Biomass-Derived Carbon Electrodes with Optimized Defects and Porosity via Regulated Carbonization Temperature for Supercapacitors

Biomass-Derived Carbon Electrodes with Optimized Defects and Porosity via Regulated Carbonization Temperature for 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 Biomass-Derived Carbon Electrodes with Optimized Defects and Porosity via Regulated Carbonization Temperature for Supercapacitors Tserenlkham Byambadorj, Jiawei Zhang, Xuzhen Lu, Yuehui Wang, and 4 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8826492/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 Biomass-derived carbons hold substantial promise for sustainable electrochemical energy storage due to their low cost, wide availability, and intrinsic heteroatom- and mineral-rich nature. However, the fundamental influence of carbonization temperature on the structural evolution of non-activated biomass-derived carbons remains insufficiently understood. In this work, corn straw-derived carbon (CS) is produced without any chemical additives to isolate the intrinsic effects of carbonization temperature on its physicochemical properties. Systematic temperature variation from 600 to 1000°C reveals pronounced changes in micro-morphology, pore development, defect density, and the ordering of the carbon matrix, all strongly governed by the inherent mineral content of corn straw. Electrochemical evaluation in alkaline electrolyte demonstrates that CS-800 delivers the highest specific capacitance of 53.8 F g⁻ 1 at 1 A g⁻ 1 in a three-electrode configuration and maintains favorable rate capability in a symmetric supercapacitor device. The symmetric coin cell supercapacitor device assembled with CS-800 as the electrodes achieved an energy density of 3.64/5.8 Wh kg⁻ 1 and power density 5200/750 W kg⁻ 1 , along with remarkable cycling stability over 30000 cycles with negligible capacitance loss. Overall, this study provides mechanistic insight into temperature-driven structural evolution in non-activated biomass carbons, offering a baseline understanding that can guide rational design and future activation strategies for high-performance, sustainable carbon electrodes. biomass-derived carbon carbonization temperature defects supercapacitor Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 1. Introduction Supercapacitors (SCs) are widely recognized as promising energy-storage devices owing to their high-power density, long cycling life, low maintenance requirements, and environmental compatibility. Achieving high‐performance SCs depends critically on the development of electrode materials with tailored nanostructures, such as hierarchical porosity, high electrical conductivity, abundant electrochemically active sites, and robust structural stability. In this context, carbon-based materials—including graphene, carbon nanotubes, carbon nanofibers, mesoporous carbons, and activated carbons—have been extensively explored and remain central to SC research [ 1 , 2 ]. Biomass has emerged as one of the most attractive precursors for carbon materials due to its abundance, renewability, and inherent heteroatom and mineral content [ 3 , 4 ]. Numerous biomass-derived carbons have demonstrated strong potential in SC applications, however, most studies rely on chemical activation or additives, which can obscure the intrinsic influence of carbonization conditions on structural and electrochemical properties. The energy storage performance of biomass-derived carbon is governed not only by specific surface area and porosity but also by defect density, surface functional groups, graphitization degree, and microstructural ordering [ 5 – 9 ]. Strategies to enhance performance include heteroatom doping (N, S, O), structural tuning (crystalline vs. amorphous carbon), pore regulation, and heat-treatment optimization (temperature, atmosphere, duration) [ 10 – 12 ]. Among these, the carbonization temperature of precursors is particularly critical in the produce of biomass-derived carbon, as it can substantially influence the final characteristics of the material. Notably, Qiu et al. suggested that modest temperature can result in more effective activation, while excessively high temperatures may lead to pore collapse, obstructing existing pores and diminishing both specific surface area (SSA) and pore volume. Simultaneously, the carbon-yield progressively diminished as a result of the elevated activation temperature and the synergistic activation influence of dopants [ 13 ]. Xue et al. observed that the diffraction peaks of the samples migrated to higher angles at increased temperature, signifying the contraction of the framework during pyrolysis process [ 14 ]. More recently, Diego et al. investigated the influence of pre-carbonization temperature at 350 to 500°C for the supercapacitor applications, suggesting that pre-carbonization temperature significantly regulate the structural and electrochemical properties [ 15 ]. Ma et al. emphasized that choosing the suitable activation temperature is favorable for production of activated carbon materials with SSA [ 16 ]. However, the employment of direct pyrolysis and two-step pre-carbonization followed by chemical activation, hydrothermal carbonization, or ball milling pre-treatment, along with activating/templating agents (KOH, ZnCl 2 , H 3 PO 4 , etc.), could have masked the direct effect of temperature. Moreover, previous studies often lack a systematic exploration of additive-free biomass carbonization, leaving the intrinsic effect of temperature insufficiently understood. Herein, corn straw-derived carbons (CS) were prepared at 600, 800, and 1000°C via a straightforward, additive-free carbonization process. The absence of chemical activation allows for the isolation of temperature effects on defect density, pore structure, surface functional groups, and crystallinity. CS-800 achieves a specific capacitance of 53.8 F g⁻ 1 at 1 A g⁻ 1 and the capacitance retention remain above 90% over 30,000 cycles in a 1 M KOH electrolyte. The superior electrochemical performances can be attributed to the suitable defects/heteratoms and pore structure, resulting in the balance between kinetics behaviour and energy storage. These results suggest that carefully adjusting the carbonization temperature is conducive to biomass-derived carbons with enhanced electrochemical performance, offering theoretical guidance for developing sustainable carbon electrodes. 2. Materials and methods 2.1. Materials Corn straw was sourced from a farm in Harbin, Heilongjiang Province, China. 1-Methyl-2pyrrolidinone, potassium hydroxide (KOH), polyvinylidene difluoride (PVDF) were purchased from Sinopharm Chemical Reagent Co. Battery-grade conductive carbon black (Super P) was purchased from Shenzhen Kejing Zhida Technology Co. Battery-grade nickel foam was purchased from Kunshan Jiayisheng Electronics Co. High purity argon (Ar ≥ 99%) was purchased from Harbin Grand Gas Co. 2.2. Synthesis of carbon materials The raw corn straw was thoroughly washed with deionized (DI) water to eliminate adhered soil and surface contaminants, then dried at 100°C for 12 h. The dried material was crushed and sieved through a 20-mesh to ensure uniform particle size. The sieved biomass was placed in an alumina boat and maintained at 2 h at 600, 800, or 1000°C with a rate of 5°C min⁻ 1 under argon atmosphere. Final, the samples were naturally cooled to room temperature to obtain carbon without de-ashing. De-ashing treatment To remove residual inorganic components that may influence the structural and electrochemical behavior of biomass carbons, the carbonized products were subjected to a de-ashing treatment. The obtained carbon were washed with 10 wt% HCl under stirring at room temperature, followed by repeated rinsing with deionized water until a neutral pH. The washed samples were then dried at 80°C. The final products were denoted as CS-600, CS-800, and CS-1000, according to their respective carbonization temperatures. Bulk mass (gravimetric) yield calculation method Raw corn straw was dried at 100°C for 12 h to constant weight, with five independent batches were prepared at each temperature to ensure statistical reliability. For each batch, a known mass of dried precursor ( m pre ) was carbonized under identical conditions. After cooling under argon, the crude carbon was weighed to obtain m char,crude . The char was then subjected to acid washing, filtration, rinsing, and drying, after which the ash-free char mass ( m char,ashed ) was recorded. This procedure was performed for samples carbonized at 600, 800, 1000°C. The mass yield was calculated according to Eqs. 1–3. Crude char yield (%)= \(\frac{{{m_{char,crude}}}}{{{m_{pre}}}} \times 100\) (1) De-ashed char yield (%)= \(\frac{{{m_{char.ashed}}}}{{{m_{pre}}}} \times 100\) (2) Ash removal (%)= \(\frac{{{m_{char,crude}} - {m_{char,ashed}}}}{{{m_{char,crude}}}} \times 100\) (3) 2.3. Structure and morphology characterization The crystal structure of the samples was acquired by X-Ray Diffraction (XRD; Malvern Panalytical Empyrean) with Cu Kα radiation (λ = 1.5406 Å) in the 2θ range of 5–90°. Raman spectroscopy (WITec Alpha 300 R) with a laser wavelength of 532 nm was used to explore the ratio of I D /I G . Surface functional groups were determined using an FT-IR spectrometer (PerkinElmer) in the transmission mode covering the range of 400–4000 cm⁻ 1 . To verify the thermal stability of the carbon materials, thermal gravimetric analysis (TGA) was performed in the temperature range RT–750°C using a NETZSCH instrument operating in an air atmosphere at a heating rate of 5°C min⁻ 1 . Brunauer-Emmett-Teller Specific Surface Area Analysis characterizes the specific surface area and pore structure (Micromeritics ASAP 2460 Version 3.01.02). The pore size distribution was determined using the Barrett-Joyner-Halenda (BJH) model, and the SSA was measured using the Brunauer-Emmett-Teller (BET) method. The morphologies of the samples were examined using a scanning electron microscope (SEM, Hitachi-SU8020) and a transmission electron microscope (TEM, JEM-F200 (URP)). The oxygen vacancy concentration of the samples was recorded by electron paramagnetic resonance (EPR) spectra using a Bruker EMXplus-6/1 instrument. The elemental composition and chemical state of materials were analyzed by X-ray photoemission spectroscopy (XPS, Thermo Scientific ESCALAB 250Xi). The binding energies were normalized considering the C 1s peak at 284.8 eV as a reference. 2.4. Electrochemical Measurements Electrochemical tests were analyzed by the CHI760E workstation. The working electrode was fabricated by mixing carbon materials, PVDF, and Super P in a mass ratio of 8:1:1. Then, NMP was used as the solvent, and the mixture was stirred at 500 rpm for 12 h to form a slurry. Finally, the slurry was uniformly coated on the nickel foam substrate and dried at 60°C for 12 h to obtain an electrode. The three-electrode system: Pt foil, Ag/AgCl, and as-prepared electrode served as the counter electrode, reference electrode, and working electrode, respectively. 