pH-Modulated Upcycling of Black Liquor into N/O Co-Doped Hierarchical Porous Carbon via Green Templating-Activation Synergy for High-Performance Supercapacitors

pH-Modulated Upcycling of Black Liquor into N/O Co-Doped Hierarchical Porous Carbon via Green Templating-Activation Synergy for High-Performance Supercapacitors | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article pH-Modulated Upcycling of Black Liquor into N/O Co-Doped Hierarchical Porous Carbon via Green Templating-Activation Synergy for High-Performance Supercapacitors Cheng zhang, Yu Chen Han, Jie Zheng, Wen Juan Wu, Yong Can Jin This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8729298/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Under the global acceleration of carbon neutrality initiatives, the green valorization of industrial waste has emerged as a critical pathway for establishing sustainable manufacturing systems. This study addresses the challenges of inefficient component regulation and restricted pore development in activated carbon derived from pulping black liquor (BL). We propose an innovative activation strategy utilizing magnesium basic carbonate (MBC) as a dual-functional template-activator. By integrating evaporation-induced self-assembly (EISA) with pH gradient fractionation technology, BL is converted into high-performance nitrogen-doped porous activated carbon. A hierarchical regulation system was established to synthesize three lignin-based activated carbons: alkaline, neutral, and acidic. Characterization reveals that AcidBLAC@MBC@N exhibits a unique lamellar porous architecture with a synergistic micro-mesoporous network (mesopore 78%). The optimized material demonstrates exceptional electrochemical performance: a specific capacitance of 305 F g⁻¹ at 0.5 A g⁻¹ and 98% capacitance retention after 10,000 cycles. This work circumvents energy-intensive pre-carbonization (> 800°C) and complex component separation steps in conventional processes. This strategy establishes an eco-efficient paradigm for black liquor valorization, offering a scalable solution for transforming industrial waste into advanced energy storage materials. The design principles may be extended to other biomass-derived systems, aligning with circular economy and carbon neutrality goals. pH-Modulated Porous carbon Lignin Electrochemical performance Supercapacitor Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Introduction The invention of papermaking in ancient China revolutionized information recording, profoundly advancing societal development and civilization. Since the 19th century, however, the rapidly expanding paper industry has generated severe environmental pollution, exerting mounting pressure on global ecosystems. Industrial scale-up has significantly degraded water resources, air quality, and ecological equilibrium(Nicholson, 2007, Li et al., 2022 ). Particularly problematic is alkaline pulping—the dominant industrial method (90% of global pulp production)—which utilizes high-temperature alkaline solutions (NaOH/Na₂S) to chemically delignify wood, separating cellulose fibers. This process annually yields > 1.3 billion tons of black liquor (BL) (Ahmad et al., 2020 ), characterized by extreme pollution potential: high chemical oxygen demand (COD > 80 g·L⁻¹) and strong alkalinity (pH 12–14), posing critical ecological threats(Ahmad et al., 2020 ). BL primarily consists of lignocellulosic residues, carbohydrates, and inorganic compounds(Morya et al., 2022 ). Among these, Lignin, a three-dimensional cross-linked macromolecular network comprising phenylpropane units linked by aryl ether and C–C bonds(Figueiredo et al., 2017 ), represents the predominant renewable aromatic polymer in nature, constituting 15–30% of lignocellulosic biomass by weight. Currently, lignin is predominantly consumed through combustion, with an annual production exceeding 50 million tons. However, its complex and heterogeneous structure severely limits further conversion and valorization, posing a major obstacle to broader applications. Despite its high carbon content and renewability, only a small fraction of lignin is utilized for producing bio-based products, failing to achieve large-scale or high-value applications. Due to its structural complexity, the efficient utilization of lignin remains hindered by numerous technical and economic challenges, resulting in relatively inefficient conversion processes and the underutilization of vast lignin resources. Supercapacitors (SCs) represent a class of energy storage devices distinguished by high power density, extended cycle life, and rapid charge/discharge capabilities(Lamba et al., 2022 ). Architecturally, a typical SC comprises current collectors, electrolyte, separator, and electrodes—with electrode materials critically determining capacitance, cycling stability, and rate performance(Youe et al., 2018 ). While diverse materials including transition metal oxides(Chen et al., 2010 ), metal composite(Mu et al., 2022 ), conducting polymers(Tian et al., 2023 ), and graphene(Dai et al., 2019 ) have been explored, activated carbon (AC) dominates commercial applications due to its high specific surface area, excellent conductivity, and cost-effectiveness(Chang et al., 2025 , Wu et al., 2024). AC precursors span fossil resources (coal(Benoy et al., 2024 ), pitch(Mondal et al., 2025)), synthetic polymers(Pandit et al., 2021), and natural biomass (e.g., sucrose(Singh et al., 2022 ), cellulose(Luo et al., 2024), and lignin(A et al., 2014)). Lignin emerges as a particularly promising renewable precursor, featuring high carbon content (> 60%) and structurally tunable microstructures(Wang et al., 2022 ), with inherent phenyl/phenolic groups providing abundant electroactive sites(Ewurum and McDonald, 2025). Derived lignin-based carbon materials (LAC) exhibit π-conjugated domains facilitating rapid electron transport, coupled with high surface area and optimized pore architectures that shorten ion diffusion pathways—collectively enabling efficient charge/discharge kinetics(Yang et al., 2023 ). Critical to optimizing symmetric EDLC performance are four material characteristics: (1) hierarchical pore architectures integrating macropores (ion reservoirs), mesopores (transport channels), and micropores (charge-accumulation domains)(Seo et al., 2023, Thines et al., 2017); (2) controlled graphitization for enhanced electron mobility; (3) heteroatom doping (notably nitrogen) to introduce pseudocapacitance via Faradaic reactions, improve wettability through polarity modification, and boost electrochemical properties(Lee et al., 2018); and (4) percolated conductive networks. Currently, the most widely adopted and effective strategy for fabricating ideal lignin-derived activated carbon (LAC) is the chemical activation method(Li et al., 2023a). Conventional activating agents used in other carbon material syntheses—such as H₃PO₄(Li et al., 2023a), H₂SO₄(Tian et al., 2017 ), Zn(NO₃)₂·6H₂O(Ma et al., 2020 ), and KOH(Chen et al., 2020 )—are also applicable to LAC preparation. However, MBC exhibits distinct advantages over these traditional activators in terms of green chemistry, environmental compatibility, cost-effectiveness, and recyclability(Mo and Wu, 2022 ). Its unique lamellar structure not only serves as a template to promote pore formation but also optimizes the pore architecture of LAC.The type and concentration of surface pores and active sites in carbon materials are critical factors influencing their electrical conductivity and electronic configuration(B et al., 2021). Pure porous carbon materials typically suffer from limited active sites, which restricts electron transfer and storage capacity, thereby reducing conductivity(Zeng et al., 2020 ). To address this limitation, this study introduces melamine to incorporate heteroatomic nitrogen into black liquor-derived activated carbon (BLAC). This nitrogen doping strategy introduces defects and active sites, effectively modifying the electronic structure of BLAC and enhancing its electrochemical activity. In this study, BL was subjected to pH fractionation, followed by the utilization of MBC as a dual-functional activation template. EISA mechanism was employed to drive the coordination between lignin and MBC, forming a lamellar composite precursor. Coupled with a nitrogen doping process, pyridinic-N and graphitic-N active sites were introduced into the carbon framework, successfully converting BL into a high-performance nitrogen-doped porous activated carbon material. The structural and chemical properties of the Raman spectroscopy, field-emission scanning electron microscopy (FE-SEM), N₂ adsorption-desorption isotherms, and X-ray photoelectron spectroscopy (XPS). Electrochemical performance was evaluated through cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS). Results demonstrate that the synthesis of lamellar porous BLAC directly from lignin in black liquor represents an environmentally benign and facile synthetic strategy, offering a novel pathway for sustainable carbon material fabrication. Experimental 2.1 Materials Poplar wood used in this work was obtained from a pulp mill in Jiangsu, China.Na 2 S, NaOH, MBC, Melamine, HCl, Graphite, PVDF, N-Methyl-2-pyrrolidone (NMP), Nickel foam, Agate mortar, Glucan, Xylose, Arabinose and H₂SO₄ used in this work were purchased from Sigma-Aldrich. All reagents were used as received without any further purification. 2.2 Preparation of BL and pH regulation The Kraft pulping process was employed using poplar wood chips (20–30 mm in length). The pulping conditions were set as follows: solid-to-liquid ratio of 1:5, alkali charge of 20% (based on Na₂O), sulfidity of 25%, heated to 170°C and maintained for 2 h. After cooking, the mixture was cooled to room temperature, and the black liquor was collected. Three 50 mL aliquots of black liquor were prepared. The pH of the samples was adjusted using HCl: Group A remained unadjusted (original pH), serving as the control. Group B was adjusted to pH 7.0 ± 0.2 with 2.2 ± 0.2 mL of HCl, and Group C to pH 1.0 ± 0.2 with 6.5 ± 0.2 mL of HCl. The solutions from Groups B and C were transferred to centrifuge tubes and centrifuged at 7000 rpm for 10 min to separate the supernatant and precipitate. Group B precipitates were washed three times with distilled water, while Group C precipitates were washed three times with 1 M HCl. The washed precipitates were then transferred to Petri dishes and vacuum-dried at 60°C. 2.3 Determination of components A 2 mL aliquot of black liquor (pH 12) was dried in an oven at 105°C until constant weight was achieved. The dried solid residue was transferred to a ceramic crucible and subjected to calcination treatment in a muffle furnace at 575°C. For hydrolysis, 3 mL of 72% sulfuric acid was added to 0.3 g of dried sample (Groups B and C). The mixture was incubated in a water bath at 30°C for 1 h, followed by dilution with 84 mL of distilled water. The solution was transferred to a sealed glass bottle and autoclaved at 121°C for 1 h. After cooling, the insoluble residue was filtered using a sintered glass crucible, dried, and weighed. The residue was further calcined at 575°C.UV-Vis spectrophotometry was performed at 205 nm(Klinke et al., 2004). A 4% sulfuric acid solution served as the blank for baseline calibration. Absorbance values of diluted samples at 205 nm were recorded in triplicate.。 Glucose, xylose, and arabinose standard solutions (0.1 mg mL⁻¹) were prepared. Separation was conducted using a Rezex RCM-Monosaccharide column (300 × 7.8 mm, 8 µm) equipped with a pre-column guard system. The mobile phase consisted of acetonitrile: water (75:25, v/v) under isocratic elution at a flow rate of 1.0 mL min⁻¹. The column temperature was maintained at 35 ± 1°C to ensure retention time stability. A peak area-concentration standard curve was constructed. Sample solutions were filtered through 0.22 µm nylon membranes prior to injection. Monosaccharide concentrations were calculated via the external standard method, with results reported as the average of three independent measurements (relative standard deviation, RSD < 2.5%). 