Response surface methodology-optimized single-batch production of herbal residue-based N,P Co-doped carbon materials for enhanced electrochemical efficiency

Abstract

N,P co-doped porous carbon (NPPC) was prepared as a high-performance electrode material for supercapacitors. NPPC materials were synthesized through a facile single-batch carbonization-activation strategy. Poria cocos residue was used as a renewable biomass precursor, potassium carbonate as the chemical activator, and melamine phosphate served as the dual N/P doping agent. A Box-Behnken design  in response surface methodology was utilized to optimize three critical process parameters: K₂CO₃ ratio, N,P co-doped ratio, and activation temperature, aiming at maximizing the specific capacitance. The morphological, structural, and electrochemical properties of the prepared carbon materials were systematically characterized by scanning electron microscopy, N₂ adsorption–desorption  isotherms, X-ray photoelectron spectroscopy, cyclic voltammetry, galvanostatic charge-discharge technique, and electrochemical impedance spectroscopy. The optimized NPPC exhibited hierarchical porous structure with a high specific surface area (reaching 2980 m²⋅g⁻¹), uniformly distributed N (12.3 at.%) and P (0.59 at.%) heteroatoms, and excellent supercapacitive performance. It achieved a maximum specific capacitance of 332 F⋅g⁻¹ at a current density of 1 A⋅g⁻¹ in a 6 M KOH electrolyte. This work realizes the high-value valorization of TCM solid waste and provides a green, cost-effective, and scalable route for the synthesis of high-performance supercapacitor electrode materials, aligning with the goals of waste recycling and carbon neutrality.

Full Article

Response Surface Methodology-optimized Single-batch Production of Herbal Residue-based N,P Co-doped Carbon Materials for Enhanced Electrochemical Efficiency

Sidan Li  ,^a,b,c Yuzhao Ma,^a,b,c Shihua Xu,^a,b,c Xueyang Huang,^a,b,c Yuehan Li,^a,b,c

Zheng Li,^a,b,c and Yuan Yuan  ^a,b,c,*

N,P co-doped porous carbon (NPPC) was prepared as a high-performance electrode material for supercapacitors. NPPC materials were synthesized through a facile single-batch carbonization-activation strategy. Poria cocos residue was used as a renewable biomass precursor, potassium carbonate as the chemical activator, and melamine phosphate served as the dual N/P doping agent. A Box-Behnken design in response surface methodology was utilized to optimize three critical process parameters: K₂CO₃ ratio, N,P co-doped ratio, and activation temperature, aiming at maximizing the specific capacitance. The morphological, structural, and electrochemical properties of the prepared carbon materials were systematically characterized by scanning electron microscopy, N₂ adsorption–desorption isotherms, X-ray photoelectron spectroscopy, cyclic voltammetry, galvanostatic charge-discharge technique, and electrochemical impedance spectroscopy. The optimized NPPC exhibited hierarchical porous structure with a high specific surface area (reaching 2980 m²⋅g⁻¹), uniformly distributed N (12.3 at.%) and P (0.59 at.%) heteroatoms, and excellent supercapacitive performance. It achieved a maximum specific capacitance of 332 F⋅g⁻¹ at a current density of 1 A⋅g⁻¹ in a 6 M KOH electrolyte. This work realizes the high-value valorization of TCM solid waste and provides a green, cost-effective, and scalable route for the synthesis of high-performance supercapacitor electrode materials, aligning with the goals of waste recycling and carbon neutrality.

DOI: 10.15376/biores.21.3.6068-6082

Keywords: Herbal residue; Carbon materials; N,P co-doped; Single-batch production; Electrochemical properties; Box–Behnken design

Contact information: a: College of Mechanical and Resource Engineering, Wuzhou University, Guangxi Wuzhou 543000, China; b: Guangxi Engineering Research Center of Comprehensive Utilization of Renewable Resources, Guangxi Wuzhou 543000, China; c: Guangxi University Engineering Research Center of Comprehensive Utilization of Renewable Resources, Guangxi Wuzhou 543000, China;

