectrum of SPC3.

Nitrogen adsorption–desorption isotherms were employed to investigate the pore size distribution of the SPCx samples. As shown in Fig. 6a, the SPCx samples exhibited composite Type I/IV isotherms. The rose sharply at low relative pressure (P/P₀ < 0.1) indicates the presence of abundant micropores. The existence of mesopores is confirmed by a distinct H4-type hysteresis loop observed in the intermediate pressure range (0.4 < P/P₀ < 0.9), while the rising adsorption tail at P/P₀ > 0.9 suggests the coexistence of macropores37,38. These results demonstrate that the SPCx materials possess a hierarchical porous structure comprising micropores, mesopores, and macropores. In this structure, micropores provide a high surface area and abundant active sites, mesopores facilitate electrolyte infiltration and ion transport, and macropores act as ion-buffering reservoirs to shorten the diffusion distance for ions from the bulk electrolyte to the meso/micropore networks.

Fig. 6
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The N2 adsorption/desorption isotherms (a) and pore size distributions (b) of SPC3.

Figure 6b displays the pore size distributions derived from density functional theory (DFT) analysis. Increasing the KOH/C ratio resulted in the following changes: (1) both the most probable pore sizes (SPC2: 0.89 nm → SPC3: 0.9 nm → SPC4: 0.94 nm) and the average pore diameter (SPC2: 1.66 nm → SPC3: 1.74 nm → SPC4: 1.81 nm) shifted towards larger size, indicating that the intensified KOH etching promoted the enlargement and formation of micropores; and (2) the total pore volume and mesopore volume increased accordingly. These results demonstrate that a higher KOH/C ratio not only created new mesopores but also further expanded the microporous system.

The surface area and pore volume parameters were systematically analyzed using both density functional theory (DFT) and Brunauer–Emmett–Teller (BET) methods, as shown in Table 1. The SPC2 sample exhibited a relatively low specific surface area (1609.6 m2 g-1) and total pore volume (0.67 cm3 g-1), with a low mesopore-to-total pore volume ratio (Vmes/Vp = 9.55%), confirming its predominantly microporous nature. The SPC3 sample showed remarkable improvements over SPC2, achieving a significantly higher surface area of 2746.5 m2 g-1 and a larger pore volume of 1.20 cm3 g-1. Notably, SPC3 exhibited the highest Vmes/Vp ratio (12%), representing an optimal balance between micropores and mesopores where micropores provide charge storage sites while mesopores enable efficient ion transport pathways, thereby enhancing ion accessibility. However, excessive KOH activation led to the collapse or merging of some micropores in the SPC4 sample, which was directly evidenced by a corresponding reduction in the micropore surface area from 1824.3 m2 g-1 to 1740.4 m2 g-1.

Table 1 Summary of specific surface area and total pore volume of SPCx.
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Electrochemical properties

The electrochemical performance of the SPC and SPCx electrodes was evaluated using a three-electrode system in 6 M KOH electrolyte. As shown in Fig. 7a, all samples exhibited nearly rectangular cyclic voltammetry (CV) curves within the potential window of -1.0 to 0 V at a scan rate of 30 mV s⁻1, indicating an energy storage mechanism dominated by electric double-layer capacitance (EDLC). Notably, the non-activated SPC electrode, due to its limited specific surface area and porosity, exhibited a much smaller CV curve, reflecting its inferior charge storage capability. In contrast, after KOH activation, more pores and a larger surface area were created, thereby providing more charge adsorption sites, and the CV area increased substantially. Among them, SPC3 showed the largest CV area, which can be attributed to its optimized pore structure with a high specific surface area (2746.5 m2 g-1) and a mesopore volume ratio (12%). This well-defined hierarchical porous structure not only provided abundant charge storage sites but also ensured efficient ion transport. When the scan rate was increased from 5 to 100 mV s-1, as illustrated in Fig. 7b, SPC3 maintained highly rectangular CV shapes, suggesting excellent rate capability and rapid charge-storage kinetics.