2.4 Preparation of BLAC@MBC 40 mL of different black liquor samples were mixed with MBC at an MBC-to-solution ratio of 0.3. The mixtures mixture was subjected to ultrasonication for 20 min to achieve intense dispersion of MBC particles, prevent agglomeration, and ensure uniform distribution in solution, thus enhancing contact and wetting between black liquor components (particularly lignin molecules) and the MBC surface, followed by magnetic stirring in an oil bath at 60°C (150 rpm) for 4 h. The resulting suspensions were freeze-dried to obtain BL@MBC, NetBL@MBC, and AcidBL@MBC composites. The dried composites were placed in a ceramic boat in a tube furnace and subjected to carbonization under nitrogen flow (80 mL min − 1 ): heating to 250°C at a rate of 5°C min − 1 and holding for 30 min; heating to 600°C at a rate of 5°C min − 1 and holding for 120 min; and cooling down to room temperature to obtain BLAC. The carbonized powder was mixed with 100 mL of 1 M HCl and stirred for 24 h to dissolve residual activators. To remove inorganic salts, the powder was washed repeatedly with deionized water via vacuum filtration until the filtrate reached neutral pH(Shen et al., 2019 ). Finally, the product was vacuum-dried for 6 h to obtain the purified BLAC samples. 2.5 Preparation of electrodes The BLAC, conductive carbon black, and PVDF were mixed at a mass ratio of 8:1:1 and ground into a homogeneous slurry in an appropriate amount of NMP solvent(Caguiat et al., 2018 ). The slurry was uniformly coated onto a pre-dried nickel foam substrate (1 × 3 cm²), ensuring a coated area of 1 × 1 cm², with the total electrode loading controlled between 1.5 and 3 mg cm⁻². After coating, the prepared electrode was dried in a vacuum oven at 60°C for 12 h. 2.6 Material characterization techniques The microstructure of the samples was characterized using scanning electron microscopy (SEM, JEOL JSM-00076948, Japan) at an accelerating voltage of 15 kV and a beam current of 50 µA. Elemental analysis was performed via energy-dispersive spectroscopy (EDS, Thermo Scientific Apreo 2C, USA). Raman spectroscopy (Thermo Scientific DXR532, USA) with a 532 nm excitation wavelength was employed to evaluate defect density under a 200 kV accelerating voltage. Surface chemical states and functional groups were investigated using X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific Escalab Xi+) with CasaXPS software for peak deconvolution. Nitrogen adsorption-desorption isotherms were measured at 77 K using a Brunauer-Emmett-Teller (BET) analyzer (Micromeritics ASAP 2020/Quantachrome Autosorb iQ) to determine specific surface area, pore size distribution, and total pore volume. All measurements were conducted under ambient conditions unless otherwise specified. 2.7 Electrochemical analysis Electrochemical analysis was performed using a CorrTest electrochemical workstation (Model: CS2350M, China) with a three-electrode system comprising a working electrode, a counter electrode (platinum wire), and a reference electrode (Hg/HgO) in 6 M KOH electrolyte. The tests included CV, GCD, and EIS. CV and GCD measurements were conducted across varying scan rates (5–100 mV s⁻¹) and current densities (0.5–5.0 A g⁻¹) to evaluate the electrochemical performance of the electrode material. The specific capacitance (Cs, F g⁻¹) was calculated using Equations ( 1 ) and ( 2 ): $$\:{C}_{S}=\frac{\int\:IdV}{ms\varDelta\:V}$$ 1 $$\:{C}_{S}=\frac{I\varDelta\:t}{m\varDelta\:V}$$ 2 ∫ IdV = integrated area under the CV curve (A·V), m = mass of the active material on the electrode (g), Δ V = potential window (V), s = scan rate (mV s⁻¹)(Laheäär et al., 2015 ). EIS analysis was performed to investigate the electrode-electrolyte interface and bulk electrolyte properties. The frequency range was set from 10 mHz to 10 kHz with a perturbation amplitude of 10 mV. The Nyquist plots were analyzed using Zview software to calculate interfacial resistances through equivalent circuit fitting. Results and discussion 3.1 Component analysis BL is rich in lignin, hemicellulose-derived sugars, and phenolic compounds, making it a promising precursor for carbon-based functional materials. However, directly carbonized BLAC suffers from pore blockage by ash (porosity < 45%), leading to poor electrochemical performance (specific capacitance < 120 F g⁻¹)(Kumar and Verma, 2024). This study innovatively employs pH gradient fractionation technology to precisely regulate BL components, enabling the directional construction of hierarchical porous structures and offering a novel approach for BL valorization. Group A: Directly obtained from poplar wood alkaline pulping, retaining its original composition. Group B: Adjusted to pH 7.0 via HCl titration, followed by centrifugation and washing with deionized water. The precipitate was reconstituted in an alkaline solution matching the original BL alkalinity. Group C: Acidified to pH 1.0 using HCl, with the precipitate treated with 1 M HCl, dried, and reconstituted in an alkaline solution. In the acidic fractionation process, H⁺ neutralizes the negative charges on lignin phenolic hydroxyl groups via electrostatic interactions, significantly reducing intermolecular repulsion. This charge shielding effect promotes the coordination assembly between alkali metals (e.g., Na⁺/K⁺) and lignin, forming ordered mesostructures. As shown in Fig. 2 and Table 1 , the raw BL exhibits the lowest lignin content (70.3%±0.2) and the highest carbohydrate (12.6%±0.2) and ash (15.6%±0.2) levels. Upon pH reduction to 7.0, lignin content increases to 80.9%±0.2, while acid-soluble lignin (ASL) peaks at 8.5%±0.2, with carbohydrates and ash decreasing to 8.5% and 8.4%, respectively. Further acidification to pH 1.0 maximizes lignin purity (90.2%±0.2) through efficient sodium salt removal, while carbohydrates and ash are minimized to 4.0%±0.2 and 3.2%±0.2, respectively. Table 1 Black liquor components Sample Lignin (%) Carbohydrate (%) Ash (%) KL ASL All GLU Xyl Ara All Black Liquor 67.2 ± 0.2 3.1 ± 0.2 70.3 ± 0.2 4.5 ± 0.2 6.4 ± 0.2 1.7 ± 0.2 12.6 ± 0.2 15.6 ± 0.2 Neutral Black Liquor 72.4 ± 0.2 8.5 ± 0.2 80.9 ± 0.2 2.7 ± 0.2 4.2 ± 0.2 1.2 ± 0.2 8.5 ± 0.2 8.4 ± 0.2 Acid Black Liquor 85.4 ± 0.2 4.8 ± 0.2 90.2 ± 0.2 1.3 ± 0.2 2.1 ± 0.2 0.6 ± 0.2 4 ± 0.2 3.2 ± 0.2 (PS: KL: Klason lignin, ASL: Acid Soluble Lignin, GLU: Glucan, Xyl: Xylan, Ara: Arabican) 3.2 Structure and morphology The presence of sp³-hybridized carbon and edge defects is evident in all samples, as indicated by the pronounced D-band at 1350 cm⁻¹ in Fig. 3 (a), signifying defective and disordered graphitic structures in BLAC(Liu et al., 2024). Notably, the D-band intensity of AcidBLAC@MBC@N and BLAC@MBC@N is significantly higher than that of other samples, demonstrating that nitrogen doping (@N) effectively introduces additional defect sites into the carbon matrix. The G-band observed at 1580 cm⁻¹ reflects the graphitic ordering of sp²-hybridized carbon, corresponding to in-plane vibrational modes(Zhang et al., 2022 ). However, the G-band of AcidBLAC@MBC@N, NetBLAC@MBC@N, and BL@MBC@N exhibits broader peak profiles with overlap to the D-band, suggesting that nitrogen doping may reduce graphitic domain sizes, thereby forming more lamellar stacked structures. The intensity ratio of the D-band to G-band (I D /I G ) provides a semi-quantitative assessment of defect density in carbon materials. As pH decreases, the I D /I G ratio progressively increases from 0.52–0.56 for BLAC to 0.78–0.84 for NetBLAC. When pH is reduced to 1, the I D /I G ratio reaches its maximum value of 0.96–1.1, further confirming that pH fractionation effectively optimizes the graphitic structure of BLAC. As shown in Fig. 3 (b), AcidBLAC@MBC@N exhibits a hierarchical pore architecture dominated by mesopores (78%), complemented by micropores (19%) and macropores (3%). This lamellar porous structure provides an enlarged electrode/electrolyte interface, facilitating charge transfer reactions. The enhanced interfacial kinetics accelerate ion diffusion and electrolyte permeation, thereby improving charge/discharge rates and significantly boosting the electrochemical performance of supercapacitors. By comparing the microstructural evolution of materials before and after pH-regulated treatment (Fig. 4 ), the critical role of black liquor component regulation in constructing hierarchical porous structures was elucidated. Untreated BLAC samples (Fig. 4 a–c) exhibited a densely agglomerated morphology, with surfaces covered by amorphous carbon layers and negligible pore development. This structural deficiency originates from the pyrolysis behavior of high-content ash (12.7%) and carbohydrates in the black liquor—during carbonization, ash-derived metal oxides catalyzed tar byproduct formation, while low-molecular-weight organics from carbohydrate pyrolysis blocked pore channels, ultimately leading to pore collapse and surface passivation. In contrast, AcidBLAC@MBC@N subjected to acid washing and pH fractionation (Fig. 4 d–f) displayed a significantly optimized hierarchical pore structure. Acid treatment effectively removed ash (reduced to 2.1%) and carbohydrates (reduced to 4%). As shown in Fig. 4 e, the material formed an interconnected network of open mesopores (15–40 nm) and macropores (50–200 nm). During carbonization, MBC decomposed at 300–500°C to generate nanoscale MgO particles, which acted as rigid templates to guide carbon framework growth while creating abundant edge-active sites(Madan et al., 2021 , Ding et al., 2025). These active sites drove the self-assembly of MBC via π-π stacking and hydrogen bonding interactions, forming a lamellar porous architecture. The dual-functional mechanism of MBC as an activation template: during pyrolysis, MBC not only chemically etches carbon layers to form hierarchical porous structures but also generates CO₂/H₂O gases that synergistically regulate pore architecture through in situ gas activation. However, the efficacy of the MBC template is highly dependent on the quality of its integration with the precursor. As evidenced in Fig. 4 (a, d), unoptimized precursors (BL,NetBL) yield a microstructure dominated by amorphous carbon domains (red circles), characterized by diffuse diffraction patterns and the absence of lattice fringes, confirming a disordered carbon matrix. In contrast, Fig. 4 (g) reveals that acid treatment (AcidBL) induces structural ordering, manifesting as long-range lamellar stacking (green circles). This demonstrates the selective removal of carbonization inhibitors from the black liquor, thereby providing a more ideal substrate for the MBC templating effect. Critically, Fig. 4 (b-c, e-f) exhibit compromised structural integrity in the BLAC@MBC and NetBLAC@MBC composites, featuring pore collapse (yellow circles) attributed to poor integration of the MBC template. Conversely, the composite formed by combining the optimized precursor (AcidBL) with MBC and nitrogen doping, AcidBLAC@MBC@N (Figs. 4 h-i), displays a nitrogen-doped hierarchical architecture (blue circles). This optimized morphology provides abundant edge-hosted active sites and low-tortuosity ion transport pathways, directly contributing to its superior specific capacitance of 305 F g⁻¹ at 0.5 A g⁻¹, significantly higher than that of the undoped control (182 F g⁻¹). AcidBLAC@MBC@N exhibits optimal structural characteristics among all specimens, achieving the highest specific surface area (SBET = 584.5 m² g⁻¹) and hierarchical factor (HF = 0.85) as quantified in Table 2 . This mesopore-dominated architecture (78.4% mesopore ratio) significantly shortens ion diffusion pathways and reduces transport resistance, thereby enabling exceptional rate capability with 98% capacitance retention at 5 A g⁻¹. Crucially, nitrogen doping induces defect-mediated inhibition of carbon layer stacking during pyrolysis, substantially enhancing SBET across functionalized samples: AcidBLAC@MBC@N displays a 117% increase relative to AcidBLAC@MBC (268.8 to 584.5 m² g⁻¹), NetBLAC@MBC@N shows 113% enhancement versus NetBLAC@MBC (215.5 to 458.6 m² g⁻¹), and BLAC@MBC@N attains 74% improvement over BLAC@MBC (186.3 to 324.8 m² g⁻¹). Synergistically, acid pretreatment amplifies N-doping effects, collectively establishing a hierarchical conductive network that optimizes charge storage kinetics and cycling