* Corresponding author: yuan__yuan2014@sina.com

INTRODUCTION

The increasing global energy crisis and the pressing demand for sustainable energy storage solutions have spurred extensive research into high-performance supercapacitors (Jia et al. 2025). The heightened interest in supercapacitors stems from their sustainable energy storage potential offering rapid charge–discharge capabilities, long-lasting cycle stability, and eco-friendliness (Zhai et al. 2016; Wu et al. 2025). The appropriate selection of electrode materials plays a crucial role in determining electrochemical performance of supercapacitors, and porous carbon materials are emerging as promising candidates due to their large specific surface area, excellent electrical conductivity, and a tunable pore structure (Cheng et al. 2024; Qiang et al. 2024). Biomass-derived materials are particularly attractive as carbon precursors because of their renewable characteristics, affordability, widespread abundance, and inherent eco-friendliness, all of which align with the goals of waste conversion and carbon neutrality (Yan et al. 2017; Tiwari et al. 2022; Liu et al. 2024; Murugan et al. 2024).

Residues from traditional Chinese medicine (TCM) are generated in significant amounts, with China alone producing over 30 million tons annually through decoction processes (Jia et al. 2025). Typically, these residues are discarded or incinerated, leading to substantial resource wastage and environmental degradation (Kang et al. 2017; Hung et al. 2022;). However, these residues are rich in carbonaceous components such as cellulose, hemicellulose, and lignin, and also contain inherent heteroatoms including nitrogen (N) and oxygen (O), making them ideal precursors for producing functional porous carbon materials (Zhou et al. 2023; Chen et al. 2024). Poria Cocos residue (PR) is a widely available, low-cost Chinese herb waste with high carbon content and favorable organic composition for carbonization. It can be readily converted into heteroatom-doped hierarchical porous carbon with large surface area. This can make it a promising precursor for energy storage and environmental applications (Chen et al. 2024). Transforming TCM residues into valuable electrode materials can not only address waste disposal problems but also provide a sustainable approach to energy storage (Wang et al. 2019).

Common strategies to improve the electrochemical performance of biomass-derived carbon materials include heteroatom doping (such as N, phosphorus (P), sulfur (S)) and chemical activation (Huang et al. 2020). Noteworthy, the co-doping of N and P in carbon materials can modify the electronic structure, introduce numerous defect sites, and induce pseudocapacitance through Faradaic redox reactions. This enhancement leads to improved conductivity, wettability, and ion adsorption capabilities (Yan et al. 2014; Li et al. 2022; Wang et al. 2025; Tahir et al. 2026). Chemical activation using agents such as potassium hydroxide (KOH) and melamine phosphate is a highly effective strategy for generating hierarchical porous structures specifically comprising micropores and mesopores, which enhance the diffusion of electrolyte ions and optimize the utilization of active sites. Moreover, response surface methodology (RSM) is a valuable tool for optimizing process parameters (Zhang et al. 2021; Teimouri et al. 2024) such as activation temperature, dopant ratio, and activator dosage, ensuring reproducible and efficient material synthesis (Shi et al. 2023).

This is the first study to employ PR as the carbon precursor for N,P co-doped porous carbon (NPPC), realizing the high-value valorization of TCM solid waste and alleviating environmental pollution. A facile single-batch carbonization-activation route was developed using K₂CO₃ as activator and melamine phosphate (C₃H₉N₆PO₄) as dual N/P dopant. The single-batch carbonization-activation strategy simplifies the synthetic process, reduces energy consumption, realizes in-situ homogeneous heteroatom doping, and avoids intermediate washing/drying steps compared with the traditional two-step method. The Box–Behnken design (BBD) in RSM was utilized to optimize key parameters, including the K₂CO₃ ratio, N and P co-doping ratio, and activation temperature, for specific capacitance. Various techniques were used to characterize the morphological, structural, and electrochemical properties of the materials. To fabricate the working electrode for supercapacitor tests, the optimized NPPC was mixed with acetylene black and polytetrafluoroethylene (PTFE) at a mass ratio of 8:1:1. As critical components of the electrode, acetylene black serves as a conductive agent to reduce internal resistance and construct efficient electron transport pathways within the electrode matrix, which is consistent with the role of conductive additives in enhancing charge transfer efficiency reported in carbon-based electrode systems (Wang et al. 2006). Meanwhile, PTFE functions as a binder to firmly fix the active material (NPPC) and conductive agent (acetylene black) into a coherent and stable structure, preventing particle detachment during electrochemical cycles and ensuring long-term operational reliability of the electrode (Tahir et al. 2026).