1 M KOH is used as the electrolyte. Cyclic voltammetry (CV) tests were conducted at operating voltages of -1 V and 0 V at different scan rates. Galvanostatic charge/discharge (GCD) tests were conducted at different current densities. Electrochemical impedance spectroscopy (EIS) measurements were performed using a voltage amplitude of 5 mV of open-circuit potential between 10⁻ 2 and 10 5 Hz. In a three-electrode system, Eq. ( 4 ) can determine the gravimetric specific capacitance ( C , F g⁻ 1 ) of the as-prepared working electrode from the constant current charge/discharge curve. $${C_g}=\frac{{I\Delta t}}{{m\Delta V}}$$ 4 where I (A), Δ t (s), Δ V (V), and m (g) represent discharge current, discharge time, voltage window, and active mass of the working electrode, respectively. The symmetrical supercapacitor device was built with CS-800 as the positive and negative electrodes in a 1 M KOH aqueous electrolyte and separator to examine potential applications for energy storage. Furthermore, the assembled device was maintained at ambient temperature for over 12 h to ensure sufficient electrolyte infiltration. In the symmetric capacitor, Eq. ( 5 ) was used to calculate the capacitance ( C , F g⁻ 1 ) of the signal electrode. Eqs. ( 6 ) and ( 7 ) were used to calculate the energy density ( E , Wh kg⁻ 1 ) and power density ( P , W kg⁻ 1 ), respectively. $$C=\frac{{I\Delta t}}{{M\Delta V}}$$ 5 $$E=\frac{{C{{\left( {\Delta V} \right)}^2}}}{2}$$ 6 $$P=\frac{{3600E}}{{\Delta t}}$$ 7 where M (g) represents the total mass of the active materials loaded on two electrodes [ 17 ]. 3. Results and discussions 3.1. Morphology and pore structure analysis The preparation of corn straw–derived carbons via direct carbonization followed by de-ashing as illustrated in Fig. 1 a. The activated agent free strategy can isolate the intrinsic effects of carbonization temperature on the evolution of the carbon framework, surface chemistry, and electrochemical behavior. During carbonization, the gradual thermal decomposition of the biomass matrix leads to the release of volatile species such as CO 2 , CO, H 2 O, and light hydrocarbons, and is accompanied by the gradual aromatization and condensation of the carbon framework [ 18 – 20 ]. XRD patterns of the CS-X materials (Fig. 1 b) exhibited a broad diffraction feature centered at ~ 22–25°, characteristic of turbostratically disordered (002) carbon, along with a weak and broadened reflection near ~ 43° corresponding to the (100) plane [ 21 ]. Superimposed on this background, sharp reflections at ~ 20.7° and ~ 26.6° are observed and are assigned to crystalline SiO 2 (quartz) marked *, suggests the presence of impurities (SiO x or ash). The SiO 2 content in biomass-derived carbon is generally related to the high ash content of the precursor and the carbonization temperature, and is abundant in the XRD pattern of the unwashed sample [ 22 ]. However, the retention of weak SiO 2 peaks after acid washing is attributed to the acid-insoluble silica present in the biomass [ 23 ]. TEM and SAED analysis confirm that the carbon matrix remains predominantly amorphous/turbostratic, indicating that these sharp peaks do not originate from crystalline carbon. Two characteristic Raman bands centered at ~ 1358 cm⁻ 1 (D band) and ~ 1592 cm⁻ 1 (G band) were observed for all CS-X (Fig. 1 c). The Raman peaks exhibited negligible variations, indicating that the impact on the graphite crystallinity of the sample is minimal within the temperature range. FT-IR spectra (Fig. 1 d) further illustrate the progressive removal of oxygen-containing functional groups with increasing temperature. CS-600 exhibits distinct absorption features near 1096 cm⁻ 1 (C–O–C vibrations) and 1599 cm⁻ 1 (C = C or C = O stretching), which is attribute to incomplete decomposition of biomass [ 14 ]. These functional groups are largely eliminated at 800 and 1000°C, consistent with thermal cleavage of ether and carbonyl bonds. As shown in Fig. 1 e, thermogravimetric analysis (TGA) was carried out in air from 100°C to 750°C to evaluate the thermal stability and inorganic residue content of the CS-X samples. All samples exhibit two distinct stages of weight loss. The first stage is associated with the physical adsorption of moisture, and the second stage is associated with the oxidative decomposition of biomass-derived carbonaceous components [ 24 ]. The total mass losses of CS-600, CS-800, and CS-1000 are approximately 55%, 54%, and 45%, respectively. The decrease in char yield and the concurrent increase in TGA residual mass with increasing carbonization temperature indicate a transition from mass retention to structural stabilization, whereby fewer, however, more thermally robust carbon domains are formed [ 25 , 26 ]. It should be noted that char yield is referenced to the initial raw corn straw mass, whereas TGA residue is referenced to the pre-carbonized sample; therefore, the two parameters reflect different aspects of the carbonization process. Furthermore, bulk mass yield calculations were performed based on gravimetric measurements. As the carbonization temperature increased from 600 to 1000°C, the gravimetric yield of the carbon products gradually decreased, reflecting enhanced thermal decomposition and volatilization of the biomass precursor. The crude char yields for CS-600, CS-800, and CS-1000 were 29.2 ± 0.8%, 28.5 ± 0.6%, and 27.5 ± 0.4%, respectively. After acid washing, the corresponding de-ashed char yields decreased to 22.1 ± 0.7%, 21.4 ± 0.5%, and 19.4 ± 0.3%, indicating an ash removal efficiency of approximately 24.5–29.5 wt%. Nitrogen adsorption–desorption measurements were conducted to investigate the pore structures of the CS-X samples. As illustrated in Fig. 1 f, the non-closed isotherm of CSC-600 can be reasonably ascribed to the high density of oxygen-containing surface functional groups, which results in adsorption irreversibility. Conversely, the non-closure of the CS-1000 isotherm is more likely caused by pore structure collapse or contraction induced by excessive carbonization and framework shrinkage at elevated temperatures. The specific surface areas (SSA) of CS-600, CS-800, and CS-1000 are 8.09, 7.85, and 6.70 m 2 g⁻ 1 , respectively. The monotonic decrease in SSA with increasing carbonization temperature is attributed to structural densification and partial collapse of thermolabile pore frameworks during high-temperature treatment [ 27 ]. Figure 1 g illustrates the pore size distribution obtained from the adsorption branch utilizing a BJH-type model. All samples exhibit a predominant mesopore distribution between roughly 2–40 nm, with minimal contribution from micropores (< 2 nm). This verifies that the carbon's porosity is predominantly mesoporous, aligning with the isotherm forms depicted in Fig. 1 f. The SEM images of CS-X materials (Figs. 2 a–f) reveal that all samples retain the strip-like morphology of the corn stalk precursor, accompanied by abundant surface wrinkles and nanoscale particles, which are beneficial for electrolyte infiltration and enlargement of the solid-liquid interfacial during electrochemical processes. TEM analysis of CS-800 (Figs. 2 g and h) shows interconnected carbon nanoparticles forming a porous architecture without long-range ordered lattice fringes. The corresponding SAED patterns displays diffuse halo rings, indicative of low-crystallinity, turbostratically disordered carbon rather than well-crystallized graphite. This observation corroborates the XRD results, confirming that the broad (002) reflection arises from disordered carbon stacking, while the sharp diffraction peaks observed in the XRD patterns do not originate from crystalline carbon yet are instead associated with trace inorganic residues. Furthermore, EDS elemental mapping (Fig. 2 i and Fig. S1 ) confirms homogeneous distribution of C, N, O, P, and S throughout the carbon matrix, demonstrating successful in-situ self-doping during carbonization, consistent with previous reports [ 28 , 29 ]. As shown in Fig. 3 a, the EPR spectra of CS-600, CS-800, and CS-1000 reveal the presence of unpaired electrons in all samples, and the characteristic EPR signal (g = 2.003) was examined. These electrons are predominantly found on carbon atoms (free radicals) and are also linked to oxygen-containing chemical groups on the surface of the material [ 30 – 32 ]. Among the samples, CS-800 has a moderate EPR intensity, indicating the optimized concentration of paramagnetic defects, which is conducive to charge storage either through the faradaic or capacitive mechanism. The decrease in the EPR signal of CS-1000 may be attributed to the healing of defects and enhanced graphitization at high pyrolysis temperatures. Although the higher EPR intensity of CS-600 indicates the presence of paramagnetic defects and surface functionality, it is not associated with improved electrochemical performance. Surface defects, volatile oxygen-containing groups, and disordered carbon structures can significantly hinder electron transfer and structural integrity. Excessive defect sites may promote irreversible side reactions or increase charge transfer resistance, resulting in lower specific capacitance and worse rate performance compared to CS-800. Thus, the presence of moderate defects may improve pseudocapacitive properties, but the highly defective surface observed in CS-600 appears to suppress the electrochemical efficiency. XPS analysis (Figs. 3 b–f) confirms the presence of C, O, N, P, and S in all CS-X samples. Increasing carbonization temperature to 800°C increases the carbon content while reducing oxygen concentration. High-resolution C 1s spectra reveal dominant sp 2 -hybridized carbon (C–C/C = C, ~ 284.6 eV), along with oxygen-containing functional group attributed to C–O/C–N (~ 286.0 eV), consistent with recent carbon material studies using XPS. Correspondingly, the O 1s spectra exhibit peaks at ~ 531.0 eV and ~ 532.5 eV, which are assigned to carbonyl oxygen (C = O) and hydroxyl/ether oxygen (C–OH/C–O–C), respectively [ 33 – 35 ]. Likewise, the N 1s spectra reveal three peaks with the binding energies correlated to pyrrolic-N (N-5), quaternary-N (N-Q), and pyridinic-N (N-6). Pyridine nitrogen provides additional electron and charge mobility to enhance electrical conductivity and provides active sites for redox reactions to enhance Faradaic reactions. Pyrrole nitrogen improves interfacial and cyclic performance, while graphitic nitrogen provides additional sites for electron transfer [ 36 , 37 ]. The retention of nitrogen functional groups in the derived carbon without the need for additional activation or doping suggests that this approach allows for self-doping through the intrinsic composition of the raw material. This property makes corn straw-derived carbon a promising candidate for high-capacity energy storage applications. 