Investigating the Effect of Pyrolysis Process Pressure on the Supercapacitive Properties of Biochar

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Biochar samples were prepared under three different pressures: 0.5, 1.0, and 1.5 atm, with constant temperature and duration. Morphological characterization of synthesized biochar was studied via SEM and BET analyses which revealed greater porosity and higher surface area at low-pressure pyrolysis. FTIR and CHN elemental analysis indicated an increase in surface functional groups at higher pressures, resulting in enhanced hydrophilicity. XRD analysis showed a progressive increase in graphitic ordering with pressure. Electrochemical evaluations using cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS) demonstrated that biochar synthesized at 0.5 atm exhibited superior specific capacitance, rate capability, and cycling stability due to its porous architecture and efficient ion transport. In contrast, samples prepared at higher pressures showed diminished electrochemical performance. The results suggest that optimizing pyrolysis pressure is crucial for tailoring biochar properties for energy storage applications, specifically for use in supercapacitor electrodes. Biochar Pyrolysis Pressure supercapacitor Biochar structure Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 8 Highlights • Low-pressure pyrolysis (0.5 atm) yields biochar with higher surface area, porosity, and superior supercapacitive performance. • Increasing pyrolysis pressure enhances surface functional groups and hydrophilicity but decreases pore volume and surface area. • Biochar synthesized at 0.5 atm demonstrates highest specific capacitance and best charge-discharge stability. • SEM, XRD, BET, FTIR, and CHN analyses reveal pressure-dependent structural and chemical evolution of biochar. • Optimizing pyrolysis pressure is key to engineering biochar for efficient energy storage applications. 1. Introduction Converting biomass into other products is highly significant, as it enables waste recycling and increases the added value of the overall industrial chain. [1, 2]. The term biomass generally denotes organic materials originating from agricultural and forestry waste. While these resources are plentiful, they are often characterized by a high water content [3, 4]. Due to the vast volume of biomass worldwide, if it is burned or left to decompose in the environment, it will oxidize and release large amounts of CO 2 into the atmosphere. [5, 6]. Therefore, it can be said stabilizing the carbon contained in biomass is essential to prevent CO 2 emissions. Among current biomass recycling and reprocessing techniques, pyrolysis is often the preferred approach, as it generates minimal pollution while yielding valuable by-products[7, 8]. In addition to fixing carbon and generating valuable biochar, pyrolysis serves as the initial step in gasification and other thermochemical conversion processes used to harness energy from biomass.[9, 10]. Accordingly, understanding how operating parameters influence the thermal conversion of biomass during pyrolysis is key to optimizing the overall process. [11, 12]. Biomass pyrolysis involves the thermal decomposition of macromolecular chemical bonds within raw biomass under an oxygen-free atmosphere, yielding biogas, bio-oil, and biochar—products characterized by compounds of significantly lower molecular weight than those present in the original material. [13, 14]. In contrast to conventional pyrolysis, fast pyrolysis utilizes a higher heating rate and a shorter residence time for hot vapors. When carried out at a moderate temperature, this process maximizes the production of liquid biofuels. [15, 16]. With its versatile properties, biochar is widely used as a carbon template for net-shaped materials and as a catalyst support. It is also a key component in electrodes for electrochemical energy storage systems [17, 18]. In particular, there has been growing interest in the application of biochar for electrodes in electrochemical capacitors.[19–23]. Furthermore, electrochemical performance is affected by several factors, including microporosity, surface chemistry, and electrical conductivity. [24]. The properties of biochar can vary significantly, as they are strongly influenced by both the biomass feedstock and the pyrolysis conditions. [22, 25]. Key parameters in the pyrolysis process include time, heating rate, temperature, and pressure (i.e., atmospheric conditions)[26]. Consequently, understanding how to optimize these parameters is essential for obtaining the desired biochar properties for specific applications[27]. With respect to pressure, three primary thermal decomposition techniques—low pressure pyrolysis, atmospheric pyrolysis and higy pressure —are commonly employed in industry [28]. Pyrolysis in low pressure is conducted in an oxygen-free environment at pressures significantly below atmospheric levels, enabling biofuel extraction. This method has comparable effects to atmospheric pyrolysis, which requires a rapid heating rate and finely sized biomass feedstocks [28, 29]. Furthermore, compared to atmospheric pyrolysis, pyrolysis in low pressure offers several advantages: (1) reduced residence time of volatile compounds, preventing pore blockage from secondary charring reactions; (2) greater sensitivity of the biochar surface to oxidation; and (3) higher carbon yield [30–33]. As a result, biochar produced under low pressure conditions is expected to possess a more open pore structure and be more suitable as a precursor for activated carbon. Nevertheless, there is currently limited research reporting on the physicochemical characteristics of biochar monoliths prepared under low pressure conditions, especially regarding their use as electrodes in electrochemical energy storage applications. K. M. Subramaniam et al. investigated feedstock variability, process parameters, and energy efficiency in biomass pyrolysis. This research assesses the pyrolysis