Fig. 7
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(a) comparison of CV curves at 30 mV/s; (b) CV curves of SPC3 with varied scan rates; (c) GCD curves of SPCx at 1 A/g; (d) GCD curves of SPC3 at different current densities; (e) specific capacitance of SPCx at different current densities; (f) Nyquist plots of SPCx; (g) Cycling stability of the SPC3 electrode.

Galvanostatic charge–discharge (GCD) measurements were performed at a current density of 1 A g⁻1 (Fig. 7c). All samples exhibited nearly symmetrical triangular-shaped charge–discharge curves, confirming their typical electric double-layer capacitive behavior and excellent electrochemical reversibility. The unactivated SPC sample showed relatively short charge–discharge durations, reflecting its lower specific capacitance. With increasing KOH/C activation ratios, the charge–discharge time significantly prolonged, which was primarily attributed to the hierarchical porous structure and remarkably enhanced specific surface area constructed through KOH activation. Among them, the SPC3 sample demonstrated the longest discharge time, consistent with the aforementioned cyclic voltammetry results. When the KOH/C.

ratio increased to 4:1, a reduction in discharge time was observed. This phenomenon could be ascribed to excessive activation leading to decreased micropore surface area (SPC3: 1824.3 m2/g → SPC4: 1740.4 m2/g) and directly diminishing the effective electrode/electrolyte contact interface. Within the current density range of 1–10 A g⁻1 (Fig. 7d), the SPC3 sample consistently maintained a typical symmetrical triangular charge–discharge profile, indicating superior charge transfer kinetics.

The specific capacitance values calculated from GCD measurements of all samples (Fig. 7e) revealed that KOH activation significantly enhanced the specific capacitance of the SPCx samples. However, compared to the SPC samples, the rate performance of the activated SPCx series generally showed a downward trend. This was mainly attributed to the etching process of KOH, which damaged the sp2 carbon network and reduced the degree of graphitization of the materials, thereby weakening their intrinsic electronic conductivity. The conclusion was supported by the Raman spectroscopy analysis: the ID/IG ratio significantly increased from 0.881 of SPC to 0.986 of SPC3. It is noteworthy that, despite the overall trend, the SPC3 sample delivered a remarkable specific capacitance of 277 F g-1 at 1 A g-1 and exhibited a high retention of 76.2% (211 F g-1) even at 10 A g-1. This superior electrochemical performance originated from its well-engineered hierarchical pore architecture: (1) ordered fiber channels that construct efficient ion transport pathways, (2) an optimized mesopore ratio (12%) balancing rapid ion transport and effective storage, (3) extremely high specific surface area that provides sufficient charge storage sites. In contrast, the SPC4 sample exhibited only 72.7% capacity retention at 10 A g⁻1, mainly due to decreased microporous specific surface area and mesopore ratio.

The charge transport properties of all samples were investigated using electrochemical impedance spectroscopy (EIS). As shown in Fig. 7f, the Nyquist diagram clearly revealed the differences in impedance characteristics among the various electrode materials, and corresponding fitted parameters were provided in Table 2 by ZView software. In the high-frequency region, the unactivated SPC electrode exhibited a low internal resistance (RS), which was primarily attributed to the naturally ordered fiber channel structure in the biomass precursor and the high degree of graphitization. The structure effectively facilitated electrolyte penetration and significantly reduced ion migration resistance. After KOH activation, the Rs values of SPC2-4 increased noticeably. The phenomenon was primarily attributed to the destruction of the original continuous carbon skeleton structure by KOH etching, which reduced the graphitization degree and consequently led to decreased electron mobility. The diameter of the semicircle in the high-frequency region represented the charge transfer resistance (Rct). The SPC3 sample exhibited the lowest Rct value, indicating that its optimized hierarchical porous structure effectively promoted the charge transfer process. Furthermore, its steepest low-frequency slope signified low ion diffusion resistance, which was quantitatively validated by the Warburg element parameters. SPC3 possessed both the lowest ion diffusion resistance (Ws-R), indicating facile ion transport, and the smallest diffusion time constant (Ws-T). Based on the Eq. 44,45 D = L2/T (where L is the constant effective diffusion thickness), the minimal Ws-T value translated directly to the highest ion diffusion coefficient (D), confirming superior ion transport kinetics facilitated by its unique porous structure.