3.2. Electrochemical performance in a three-electrode system The electrochemical performance of CS-600, CS-800, and CS-1000 was evaluated in 1 M KOH electrolyte using a three-electrode configuration. Figure 4 a presents the cyclic voltammetry (CV) curves recorded at a scan rate of 100 mV s⁻ 1 within the potential window of − 1.0 to 0 V. All samples exhibit quasi-rectangular CV profiles, indicating typical electric double-layer capacitance (EDLC) behavior with additional pseudocapacitive contributions. The latter arise from surface defects and oxygen-containing functional groups, such as hydroxyl, carboxyl, and phenolic species, which facilitate reversible Faradaic reactions at the electrode/electrolyte interface [ 38 ]. Among the three samples, CS-800 demonstrates the highest current response, suggesting superior charge storage capability. The galvanostatic charge–discharge (GCD) curves shown in Fig. 4 b further corroborate the CV results. CS-800 exhibits the longest discharge time under identical current conditions, reflecting its higher capacitance. Based on the GCD analysis, the specific capacitances of CS-600, CS-800, and CS-1000 are 21.2, 53.8, and 36.7 F g⁻ 1 at 1 A g ⁻ 1 , respectively. The significantly enhanced capacitance of CS-800 is attributed to the optimized balance among defect density, pore structure, and graphitization degree. Although CS-600 possesses a slightly larger specific surface area, its lower carbonization temperature results in a higher proportion of unstable functional groups and insufficiently developed conductive pathways. In contrast, CS-1000 exhibits improved conductivity but reduced surface area due to structural densification at elevated temperatures. CS-800 offers the most favorable combination of moderate graphitization, abundant defects, and mesoporous architecture, thereby enhancing ion accessibility and charge transport. These results confirm that specific capacitance is not determined solely by surface area; rather, it is governed by the coupled effects of pore size distribution, defect concentration, and structural ordering, all of which are modulated by the carbonization temperature. Figure 4 c shows the CV curves of CS-800 recorded at scan rates from 20 to 100 mV s⁻ 1 . All curves retain a quasi-rectangular shape without noticeable distortion, indicating that CS-800 maintains rapid charge–discharge kinetics and robust capacitive behavior even at high scan rates. Compared with CS-600 (Fig. S2a) and CS-1000 (Fig. S3a), CS-800 exhibits a significantly higher current response across all scan rates, highlighting its superior electrochemical activity. The slight deviation from an ideal rectangular profile is attributed to pseudocapacitive contributions superimposed on the dominant EDLC mechanism. The GCD curves (Fig. 4 d) display symmetric triangular shapes without discernible potential plateaus, even when the current density increases sevenfold from 1 to 7 A g⁻ 1 . This confirms the fast and reversible charge storage process of CS-800. As shown in Fig. 4 e, the CS-800 electrode delivers a specific capacitance of 31.5 F g⁻ 1 at 7 A g⁻ 1 , corresponding to a capacitance retention of 58.55% relative to its value at 1 A g⁻ 1 . This performance is substantially higher than those of CS-600 (7.7 F g⁻ 1 , Fig. S2b) and CS-1000 (18.9 F g⁻ 1 , Fig. S3b). The decline in capacitance at higher current densities observed for all CS-X samples is attributed to the limited diffusion of electrolyte ions into the internal pore structure under fast charge–discharge conditions. Figure 4 f presents the capacitive and diffusive contributions extracted from the CV curve at 20 mV s⁻ 1 , with additional deconvoluted profiles at 40 and 100 mV s⁻ 1 shown in Fig. S4. Figure 4 g summarizes the specific capacitance at current densities from 1 to 7 A g⁻ 1 , partitioned into surface-controlled (capacitive) and diffusion-controlled components. The diffusion-limited contribution decreases consistently with increasing current density, whereas the capacitive contribution increases from 1 to 3 A g⁻ 1 and then declines at higher current densities (5–7 A g⁻ 1 ). These results indicate that at elevated scan rates, charge storage in CS-800 is dominated by rapid surface-controlled (capacitive or pseudocapacitive) kinetics, whereas slower diffusion-controlled faradaic processes become progressively less accessible. Figures 4 h and 4 j show the Nyquist and Bode plots of CS-X electrodes fitted with the corresponding equivalent circuit model. The series resistances (R s ≈ 137, 123, and 116 mΩ for CS-600, CS-800, and CS-1000, respectively) indicate low intrinsic resistance and minimal IR drop during operation. Additionally, the electrodes exhibit relatively small charge-transfer resistance (R ct ), signifying efficient interfacial charge transfer and favorable redox kinetics. The corresponding 2D Bode analysis (Fig. 4 i) provides further insight into the rate-limiting processes across the measured frequency spectrum. 3.3. Energy storage performances of the symmetric supercapacitor The electrochemical performance of CS-800 was further evaluated in a symmetric supercapacitor (CS-800//CS-800 SSC), using 1 M KOH as the electrolyte and glass fiber as the separator (Fig. 5 a). The CV curves recorded at various scan rates (Fig. 5 b) exhibit characteristic quasi-rectangular shapes without significant distortion, indicating efficient ion transport and capacitive behavior. Correspondingly, the GCD curves at multiple current densities (Fig. 5 c) present nearly symmetric triangular profiles, confirming the excellent rate capability and reversible charge storage of the device. As shown in Fig. 5 d, the CS-800//CS-800 SSC delivers specific capacitances of 18.6, 16.3, 14.8, 13.0, and 11.7 F g − 1 at current densities of 1, 2, 3, 5, and 7 A g⁻ 1 , respectively. The gradual decline in capacitance at higher current densities reflects the expected limitations in ion diffusion within the electrode structure during rapid charge–discharge processes. Overall, these results demonstrate that CS-800 enables stable capacitive behavior and a favorable rate response in a practical device configuration. The CS-800//CS-800 symmetric supercapacitor (SSC) exhibits exceptional cycling stability, maintaining capacitance retention of nearly 93,75% after 30,000 cycles at 2 A g⁻ 1 (Fig. 5 e), indicative of highly reversible charge storage and structural robustness. The inset shows the galvanostatic charge–discharge curves of the first five and last five cycles, highlighting the maintained charge–discharge symmetry and reversibility. The Ragone plot (Fig. 5 f) shows that the SSC achieves a maximum energy density of 5.8 Wh kg⁻ 1 at a power density of 750 W kg⁻ 1 and sustains a maximum power density of 5,200 W kg⁻ 1 at an energy density of 3.64 Wh kg⁻ 1 . The progressive reduction in energy density alongside rising power density exemplifies the conventional power–energy trade-off seen in electrochemical supercapacitors, wherein elevated current densities diminish discharge durations and amplify resistive losses. Nonetheless, the gadget exhibited a comparatively high energy density even with increased power density. These values are equivalent to or above those of other biomass-derived activated carbons recently reported in the literature [ 39 – 46 ]. To evaluate practical applications, two identical CS-800//CS-800 SSC cells were connected in parallel and series topologies, respectively (Fig. 5 g). The CV curves (Fig. 5 h) show that the voltage range of the series configuration is almost twice that of a single cell, while the parallel configuration increases the effective capacitance, consistent with the theoretical principles of capacitors. Figure 5 i compares the specific capacitance of the two setups. While the parallel assembly provides higher capacitance at a given operating voltage, the series connection is advantageous for applications requiring an expanded voltage window, highlighting the intrinsic trade-off between capacitance and operating voltage in device integration. 4. Conclusion In summary, porous carbons were successfully derived from corn straw at different carbonization temperatures, resulting in distinct defect densities, pore structures, specific surface areas, and degrees of graphitization. These structural variations collectively govern the electrochemical performance, demonstrating that energy storage behavior is not determined solely by surface area or porosity but also by defects, functional groups, and other intrinsic structural features. Among the samples, CS-800 exhibits the highest specific capacitance of 53.8 F g⁻ 1 at 1 A g⁻ 1 , reflecting an optimal balance between defect density, pore accessibility, and partial graphitization. The symmetric CS-800//CS-800 supercapacitor delivers robust cycling stability, high rate capability, and a maximum energy density of 5.8 Wh kg⁻ 1 at a power density of 750 W kg⁻ 1 in 1 M KOH, highlighting its practical applicability. This study provides mechanistic insight into the influence of carbonization temperature on the structural evolution and electrochemical properties of additive-free biomass-derived carbons, offering guidance for the rational design of sustainable carbon-based electrodes for high-performance supercapacitors. Declarations Conflict of Interest The authors declare no conflict of interest. Author Contribution Tserenlkham Byambadorj: Formal analysis; Investigation; Writing-original draft Jiawei Zhang: Conceptualization; Funding acquisition; Writing-review&editingXuzhen Lu: Data Curation; Writing-review&editingYuehui Wang: Validation; Writing-review&editingFan Wang: Methodology; Writing-review&editingQian Liu: Formal analysis; Writing-review&editingYu Li: Visualization; Writing-review&editingMinghua Chen: Funding acquisition; Writing-review&editing Acknowledgments This work is supported by Heilongjiang Provincial Natural Science Foundation of China (2024ZX02C23); Education Department of Heilongjiang Province (LJYXL2023-092). Data Availability The data that support the findings of this study are available from the corresponding author upon reasonable request. References Wang Y, Zhang M, Shen X et al (2021) Biomass-derived carbon materials: controllable preparation and versatile applications. 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Biom Bioener 193:107574. https://doi.org/10.1016/j.biombioe.2024.107574 Ma T, Xu S, Zhu M (2024) Porous carbon from verbena straw with self-doped O/N and its high-performance aqueous and flexible all-solid-state supercapacitors. J Pow Sour 597:234147. https://doi.org/10.1016/j.jpowsour.2024.234147 Duan G, Xiao J, Tian Z et al (2024) Nano-CaCO3 templated porous carbon enable high-rate and ultralong cycle performance supercapacitor. J Ener Stor 78:109934. https://doi.org/10.1016/j.est.2023.109934 Yang H, Yan R, Chen H et al (2007) Characteristics of hemicellulose, cellulose and lignin pyrolysis. Fuel 86:1781–1788. https://doi.org/10.1016/j.fuel.2006.12.013 Collard F-X, Blin J (2014) A review on pyrolysis of biomass constituents: Mechanisms and composition of the products obtained from the conversion of cellulose, hemicelluloses and lignin. 