This study endeavored to establish a practical and sustainable method for transforming TCM residues into high-performance electrode materials for supercapacitors, thereby advancing the generation of environmentally-friendly energy storage systems. This work is the first to fabricate N,P co-doped hierarchical porous carbon from Poria cocos residue via a single-batch carbonization-activation route optimized by BBD-based RSM. It realizes the high-value conversion of TCM waste and provides a green method for high-performance supercapacitor electrode preparation.

EXPERIMENTAL

Materials

Poria cocos residue (PR) was acquired from a TCM processing factory. PR was first rinsed with purified water to remove impurities, thoroughly dried at 80 ℃, and then crushed into fine powder. K₂CO₃, C₃H₉N₆PO₄, HCl, and anhydrous ethanol, were all of analytical purity and were purchased from Guoyao Group Chemical Reagent Co., Ltd. Acetylene black and polytetrafluoroethylene (PTFE, 5 wt.%) were sourced from Dongguan Hongcheng Plastic Raw Materials Co., Ltd. Foam nickel (1 cm × 2 cm, 110 pores per inch, thickness 1.5 mm, 99.9% purity) was procured from Shanghai Yuezhi Electronic Technology Co., Ltd., while nitrogen gas (>99.999% purity) was supplied by Guangzhou Yuejia Gas Co., Ltd.

Synthesis of N,P Co-Doped Porous Carbon Materials

For synthesizing NPPC, aliquots of PR, K₂CO₃, and C₃H₉N₆PO₄ were weighed in varying mass ratios, mixed and ground uniformly, and transferred into a quartz tube as shown in Fig. 1. Single-batch carbonization–activation treatment was conducted under N₂ atmosphere (5 °C⋅min⁻¹). Specifically, the temperature was raised to 300 °C at a heating rate of 5 °C⋅min⁻¹ and maintained for 30 min. This is a pre-carbonation stage in a continuous heating process without intermediate cooling, washing or reagent addition. Subsequently, it was further increased to the preset activation temperatures (500 to 700 °C) at a heating rate of 10 °C⋅min⁻¹ with a holding time of 2 h. After cooling, the obtained products were thoroughly rinsed with dilute HCl and then with purified water until the pH approached neutral. Then, the as-prepared sample was dried at 105 °C for 6 h to obtain the final material, which was denoted as NPPC. For comparative analysis, the sample obtained under only K₂CO₃ activation was used as a control and referred to as PC.

Fig. 1. A schematic diagram of the single-batch synthesis of NPPC

Characterization of Materials

The morphology of the samples was characterized by scanning electron microscopy (SEM, FEI, Hillsboro, The Netherlands). The surface area and pore size distribution of the product were determined by Brunauer–Emmett–Teller (BET) analysis (Quantachrome Autosorb ASAP 2460, USA). The crystal structure of the products was analyzed by X-ray diffraction (XRD, smartlab9, Japan) and Raman spectroscopy (Confocal Laser MicroRaman spectrometer, Horiba LabRAM HR Evolution, Japan). The elemental valence states of the samples were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Escalab 250xi, USA).