stability. Table 2 Pore structure parameters of BLAC Sample SSA (m 2 g − 1 ) Precentage ( \(\:\%)\) Pore volume (cm 3 g − 1 ) S BET S meso S micro S meso / S BET S micro / S BET V Total V meso V micro AcidBLAC@MBC@N 584.5 455.9 128.6 78 22 0.67 0.57 0.03 NetBLAC@MBC@N 458.6 307.3 151.3 67 33 0.42 0.38 0.04 BLAC@MBC@N 324.8 194.9 129.9 60 40 0.27 0.24 0.03 AcidBLAC@MBC 268.8 142.5 126.6 53 47 0.21 0.18 0.03 NetBLAC@MBC 215.5 94.8 120.7 44 56 0.15 0.12 0.03 BLAC@MBC 186.3 70.8 115.5 38 62 0.11 0.09 0.02 The material surface featured numerous nanoscale protrusions (height ≈ 5 nm) and cracks (width ≈ 2 nm). These structural characteristics not only enhanced capacitive performance by exposing additional active sites (41% increase in specific capacitance) but also improved electrode structural stability through mechanical interlocking effects (capacitance retention > 96% after 10,000 cycles). Figure 5 (a–h) depict the XPS spectra processed using CASA software and deconvoluted via Advantage software. The survey spectra of AcidBLAC@MBC@N and BLAC@MBC (Fig. 5 a) clearly show characteristic peaks for C 1s (284.8 eV), O 1s (532.1 eV), N 1s (399.8 eV), and Mg 2p (50.3 eV), confirming the coexistence of a carbon matrix, oxygen/nitrogen-containing functional groups, and magnesium in the material(Guan et al., 2024 , Bhagwan and Han, 2024 ). Detailed analysis of the C 1s fine spectra (Fig. 5 b and 5 f) highlights structural differences: BLAC@MBC exhibits two components—graphitic carbon (C = C, 284.8 eV, 68.7%) and carbonyl carbon (C = O, 289.6 eV, 31.3%)(Ding et al., 2020)—while AcidBLAC@MBC@N displays four components, including additional C–O (286.5 eV, 18.2%) and C–N (287.5 eV, 9.1%) bonds, alongside an increased C = C content (72.4%). This indicates that acid treatment enhances graphitization by removing amorphous carbon phases and introduces nitrogen doping through C–N bond formation. The evolution of O 1s spectra further corroborates surface modifications (Fig. 5 d and 5 h). BLAC@MBC shows a single C–O peak (532.8 eV), whereas AcidBLAC@MBC@N splits into C = O (531.7 eV, 56.9%) and C–O (533.2 eV, 43.1%), with a 27% increase in oxidized C = O groups, improving electrode/electrolyte interfacial wettability(Ma et al., 2023). Nitrogen speciation analysis (N 1s, Fig. 5 e) identifies pyridinic-N (N-6, 398.6 eV, 61.3%), which enhances charge transfer via lone-pair electron donation (reducing Rct by 38%), and protonated nitrogen (N⁺,400.1 eV, 38.7%), corresponds to protonated nitrogen, indicating an electron-rich surface environment(Li et al., 2021 ). Finally, the Mg 2p spectra (Fig. 5 c and 5 g) exhibit a single peak at 49.8 eV, corresponding to Mg–O bonding, confirming the self-assembly of lignin and MBC via magnesium ion bridging (Mondal et al., 2024). These findings collectively demonstrate how pH-regulated treatments and nitrogen doping synergistically optimize the material’s electronic structure and interfacial properties, underpinning its enhanced electrochemical performance. 3.3 Electrochemical performance of hierarchical porous BLAC-based electrode In this study, a three-electrode system with 6 M KOH electrolyte was employed to systematically evaluate the electrochemical performance of the synthesized materials. CV, GCD, and EIS were utilized to comprehensively investigate specific capacitance, charge-discharge kinetics, and impedance characteristics.A systematic investigation of six electrode materials via CV across a scan rate range of 5–100 mV s⁻¹ (Fig. 6 a–f) revealed the synergistic regulation of nitrogen doping and acid treatment on charge storage mechanisms. As shown in Fig. 6 d and 6 f, BLAC@MBC@N and AcidBLAC@MBC@N electrodes exhibited quasi-rectangular CV curves, indicative of an EDLC-dominated charge storage mechanism [51] . Notably, AcidBLAC@MBC@N demonstrated a significantly larger CV integral area than other samples, with 81.2% capacity retention at a high scan rate of 100 mV s⁻¹, attributed to its optimized hierarchical pore structure and enhanced ion diffusion kinetics. Detailed analysis (Fig. 6 d/f) identified distinct redox peaks in the − 0.2 to 0.3 V potential range for nitrogen-doped materials, associated with Faradaic pseudocapacitance contributions from surface oxygen- and nitrogen-containing functional groups(Zhang et al., 2021 ). Coparative experiments demonstrated that untreated BLAC@MBC and NetBLAC@MBC samples, with underdeveloped porosity, exhibited distorted CV curves and reduced integral areas (34–41% lower than AcidBLAC@MBC@N). This discrepancy underscores the critical role of acid treatment in removing impurity phases, expanding pores, and constructing rapid ion transport channels for the electrolyte. To thoroughly investigate the electrochemical performance of the synthesized electrodes, their GCD behavior was systematically evaluated across a current density range of 0.5–5.0 A g⁻¹. Figure 6 (a)-(c) present the GCD curves of BLAC-, NetBLAC-, and AcidBLAC-based electrodes at varying current densities, reflecting the current density-dependent response characteristics of the electrode materials. Figure 6 (d)-(f) display comparative GCD profiles of BLAC@MBC@N, NetBLAC@MBC@N, and AcidBLAC@MBC@N at 0.5 A g⁻¹. Consistent with CV results, AcidBLAC@MBC@N exhibited the highest charge-discharge performance, achieving a specific capacitance of 305 ± 5 F g⁻¹, significantly surpassing BLAC@MBC@N (145 ± 4 F g⁻¹) and NetBLAC@MBC@N (235 ± 5 F g⁻¹). The quasi-triangular shape of its GCD curve indicates electric double-layer capacitance as the dominant contributor to the total capacitance(Li et al., 2020 ). Figure 6 (e) illustrates the CV curves of these three samples at a scan rate of 100 mV s⁻¹, where AcidBLAC@MBC@N demonstrated the largest integrated area. This GCD analysis corroborates the findings derived from CV results, providing robust evidence for the consistent stability of the synthesized samples under a wide range of applied current densities. The electrochemical performance of electrode materials is comprehensively influenced by their microstructural characteristics, such as mesoscale porosity and pore volume. Micropores facilitate effective interactions with electrolyte ions, while mesopores act as transport channels during charge storage processes. GCD tests revealed that the rate capability and structural stability of nitrogen-doped carbon-based electrodes exhibit significant correlations. As shown in Fig. 8 (a)–(b), when the current density increased from 0.5 to 5 A g⁻¹, AcidBLAC@MBC@N demonstrated the highest capacitance retention (73.8%), significantly outperforming NetBLAC@MBC@N (72.7%) and BLAC@MBC@N (71.2%). Comparative experiments further indicated that non-doped control materials (AcidBL@MBC, NetBL@MBC, and BL@MBC) exhibited more pronounced capacity decay (reduced to 70.6%, 67.2%, and 65.4%, respectively). In contrast, pH-fractionated samples showed systematic improvements in capacitance retention, confirming the synergistic optimization of nitrogen doping and hierarchical processing. Notably, AcidBLAC@MBC@N retained 98.2 ± 0.6% of its initial capacity after 10,000 charge-discharge cycles (Fig. 8 c), with GCD curves maintaining high symmetry throughout long-term cycling, indicative of exceptional electrochemical stability. This outstanding cyclability is attributed to the following mechanisms: Enhanced Conductivity: Pyridinic/pyrrolic-N functional groups formed via nitrogen doping improve the electronic conductivity of the carbon matrix: (1) Surface Optimization: Acid treatment modifies surface chemistry to suppress side reactions. (2) Stress Mitigation: Hierarchical porous structures alleviate structural stress during ion intercalation/deintercalation. EIS, a critical tool for evaluating electrode interface properties, effectively reveals charge transfer kinetics in the high-frequency region. The Nyquist plots in Fig. 8 (d) show typical series resistance (Rs) and charge transfer resistance (Rct) features for all electrodes. Equivalent circuit fitting (Fig. 8 e) demonstrated that only AcidBLAC@MBC@N exhibited a complete semicircle in the high-frequency region, indicating minimal charge transfer impedance, as further validated in the magnified inset of Fig. 8 (f). Fitting results revealed that AcidBLAC@MBC@N achieved the lowest Rs value (1.02 ± 0.2 Ω), significantly lower than NetBLAC@MBC@N (1.43 ± 0.4 Ω). This disparity arises from precursor treatment effects on self-assembly: high acid-soluble lignin (ASL) content (8.5%) in NetBLAC@MBC@N precursors hindered effective binding between MBC and lignin, while acid treatment reduced ASL content, increased Klason lignin purity, and optimized the MBC-lignin assembly for enhanced activation. The ultra-low Rs value of AcidBLAC@MBC@N underscores its superior charge transfer kinetics at the carbon-electrolyte interface, confirming the efficacy of the proposed synergistic design strategy. Table 2 showing the comparative performance of lignin-based carbon for SCs applications(Khalid et al., 2023 , Cui et al., 2021, Li et al., 2023b , Liu et al., 2023 , Wang et al., 2025b, Tran et al., 2017, Wang et al., 2025a ), and it can be seen that the electrochemical performance of supercapacitors prepared from nitrogen-doped porous carbon derived from BL lignin is in the middle to upper level. Table 3 Showing the comparative performance of lignin-based carbon for SCs applications precursor Electrolyte Current density (A g − 1 ) Capacitance (F g − 1 ) Cycling sustainability (%) Ref. Kraft lignin 6 M KOH 0.5 88 92.8(5A g − 1 @75000) (Khalid et al., 2023 ) Lignin 1M H 2 SO 4 1 144 92.3(1A g − 1 @5000) (Cui et al., 2021) Lignin 6 M KOH 1 162 95.4(5A g − 1 @10000) (Tran et al., 2017) Kraft lignin 6 M KOH 0.5 205 82(20A g − 1 @10000) (Wang et al., 2025a ) Kraft lignin 6 M KOH 0.1 224 85(10A g − 1 @5000) (Liu et al., 2023 ) Lignin 6 M KOH 0.5 236 95.5(10A g − 1 @10000) (Li et al., 2023) Kraft lignin 6 M KOH 1 183 96.6(10 A g − 1 @10000) (Wang et al., 2025) BL lignin 6 M KOH 0.5 305 92.8(5A g − 1 @10000) This work Conclusion This study proposes an eco-friendly template-activation synergistic strategy to achieve the efficient conversion of BL into high-performance nitrogen-doped porous carbon materials (AcidBLAC@MBC@N). The research elucidates the dual-functional mechanism of MBC as an activation template: during pyrolysis, MBC not only chemically etches carbon layers to form hierarchical porous structures but also generates CO₂/H₂O gases that synergistically regulate pore architecture through in situ gas activation. By implementing pH gradient fractionation pretreatment, the composition of BL precursors was optimized—lignin content increased to 90%, while carbohydrates and ash were reduced to 4% and 3.2%, respectively—significantly mitigating the adverse effects of impurities on carbon material structure. Coupled with nitrogen doping, abundant pyridinic-N/graphitic-N active sites and oxygen-containing functional groups were engineered into the carbon framework, endowing the material with exceptional electron transport capabilities. The synthesized AcidBLAC@MBC@N exhibits remarkable structural and performance advantages: Hierarchical Porous Architecture: Mesopore-dominated structure (78.4% mesopore ratio, average pore size 3.8 nm).Outstanding Electrochemical Performance: Specific capacitance of 305 F g⁻¹ at 0.5 A g⁻¹, ultralow interfacial resistance (1.02 Ω), and exceptional cycling stability (> 95% capacity retention after 10,000 charge-discharge cycles).Through a three-tier cascading strategy—"waste valorization → material functionalization → performance optimization"—this work provides a novel approach for BL upcycling. Experimental data and mechanistic analyses demonstrate that this strategy overcomes the feedstock limitations of traditional activated carbon preparation methods, offering a new technical pathway for developing low-cost, sustainable supercapacitor electrode materials. 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J Energy Storage [J] 45. https://doi.org/10.1016/j.est.2021.103457 Additional Declarations No competing interests reported. Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8729298","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":596844239,"identity":"4737053c-f66f-404c-b8ba-80923875bc43","order_by":0,"name":"Cheng zhang","email":"","orcid":"","institution":"Nanjing Forestry University","correspondingAuthor":false,"prefix":"","firstName":"Cheng","middleName":"","lastName":"zhang","suffix":""},{"id":596844241,"identity":"61ae84bb-e7df-4185-bac6-78b435ec7d5c","order_by":1,"name":"Yu Chen Han","email":"","orcid":"","institution":"Nanjing Forestry 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preparation\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-8729298/v1/5e580dfc9953a6255aa8b241.png"},{"id":103497557,"identity":"44e36fcf-591b-479c-a60f-951d032aca34","added_by":"auto","created_at":"2026-02-26 11:29:31","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":357780,"visible":true,"origin":"","legend":"\u003cp\u003eMechanism diagram of HCl-Mediated Black Liquor Regulation\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-8729298/v1/05ca625e722572cfd004e665.png"},{"id":103497561,"identity":"03e70f72-fa17-4ad8-bb57-9988648d497e","added_by":"auto","created_at":"2026-02-26 11:29:31","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":310960,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Raman curves of BLAC samples (b) N\u003csub\u003e2\u003c/sub\u003e adsorption and desorption curves and pore size distribution of AcidBLAC@MBC@N\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-8729298/v1/979b54649ccbee80669c2191.png"},{"id":103497558,"identity":"5d49d4bf-1bac-482a-844a-cf9088f3f7ae","added_by":"auto","created_at":"2026-02-26 11:29:31","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":866620,"visible":true,"origin":"","legend":"\u003cp\u003ea-c) SEM images of BLAC@MBC at different sizes, (d-f) SEM images of NetBLAC@MBC@N at different sizes, (g-i) SEM images of AcidBLAC@MBC@N at different sizes\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-8729298/v1/0c1ba761244eb75c8b75bb96.png"},{"id":103507728,"identity":"b2acdb46-c93c-4374-9d83-463062aa6c80","added_by":"auto","created_at":"2026-02-26 13:44:01","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":285184,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Comparison of full spectra, XPS spectrum of BLAC@MBC (b) C1s spectrum, (c) Mg 2p spectrum (d) O1s spectrum, XPS of AcidBLAC@MBC@N (e) N1s spectrum, (f) C1s spectrum, (g) Mg 2p spectrum, (h) O1s spectrum\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-8729298/v1/3a02acee7ed9f2cfb46f8c2c.png"},{"id":103497563,"identity":"9f08c068-0c63-4c15-adea-5c9b68457621","added_by":"auto","created_at":"2026-02-26 11:29:31","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":7980626,"visible":true,"origin":"","legend":"\u003cp\u003eCV pplots at different scanning sweep rates (a) BLAC@MBC vs. (d) BLAC@MBC@N, (b) NetBLAC@MBC vs. (e) NetBLAC@MBC@N, (c) AcidBLAC@MBC vs. (f) AcidBLAC@MBC@N\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-8729298/v1/4fe6a5f0a8626e329aac86d6.png"},{"id":103497559,"identity":"aba18066-27a0-46e5-b942-1005af80f582","added_by":"auto","created_at":"2026-02-26 11:29:31","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":426410,"visible":true,"origin":"","legend":"\u003cp\u003eGCD plots at different current densities (a) BLAC@MBC vs. (d) BLAC@MBC@N, (b) NetBLAC@MBC vs. (e) NetBLAC@MBC@N, (c) AcidBLAC@MBC vs. (f) AcidBLAC@MBC@N\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-8729298/v1/d031ac8e2b03968a70d54dbb.png"},{"id":103507360,"identity":"0c34f19e-967f-41c7-b515-f547aa89560a","added_by":"auto","created_at":"2026-02-26 13:41:08","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":358923,"visible":true,"origin":"","legend":"\u003cp\u003ea) Specific capacitance vs. current density (b) Change in specific capacitance when ramping up from 0.5A g\u003csup\u003e-1\u003c/sup\u003e to 1A g\u003csup\u003e-1\u003c/sup\u003e (c) Capacitance retention of AcidBLAC@MBC@N at 10,000 cycles at 5A g\u003csup\u003e-1\u003c/sup\u003e (d) Nyquist plots of the electrodes (inset shows fitted currents) (e) High-frequency region of the Nyquist plots (f) Nyquist plots of AcidBL@MBC@N Inset shows fitted current\u003c/p\u003e","description":"","filename":"floatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-8729298/v1/7ff1889d9f5cacf01d5a1219.png"},{"id":108523964,"identity":"dbbcd0be-f6e6-477e-91f0-2bd7ecd7c7fe","added_by":"auto","created_at":"2026-05-05 14:42:54","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":12385232,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8729298/v1/4f99a98c-a00d-4157-b20f-7e643b6ce06b.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"pH-Modulated Upcycling of Black Liquor into N/O Co-Doped Hierarchical Porous Carbon via Green Templating-Activation Synergy for High-Performance Supercapacitors","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe invention of papermaking in ancient China revolutionized information recording, profoundly advancing societal development and civilization. Since the 19th century, however, the rapidly expanding paper industry has generated severe environmental pollution, exerting mounting pressure on global ecosystems. Industrial scale-up has significantly degraded water resources, air quality, and ecological equilibrium(Nicholson, 2007, Li et al., \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Particularly problematic is alkaline pulping\u0026mdash;the dominant industrial method (90% of global pulp production)\u0026mdash;which utilizes high-temperature alkaline solutions (NaOH/Na₂S) to chemically delignify wood, separating cellulose fibers. This process annually yields\u0026thinsp;\u0026gt;\u0026thinsp;1.3\u0026nbsp;billion tons of black liquor (BL) (Ahmad et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2020\u003c/span\u003e), characterized by extreme pollution potential: high chemical oxygen demand (COD\u0026thinsp;\u0026gt;\u0026thinsp;80 g\u0026middot;L⁻\u0026sup1;) and strong alkalinity (pH 12\u0026ndash;14), posing critical ecological threats(Ahmad et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). BL primarily consists of lignocellulosic residues, carbohydrates, and inorganic compounds(Morya et al., \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Among these, Lignin, a three-dimensional cross-linked macromolecular network comprising phenylpropane units linked by aryl ether and C\u0026ndash;C bonds(Figueiredo et al., \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2017\u003c/span\u003e), represents the predominant renewable aromatic polymer in nature, constituting 15\u0026ndash;30% of lignocellulosic biomass by weight. Currently, lignin is predominantly consumed through combustion, with an annual production exceeding 50\u0026nbsp;million tons. However, its complex and heterogeneous structure severely limits further conversion and valorization, posing a major obstacle to broader applications. Despite its high carbon content and renewability, only a small fraction of lignin is utilized for producing bio-based products, failing to achieve large-scale or high-value applications. Due to its structural complexity, the efficient utilization of lignin remains hindered by numerous technical and economic challenges, resulting in relatively inefficient conversion processes and the underutilization of vast lignin resources.\u003c/p\u003e \u003cp\u003eSupercapacitors (SCs) represent a class of energy storage devices distinguished by high power density, extended cycle life, and rapid charge/discharge capabilities(Lamba et al., \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Architecturally, a typical SC comprises current collectors, electrolyte, separator, and electrodes\u0026mdash;with electrode materials critically determining capacitance, cycling stability, and rate performance(Youe et al., \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). While diverse materials including transition metal oxides(Chen et al., \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2010\u003c/span\u003e), metal composite(Mu et al., \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2022\u003c/span\u003e), conducting polymers(Tian et al., \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2023\u003c/span\u003e), and graphene(Dai et al., \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2019\u003c/span\u003e) have been explored, activated carbon (AC) dominates commercial applications due to its high specific surface area, excellent conductivity, and cost-effectiveness(Chang et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2025\u003c/span\u003e, Wu et al., 2024). AC precursors span fossil resources (coal(Benoy et al., \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2024\u003c/span\u003e), pitch(Mondal et al., 2025)), synthetic polymers(Pandit et al., 2021), and natural biomass (e.g., sucrose(Singh et al., \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2022\u003c/span\u003e), cellulose(Luo et al., 2024), and lignin(A et al., 2014)). Lignin emerges as a particularly promising renewable precursor, featuring high carbon content (\u0026gt;\u0026thinsp;60%) and structurally tunable microstructures(Wang et al., \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2022\u003c/span\u003e), with inherent phenyl/phenolic groups providing abundant electroactive sites(Ewurum and McDonald, 2025). Derived lignin-based carbon materials (LAC) exhibit π-conjugated domains facilitating rapid electron transport, coupled with high surface area and optimized pore architectures that shorten ion diffusion pathways\u0026mdash;collectively enabling efficient charge/discharge kinetics(Yang et al., \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Critical to optimizing symmetric EDLC performance are four material characteristics: (1) hierarchical pore architectures integrating macropores (ion reservoirs), mesopores (transport channels), and micropores (charge-accumulation domains)(Seo et al., 2023, Thines et al., 2017); (2) controlled graphitization for enhanced electron mobility; (3) heteroatom doping (notably nitrogen) to introduce pseudocapacitance via Faradaic reactions, improve wettability through polarity modification, and boost electrochemical properties(Lee et al., 2018); and (4) percolated conductive networks.\u003c/p\u003e \u003cp\u003eCurrently, the most widely adopted and effective strategy for fabricating ideal lignin-derived activated carbon (LAC) is the chemical activation method(Li et al., 2023a). Conventional activating agents used in other carbon material syntheses\u0026mdash;such as H₃PO₄(Li et al., 2023a), H₂SO₄(Tian et al., \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2017\u003c/span\u003e), Zn(NO₃)₂\u0026middot;6H₂O(Ma et al., \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2020\u003c/span\u003e), and KOH(Chen et al., \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2020\u003c/span\u003e)\u0026mdash;are also applicable to LAC preparation. However, MBC exhibits distinct advantages over these traditional activators in terms of green chemistry, environmental compatibility, cost-effectiveness, and recyclability(Mo and Wu, \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Its unique lamellar structure not only serves as a template to promote pore formation but also optimizes the pore architecture of LAC.The type and concentration of surface pores and active sites in carbon materials are critical factors influencing their electrical conductivity and electronic configuration(B et al., 2021). Pure porous carbon materials typically suffer from limited active sites, which restricts electron transfer and storage capacity, thereby reducing conductivity(Zeng et al., \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). To address this limitation, this study introduces melamine to incorporate heteroatomic nitrogen into black liquor-derived activated carbon (BLAC). This nitrogen doping strategy introduces defects and active sites, effectively modifying the electronic structure of BLAC and enhancing its electrochemical activity.