Table 2 Fitting parameters of fitting circuit.
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These results further confirm that the optimized synergistic combination of naturally aligned channels and hierarchical porous structures enables the construction of efficient ion transport channels, significantly lowers the ion diffusion barrier, and thereby enhances the electrochemical performance. Meanwhile, the structural advantage was further confirmed by cycling stability tests. As depicted in Fig. 7g, after 10,000 charge/discharge cycles at a high current density of 10 A g-1, the device exhibited an impressive capacitance retention of 95.6%, which indicates that the optimized structure can maintain excellent stability and durability even under long-term high current density.

To evaluate the capacitive contribution to charge storage, a detailed kinetic analysis of the CV curves for SPC3 was performed. As showed in Fig. 8b, the b-value derived from the relationship (i = avᵇ) was approximately 0.94, indicating that the electrochemical process was primarily controlled by capacitive mechanisms46. The charge storage contributions were further quantified using the Dunn method20, with the results shown in Fig. 8(c-d). It was demonstrated that the capacitive contribution accounted for 78% of the total charge storage at a scan rate of 5 mV s-1, and increased to 95% at 100 mV s-1. The dominant capacitive behavior was attributed to a synergistic effect: the oxygen functional groups were found to enhance surface wettability and induce pseudocapacitance through rapid, reversible redox reactions49, while the hierarchical porous structure with naturally aligned channels ensured efficient ion access to the active surfaces and promoted rapid surface-controlled kinetics, thereby suppressing diffusion-limited processes.

Fig. 8
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Kinetic analysis of the SPC3 electrode. (a) CV curves of SPC3 electrode at different scan rates; (b) Plots for b-value determination; (c) capacitive contribution at 30 mV s-1; (d) capacitive contribution ratio at different scan rates.

To thoroughly investigate the practical energy storage performance of the SPC3 electrode material, a symmetric supercapacitor (SPC3//SPC3) was constructed using SPC3 as both the cathode and anode, and its electrochemical behavior was systematically evaluated in a 6 M KOH electrolyte system. As shown in Fig. 9a, when the voltage window was expanded from 0–1 V to 0–1.6 V at a scan rate of 100 mV s-1, the CV curve exhibited significant polarization at 1.4 V, indicating the occurrence of irreversible side reactions at this voltage. Based on this observation, an optimal working voltage window of 0–1.2 V was determined. Figure 9b shows that within the 0–1.2 V voltage range, the CV curves of the SPC3//SPC3 symmetric supercapacitor retained a nearly ideal rectangular shape even at a high scan rate of 100 mV s-1, confirming its exceptional rate capability and rapid charge transfer kinetics. Galvanostatic charge–discharge (GCD) measurements (Fig. 9c-d) further revealed that all GCD curves exhibited typical isosceles triangular profiles across current densities ranging from 1 to 10 A g-1. A specific capacitance of 104.5 F g-1 was achieved at 1 A g-1. Notably, when the current density was increased to 10 A g-1, the capacitance retention reached a high value of 80.6%. The outstanding performance was attributed to the aligned ion transport channels and balanced hierarchical porous structure of the SPC3//SPC3 symmetric supercapacitor, which effectively shorten ion diffusion pathways and enhance the adsorption/desorption kinetics of electrolyte ions. After 10,000 cycles at a high current density of 10 A g-1 and a wide voltage window of 0–1.2 V, the device exhibits a capacitance retention of 70.1% (Fig. 9e). This performance decay correlates with significant structural degradation, wherein the aligned channel structure is disrupted and the hierarchical pores collapse and coalesce, as conclusively demonstrated by the SEM image in Figure S1.