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Chem Eng J 240:574–578. https://doi.org/10.1016/j.cej.2013.10.081 Fan Y, Fu F, Yang D et al (2023) Balancing the gravimetric and volumetric capacitance of nitrogen-enriched lignin porous carbon for high performance supercapacitors. J Ener Stor 63:106947. https://doi.org/10.1016/j.est.2023.106947 Liu Q, Wu D, Wang T et al (2023) Polysaccharide of agar based ultra-high specific surface area porous carbon for superior supercapacitor. Int J Bio Macro 228:40–47. https://doi.org/10.1016/j.ijbiomac.2022.12.126 Zhu Y, Wang T, Ma Y et al (2025) Tailoring porous carbon from optimized coal blends for high-performance supercapacitor electrodes. Chem Eng J 162989. https://doi.org/10.1016/j.cej.2025.162989 Yang M, Cui J, Daboczi M et al (2023) Interplay between Collective and Localized Effects of Point Defects on Photoelectrochemical Performance of TiO2 Photoanodes for Oxygen Evolution. Adv Mat Int 10:2300595. https://doi.org/10.1002/admi.202300595 Li J, Cai J, Zhang Y et al (2024) Highly Active and Stable Pd/MgAl2O4 Catalysts for Methane Catalytic Combustion. Adv Ener Sus Res 5:2400044. https://doi.org/10.1002/aesr.202400044 Chen B, Wu D, Wang T et al (2023) Rapid preparation of porous carbon by flame burning carbonization method for supercapacitor. Chem Eng J 462:142163. https://doi.org/10.1016/j.cej.2023.142163 Li J, Wang R, Han L et al (2025) Unveiling the neglected role of oxygen doping in nitrogen-doped carbon for enhanced capacitive deionization performance, Nat. Comm. 16 1996. https://doi.org/10.1038/s41467-025-56694-0 Li R, Zhou Y, Li W et al (2020) Structure engineering in biomass-derived carbon materials for electrochemical energy storage, Research. https://doi.org/10.34133/2020/8685436 Devi M, Shikhar J, Sharma S (2025) Effect of nitrogen content on performance of supercapacitors composed of nitrogen–carbon materials. J Mater Chem A 13:42343–42354. https://doi.org/10.1039/D5TA06469D Mohammed SJ, Mohammed AS, Abdalla KK et al (2025) Advances in nitrogen-doped carbon dots for electrochemical energy storage: from synthesis to applications. Mater Adv 6:8740–8773. https://doi.org/10.1039/D5MA00927H Han G, Jia J, Liu Q et al (2022) Template-activated bifunctional soluble salt ZnCl2 assisted synthesis of coal-based hierarchical porous carbon for high-performance supercapacitors. Carbon 186:380–390. https://doi.org/10.1016/j.carbon.2021.10.042 Abbas SC, Lin C, Hua Z et al (2022) Bamboo-derived carbon material inherently doped with SiC and nitrogen for flexible supercapacitors. Chem Eng J 433:133738. https://doi.org/10.1016/j.cej.2021.133738 Jain A, Ghosh M, Krajewski M et al (2021) Biomass-derived activated carbon material from native European deciduous trees as an inexpensive and sustainable energy material for supercapacitor application. J Energy Storage 34:102178. https://doi.org/10.1016/j.est.2020.102178 Deshpande A, Rawat S, Patil IM et al (2023) Converting renewable saccharides to heteroatom doped porous carbons as supercapacitor electrodes. Carbon 214:118368. https://doi.org/10.1016/j.carbon.2023.118368 Karamanova B, Mladenova E, Thomas M et al (2023) Electrochemical performance of symmetric solid-state supercapacitors based on carbon xerogel electrodes and solid polymer electrolytes. Gels 9:983. https://doi.org/10.3390/gels9120983 Zhang W, Pang Y, Wang B et al (2024) Wood-derived carbon electrodes prepared via simple surface treatment for high performance supercapacitors. Diam Relat Mater 141:110684. https://doi.org/10.1016/j.diamond.2023.110684 Sumangala Devi N, Vivekanandhan S (2024) Effect of carbonization temperatures on the synthesis of biocarbon from Borassus flabellifer fruit fiber for capacitive energy storage. Appl Res 3:e202400005. https://doi.org/10.1002/appl.202400005 Li H, Ma R, Chen F et al (2023) Constructing interconnected microporous structures in carbon by homogeneous activation as a sustainable electrode material for high-performance supercapacitors. Molecules 28:6851. https://doi.org/10.3390/molecules28196851 Chaiammart N, Vignesh V, Thu MM et al (2025) Chemically activated carbons derived from cashew nut shells as potential electrode materials for electrochemical supercapacitors. Carbon Resour Convers 8:100267. https://doi.org/10.1016/j.crcon.2024.100267 Additional Declarations No competing interests reported. Supplementary Files SupportingInformation.docx 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. 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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-8826492","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":593857614,"identity":"0dcdef8c-17a5-4f0f-98b1-96dabc5b38a6","order_by":0,"name":"Tserenlkham Byambadorj","email":"","orcid":"","institution":"Harbin University of Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Tserenlkham","middleName":"","lastName":"Byambadorj","suffix":""},{"id":593857615,"identity":"12281515-7309-4a3b-9f11-8f2f6a3964c5","order_by":1,"name":"Jiawei Zhang","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA7ElEQVRIiWNgGAWjYBACxgYGNhAtZ8DAAxU6QKQWY4iWBCK0AAFYS+IGorUwz0h/9uDnjtr07dJnj0n+/MEgx3cjgfFzAT6HzUhIN+w9czx3Z19emjRPAoOx5I0EZukZ+LTMTjgmwdt2LHfDGR4zaaDDEjfcSGBj5sGrJbFN8m/bsXQDoBbJHwkM9URoSWaT5m2rSQBpkQA6LMGAoJb5z9ikZdsOGG44w5dszZMmYTjzzMNmaXxaDHuOP5N821Ynb3CG9+DNHzY28nzHkw9+xqulAUwdhvElGMDRiw/IQ6g6vIpGwSgYBaNghAMAGEhK2uvOOvgAAAAASUVORK5CYII=","orcid":"","institution":"Harbin University of Science and Technology","correspondingAuthor":true,"prefix":"","firstName":"Jiawei","middleName":"","lastName":"Zhang","suffix":""},{"id":593857616,"identity":"61808e4d-f39e-4b79-abcd-520f799d8863","order_by":2,"name":"Xuzhen Lu","email":"","orcid":"","institution":"Harbin University of Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Xuzhen","middleName":"","lastName":"Lu","suffix":""},{"id":593857617,"identity":"b9296f1b-caec-4a1c-baaa-2a6a48b2990b","order_by":3,"name":"Yuehui Wang","email":"","orcid":"","institution":"Harbin University of Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Yuehui","middleName":"","lastName":"Wang","suffix":""},{"id":593857618,"identity":"bcf37793-a05f-485d-90f0-92f4baeb81da","order_by":4,"name":"Fan Wang","email":"","orcid":"","institution":"Harbin University of Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Fan","middleName":"","lastName":"Wang","suffix":""},{"id":593857619,"identity":"2a37b034-67d4-45d7-a87b-b2269f048818","order_by":5,"name":"Qian Liu","email":"","orcid":"","institution":"Harbin University of Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Qian","middleName":"","lastName":"Liu","suffix":""},{"id":593857620,"identity":"0f823d1a-36d4-4995-805c-ccf5d9e864e1","order_by":6,"name":"Yu Li","email":"","orcid":"","institution":"Harbin University of Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Yu","middleName":"","lastName":"Li","suffix":""},{"id":593857621,"identity":"b5261111-6bd9-42d2-8166-f268a8a6cf8b","order_by":7,"name":"Minghua Chen","email":"","orcid":"","institution":"Harbin University of Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Minghua","middleName":"","lastName":"Chen","suffix":""}],"badges":[],"createdAt":"2026-02-09 06:23:31","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8826492/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8826492/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":103056493,"identity":"0184f525-4e8f-4fdc-b406-9b6ebccd1c9f","added_by":"auto","created_at":"2026-02-20 09:11:58","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":919879,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eThe analysis of structure and pore distribution of CS-X samples. \u003c/strong\u003e(a)\u003cstrong\u003e \u003c/strong\u003eSchematic diagram illustrating the preparation process, (b) XRD patterns, (c) Raman spectrum, (d) FT-IR spectra, (e) TGA curves, (f) N\u003csub\u003e2\u003c/sub\u003e adsorption-desorption isotherm, and (g) Pore size distribution (inset: pore volume histogram) of the CS-X samples.\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-8826492/v1/9fbe485644880e5a8134cf26.png"},{"id":104779144,"identity":"83e0e0fb-949a-41bf-aae9-8fc9564e819b","added_by":"auto","created_at":"2026-03-17 07:35:42","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":889212,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eThe morphology characterization of CS-X: SEM images of \u003c/strong\u003e(a, d) CS-600, (b, e) CS-800, and (c, f) CS-1000, respectively. (g) TEM image, (h) SAED patterns, and (i) EDS mapping of CS-800.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-8826492/v1/ec974d78b304194a5dac821f.png"},{"id":103056508,"identity":"64d246e2-29fb-4acb-8f43-077cbbe3b649","added_by":"auto","created_at":"2026-02-20 09:12:32","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":4670863,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eThe element analysis of CS-X samples.\u003c/strong\u003e (a) EPR plot, (b) XPS survey spectra, (c) Elemental distribution, and high-resolution XPS spectra of C 1s (d), O 1s (e), and N 1s (f) of as-prepared materials.\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-8826492/v1/140281122cebe3983ec85b31.png"},{"id":103056444,"identity":"ac6d34cb-bf5b-4960-a3c5-7d4e02999de3","added_by":"auto","created_at":"2026-02-20 09:10:35","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":3146934,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eElectrochemical performances of CS-X electrodes:\u003c/strong\u003e (a) CV curves at scan rate of 100 mV s\u003cstrong\u003e⁻\u003c/strong\u003e\u003csup\u003e\u003cstrong\u003e1\u003c/strong\u003e\u003c/sup\u003e, (b) GCD curves at a current density of 1 A g\u003cstrong\u003e⁻\u003c/strong\u003e\u003csup\u003e\u003cstrong\u003e1\u003c/strong\u003e\u003c/sup\u003e, (c) CV curves at various scan rates of the CS-800, (d) GCD curves at different current densities of the CS-800, (e) Specific capacitance at various current densities, (f) Deconvoluted voltammograms into capacitive and diffusive regions scanned at 20 mV s\u003cstrong\u003e⁻\u003c/strong\u003e\u003csup\u003e\u003cstrong\u003e1\u003c/strong\u003e\u003c/sup\u003e, (g) Specific capacitance values separated into capacitive and diffusive components at various current densities (1-7 A g\u003cstrong\u003e⁻\u003c/strong\u003e\u003csup\u003e\u003cstrong\u003e1\u003c/strong\u003e\u003c/sup\u003e), (h) Nyquist curve with an equivalent circuit, and (i) Bode graph.