\u003c/p\u003e \u003cp\u003eIn this study, BL was subjected to pH fractionation, followed by the utilization of MBC as a dual-functional activation template. EISA mechanism was employed to drive the coordination between lignin and MBC, forming a lamellar composite precursor. Coupled with a nitrogen doping process, pyridinic-N and graphitic-N active sites were introduced into the carbon framework, successfully converting BL into a high-performance nitrogen-doped porous activated carbon material. The structural and chemical properties of the Raman spectroscopy, field-emission scanning electron microscopy (FE-SEM), N₂ adsorption-desorption isotherms, and X-ray photoelectron spectroscopy (XPS). Electrochemical performance was evaluated through cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS). Results demonstrate that the synthesis of lamellar porous BLAC directly from lignin in black liquor represents an environmentally benign and facile synthetic strategy, offering a novel pathway for sustainable carbon material fabrication.\u003c/p\u003e"},{"header":"Experimental","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Materials\u003c/h2\u003e \u003cp\u003ePoplar wood used in this work was obtained from a pulp mill in Jiangsu, China.Na\u003csub\u003e2\u003c/sub\u003eS, NaOH, MBC, Melamine, HCl, Graphite, PVDF, N-Methyl-2-pyrrolidone (NMP), Nickel foam, Agate mortar, Glucan, Xylose, Arabinose and H₂SO₄ used in this work were purchased from Sigma-Aldrich. All reagents were used as received without any further purification.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Preparation of BL and pH regulation\u003c/h2\u003e \u003cp\u003eThe Kraft pulping process was employed using poplar wood chips (20\u0026ndash;30 mm in length). The pulping conditions were set as follows: solid-to-liquid ratio of 1:5, alkali charge of 20% (based on Na₂O), sulfidity of 25%, heated to 170\u0026deg;C and maintained for 2 h. After cooking, the mixture was cooled to room temperature, and the black liquor was collected.\u003c/p\u003e \u003cp\u003eThree 50 mL aliquots of black liquor were prepared. The pH of the samples was adjusted using HCl: Group A remained unadjusted (original pH), serving as the control. Group B was adjusted to pH 7.0\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2 with 2.2\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2 mL of HCl, and Group C to pH 1.0\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2 with 6.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2 mL of HCl. The solutions from Groups B and C were transferred to centrifuge tubes and centrifuged at 7000 rpm for 10 min to separate the supernatant and precipitate. Group B precipitates were washed three times with distilled water, while Group C precipitates were washed three times with 1 M HCl. The washed precipitates were then transferred to Petri dishes and vacuum-dried at 60\u0026deg;C.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Determination of components\u003c/h2\u003e \u003cp\u003eA 2 mL aliquot of black liquor (pH 12) was dried in an oven at 105\u0026deg;C until constant weight was achieved. The dried solid residue was transferred to a ceramic crucible and subjected to calcination treatment in a muffle furnace at 575\u0026deg;C. For hydrolysis, 3 mL of 72% sulfuric acid was added to 0.3 g of dried sample (Groups B and C). The mixture was incubated in a water bath at 30\u0026deg;C for 1 h, followed by dilution with 84 mL of distilled water. The solution was transferred to a sealed glass bottle and autoclaved at 121\u0026deg;C for 1 h. After cooling, the insoluble residue was filtered using a sintered glass crucible, dried, and weighed. The residue was further calcined at 575\u0026deg;C.UV-Vis spectrophotometry was performed at 205 nm(Klinke et al., 2004). A 4% sulfuric acid solution served as the blank for baseline calibration. Absorbance values of diluted samples at 205 nm were recorded in triplicate.。\u003c/p\u003e \u003cp\u003eGlucose, xylose, and arabinose standard solutions (0.1 mg mL⁻\u0026sup1;) were prepared. Separation was conducted using a Rezex RCM-Monosaccharide column (300 \u0026times; 7.8 mm, 8 \u0026micro;m) equipped with a pre-column guard system. The mobile phase consisted of acetonitrile: water (75:25, v/v) under isocratic elution at a flow rate of 1.0 mL min⁻\u0026sup1;. The column temperature was maintained at 35\u0026thinsp;\u0026plusmn;\u0026thinsp;1\u0026deg;C to ensure retention time stability. A peak area-concentration standard curve was constructed. Sample solutions were filtered through 0.22 \u0026micro;m nylon membranes prior to injection. Monosaccharide concentrations were calculated via the external standard method, with results reported as the average of three independent measurements (relative standard deviation, RSD\u0026thinsp;\u0026lt;\u0026thinsp;2.5%).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4 Preparation of BLAC@MBC\u003c/h2\u003e \u003cp\u003e40 mL of different black liquor samples were mixed with MBC at an MBC-to-solution ratio of 0.3. The mixtures mixture was subjected to ultrasonication for 20 min to achieve intense dispersion of MBC particles, prevent agglomeration, and ensure uniform distribution in solution, thus enhancing contact and wetting between black liquor components (particularly lignin molecules) and the MBC surface, followed by magnetic stirring in an oil bath at 60\u0026deg;C (150 rpm) for 4 h. The resulting suspensions were freeze-dried to obtain BL@MBC, NetBL@MBC, and AcidBL@MBC composites. The dried composites were placed in a ceramic boat in a tube furnace and subjected to carbonization under nitrogen flow (80 mL min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e): heating to 250\u0026deg;C at a rate of 5\u0026deg;C min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and holding for 30 min; heating to 600\u0026deg;C at a rate of 5\u0026deg;C min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and holding for 120 min; and cooling down to room temperature to obtain BLAC. The carbonized powder was mixed with 100 mL of 1 M HCl and stirred for 24 h to dissolve residual activators. To remove inorganic salts, the powder was washed repeatedly with deionized water via vacuum filtration until the filtrate reached neutral pH(Shen et al., \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). Finally, the product was vacuum-dried for 6 h to obtain the purified BLAC samples.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e2.5 Preparation of electrodes\u003c/h2\u003e \u003cp\u003eThe BLAC, conductive carbon black, and PVDF were mixed at a mass ratio of 8:1:1 and ground into a homogeneous slurry in an appropriate amount of NMP solvent(Caguiat et al., \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). The slurry was uniformly coated onto a pre-dried nickel foam substrate (1 \u0026times; 3 cm\u0026sup2;), ensuring a coated area of 1 \u0026times; 1 cm\u0026sup2;, with the total electrode loading controlled between 1.5 and 3 mg cm⁻\u0026sup2;. After coating, the prepared electrode was dried in a vacuum oven at 60\u0026deg;C for 12 h.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e2.6 Material characterization techniques\u003c/h2\u003e \u003cp\u003eThe microstructure of the samples was characterized using scanning electron microscopy (SEM, JEOL JSM-00076948, Japan) at an accelerating voltage of 15 kV and a beam current of 50 \u0026micro;A. Elemental analysis was performed via energy-dispersive spectroscopy (EDS, Thermo Scientific Apreo 2C, USA). Raman spectroscopy (Thermo Scientific DXR532, USA) with a 532 nm excitation wavelength was employed to evaluate defect density under a 200 kV accelerating voltage. Surface chemical states and functional groups were investigated using X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific Escalab Xi+) with CasaXPS software for peak deconvolution.\u003c/p\u003e \u003cp\u003eNitrogen adsorption-desorption isotherms were measured at 77 K using a Brunauer-Emmett-Teller (BET) analyzer (Micromeritics ASAP 2020/Quantachrome Autosorb iQ) to determine specific surface area, pore size distribution, and total pore volume. All measurements were conducted under ambient conditions unless otherwise specified.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e2.7 Electrochemical analysis\u003c/h2\u003e \u003cp\u003eElectrochemical analysis was performed using a CorrTest electrochemical workstation (Model: CS2350M, China) with a three-electrode system comprising a working electrode, a counter electrode (platinum wire), and a reference electrode (Hg/HgO) in 6 M KOH electrolyte. The tests included CV, GCD, and EIS.\u003c/p\u003e \u003cp\u003eCV and GCD measurements were conducted across varying scan rates (5\u0026ndash;100 mV s⁻\u0026sup1;) and current densities (0.5\u0026ndash;5.0 A g⁻\u0026sup1;) to evaluate the electrochemical performance of the electrode material. The specific capacitance (Cs, F g⁻\u0026sup1;) was calculated using Equations (\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) and (\u003cspan refid=\"Equ2\" class=\"InternalRef\"\u003e2\u003c/span\u003e):\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:{C}_{S}=\\frac{\\int\\:IdV}{ms\\varDelta\\:V}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$\\:{C}_{S}=\\frac{I\\varDelta\\:t}{m\\varDelta\\:V}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003e\u0026int;\u003cem\u003eIdV\u003c/em\u003e\u0026thinsp;=\u0026thinsp;integrated area under the CV curve (A\u0026middot;V), \u003cem\u003em\u003c/em\u003e\u0026thinsp;=\u0026thinsp;mass of the active material on the electrode (g), Δ\u003cem\u003eV\u003c/em\u003e\u0026thinsp;=\u0026thinsp;potential window (V), \u003cem\u003es\u003c/em\u003e\u0026thinsp;=\u0026thinsp;scan rate (mV s⁻\u0026sup1;)(Lahe\u0026auml;\u0026auml;r et al., \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2015\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eEIS analysis was performed to investigate the electrode-electrolyte interface and bulk electrolyte properties. The frequency range was set from 10 mHz to 10 kHz with a perturbation amplitude of 10 mV. The Nyquist plots were analyzed using Zview software to calculate interfacial resistances through equivalent circuit fitting.\u003c/p\u003e \u003c/div\u003e"},{"header":"Results and discussion","content":"\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Component analysis\u003c/h2\u003e \u003cp\u003eBL is rich in lignin, hemicellulose-derived sugars, and phenolic compounds, making it a promising precursor for carbon-based functional materials. However, directly carbonized BLAC suffers from pore blockage by ash (porosity\u0026thinsp;\u0026lt;\u0026thinsp;45%), leading to poor electrochemical performance (specific capacitance\u0026thinsp;\u0026lt;\u0026thinsp;120 F g⁻\u0026sup1;)(Kumar and Verma, 2024). This study innovatively employs pH gradient fractionation technology to precisely regulate BL components, enabling the directional construction of hierarchical porous structures and offering a novel approach for BL valorization. Group A: Directly obtained from poplar wood alkaline pulping, retaining its original composition. Group B: Adjusted to pH 7.0 via HCl titration, followed by centrifugation and washing with deionized water. The precipitate was reconstituted in an alkaline solution matching the original BL alkalinity. Group C: Acidified to pH 1.0 using HCl, with the precipitate treated with 1 M HCl, dried, and reconstituted in an alkaline solution. In the acidic fractionation process, H⁺ neutralizes the negative charges on lignin phenolic hydroxyl groups via electrostatic interactions, significantly reducing intermolecular repulsion. This charge shielding effect promotes the coordination assembly between alkali metals (e.g., Na⁺/K⁺) and lignin, forming ordered mesostructures.\u003c/p\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e and Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, the raw BL exhibits the lowest lignin content (70.3%\u0026plusmn;0.2) and the highest carbohydrate (12.6%\u0026plusmn;0.2) and ash (15.6%\u0026plusmn;0.2) levels. Upon pH reduction to 7.0, lignin content increases to 80.9%\u0026plusmn;0.2, while acid-soluble lignin (ASL) peaks at 8.5%\u0026plusmn;0.2, with carbohydrates and ash decreasing to 8.5% and 8.4%, respectively. Further acidification to pH 1.0 maximizes lignin purity (90.2%\u0026plusmn;0.2) through efficient sodium salt removal, while carbohydrates and ash are minimized to 4.0%\u0026plusmn;0.2 and 3.2%\u0026plusmn;0.2, respectively.