Fig. 9
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(a) CV curves of the SPC3//SPC3 at the different voltage window; (b) CV curves of the SPC3//SPC3 atthe different scan rates; (c) GCD curves; (d) the specific capacitance of the SPC3//SPC3 at different current densities; (e) Cycling stability of the asymmetric supercapacitor; (f) Comparison of power and energy densities.

Based on the above electrochemical performance tests, the energy density and power density of the SPC3//SPC3 symmetric supercapacitor were calculated using the following equations:

$$E = \frac{{C \times (\Delta V)^{2} }}{7.2}$$
$$P = \frac{3600 \times E}{{\Delta t}}$$

E stands for the energy density (Wh kg-1), whereas C signifies the specific capacitance of SPC3//SPC3 (F g-1). ΔV denotes the discharge potential window (V), P represents the power density (W kg-1), and Δt is the discharge time (s).

The corresponding calculation results were shown in Fig. 9f. The device displayed an energy density of 20.9 Wh kg-1 while maintaining a power density of 600 W kg-1. The performance parameters of SPC3//SPC3 symmetric supercapacitor were significantly superior to most reported carbon-based symmetric supercapacitor systems. Remarkably, even at an elevated power density of 6000 W kg⁻1, the SPC3-based device retained a high energy density of 16.83 Wh kg⁻1. This performance outperformed engineered comparative materials, such as self-assembled Co–Ni porous carbon spheres and N, P co-doped hierarchical porous carbon, which typically suffer from a sharp decline in energy density under high-power conditions 31,32,39,40,41,42,43. It is crucial to note that the SPC3 device was fabricated from pure biomass-derived carbon without any metal doping or complex modification. Impressively, even with this simple composition, it delivered a highly competitive energy density at a substantially higher power density.

The superior performance was further contextualized by a comprehensive comparison with other biomass-derived carbons of comparable specific surface area prepared under similar KOH activation conditions (Table 3). The data show that the SPC3 device maintained a significantly higher energy density at high power densities than most counterparts. We conclusively attribute this advantage to the naturally aligned and hierarchical porous channels inherited from the strawberry straw precursor. This unique architecture ensured extremely fast ion transport kinetics, enabling the device to deliver high power with minimal sacrifice in energy storage—a critical advantage for practical applications.

Table 3 Summary of supercapacitor performance of biomass-derived carbon materials.
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Conclusion

In summary, the study successfully prepared biomass-derived carbon with both aligned fibrous channels and a hierarchical porous structure from strawberry straw. The effects of KOH/C activation ratios on the structure and electrochemical performance were systematically investigated. The optimal performance was achieved at a KOH/C ratio of 3:1 (SPC3), exhibiting an ultrahigh specific surface area of 2746.5 m2 g-1 and an optimized mesopore proportion of 12%. This structure delivers remarkable synergistic benefits: aligned fibrous channels ensure efficient ion transport, while the hierarchical porous architecture enables optimal electrolyte ion diffusion and effective charge storage. Consequently, SPC3 achieves a high specific capacitance of 277 F/g at a current density of 1 A/g, and retains 76.2% of its capacitance even at a high current density of 10 A/g. Furthermore, the SPC3//SPC3 symmetric supercapacitor exhibited outstanding energy storage characteristics, achieving an energy density of 16.83 Wh kg-1 at a high power density of 6000 W kg-1. The work confirms that precursor materials with an innate, well-aligned channel structure are ideal for designing high-power supercapacitors, as such biological architecture provides highway-like pathways for rapid ion transport. Our study not only presents a novel strategy for the high-value utilization of agricultural waste but also demonstrates a viable approach to synergistically optimizing ion transport kinetics and charge storage capacity. The superior performance of the SPC3 device underscores its significant potential for applications in fast-charging and high-power-output systems.

Declaration