Electrochemical Measurements

For the fabrication of working electrodes for supercapacitor electrochemical measurements, NPPC was mixed with acetylene black and PTFE at a mass ratio of 8:1:1. An appropriate amount of anhydrous ethanol was added dropwise to the mixture while stirring magnetically at 300 rpm for 30 min to form a homogeneous viscous slurry with a solid content of ~25 wt%. The slurry was then subjected to ultrasonic dispersion for 10 min to eliminate agglomerates and to ensure uniform distribution of components. Prior to coating, nickel foam was pre-treated by sequential immersion in 3 M HCl solution for 15 min (to remove surface oxides), deionized water, and anhydrous ethanol for ultrasonic cleaning (each for 10 min), followed by drying at 100 ℃ for 2 h to ensure surface cleanliness and conductivity. The well-dispersed slurry was uniformly coated onto the pre-treated nickel foam using a microsyringe (100 μL) with a coating area precisely controlled at 1 cm × 1 cm. The coated nickel foam was transferred to a vacuum oven and dried at 80 ℃ for 6 h under a pressure of -0.09 MPa to remove residual solvent and enhance interparticle adhesion. After drying, the electrode was pressed at a pressure of 10 MPa for 30 s using a tablet press to improve the contact between the active material layer and the nickel foam current collector, reducing interfacial resistance. The actual mass loading of NPPC on each electrode was weighed for the nickel foam before and after coating/drying, with an average loading of 4.5 ± 0.2 mg to ensure experimental reproducibility. All electrodes were stored in a desiccator under vacuum prior to electrochemical testing.

The three-electrode tests were conducted on an electrochemical workstation (Chenhua CHI660E), including cyclic voltammenty (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS). Among them, the potential window of the CV test was -1 to 0 V, and the scanning rate was 10 to 100 mV/s. The potential window for GCD was -1 to 0 V, and the current density was 1 to 10 A/g. The EIS test was conducted under an open-circuit voltage of 5 mV, with a frequency range of 10 mHz to 100 kHz. The electrode material, which was a platinum sheet, and HgO/Hg electrode were employed as working electrode, counter electrode, and reference electrode, respectively. A 6 M KOH solution was used as electrolyte.

The specific capacitance of the electrode material was calculated by Eq. 1,

 (1)

where C is the specific capacitance (F⋅g⁻¹); I denotes the discharge current (A⋅g⁻¹); Δt represents the discharge time (s); m is the mass of the electrode material (g); and ΔV denotes the voltage difference (V).

Experimental Design

A BBD with three independent parameters and three levels was generated using Design-Expert 8.0.6 software (Stat-Ease Inc., Minneapolis, USA). In total, 17 experiments were conducted on a central location to determine the variables that impact specific capacitance. This approach allows for the establishment of statistical relationships between experimental variables and response variables, which can characterize the response surface and determine the optimal manufacturing conditions (Yuan et al. 2024). These features can enable the prediction of the optimal board properties. Table 1 presents the design matrix and the specific capacitance values of NPPC.

Table 1. BBD for Coded Factors and Results for Specific Capacitance of NPPC

The specific capacitance of NPPC was affected by three critical parameters: the K₂CO₃ ratio (A), the N,P co-doped ratio (B), and the activation temperature (C). These parameters were selected as independent variables based on preliminary experimental results. An analysis of variance (ANOVA) was conducted at a 95% confidence level. All data were presented by taking the average of three replicates and their coefficient of variations (C.V.s) (Yuan et al. 2024). The C.V. observations for the NPPC samples and the PC control sample were found to be consistent.

RESULTS AND DISCUSSION

Data Analysis and Regression Models

Table 2 lists the ANOVA p-values for specific capacitance. All p-values below 0.05 were significant, and those above 0.05 indicated insignificant model terms (Alslaibi et al. 2013). Furthermore, p-values below 0.01 suggest that the model for specific capacitance is highly statistically significant, with only a 0.01% probability that these values are a result of random noise. The significant model terms in this scenario included A, B, AB, BC, A², B², and C².

Table 2. Analysis of Variance for Parameters and their Interactions

The regression equation is as follows:

specific capacitance of NPPC = 334.40 + 8.75A − 11.25B − 3.50C

+ 23.25AB + 4.25AC + 20.75BC − 27.58 A² − 27.08 B² − 14.07C²             (2)

The models were a good fit, with an R² values of 0.9905 for all. The predicted R² value (0.9235) was consistent with the adjusted R² value (0.9783). The Lack of Fit F-value of 1.10 suggested that the model was not significant. The low coefficient of variation (C.V.% = 1.43) validated the accuracy and reliability of the experimental values in the regression model (Yuan et al. 2024).