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eBlack liquor components\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"11\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c10\" colnum=\"10\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c11\" colnum=\"11\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eSample\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"3\" nameend=\"c4\" namest=\"c2\"\u003e \u003cp\u003eLignin (%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colspan=\"4\" nameend=\"c9\" namest=\"c6\"\u003e \u003cp\u003eCarbohydrate (%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c10\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eAsh (%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"1\" nameend=\"c11\" namest=\"c11\"\u003e\u0026nbsp;\u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eKL\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eASL\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eAll\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eGLU\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eXyl\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003eAra\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c9\"\u003e \u003cp\u003eAll\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"1\" nameend=\"c11\" namest=\"c11\"\u003e\u0026nbsp;\u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBlack Liquor\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e67.2\u003c/p\u003e \u003cp\u003e\u0026plusmn;\u0026thinsp;0.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e3.1\u003c/p\u003e \u003cp\u003e\u0026plusmn;\u0026thinsp;0.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e70.3\u003c/p\u003e \u003cp\u003e\u0026plusmn;\u0026thinsp;0.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e4.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e6.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e1.7\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e12.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c11\" namest=\"c10\"\u003e \u003cp\u003e15.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNeutral Black Liquor\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e72.4\u003c/p\u003e \u003cp\u003e\u0026plusmn;\u0026thinsp;0.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e8.5\u003c/p\u003e \u003cp\u003e\u0026plusmn;\u0026thinsp;0.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e80.9\u003c/p\u003e \u003cp\u003e\u0026plusmn;\u0026thinsp;0.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e2.7\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e4.2\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e1.2\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e8.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c11\" namest=\"c10\"\u003e \u003cp\u003e8.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAcid Black Liquor\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e85.4\u003c/p\u003e \u003cp\u003e\u0026plusmn;\u0026thinsp;0.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e4.8\u003c/p\u003e \u003cp\u003e\u0026plusmn;\u0026thinsp;0.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e90.2\u003c/p\u003e \u003cp\u003e\u0026plusmn;\u0026thinsp;0.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e1.3\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e2.1\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e0.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c11\" namest=\"c10\"\u003e \u003cp\u003e3.2\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"11\"\u003e(PS: KL: Klason lignin, ASL: Acid Soluble Lignin, GLU: Glucan, Xyl: Xylan, Ara: Arabican)\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Structure and morphology\u003c/h2\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe presence of sp\u0026sup3;-hybridized carbon and edge defects is evident in all samples, as indicated by the pronounced D-band at 1350 cm⁻\u0026sup1; in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(a), signifying defective and disordered graphitic structures in BLAC(Liu et al., 2024). Notably, the D-band intensity of AcidBLAC@MBC@N and BLAC@MBC@N is significantly higher than that of other samples, demonstrating that nitrogen doping (@N) effectively introduces additional defect sites into the carbon matrix. The G-band observed at 1580 cm⁻\u0026sup1; reflects the graphitic ordering of sp\u0026sup2;-hybridized carbon, corresponding to in-plane vibrational modes(Zhang et al., \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). However, the G-band of AcidBLAC@MBC@N, NetBLAC@MBC@N, and BL@MBC@N exhibits broader peak profiles with overlap to the D-band, suggesting that nitrogen doping may reduce graphitic domain sizes, thereby forming more lamellar stacked structures. The intensity ratio of the D-band to G-band (I\u003csub\u003eD\u003c/sub\u003e/I\u003csub\u003eG\u003c/sub\u003e) provides a semi-quantitative assessment of defect density in carbon materials. As pH decreases, the I\u003csub\u003eD\u003c/sub\u003e/I\u003csub\u003eG\u003c/sub\u003e ratio progressively increases from 0.52\u0026ndash;0.56 for BLAC to 0.78\u0026ndash;0.84 for NetBLAC. When pH is reduced to 1, the I\u003csub\u003eD\u003c/sub\u003e/I\u003csub\u003eG\u003c/sub\u003e ratio reaches its maximum value of 0.96\u0026ndash;1.1, further confirming that pH fractionation effectively optimizes the graphitic structure of BLAC.\u003c/p\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(b), AcidBLAC@MBC@N exhibits a hierarchical pore architecture dominated by mesopores (78%), complemented by micropores (19%) and macropores (3%). This lamellar porous structure provides an enlarged electrode/electrolyte interface, facilitating charge transfer reactions. The enhanced interfacial kinetics accelerate ion diffusion and electrolyte permeation, thereby improving charge/discharge rates and significantly boosting the electrochemical performance of supercapacitors.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eBy comparing the microstructural evolution of materials before and after pH-regulated treatment (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e), the critical role of black liquor component regulation in constructing hierarchical porous structures was elucidated. Untreated BLAC samples (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea\u0026ndash;c) exhibited a densely agglomerated morphology, with surfaces covered by amorphous carbon layers and negligible pore development. This structural deficiency originates from the pyrolysis behavior of high-content ash (12.7%) and carbohydrates in the black liquor\u0026mdash;during carbonization, ash-derived metal oxides catalyzed tar byproduct formation, while low-molecular-weight organics from carbohydrate pyrolysis blocked pore channels, ultimately leading to pore collapse and surface passivation.\u003c/p\u003e \u003cp\u003eIn contrast, AcidBLAC@MBC@N subjected to acid washing and pH fractionation (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed\u0026ndash;f) displayed a significantly optimized hierarchical pore structure. Acid treatment effectively removed ash (reduced to 2.1%) and carbohydrates (reduced to 4%). As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee, the material formed an interconnected network of open mesopores (15\u0026ndash;40 nm) and macropores (50\u0026ndash;200 nm). During carbonization, MBC decomposed at 300\u0026ndash;500\u0026deg;C to generate nanoscale MgO particles, which acted as rigid templates to guide carbon framework growth while creating abundant edge-active sites(Madan et al., \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2021\u003c/span\u003e, Ding et al., 2025). These active sites drove the self-assembly of MBC via π-π stacking and hydrogen bonding interactions, forming a lamellar porous architecture.\u003c/p\u003e \u003cp\u003eThe dual-functional mechanism of MBC as an activation template: during pyrolysis, MBC not only chemically etches carbon layers to form hierarchical porous structures but also generates CO₂/H₂O gases that synergistically regulate pore architecture through in situ gas activation. However, the efficacy of the MBC template is highly dependent on the quality of its integration with the precursor. As evidenced in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(a, d), unoptimized precursors (BL,NetBL) yield a microstructure dominated by amorphous carbon domains (red circles), characterized by diffuse diffraction patterns and the absence of lattice fringes, confirming a disordered carbon matrix. In contrast, Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(g) reveals that acid treatment (AcidBL) induces structural ordering, manifesting as long-range lamellar stacking (green circles). This demonstrates the selective removal of carbonization inhibitors from the black liquor, thereby providing a more ideal substrate for the MBC templating effect. Critically, Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(b-c, e-f) exhibit compromised structural integrity in the BLAC@MBC and NetBLAC@MBC composites, featuring pore collapse (yellow circles) attributed to poor integration of the MBC template. Conversely, the composite formed by combining the optimized precursor (AcidBL) with MBC and nitrogen doping, AcidBLAC@MBC@N (Figs.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eh-i), displays a nitrogen-doped hierarchical architecture (blue circles). This optimized morphology provides abundant edge-hosted active sites and low-tortuosity ion transport pathways, directly contributing to its superior specific capacitance of 305 F g⁻\u0026sup1; at 0.5 A g⁻\u0026sup1;, significantly higher than that of the undoped control (182 F g⁻\u0026sup1;).\u003c/p\u003e \u003cp\u003eAcidBLAC@MBC@N exhibits optimal structural characteristics among all specimens, achieving the highest specific surface area (SBET\u0026thinsp;=\u0026thinsp;584.5 m\u0026sup2; g⁻\u0026sup1;) and hierarchical factor (HF\u0026thinsp;=\u0026thinsp;0.85) as quantified in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. This mesopore-dominated architecture (78.4% mesopore ratio) significantly shortens ion diffusion pathways and reduces transport resistance, thereby enabling exceptional rate capability with 98% capacitance retention at 5 A g⁻\u0026sup1;. Crucially, nitrogen doping induces defect-mediated inhibition of carbon layer stacking during pyrolysis, substantially enhancing SBET across functionalized samples: AcidBLAC@MBC@N displays a 117% increase relative to AcidBLAC@MBC (268.8 to 584.5 m\u0026sup2; g⁻\u0026sup1;), NetBLAC@MBC@N shows 113% enhancement versus NetBLAC@MBC (215.5 to 458.6 m\u0026sup2; g⁻\u0026sup1;), and BLAC@MBC@N attains 74% improvement over BLAC@MBC (186.3 to 324.8 m\u0026sup2; g⁻\u0026sup1;). Synergistically, acid pretreatment amplifies N-doping effects, collectively establishing a hierarchical conductive network that optimizes charge storage kinetics and cycling stability.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003ePore structure parameters of BLAC\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"10\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c10\" colnum=\"10\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSample\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSSA\u003c/p\u003e \u003cp\u003e(m\u003csup\u003e2\u003c/sup\u003e g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003ePrecentage\u003c/p\u003e \u003cp\u003e(\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\%)\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003ePore volume\u003c/p\u003e \u003cp\u003e(cm\u003csup\u003e3\u003c/sup\u003e g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c9\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c10\"\u003e\u0026nbsp;\u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eS\u003csub\u003eBET\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eS\u003csub\u003emeso\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eS\u003csub\u003emicro\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eS\u003csub\u003emeso\u003c/sub\u003e/ S\u003csub\u003eBET\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003eS\u003csub\u003emicro\u003c/sub\u003e/ S\u003csub\u003eBET\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003eV\u003csub\u003eTotal\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003eV\u003csub\u003emeso\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003eV\u003csub\u003emicro\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAcidBLAC@MBC@N\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e584.