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-8826492/v1/39bdbaacceedae32a54f537e.png"},{"id":103053431,"identity":"486d42ad-82cd-4460-9a26-f80ce9e646e2","added_by":"auto","created_at":"2026-02-20 08:15:04","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":1095036,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eElectrochemical performances of CS-800//CS-800 SSC:\u003c/strong\u003e(a) Schematic diagram of the SSC device, (b) CV curves at various scan rates, (c) GCD curves at different current densities, (d) rate performance, (e) cycling stability, (f) Ragone plots of this work compared with publications [39-46], (g) device configuration and electrical connections, (h) CV curves of parallel vs. series connections, and (i) Specific capacitance comparison.\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-8826492/v1/edf55a8f426b307e9e89c457.png"},{"id":107913251,"identity":"8fa80212-b352-44fc-89c1-e7260aea1f28","added_by":"auto","created_at":"2026-04-27 13:43:21","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":11028250,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8826492/v1/4688a6c7-4598-46af-bb3e-d793427b2836.pdf"},{"id":103056430,"identity":"0e0ccab6-5c3e-49c1-9cfa-eb032c0a0acd","added_by":"auto","created_at":"2026-02-20 09:10:16","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":166459,"visible":true,"origin":"","legend":"","description":"","filename":"SupportingInformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-8826492/v1/4ec92741cc3ba098e8446acf.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Biomass-Derived Carbon Electrodes with Optimized Defects and Porosity via Regulated Carbonization Temperature for Supercapacitors","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eSupercapacitors (SCs) are widely recognized as promising energy-storage devices owing to their high-power density, long cycling life, low maintenance requirements, and environmental compatibility. Achieving high‐performance SCs depends critically on the development of electrode materials with tailored nanostructures, such as hierarchical porosity, high electrical conductivity, abundant electrochemically active sites, and robust structural stability. In this context, carbon-based materials\u0026mdash;including graphene, carbon nanotubes, carbon nanofibers, mesoporous carbons, and activated carbons\u0026mdash;have been extensively explored and remain central to SC research [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. Biomass has emerged as one of the most attractive precursors for carbon materials due to its abundance, renewability, and inherent heteroatom and mineral content [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. Numerous biomass-derived carbons have demonstrated strong potential in SC applications, however, most studies rely on chemical activation or additives, which can obscure the intrinsic influence of carbonization conditions on structural and electrochemical properties.\u003c/p\u003e \u003cp\u003eThe energy storage performance of biomass-derived carbon is governed not only by specific surface area and porosity but also by defect density, surface functional groups, graphitization degree, and microstructural ordering [\u003cspan additionalcitationids=\"CR6 CR7 CR8\" citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. Strategies to enhance performance include heteroatom doping (N, S, O), structural tuning (crystalline vs. amorphous carbon), pore regulation, and heat-treatment optimization (temperature, atmosphere, duration) [\u003cspan additionalcitationids=\"CR11\" citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. Among these, the carbonization temperature of precursors is particularly critical in the produce of biomass-derived carbon, as it can substantially influence the final characteristics of the material. Notably, Qiu et al. suggested that modest temperature can result in more effective activation, while excessively high temperatures may lead to pore collapse, obstructing existing pores and diminishing both specific surface area (SSA) and pore volume. Simultaneously, the carbon-yield progressively diminished as a result of the elevated activation temperature and the synergistic activation influence of dopants [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. Xue et al. observed that the diffraction peaks of the samples migrated to higher angles at increased temperature, signifying the contraction of the framework during pyrolysis process [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. More recently, Diego et al. investigated the influence of pre-carbonization temperature at 350 to 500\u0026deg;C for the supercapacitor applications, suggesting that pre-carbonization temperature significantly regulate the structural and electrochemical properties [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. Ma et al. emphasized that choosing the suitable activation temperature is favorable for production of activated carbon materials with SSA [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. However, the employment of direct pyrolysis and two-step pre-carbonization followed by chemical activation, hydrothermal carbonization, or ball milling pre-treatment, along with activating/templating agents (KOH, ZnCl\u003csub\u003e2\u003c/sub\u003e, H\u003csub\u003e3\u003c/sub\u003ePO\u003csub\u003e4\u003c/sub\u003e, etc.), could have masked the direct effect of temperature. Moreover, previous studies often lack a systematic exploration of additive-free biomass carbonization, leaving the intrinsic effect of temperature insufficiently understood.\u003c/p\u003e \u003cp\u003eHerein, corn straw-derived carbons (CS) were prepared at 600, 800, and 1000\u0026deg;C via a straightforward, additive-free carbonization process. The absence of chemical activation allows for the isolation of temperature effects on defect density, pore structure, surface functional groups, and crystallinity. CS-800 achieves a specific capacitance of 53.8 F g⁻\u003csup\u003e1\u003c/sup\u003e at 1 A g⁻\u003csup\u003e1\u003c/sup\u003e and the capacitance retention remain above 90% over 30,000 cycles in a 1 M KOH electrolyte. The superior electrochemical performances can be attributed to the suitable defects/heteratoms and pore structure, resulting in the balance between kinetics behaviour and energy storage. These results suggest that carefully adjusting the carbonization temperature is conducive to biomass-derived carbons with enhanced electrochemical performance, offering theoretical guidance for developing sustainable carbon electrodes.\u003c/p\u003e"},{"header":"2. Materials and methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1. Materials\u003c/h2\u003e \u003cp\u003eCorn straw was sourced from a farm in Harbin, Heilongjiang Province, China. 1-Methyl-2pyrrolidinone, potassium hydroxide (KOH), polyvinylidene difluoride (PVDF) were purchased from Sinopharm Chemical Reagent Co. Battery-grade conductive carbon black (Super P) was purchased from Shenzhen Kejing Zhida Technology Co. Battery-grade nickel foam was purchased from Kunshan Jiayisheng Electronics Co. High purity argon (Ar\u0026thinsp;\u0026ge;\u0026thinsp;99%) was purchased from Harbin Grand Gas Co.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2. Synthesis of carbon materials\u003c/h2\u003e \u003cp\u003eThe raw corn straw was thoroughly washed with deionized (DI) water to eliminate adhered soil and surface contaminants, then dried at 100\u0026deg;C for 12 h. The dried material was crushed and sieved through a 20-mesh to ensure uniform particle size. The sieved biomass was placed in an alumina boat and maintained at 2 h at 600, 800, or 1000\u0026deg;C with a rate of 5\u0026deg;C min⁻\u003csup\u003e1\u003c/sup\u003e under argon atmosphere. Final, the samples were naturally cooled to room temperature to obtain carbon without de-ashing.\u003c/p\u003e \u003cp\u003e \u003cstrong\u003eDe-ashing treatment\u003c/strong\u003e \u003cp\u003eTo remove residual inorganic components that may influence the structural and electrochemical behavior of biomass carbons, the carbonized products were subjected to a de-ashing treatment. The obtained carbon were washed with 10 wt% HCl under stirring at room temperature, followed by repeated rinsing with deionized water until a neutral pH. The washed samples were then dried at 80\u0026deg;C. The final products were denoted as CS-600, CS-800, and CS-1000, according to their respective carbonization temperatures.\u003c/p\u003e \u003c/p\u003e \u003cp\u003e \u003cstrong\u003eBulk mass (gravimetric) yield calculation method\u003c/strong\u003e \u003cp\u003eRaw corn straw was dried at 100\u0026deg;C for 12 h to constant weight, with five independent batches were prepared at each temperature to ensure statistical reliability. For each batch, a known mass of dried precursor (\u003cem\u003em\u003c/em\u003e\u003csub\u003e\u003cem\u003epre\u003c/em\u003e\u003c/sub\u003e) was carbonized under identical conditions. After cooling under argon, the crude carbon was weighed to obtain \u003cem\u003em\u003c/em\u003e\u003csub\u003e\u003cem\u003echar,crude\u003c/em\u003e\u003c/sub\u003e. The char was then subjected to acid washing, filtration, rinsing, and drying, after which the ash-free char mass (\u003cem\u003em\u003c/em\u003e\u003csub\u003e\u003cem\u003echar,ashed\u003c/em\u003e\u003c/sub\u003e) was recorded. This procedure was performed for samples carbonized at 600, 800, 1000\u0026deg;C. The mass yield was calculated according to Eqs.\u0026nbsp;1\u0026ndash;3.\u003c/p\u003e \u003c/p\u003e \u003cp\u003eCrude char yield (%)=\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\frac{{{m_{char,crude}}}}{{{m_{pre}}}} \\times 100\\)\u003c/span\u003e\u003c/span\u003e (1)\u003c/p\u003e \u003cp\u003eDe-ashed char yield (%)=\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\frac{{{m_{char.ashed}}}}{{{m_{pre}}}} \\times 100\\)\u003c/span\u003e\u003c/span\u003e (2)\u003c/p\u003e \u003cp\u003eAsh removal (%)=\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\frac{{{m_{char,crude}} - {m_{char,ashed}}}}{{{m_{char,crude}}}} \\times 100\\)\u003c/span\u003e\u003c/span\u003e (3)\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3. Structure and morphology characterization\u003c/h2\u003e \u003cp\u003eThe crystal structure of the samples was acquired by X-Ray Diffraction (XRD; Malvern Panalytical Empyrean) with Cu Kα radiation (λ\u0026thinsp;=\u0026thinsp;1.5406 \u0026Aring;) in the 2θ range of 5\u0026ndash;90\u0026deg;. Raman spectroscopy (WITec Alpha 300 R) with a laser wavelength of 532 nm was used to explore the ratio of I\u003csub\u003eD\u003c/sub\u003e/I\u003csub\u003eG\u003c/sub\u003e. Surface functional groups were determined using an FT-IR spectrometer (PerkinElmer) in the transmission mode covering the range of 400\u0026ndash;4000 cm⁻\u003csup\u003e1\u003c/sup\u003e. To verify the thermal stability of the carbon materials, thermal gravimetric analysis (TGA) was performed in the temperature range RT\u0026ndash;750\u0026deg;C using a NETZSCH instrument operating in an air atmosphere at a heating rate of 5\u0026deg;C min⁻\u003csup\u003e1\u003c/sup\u003e. Brunauer-Emmett-Teller Specific Surface Area Analysis characterizes the specific surface area and pore structure (Micromeritics ASAP 2460 Version 3.01.02). The pore size distribution was determined using the Barrett-Joyner-Halenda (BJH) model, and the SSA was measured using the Brunauer-Emmett-Teller (BET) method. The morphologies of the samples were examined using a scanning electron microscope (SEM, Hitachi-SU8020) and a transmission electron microscope (TEM, JEM-F200 (URP)). The oxygen vacancy concentration of the samples was recorded by electron paramagnetic resonance (EPR) spectra using a Bruker EMXplus-6/1 instrument. The elemental composition and chemical state of materials were analyzed by X-ray photoemission spectroscopy (XPS, Thermo Scientific ESCALAB 250Xi). The binding energies were normalized considering the C 1s peak at 284.8 eV as a reference.