Response Surface Interaction Analysis

Figure 1 exhibits the significant interaction combinations AB and BC that were selected to analyze their interactive effects on the specific capacitance of NPPC. The interaction effect between the K₂CO₃ ratio (A) and N,P co-doped ratio (B) was greater than that between the N,P co-doped ratio (B) and activation temperature (C). Figure 1a illustrates that there was a significant positive synergistic interaction between A and B. The optimization of the K₂CO₃ ratio was able to maximize the specific capacitance only when the N,P co-doped ratio was moderate. Conversely, only when the K₂CO₃ ratio was moderate was the optimization of the co-doping ratio able to achieve the best effect. Moreover, there was also a notable positive synergy between B and C (Fig. 1b). Only when the activation temperature was moderate was the optimization of the co-doping ratio able to maximize the specific capacitance. By contrast, only when the co-doping ratio was moderate was the optimization of activation temperature able to achieve the maximum effect. This indicates that an increase in temperature caused continuous pore opening and expansion in the material, resulting in activated carbon with a well-developed pore structure. However, if the activation temperature is too high, the carbon skeleton may become overly etched, leading to the continuous transformation of micropores into mesopores and macropores. This can also result in the collapse and damage of some pores, negatively impacting the electrolyte ion adsorption and eventually decreasing the specific capacitance.

Fig. 1. The response surface plots and response contour plots of (a) interactive effect of K₂CO₃ ratio (A) and N,P co-doped ratio (B) on specific capacitance; (b) interactive effect of N,P co-doped ratio (B) and activation temperature (C) on specific capacitance

Optimization and Verification Experiment

After the optimization analysis was completed, verification experiments were carried out employing the identical methodology under the determined optimal conditions: a K₂CO₃ ratio (A) of 1.97, an N, P co-doped ratio (B) of 0.42, and an activation temperature (C) of 579 ℃. Under these conditions, the predicted specific capacitance reached 339 F⋅g⁻¹. The experiment was optimized for convenience by adjusting the process parameters as follows: K₂CO₃ ratio (A) is 2.0, N, P co-doped ratio (B) is 0.4, and activation temperature (C) is 580 ℃. Under these conditions, the observed NNPC specific capacitance was 332 F⋅g⁻¹, which shows that the model had high reliability and good reproducibility for the NPPC specific capacitance.

Electrochemical Analysis

Figure 2a exhibits the CV curves of PC, showing narrower integral areas and significant triangular distortion at scan rates >50 mV⋅s⁻¹, but at 10 to 10 mV⋅s⁻¹, the CV curves became more pronounced.

Fig. 2. The responses of (a) PC and (b) NPPC; and GCD profiles responses of (c) PC and (d) NPPC at different current densities

Figure 2b demonstrates that NPPC exhibited nearly rectangular profiles with minimal distortion, even at a scan rate of 100 mV⋅s⁻¹, suggesting a synergistic integration between electric double-layer capacitance and pseudocapacitance. The capacitance at 1 A⋅g⁻¹, as determined from GCD tests and depicted in Figs. 2c and 2d, indicates that NPPC achieved a value of 332 F⋅g⁻¹, which was 1.47 times higher than that of PC (226 F⋅g⁻¹). The minor deviation from an ideal rectangular shape observed at low scan rates can be attributed to Faradaic redox reactions involving pyridinic nitrogen (N–H₂↔N+H₂) and phosphorus-containing groups (P–O↔P=O)(Qiang et al. 2024).

Figure 3 illustrates that each sample exhibits a high-frequency semicircle related to charge transfer resistance (R_ct). The measured series resistance (R_s) values were 0.79 Ω for PC and 0.62 Ω for NPPC. These R_s values were fairly consistent in both samples, with slight variations possibly due to intrinsic resistance differences in the carbon materials (Chen et al. 2025). The R_ct values for PC and NPPC were 0.76 and 0.35 Ω, respectively. The Warburg resistance (Z_W) in NPPC also exhibited a steeper low-frequency slope, suggesting an accelerated ion diffusion process within the hierarchical porous structure.