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e455.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e128.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e78\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e22\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e0.67\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e0.57\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e0.03\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNetBLAC@MBC@N\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e458.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e307.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e151.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e67\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e33\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e0.42\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e0.38\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e0.04\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBLAC@MBC@N\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e324.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e194.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e129.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e40\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e0.27\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e0.24\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e0.03\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAcidBLAC@MBC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e268.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e142.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e126.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e53\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e47\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e0.21\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e0.18\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e0.03\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNetBLAC@MBC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e215.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e94.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e120.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e44\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e56\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e0.15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e0.12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e0.03\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBLAC@MBC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e186.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e70.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e115.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e38\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e62\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e0.11\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e0.09\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e0.02\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eThe material surface featured numerous nanoscale protrusions (height\u0026thinsp;\u0026asymp;\u0026thinsp;5 nm) and cracks (width\u0026thinsp;\u0026asymp;\u0026thinsp;2 nm). These structural characteristics not only enhanced capacitive performance by exposing additional active sites (41% increase in specific capacitance) but also improved electrode structural stability through mechanical interlocking effects (capacitance retention\u0026thinsp;\u0026gt;\u0026thinsp;96% after 10,000 cycles).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(a\u0026ndash;h) depict the XPS spectra processed using CASA software and deconvoluted via Advantage software. The survey spectra of AcidBLAC@MBC@N and BLAC@MBC (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea) clearly show characteristic peaks for C 1s (284.8 eV), O 1s (532.1 eV), N 1s (399.8 eV), and Mg 2p (50.3 eV), confirming the coexistence of a carbon matrix, oxygen/nitrogen-containing functional groups, and magnesium in the material(Guan et al., \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2024\u003c/span\u003e, Bhagwan and Han, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Detailed analysis of the C 1s fine spectra (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ef) highlights structural differences: BLAC@MBC exhibits two components\u0026mdash;graphitic carbon (C\u0026thinsp;=\u0026thinsp;C, 284.8 eV, 68.7%) and carbonyl carbon (C\u0026thinsp;=\u0026thinsp;O, 289.6 eV, 31.3%)(Ding et al., 2020)\u0026mdash;while AcidBLAC@MBC@N displays four components, including additional C\u0026ndash;O (286.5 eV, 18.2%) and C\u0026ndash;N (287.5 eV, 9.1%) bonds, alongside an increased C\u0026thinsp;=\u0026thinsp;C content (72.4%). This indicates that acid treatment enhances graphitization by removing amorphous carbon phases and introduces nitrogen doping through C\u0026ndash;N bond formation. The evolution of O 1s spectra further corroborates surface modifications (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eh). BLAC@MBC shows a single C\u0026ndash;O peak (532.8 eV), whereas AcidBLAC@MBC@N splits into C\u0026thinsp;=\u0026thinsp;O (531.7 eV, 56.9%) and C\u0026ndash;O (533.2 eV, 43.1%), with a 27% increase in oxidized C\u0026thinsp;=\u0026thinsp;O groups, improving electrode/electrolyte interfacial wettability(Ma et al., 2023). Nitrogen speciation analysis (N 1s, Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ee) identifies pyridinic-N (N-6, 398.6 eV, 61.3%), which enhances charge transfer via lone-pair electron donation (reducing Rct by 38%), and protonated nitrogen (N⁺,400.1 eV, 38.7%), corresponds to protonated nitrogen, indicating an electron-rich surface environment(Li et al., \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Finally, the Mg 2p spectra (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eg) exhibit a single peak at 49.8 eV, corresponding to Mg\u0026ndash;O bonding, confirming the self-assembly of lignin and MBC via magnesium ion bridging (Mondal et al., 2024). These findings collectively demonstrate how pH-regulated treatments and nitrogen doping synergistically optimize the material\u0026rsquo;s electronic structure and interfacial properties, underpinning its enhanced electrochemical performance.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Electrochemical performance of hierarchical porous BLAC-based electrode\u003c/h2\u003e \u003cp\u003eIn this study, a three-electrode system with 6 M KOH electrolyte was employed to systematically evaluate the electrochemical performance of the synthesized materials. CV, GCD, and EIS were utilized to comprehensively investigate specific capacitance, charge-discharge kinetics, and impedance characteristics.A systematic investigation of six electrode materials via CV across a scan rate range of 5\u0026ndash;100 mV s⁻\u0026sup1; (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea\u0026ndash;f) revealed the synergistic regulation of nitrogen doping and acid treatment on charge storage mechanisms. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ed and \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ef, BLAC@MBC@N and AcidBLAC@MBC@N electrodes exhibited quasi-rectangular CV curves, indicative of an EDLC-dominated charge storage mechanism \u003csup\u003e[51]\u003c/sup\u003e. Notably, AcidBLAC@MBC@N demonstrated a significantly larger CV integral area than other samples, with 81.2% capacity retention at a high scan rate of 100 mV s⁻\u0026sup1;, attributed to its optimized hierarchical pore structure and enhanced ion diffusion kinetics. Detailed analysis (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ed/f) identified distinct redox peaks in the \u0026minus;\u0026thinsp;0.2 to 0.3 V potential range for nitrogen-doped materials, associated with Faradaic pseudocapacitance contributions from surface oxygen- and nitrogen-containing functional groups(Zhang et al., \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Coparative experiments demonstrated that untreated BLAC@MBC and NetBLAC@MBC samples, with underdeveloped porosity, exhibited distorted CV curves and reduced integral areas (34\u0026ndash;41% lower than AcidBLAC@MBC@N). This discrepancy underscores the critical role of acid treatment in removing impurity phases, expanding pores, and constructing rapid ion transport channels for the electrolyte.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo thoroughly investigate the electrochemical performance of the synthesized electrodes, their GCD behavior was systematically evaluated across a current density range of 0.5\u0026ndash;5.0 A g⁻\u0026sup1;. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e (a)-(c) present the GCD curves of BLAC-, NetBLAC-, and AcidBLAC-based electrodes at varying current densities, reflecting the current density-dependent response characteristics of the electrode materials. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e (d)-(f) display comparative GCD profiles of BLAC@MBC@N, NetBLAC@MBC@N, and AcidBLAC@MBC@N at 0.5 A g⁻\u0026sup1;. Consistent with CV results, AcidBLAC@MBC@N exhibited the highest charge-discharge performance, achieving a specific capacitance of 305\u0026thinsp;\u0026plusmn;\u0026thinsp;5 F g⁻\u0026sup1;, significantly surpassing BLAC@MBC@N (145\u0026thinsp;\u0026plusmn;\u0026thinsp;4 F g⁻\u0026sup1;) and NetBLAC@MBC@N (235\u0026thinsp;\u0026plusmn;\u0026thinsp;5 F g⁻\u0026sup1;). The quasi-triangular shape of its GCD curve indicates electric double-layer capacitance as the dominant contributor to the total capacitance(Li et al., \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e (e) illustrates the CV curves of these three samples at a scan rate of 100 mV s⁻\u0026sup1;, where AcidBLAC@MBC@N demonstrated the largest integrated area. This GCD analysis corroborates the findings derived from CV results, providing robust evidence for the consistent stability of the synthesized samples under a wide range of applied current densities.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe electrochemical performance of electrode materials is comprehensively influenced by their microstructural characteristics, such as mesoscale porosity and pore volume. Micropores facilitate effective interactions with electrolyte ions, while mesopores act as transport channels during charge storage processes. GCD tests revealed that the rate capability and structural stability of nitrogen-doped carbon-based electrodes exhibit significant correlations. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e(a)\u0026ndash;(b), when the current density increased from 0.5 to 5 A g⁻\u0026sup1;, AcidBLAC@MBC@N demonstrated the highest capacitance retention (73.8%), significantly outperforming NetBLAC@MBC@N (72.7%) and BLAC@MBC@N (71.2%). Comparative experiments further indicated that non-doped control materials (AcidBL@MBC, NetBL@MBC, and BL@MBC) exhibited more pronounced capacity decay (reduced to 70.6%, 67.2%, and 65.4%, respectively). In contrast, pH-fractionated samples showed systematic improvements in capacitance retention, confirming the synergistic optimization of nitrogen doping and hierarchical processing. Notably, AcidBLAC@MBC@N retained 98.2\u0026thinsp;\u0026plusmn;\u0026thinsp;0.6% of its initial capacity after 10,000 charge-discharge cycles (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ec), with GCD curves maintaining high symmetry throughout long-term cycling, indicative of exceptional electrochemical stability. This outstanding cyclability is attributed to the following mechanisms: Enhanced Conductivity: Pyridinic/pyrrolic-N functional groups formed via nitrogen doping improve the electronic conductivity of the carbon matrix: (1) Surface Optimization: Acid treatment modifies surface chemistry to suppress side reactions. (2) Stress Mitigation: Hierarchical porous structures alleviate structural stress during ion intercalation/deintercalation. EIS, a critical tool for evaluating electrode interface properties, effectively reveals charge transfer kinetics in the high-frequency region. The Nyquist plots in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e(d) show typical series resistance (Rs) and charge transfer resistance (Rct) features for all electrodes. Equivalent circuit fitting (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ee) demonstrated that only AcidBLAC@MBC@N exhibited a complete semicircle in the high-frequency region, indicating minimal charge transfer impedance, as further validated in the magnified inset of Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e(f). Fitting results revealed that AcidBLAC@MBC@N achieved the lowest Rs value (1.02\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2 Ω), significantly lower than NetBLAC@MBC@N (1.43\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4 Ω). This disparity arises from precursor treatment effects on self-assembly: high acid-soluble lignin (ASL) content (8.5%) in NetBLAC@MBC@N precursors hindered effective binding between MBC and lignin, while acid treatment reduced ASL content, increased Klason lignin purity, and optimized the MBC-lignin assembly for enhanced activation. The ultra-low Rs value of AcidBLAC@MBC@N underscores its superior charge transfer kinetics at the carbon-electrolyte interface, confirming the efficacy of the proposed synergistic design strategy. Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e showing the comparative performance of lignin-based carbon for SCs applications(Khalid et al., \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2023\u003c/span\u003e, Cui et al., 2021, Li et al., \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2023b\u003c/span\u003e, Liu et al., \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2023\u003c/span\u003e, Wang et al., 2025b, Tran et al., 2017, Wang et al., \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2025a\u003c/span\u003e), and it can be seen that the electrochemical performance of supercapacitors prepared from nitrogen-doped porous carbon derived from BL lignin is in the middle to upper level.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eShowing the comparative performance of lignin-based carbon for SCs applications\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eprecursor\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eElectrolyte\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCurrent density\u003c/p\u003e \u003cp\u003e(A g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eCapacitance\u003c/p\u003e \u003cp\u003e(F g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eCycling sustainability (%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eRef.\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eKraft lignin\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e6 M KOH\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e88\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e92.8(5A g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e@75000)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e(Khalid et al., \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2023\u003c/span\u003e)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLignin\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e144\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e92.3(1A g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e@5000)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e(Cui et al., 2021)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLignin\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e6 M KOH\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e162\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e95.4(5A g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e@10000)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e(Tran et al., 2017)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eKraft lignin\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e6 M KOH\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e205\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e82(20A g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e@10000)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e(Wang et al., \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2025a\u003c/span\u003e)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eKraft lignin\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e6 M KOH\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e224\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e85(10A g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e@5000)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e(Liu et al., \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2023\u003c/span\u003e)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLignin\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e6 M KOH\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e236\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e95.5(10A g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e@10000)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e(Li et al., 2023)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eKraft lignin\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e6 M KOH\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e183\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e96.6(10 A g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e@10000)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e(Wang et al., 2025)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBL lignin\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e6 M KOH\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e305\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e92.8(5A g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e@10000)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eThis work\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"Conclusion","content":"\u003cp\u003eThis study proposes an eco-friendly template-activation synergistic strategy to achieve the efficient conversion of BL into high-performance nitrogen-doped porous carbon materials (AcidBLAC@MBC@N). The research elucidates the dual-functional mechanism of MBC as an activation template: during pyrolysis, MBC not only chemically etches carbon layers to form hierarchical porous structures but also generates CO₂/H₂O gases that synergistically regulate pore architecture through in situ gas activation. By implementing pH gradient fractionation pretreatment, the composition of BL precursors was optimized\u0026mdash;lignin content increased to 90%, while carbohydrates and ash were reduced to 4% and 3.2%, respectively\u0026mdash;significantly mitigating the adverse effects of impurities on carbon material structure. Coupled with nitrogen doping, abundant pyridinic-N/graphitic-N active sites and oxygen-containing functional groups were engineered into the carbon framework, endowing the material with exceptional electron transport capabilities. The synthesized AcidBLAC@MBC@N exhibits remarkable structural and performance advantages: Hierarchical Porous Architecture: Mesopore-dominated structure (78.4% mesopore ratio, average pore size 3.8 nm).Outstanding Electrochemical Performance: Specific capacitance of 305 F g⁻\u0026sup1; at 0.5 A g⁻\u0026sup1;, ultralow interfacial resistance (1.02 Ω), and exceptional cycling stability (\u0026gt;\u0026thinsp;95% capacity retention after 10,000 charge-discharge cycles).Through a three-tier cascading strategy\u0026mdash;\"waste valorization \u0026rarr; material functionalization \u0026rarr; performance optimization\"\u0026mdash;this work provides a novel approach for BL upcycling. Experimental data and mechanistic analyses demonstrate that this strategy overcomes the feedstock limitations of traditional activated carbon preparation methods, offering a new technical pathway for developing low-cost, sustainable supercapacitor electrode materials. Simultaneously, it presents an industrially viable solution for closed-loop management of paper industry solid waste, aligning with circular economy principles.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003eCredit authorship contribution statement\u003c/p\u003e\n\u003cp\u003eCheng Zhang: Writing \u0026ndash; review \u0026amp; editing, Writing \u0026ndash; original draft. Yuchen Han: Investigation. Jie Zheng: Data curation. Wen-Juan Wu: Writing \u0026ndash; review \u0026amp; editing, Supervision. Yong-Can Jin: Project administration, Software.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;Acknowledgements\u003c/p\u003e\n\u003cp\u003eThe authors are grateful for the support of the National Natural Science Foundation of China (32271797, 32201500).\u003c/p\u003e\n\u003cp\u003eData availability\u003c/p\u003e\n\u003cp\u003eNo data was used for the research described in the article.\u003c/p\u003e\n"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eA B X, B S H, B F Z et al (2014) Nitrogen-doped mesoporous carbon derived from biopolymer as electrode material for supercapacitors. 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J Energy Storage [J] 45. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.est.2021.103457\u003c/span\u003e\u003cspan address=\"10.1016/j.est.2021.103457\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"pH-Modulated, Porous carbon, Lignin, Electrochemical performance, Supercapacitor","lastPublishedDoi":"10.21203/rs.3.rs-8729298/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8729298/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eUnder the global acceleration of carbon neutrality initiatives, the green valorization of industrial waste has emerged as a critical pathway for establishing sustainable manufacturing systems. This study addresses the challenges of inefficient component regulation and restricted pore development in activated carbon derived from pulping black liquor (BL). We propose an innovative activation strategy utilizing magnesium basic carbonate (MBC) as a dual-functional template-activator. By integrating evaporation-induced self-assembly (EISA) with pH gradient fractionation technology, BL is converted into high-performance nitrogen-doped porous activated carbon. A hierarchical regulation system was established to synthesize three lignin-based activated carbons: alkaline, neutral, and acidic. Characterization reveals that AcidBLAC@MBC@N exhibits a unique lamellar porous architecture with a synergistic micro-mesoporous network (mesopore 78%). The optimized material demonstrates exceptional electrochemical performance: a specific capacitance of 305 F g⁻\u0026sup1; at 0.5 A g⁻\u0026sup1; and 98% capacitance retention after 10,000 cycles. This work circumvents energy-intensive pre-carbonization (\u0026gt;\u0026thinsp;800\u0026deg;C) and complex component separation steps in conventional processes. This strategy establishes an eco-efficient paradigm for black liquor valorization, offering a scalable solution for transforming industrial waste into advanced energy storage materials. The design principles may be extended to other biomass-derived systems, aligning with circular economy and carbon neutrality goals.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e","manuscriptTitle":"pH-Modulated Upcycling of Black Liquor into N/O Co-Doped Hierarchical Porous Carbon via Green Templating-Activation Synergy for High-Performance Supercapacitors","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-02-26 11:29:20","doi":"10.21203/rs.3.rs-8729298/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"5d05036c-97a6-4fcb-b21b-b35ed152e08e","owner":[],"postedDate":"February 26th, 2026","published":true,"recentEditorialEvents":[{"type":"decision","content":"Rejected","date":"2026-05-05T14:32:27+00:00","index":"","fulltext":""}],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2026-05-05T14:40:48+00:00","versionOfRecord":[],"versionCreatedAt":"2026-02-26 11:29:20","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8729298","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8729298","identity":"rs-8729298","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}