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4. Electrochemical Measurements\u003c/h2\u003e \u003cp\u003eElectrochemical tests were analyzed by the CHI760E workstation. The working electrode was fabricated by mixing carbon materials, PVDF, and Super P in a mass ratio of 8:1:1. Then, NMP was used as the solvent, and the mixture was stirred at 500 rpm for 12 h to form a slurry. Finally, the slurry was uniformly coated on the nickel foam substrate and dried at 60\u0026deg;C for 12 h to obtain an electrode. The three-electrode system: Pt foil, Ag/AgCl, and as-prepared electrode served as the counter electrode, reference electrode, and working electrode, respectively. 1 M KOH is used as the electrolyte. Cyclic voltammetry (CV) tests were conducted at operating voltages of -1 V and 0 V at different scan rates. Galvanostatic charge/discharge (GCD) tests were conducted at different current densities. Electrochemical impedance spectroscopy (EIS) measurements were performed using a voltage amplitude of 5 mV of open-circuit potential between 10⁻\u003csup\u003e2\u003c/sup\u003e and 10\u003csup\u003e5\u003c/sup\u003e Hz.\u003c/p\u003e \u003cp\u003eIn a three-electrode system, Eq.\u0026nbsp;(\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e4\u003c/span\u003e) can determine the gravimetric specific capacitance (\u003cem\u003eC\u003c/em\u003e, F g⁻\u003csup\u003e1\u003c/sup\u003e) of the as-prepared working electrode from the constant current charge/discharge curve.\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$${C_g}=\\frac{{I\\Delta t}}{{m\\Delta V}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e4\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere \u003cem\u003eI\u003c/em\u003e (A), Δ\u003cem\u003et\u003c/em\u003e (s), Δ\u003cem\u003eV\u003c/em\u003e (V), and \u003cem\u003em\u003c/em\u003e (g) represent discharge current, discharge time, voltage window, and active mass of the working electrode, respectively.\u003c/p\u003e \u003cp\u003eThe symmetrical supercapacitor device was built with CS-800 as the positive and negative electrodes in a 1 M KOH aqueous electrolyte and separator to examine potential applications for energy storage. Furthermore, the assembled device was maintained at ambient temperature for over 12 h to ensure sufficient electrolyte infiltration. In the symmetric capacitor, Eq.\u0026nbsp;(\u003cspan refid=\"Equ2\" class=\"InternalRef\"\u003e5\u003c/span\u003e) was used to calculate the capacitance (\u003cem\u003eC\u003c/em\u003e, F g⁻\u003csup\u003e1\u003c/sup\u003e) of the signal electrode. Eqs.\u0026nbsp;(\u003cspan refid=\"Equ3\" class=\"InternalRef\"\u003e6\u003c/span\u003e) and (\u003cspan refid=\"Equ4\" class=\"InternalRef\"\u003e7\u003c/span\u003e) were used to calculate the energy density (\u003cem\u003eE\u003c/em\u003e, Wh kg⁻\u003csup\u003e1\u003c/sup\u003e) and power density (\u003cem\u003eP\u003c/em\u003e, W kg⁻\u003csup\u003e1\u003c/sup\u003e), respectively.\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$C=\\frac{{I\\Delta t}}{{M\\Delta V}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e5\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e\n$$E=\\frac{{C{{\\left( {\\Delta V} \\right)}^2}}}{2}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e6\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ4\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ4\" name=\"EquationSource\"\u003e\n$$P=\\frac{{3600E}}{{\\Delta t}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e7\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere \u003cem\u003eM\u003c/em\u003e (g) represents the total mass of the active materials loaded on two electrodes [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and discussions","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e3.1. Morphology and pore structure analysis\u003c/h2\u003e \u003cp\u003eThe preparation of corn straw\u0026ndash;derived carbons via direct carbonization followed by de-ashing as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea. The activated agent free strategy can isolate the intrinsic effects of carbonization temperature on the evolution of the carbon framework, surface chemistry, and electrochemical behavior. During carbonization, the gradual thermal decomposition of the biomass matrix leads to the release of volatile species such as CO\u003csub\u003e2\u003c/sub\u003e, CO, H\u003csub\u003e2\u003c/sub\u003eO, and light hydrocarbons, and is accompanied by the gradual aromatization and condensation of the carbon framework [\u003cspan additionalcitationids=\"CR19\" citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. XRD patterns of the CS-X materials (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb) exhibited a broad diffraction feature centered at ~\u0026thinsp;22\u0026ndash;25\u0026deg;, characteristic of turbostratically disordered (002) carbon, along with a weak and broadened reflection near ~\u0026thinsp;43\u0026deg; corresponding to the (100) plane [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. Superimposed on this background, sharp reflections at ~\u0026thinsp;20.7\u0026deg; and ~\u0026thinsp;26.6\u0026deg; are observed and are assigned to crystalline SiO\u003csub\u003e2\u003c/sub\u003e (quartz) marked *, suggests the presence of impurities (SiO\u003csub\u003ex\u003c/sub\u003e or ash). The SiO\u003csub\u003e2\u003c/sub\u003e content in biomass-derived carbon is generally related to the high ash content of the precursor and the carbonization temperature, and is abundant in the XRD pattern of the unwashed sample [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. However, the retention of weak SiO\u003csub\u003e2\u003c/sub\u003e peaks after acid washing is attributed to the acid-insoluble silica present in the biomass [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. TEM and SAED analysis confirm that the carbon matrix remains predominantly amorphous/turbostratic, indicating that these sharp peaks do not originate from crystalline carbon. Two characteristic Raman bands centered at ~\u0026thinsp;1358 cm⁻\u003csup\u003e1\u003c/sup\u003e (D band) and ~\u0026thinsp;1592 cm⁻\u003csup\u003e1\u003c/sup\u003e (G band) were observed for all CS-X (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec). The Raman peaks exhibited negligible variations, indicating that the impact on the graphite crystallinity of the sample is minimal within the temperature range. FT-IR spectra (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed) further illustrate the progressive removal of oxygen-containing functional groups with increasing temperature. CS-600 exhibits distinct absorption features near 1096 cm⁻\u003csup\u003e1\u003c/sup\u003e (C\u0026ndash;O\u0026ndash;C vibrations) and 1599 cm⁻\u003csup\u003e1\u003c/sup\u003e (C\u0026thinsp;=\u0026thinsp;C or C\u0026thinsp;=\u0026thinsp;O stretching), which is attribute to incomplete decomposition of biomass [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. These functional groups are largely eliminated at 800 and 1000\u0026deg;C, consistent with thermal cleavage of ether and carbonyl bonds.\u003c/p\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ee, thermogravimetric analysis (TGA) was carried out in air from 100\u0026deg;C to 750\u0026deg;C to evaluate the thermal stability and inorganic residue content of the CS-X samples. All samples exhibit two distinct stages of weight loss. The first stage is associated with the physical adsorption of moisture, and the second stage is associated with the oxidative decomposition of biomass-derived carbonaceous components [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. The total mass losses of CS-600, CS-800, and CS-1000 are approximately 55%, 54%, and 45%, respectively. The decrease in char yield and the concurrent increase in TGA residual mass with increasing carbonization temperature indicate a transition from mass retention to structural stabilization, whereby fewer, however, more thermally robust carbon domains are formed [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. It should be noted that char yield is referenced to the initial raw corn straw mass, whereas TGA residue is referenced to the pre-carbonized sample; therefore, the two parameters reflect different aspects of the carbonization process. Furthermore, bulk mass yield calculations were performed based on gravimetric measurements. As the carbonization temperature increased from 600 to 1000\u0026deg;C, the gravimetric yield of the carbon products gradually decreased, reflecting enhanced thermal decomposition and volatilization of the biomass precursor. The crude char yields for CS-600, CS-800, and CS-1000 were 29.2\u0026thinsp;\u0026plusmn;\u0026thinsp;0.8%, 28.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.6%, and 27.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4%, respectively. After acid washing, the corresponding de-ashed char yields decreased to 22.1\u0026thinsp;\u0026plusmn;\u0026thinsp;0.7%, 21.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5%, and 19.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3%, indicating an ash removal efficiency of approximately 24.5\u0026ndash;29.5 wt%.