Fig. 3. Nyquist plots of PC and NPPC in 6 M KOH

Morphological and Structural Characteristics

Figures 4(a and b) exhibit SEM images of the synthesized electrode materials, revealing their microscopic morphological characteristics and microstructural evolution. Both the figures show a three-dimensional interconnection framework with a clear porous structure. Figure 4a exhibits a honeycomb-like structure of PC, produced by the pore-forming effect of CO, CO₂, and K, obtained by decomposition of K₂CO₃. The CO and CO₂ can etch carbon to form micropores, while K vapor can get incorporated into the graphene layer, resulting in the disruption of the carbon microstructure (Qiang et al. 2024). Comparatively, NPPC shows a large-diameter flower-shaped thin nanosheet structure with C₃H₉N₆PO₄ (Fig. 4b), which is possibly due to the combined effect of physico–chemical double active agents with unique pore-forming capabilities (Wang et al. 2025; Liu et al. 2026).

Fig. 4. SEM images of (a) PC and (b) NPPC, and (c) nitrogen adsorption–desorption isotherms of PC and NPPC

As shown in Fig. 4c, NPPC had a hierarchical porous structure (with micropores and mesopores), as evident from the type IV N₂ adsorption–desorption isotherms with distinct hysteresis loops (at P/P₀ > 0.5). It exhibited a significantly higher specific surface area (S_BET) and pore volume (V_total) compared to PC, as presented in Table 3. For instance, the PC had an S_BET of 2061.3 m²⋅g⁻¹ and V_total of 0.95 cm³⋅g⁻¹, whereas NPPC achieved an ultrahigh S_BET of 2980.6 m²⋅g⁻¹ and V_total of 1.24 cm³⋅g⁻¹. The pore size distribution was well-balanced, with micropores (≤2 nm) providing numerous adsorption sites, and mesopores (2 to 50 nm) facilitating ion diffusion (Zhang et al. 2025). An important aspect is that the mesopore ratio (V_meso/V_total) of NPPC can reach up to 24.2%, which enhances the mass transport efficiency.

Table 3. Pore Structure for PC and NPPC

Crystal Structure and Defect Density

Figure 5a exhibits XRD patterns of NPPC and PC. These displayed amorphous carbon characteristics, with broad peaks at approximately 23° and 40°, corresponding to the (002) and (101) crystal planes of graphite, respectively. However, the XRD pattern of NPPC displayed a slightly weaker peak intensity, suggesting a lower degree of crystallinity due to N/P co-doping. Raman spectroscopy identified two distinct peaks (Fig. 5b): the D band, observed at around 1,345 cm⁻¹ and associated with structural defects and disorder; and the G band, present at around 1,589 cm⁻¹ and representing sp² hybridized graphitic carbon. The intensity ratio of D to G bands (I_D/I_G) can be employed to assess the graphitization degree, with NPPC and PC exhibiting the values of 1.02 and 0.97, respectively. A higher I_D/I_G value directly signifies an increase in structural defects with numerous functionalized surface active sites, suggesting that N/P co-doping introduces more defects and disordered structures, which correspond to active sites for ion adsorption and charge storage.

Fig. 5. (a) XRD patterns and (b) Raman spectroscopy results of PC and NPPC

Chemical Composition and Bonding Environment

The elemental composition of PC and NPPC was determined by XPS, and full spectrum plots are shown in Fig. 6a. Figure 6b shows that the high-resolution N 1s spectra of NPPC could be decomposed into various functional configurations, such as pyridinic N (around 398.3 eV), pyrrolic N (around 400.2 eV), and graphitic N (around 401.1 eV), with pyridinic N contributes to pseudocapacitance via Faradaic redox reactions. Pyrrolic N improves electrode wettability and graphitic N enhances the electrical conductivity of carbon materials. The P 2p spectrum (Fig. 6c) is divided into C–P bonds (around 13.2 eV) and P–O bonds (around 134.3 eV), with a larger peak area for C–P, suggesting a stable integration of P into the carbon matrix of NPPC.

Fig. 6. XPS survey spectra of (a) PC and NPPC; and (b) N 1s and (c) P 2p spectra of NPPC

Table 4 presents the elemental content of PC and NPPC. Apparently, NPPC was characterized by intentional and effective co-doping of N and P heteroatoms into the carbon framework. NPPC exhibited a higher N content (12.33%) and P content (0.59%) than PC (4.79% and 0.03%, respectively). These findings indicate that NPPC exhibited lower charge transfer resistance, which can be attributed to the hierarchical porous structure and efficient N/P co-doping. These heteroatoms can introduce active sites on the surface of the carbon material, promoting redox reactions and thereby enhancing the contribution of pseudocapacitance (Chen et al. 2025).

Table 4. Elemental Contents of PC and NPPC

CONCLUSIONS

To address the high-value utilization of traditional Chinese medicine (TCM) solid waste and the demand for sustainable high-performance electrode materials in supercapacitors, this work focuses on the development of N,P co-doped porous carbon (NPPC) using Poria cocos residue as the renewable carbon precursor. Through a facile single-batch carbonization-activation strategy with K₂CO₃ as the activator and melamine phosphate as the dual N/P dopant, combined with Box-Behnken design (BBD)-based response surface methodology (RSM) for precise parameter optimization, we successfully fabricated a hierarchical porous carbon material with exceptional electrochemical properties. The following conclusions are drawn based on systematic experimental investigations and characterizations:

1. A straightforward single-batch carbonization-activation approach was used to effectively fabricate N,P co-doped porous carbon materials from Poria cocos residues, thereby valorizing traditional Chinese medicine (TCM) residues and mitigating pollution. The key parameters were optimized through Box–Behnken design-based response surface methodology, identifying the optimal conditions as follows: a temperature of 580 °C, a K₂CO₃ ratio of 2.0, and an N,P co-doped ratio of 0.4. The optimized sample exhibited a superior electrochemical performance of 332 F⋅g⁻¹ at 1 A⋅g⁻¹ in 6 M KOH.
2. Synergistic K₂CO₃ activation and melamine phosphate doping enabled the formation of a highly optimized carbon material with a hierarchical porous structure, comprising both micropores and mesopores, and a uniform distribution of N (12.3%) and P (0.59%) heteroatoms. This resulted in enhanced active sites, improved ion diffusion, electronic structure tuning, and improved electrode wettability.
3. This cost-effective and environmentally sustainable approach provides a viable pathway for utilizing TCM residues, enabling the production of high-performance supercapacitor electrodes that can be extended to other types of biomass waste.

ACKNOWLEDGMENTS

The authors are grateful for the support of Youth Science Project of Guangxi (Grant No. 2023GXNSFBA026289), Special Project for Young Innovative Talents in Project of Guangxi Science and Technology Base and Special Talent (Grant No. Guike AD22080019), Doctoral Foundation of Scientific Research Project of Wuzhou University (Green Construction and Electrochemical Performance of Carbon Film based on Chinese Medicine Residue, Grant No. 2022A001), Key Project of Wuzhou University (Design and Supercapacitive Performance of Biomass-Derived Carbon Materials/MXene Flexible Films, Grant No. 2024ZD005), Wuzhou Science and Technology Plan Project (Research on Synergistic Removal of Binary Pollutants in Water by Magnetic Biomass Aerogels, Grant No. 202402027), and Education and Teaching Reform Project of Wuzhou University (Exploration and Practice of “Craftsman-type” Talent Training Mode in Applied Undergraduate Universities from the Perspective of Emerging Engineering Education, Grant No. Wyjg2023A053).

Use of Generative AI

The authors used Douba AI for literature organization in the introduction section and MogoEdit (https://www.mogoedit.com) for its English editing during the preparation of this manuscript.

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Article submitted: March 11, 2026; Peer review completed: May 3, 2026; Revised version received and accepted: May 5, 2026; Published: May 18, 2026.

DOI: 10.15376/biores.21.3.6068-6082