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eNitrogen adsorption\u0026ndash;desorption measurements were conducted to investigate the pore structures of the CS-X samples. As illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ef, the non-closed isotherm of CSC-600 can be reasonably ascribed to the high density of oxygen-containing surface functional groups, which results in adsorption irreversibility. Conversely, the non-closure of the CS-1000 isotherm is more likely caused by pore structure collapse or contraction induced by excessive carbonization and framework shrinkage at elevated temperatures. The specific surface areas (SSA) of CS-600, CS-800, and CS-1000 are 8.09, 7.85, and 6.70 m\u003csup\u003e2\u003c/sup\u003e g⁻\u003csup\u003e1\u003c/sup\u003e, respectively. The monotonic decrease in SSA with increasing carbonization temperature is attributed to structural densification and partial collapse of thermolabile pore frameworks during high-temperature treatment [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eg illustrates the pore size distribution obtained from the adsorption branch utilizing a BJH-type model. All samples exhibit a predominant mesopore distribution between roughly 2\u0026ndash;40 nm, with minimal contribution from micropores (\u0026lt;\u0026thinsp;2 nm). This verifies that the carbon's porosity is predominantly mesoporous, aligning with the isotherm forms depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ef.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe SEM images of CS-X materials (Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea\u0026ndash;f) reveal that all samples retain the strip-like morphology of the corn stalk precursor, accompanied by abundant surface wrinkles and nanoscale particles, which are beneficial for electrolyte infiltration and enlargement of the solid-liquid interfacial during electrochemical processes. TEM analysis of CS-800 (Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eg and h) shows interconnected carbon nanoparticles forming a porous architecture without long-range ordered lattice fringes. The corresponding SAED patterns displays diffuse halo rings, indicative of low-crystallinity, turbostratically disordered carbon rather than well-crystallized graphite. This observation corroborates the XRD results, confirming that the broad (002) reflection arises from disordered carbon stacking, while the sharp diffraction peaks observed in the XRD patterns do not originate from crystalline carbon yet are instead associated with trace inorganic residues. Furthermore, EDS elemental mapping (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ei and Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e) confirms homogeneous distribution of C, N, O, P, and S throughout the carbon matrix, demonstrating successful in-situ self-doping during carbonization, consistent with previous reports [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea, the EPR spectra of CS-600, CS-800, and CS-1000 reveal the presence of unpaired electrons in all samples, and the characteristic EPR signal (g\u0026thinsp;=\u0026thinsp;2.003) was examined. These electrons are predominantly found on carbon atoms (free radicals) and are also linked to oxygen-containing chemical groups on the surface of the material [\u003cspan additionalcitationids=\"CR31\" citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]. Among the samples, CS-800 has a moderate EPR intensity, indicating the optimized concentration of paramagnetic defects, which is conducive to charge storage either through the faradaic or capacitive mechanism. The decrease in the EPR signal of CS-1000 may be attributed to the healing of defects and enhanced graphitization at high pyrolysis temperatures. Although the higher EPR intensity of CS-600 indicates the presence of paramagnetic defects and surface functionality, it is not associated with improved electrochemical performance. Surface defects, volatile oxygen-containing groups, and disordered carbon structures can significantly hinder electron transfer and structural integrity. Excessive defect sites may promote irreversible side reactions or increase charge transfer resistance, resulting in lower specific capacitance and worse rate performance compared to CS-800. Thus, the presence of moderate defects may improve pseudocapacitive properties, but the highly defective surface observed in CS-600 appears to suppress the electrochemical efficiency.\u003c/p\u003e \u003cp\u003eXPS analysis (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb\u0026ndash;f) confirms the presence of C, O, N, P, and S in all CS-X samples. Increasing carbonization temperature to 800\u0026deg;C increases the carbon content while reducing oxygen concentration. High-resolution C 1s spectra reveal dominant sp\u003csup\u003e2\u003c/sup\u003e-hybridized carbon (C\u0026ndash;C/C\u0026thinsp;=\u0026thinsp;C, ~\u0026thinsp;284.6 eV), along with oxygen-containing functional group attributed to C\u0026ndash;O/C\u0026ndash;N (~\u0026thinsp;286.0 eV), consistent with recent carbon material studies using XPS. Correspondingly, the O 1s spectra exhibit peaks at ~\u0026thinsp;531.0 eV and ~\u0026thinsp;532.5 eV, which are assigned to carbonyl oxygen (C\u0026thinsp;=\u0026thinsp;O) and hydroxyl/ether oxygen (C\u0026ndash;OH/C\u0026ndash;O\u0026ndash;C), respectively [\u003cspan additionalcitationids=\"CR34\" citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]. Likewise, the N 1s spectra reveal three peaks with the binding energies correlated to pyrrolic-N (N-5), quaternary-N (N-Q), and pyridinic-N (N-6). Pyridine nitrogen provides additional electron and charge mobility to enhance electrical conductivity and provides active sites for redox reactions to enhance Faradaic reactions. Pyrrole nitrogen improves interfacial and cyclic performance, while graphitic nitrogen provides additional sites for electron transfer [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e, \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]. The retention of nitrogen functional groups in the derived carbon without the need for additional activation or doping suggests that this approach allows for self-doping through the intrinsic composition of the raw material. This property makes corn straw-derived carbon a promising candidate for high-capacity energy storage applications.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.2. Electrochemical performance in a three-electrode system\u003c/h2\u003e \u003cp\u003eThe electrochemical performance of CS-600, CS-800, and CS-1000 was evaluated in 1 M KOH electrolyte using a three-electrode configuration. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea presents the cyclic voltammetry (CV) curves recorded at a scan rate of 100 mV s⁻\u003csup\u003e1\u003c/sup\u003e within the potential window of \u0026minus;\u0026thinsp;1.0 to 0 V. All samples exhibit quasi-rectangular CV profiles, indicating typical electric double-layer capacitance (EDLC) behavior with additional pseudocapacitive contributions. The latter arise from surface defects and oxygen-containing functional groups, such as hydroxyl, carboxyl, and phenolic species, which facilitate reversible Faradaic reactions at the electrode/electrolyte interface [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]. Among the three samples, CS-800 demonstrates the highest current response, suggesting superior charge storage capability. The galvanostatic charge\u0026ndash;discharge (GCD) curves shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb further corroborate the CV results. CS-800 exhibits the longest discharge time under identical current conditions, reflecting its higher capacitance. Based on the GCD analysis, the specific capacitances of CS-600, CS-800, and CS-1000 are 21.2, 53.8, and 36.7 F g⁻\u003csup\u003e1\u003c/sup\u003e at 1 A g\u003cb\u003e⁻\u003c/b\u003e\u003csup\u003e1\u003c/sup\u003e, respectively. The significantly enhanced capacitance of CS-800 is attributed to the optimized balance among defect density, pore structure, and graphitization degree. Although CS-600 possesses a slightly larger specific surface area, its lower carbonization temperature results in a higher proportion of unstable functional groups and insufficiently developed conductive pathways. In contrast, CS-1000 exhibits improved conductivity but reduced surface area due to structural densification at elevated temperatures. CS-800 offers the most favorable combination of moderate graphitization, abundant defects, and mesoporous architecture, thereby enhancing ion accessibility and charge transport. These results confirm that specific capacitance is not determined solely by surface area; rather, it is governed by the coupled effects of pore size distribution, defect concentration, and structural ordering, all of which are modulated by the carbonization temperature.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec shows the CV curves of CS-800 recorded at scan rates from 20 to 100 mV s⁻\u003csup\u003e1\u003c/sup\u003e. All curves retain a quasi-rectangular shape without noticeable distortion, indicating that CS-800 maintains rapid charge\u0026ndash;discharge kinetics and robust capacitive behavior even at high scan rates. Compared with CS-600 (Fig. S2a) and CS-1000 (Fig. S3a), CS-800 exhibits a significantly higher current response across all scan rates, highlighting its superior electrochemical activity. The slight deviation from an ideal rectangular profile is attributed to pseudocapacitive contributions superimposed on the dominant EDLC mechanism. The GCD curves (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed) display symmetric triangular shapes without discernible potential plateaus, even when the current density increases sevenfold from 1 to 7 A g⁻\u003csup\u003e1\u003c/sup\u003e. This confirms the fast and reversible charge storage process of CS-800. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee, the CS-800 electrode delivers a specific capacitance of 31.5 F g⁻\u003csup\u003e1\u003c/sup\u003e at 7 A g⁻\u003csup\u003e1\u003c/sup\u003e, corresponding to a capacitance retention of 58.55% relative to its value at 1 A g⁻\u003csup\u003e1\u003c/sup\u003e. This performance is substantially higher than those of CS-600 (7.7 F g⁻\u003csup\u003e1\u003c/sup\u003e, Fig. S2b) and CS-1000 (18.9 F g⁻\u003csup\u003e1\u003c/sup\u003e, Fig. S3b). The decline in capacitance at higher current densities observed for all CS-X samples is attributed to the limited diffusion of electrolyte ions into the internal pore structure under fast charge\u0026ndash;discharge conditions.\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ef presents the capacitive and diffusive contributions extracted from the CV curve at 20 mV s⁻\u003csup\u003e1\u003c/sup\u003e, with additional deconvoluted profiles at 40 and 100 mV s⁻\u003csup\u003e1\u003c/sup\u003e shown in Fig. S4. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eg summarizes the specific capacitance at current densities from 1 to 7 A g⁻\u003csup\u003e1\u003c/sup\u003e, partitioned into surface-controlled (capacitive) and diffusion-controlled components. The diffusion-limited contribution decreases consistently with increasing current density, whereas the capacitive contribution increases from 1 to 3 A g⁻\u003csup\u003e1\u003c/sup\u003e and then declines at higher current densities (5\u0026ndash;7 A g⁻\u003csup\u003e1\u003c/sup\u003e). These results indicate that at elevated scan rates, charge storage in CS-800 is dominated by rapid surface-controlled (capacitive or pseudocapacitive) kinetics, whereas slower diffusion-controlled faradaic processes become progressively less accessible. Figures\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eh and \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ej show the Nyquist and Bode plots of CS-X electrodes fitted with the corresponding equivalent circuit model. The series resistances (R\u003csub\u003es\u003c/sub\u003e \u0026asymp; 137, 123, and 116 mΩ for CS-600, CS-800, and CS-1000, respectively) indicate low intrinsic resistance and minimal IR drop during operation. Additionally, the electrodes exhibit relatively small charge-transfer resistance (R\u003csub\u003ect\u003c/sub\u003e), signifying efficient interfacial charge transfer and favorable redox kinetics. The corresponding 2D Bode analysis (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ei) provides further insight into the rate-limiting processes across the measured frequency spectrum.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e3.3. Energy storage performances of the symmetric supercapacitor\u003c/h2\u003e \u003cp\u003eThe electrochemical performance of CS-800 was further evaluated in a symmetric supercapacitor (CS-800//CS-800 SSC), using 1 M KOH as the electrolyte and glass fiber as the separator (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea). The CV curves recorded at various scan rates (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb) exhibit characteristic quasi-rectangular shapes without significant distortion, indicating efficient ion transport and capacitive behavior. Correspondingly, the GCD curves at multiple current densities (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec) present nearly symmetric triangular profiles, confirming the excellent rate capability and reversible charge storage of the device. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed, the CS-800//CS-800 SSC delivers specific capacitances of 18.6, 16.3, 14.8, 13.0, and 11.7 F g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at current densities of 1, 2, 3, 5, and 7 A g⁻\u003csup\u003e1\u003c/sup\u003e, respectively. The gradual decline in capacitance at higher current densities reflects the expected limitations in ion diffusion within the electrode structure during rapid charge\u0026ndash;discharge processes. Overall, these results demonstrate that CS-800 enables stable capacitive behavior and a favorable rate response in a practical device configuration.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe CS-800//CS-800 symmetric supercapacitor (SSC) exhibits exceptional cycling stability, maintaining capacitance retention of nearly 93,75% after 30,000 cycles at 2 A g⁻\u003csup\u003e1\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ee), indicative of highly reversible charge storage and structural robustness. The inset shows the galvanostatic charge\u0026ndash;discharge curves of the first five and last five cycles, highlighting the maintained charge\u0026ndash;discharge symmetry and reversibility. The Ragone plot (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ef) shows that the SSC achieves a maximum energy density of 5.8 Wh kg⁻\u003csup\u003e1\u003c/sup\u003e at a power density of 750 W kg⁻\u003csup\u003e1\u003c/sup\u003e and sustains a maximum power density of 5,200 W kg⁻\u003csup\u003e1\u003c/sup\u003e at an energy density of 3.64 Wh kg⁻\u003csup\u003e1\u003c/sup\u003e. The progressive reduction in energy density alongside rising power density exemplifies the conventional power\u0026ndash;energy trade-off seen in electrochemical supercapacitors, wherein elevated current densities diminish discharge durations and amplify resistive losses. Nonetheless, the gadget exhibited a comparatively high energy density even with increased power density. These values are equivalent to or above those of other biomass-derived activated carbons recently reported in the literature [\u003cspan additionalcitationids=\"CR40 CR41 CR42 CR43 CR44 CR45\" citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e]. To evaluate practical applications, two identical CS-800//CS-800 SSC cells were connected in parallel and series topologies, respectively (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eg). The CV curves (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eh) show that the voltage range of the series configuration is almost twice that of a single cell, while the parallel configuration increases the effective capacitance, consistent with the theoretical principles of capacitors. Figure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ei compares the specific capacitance of the two setups. While the parallel assembly provides higher capacitance at a given operating voltage, the series connection is advantageous for applications requiring an expanded voltage window, highlighting the intrinsic trade-off between capacitance and operating voltage in device integration.\u003c/p\u003e \u003c/div\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eIn summary, porous carbons were successfully derived from corn straw at different carbonization temperatures, resulting in distinct defect densities, pore structures, specific surface areas, and degrees of graphitization. These structural variations collectively govern the electrochemical performance, demonstrating that energy storage behavior is not determined solely by surface area or porosity but also by defects, functional groups, and other intrinsic structural features. Among the samples, CS-800 exhibits the highest specific capacitance of 53.8 F g⁻\u003csup\u003e1\u003c/sup\u003e at 1 A g⁻\u003csup\u003e1\u003c/sup\u003e, reflecting an optimal balance between defect density, pore accessibility, and partial graphitization. The symmetric CS-800//CS-800 supercapacitor delivers robust cycling stability, high rate capability, and a maximum energy density of 5.8 Wh kg⁻\u003csup\u003e1\u003c/sup\u003e at a power density of 750 W kg⁻\u003csup\u003e1\u003c/sup\u003e in 1 M KOH, highlighting its practical applicability. This study provides mechanistic insight into the influence of carbonization temperature on the structural evolution and electrochemical properties of additive-free biomass-derived carbons, offering guidance for the rational design of sustainable carbon-based electrodes for high-performance supercapacitors.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eConflict of Interest\u003c/h2\u003e \u003cp\u003eThe authors declare no conflict of interest.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eTserenlkham Byambadorj: Formal analysis; Investigation; Writing-original draft Jiawei Zhang: Conceptualization; Funding acquisition; Writing-review\u0026amp;editingXuzhen Lu: Data Curation; Writing-review\u0026amp;editingYuehui Wang: Validation; Writing-review\u0026amp;editingFan Wang: Methodology; Writing-review\u0026amp;editingQian Liu: Formal analysis; Writing-review\u0026amp;editingYu Li: Visualization; Writing-review\u0026amp;editingMinghua Chen: Funding acquisition; Writing-review\u0026amp;editing\u003c/p\u003e\u003ch2\u003eAcknowledgments\u003c/h2\u003e \u003cp\u003eThis work is supported by Heilongjiang Provincial Natural Science Foundation of China (2024ZX02C23); Education Department of Heilongjiang Province (LJYXL2023-092).\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eThe data that support the findings of this study are available from the corresponding author upon reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eWang Y, Zhang M, Shen X et al (2021) Biomass-derived carbon materials: controllable preparation and versatile applications. 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Molecules 28:6851. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3390/molecules28196851\u003c/span\u003e\u003cspan address=\"10.3390/molecules28196851\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eChaiammart N, Vignesh V, Thu MM et al (2025) Chemically activated carbons derived from cashew nut shells as potential electrode materials for electrochemical supercapacitors. Carbon Resour Convers 8:100267. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.crcon.2024.100267\u003c/span\u003e\u003cspan address=\"10.1016/j.crcon.2024.100267\" 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":"biomass-derived carbon, carbonization temperature, defects, supercapacitor","lastPublishedDoi":"10.21203/rs.3.rs-8826492/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8826492/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eBiomass-derived carbons hold substantial promise for sustainable electrochemical energy storage due to their low cost, wide availability, and intrinsic heteroatom- and mineral-rich nature. However, the fundamental influence of carbonization temperature on the structural evolution of non-activated biomass-derived carbons remains insufficiently understood. In this work, corn straw-derived carbon (CS) is produced without any chemical additives to isolate the intrinsic effects of carbonization temperature on its physicochemical properties. Systematic temperature variation from 600 to 1000\u0026deg;C reveals pronounced changes in micro-morphology, pore development, defect density, and the ordering of the carbon matrix, all strongly governed by the inherent mineral content of corn straw. Electrochemical evaluation in alkaline electrolyte demonstrates that CS-800 delivers the highest specific capacitance of 53.8 F g⁻\u003csup\u003e1\u003c/sup\u003e at 1 A g⁻\u003csup\u003e1\u003c/sup\u003e in a three-electrode configuration and maintains favorable rate capability in a symmetric supercapacitor device. The symmetric coin cell supercapacitor device assembled with CS-800 as the electrodes achieved an energy density of 3.64/5.8 Wh kg⁻\u003csup\u003e1\u003c/sup\u003e and power density 5200/750 W kg⁻\u003csup\u003e1\u003c/sup\u003e, along with remarkable cycling stability over 30000 cycles with negligible capacitance loss. Overall, this study provides mechanistic insight into temperature-driven structural evolution in non-activated biomass carbons, offering a baseline understanding that can guide rational design and future activation strategies for high-performance, sustainable carbon electrodes.\u003c/p\u003e","manuscriptTitle":"Biomass-Derived Carbon Electrodes with Optimized Defects and Porosity via Regulated Carbonization Temperature for Supercapacitors","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-02-20 08:14:59","doi":"10.21203/rs.3.rs-8826492/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":"5816ae83-514f-469d-933b-7e94380cd137","owner":[],"postedDate":"February 20th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2026-04-27T13:42:00+00:00","versionOfRecord":[],"versionCreatedAt":"2026-02-20 08:14:59","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8826492","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8826492","identity":"rs-8826492","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}