characteristics of five types of agricultural wastes—rice husk, corn stalk, palm kernel shell, coconut shell, and wheat straw—using a fixed-bed reactor. The study examined temperature (300 to 700°C), heating rate (5–20°C/min), and residence time (30 to 120 min) as key process variables. The biochar and bio-oil yields from the pyrolysis of different biomass feedstocks were analyzed. It was observed that biochar yield increased with rising pyrolysis temperature, with the magnitude of this effect being feedstock-dependent. Furthermore, an increase in pyrolysis temperature altered the bio-oil composition, resulting in elevated ketone (7.8%) and ester (5.1%) concentrations and a reduction in phenolic content from 15.2% to 13.0%[34]. This study aims to investigate the influence of pyrolysis pressure on the microstructure and consequently supercapacitive properties (specifically, electrical double layer capacitance) of biochar derived from plantain leaves. Biochar samples were obtained from pyrolysis prosecc under three different pressures—0.5, 1, and 1.5 atmospheres— with fixed other parameters to assess the resulting changes in material properties and electrochemical performance. 2. Experimental 2.1 Materials: The biomass used in this research was plane tree leaves, which were collected in autumn and dried in the atmosphere for 4 months. PVDF pelletlet (Kynar®1000HD) was provided by Atofina Co. N-Methyl-2-pyrrolidone and concentrated Sulfuric acid were obtained from Merck Co. 2.2 Feedstock and Biochar Preparation: The collected leaves were powdered in the same conditions with a butterfly mill. Finally, these powders entered the reactor and were dried at 90°C for 4 hours. Then 3 kg of it was weighed and entered into the reactor and the main pyrolysis operation started. The pyrolysis temperature was 300°C and continued for 3 hours after the reactor temperature reached 300°C. In this research, three biochar preparation experiments were conducted at three different pressures of 0.5, 0.1, and 1.5 atmospheres and the same conditions. All production processes were carried out in a stainless steel reactor that had no openings and only contained one outlet and was designed for this purpose. A vacuum pump was used for a pressure of 0.5 atm, a cheek valve was used for a pressure of 1.0 atm to prevent the entry of oxygen, and a pressure relief valve was used for a pressure of 1.5 atm that was adjusted for this pressure. 2.3. Electrochemical studies: To evaluate the electrochemical performance of the synthesized samples, cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) and electrochemical impedance spectroscopy (EIS) were carried out at room temperature in a three-electrode system by utilizing an Ivium-Vertex PGSTAT. The working electrode was prepared by containing 1 mg of active material consisting of 90% biochar and 10% polyvinylidene fluoride (PVDF) dispersed in N-Methyl-2-pyrrolidone (NMP) solvent. Then, the mixture was sonicated for 45 minutes, and three drops from the resulting slurry were coated on a 2 mm glassy carbon electrode. Finally, the specimens were dried for 30 minutes at room temperature and deployed as working electrodes. A saturated calomel electrode (SCE) with a potential of 0.242 V was used as a reference electrode, and a platinum foil (4 cm 2 ) was exploited as the counter electrode. Cyclic voltammetry tests were performed with scan rates ranging from 10 to 70 mV s − 1 , and GCD curves were obtained through various current densities (0.3, 0.6, 1.2, 2.4 and 4.8 A g − 1 ) in a range of 0–1 V potential window. The EIS tests were performed from 1mHz to 100 kHz frequency range at 0 V vs. SCE. All experiments were conducted in 0.5 M H 2 SO 4 as the aqueous electrolyte. The specific capacitance values for CV of the electrodes are obtained using the following Eq. ( 1 )[35]: where Csp (F g − 1 ) is the specific capacitance, I (A) is the voltametric current, m (g) is the mass of the electrode material, ʋ (V s − 1 ) is the scan rate, ∆V (V) is the potential window, and the integral shows the area under the curves. The specific capacitance values for GCD of the electrodes are calculated as following Eq. ( 2 ): $$\:{C}_{sp}=\:\frac{I\times\:t}{m\:\varDelta\:V}$$ 2 where Cs (F g − 1 ), I (A), t (s), m (g) and ∆V (V) are the specific capacity, discharge current, discharge time, the mass of the active material and potential window, respectively. 2.4. Characterization Scanning Electron Microscopy (SEM) analysis was performed on FT- SRM, model MIRA3-TESCAN instrument to examine the surface structure of the biochar particles. The microscope was adjusted to an acceleration voltage of 15 kV and a magnification range of 500–1000 times. FTIR (FT/IR-4600; JASCO Co., Japan) was utilized to identify the functional groups and chemistry of synthesized biochar. The elemental (Carbon, hydrogen, nitrogen, and oxygen) analyzer (CHN) (Vario CHNEL-2) was employed to determine the elemental compositions of the biochar. X-ray diffraction analysis (XRD), and Branueur–Emmett–Teller (BET) analyses. XRD pattern was obtained using an X-ray diffraction (XRD) spectroscopy (model: APD 2000, G.N.R. srl, Novara, Italy) with Cu Kα radiation (λ = 1.5405 Å) over the angular range 15° ≤ 2θ ≤ 60°, operating at 30 kV and 10 mA. The surface area and pore width of the biochar were measured using a BET analyzer (Micromeritics Gemini 2375 V4). The biochar samples' surface area was determined from the nitrogen adsorption-desorption isotherm using the BET theory. Also, the total pore volume and pore diameter by using the Barrett–Joyner–Halenda (BJH) method was employed to calculate them in the range of mesopores and small macropores from experimental N2 isotherms (desorption branch) using the Kelvin model of pore filling. The water contact angle measurement was conducted using the images of 10 µL water droplets on four different points of the biochar surface using a digital microscope. 3. Results and Discussion 3.1.3. Fourier Transform-Infrared (FT-IR) Spectra: In order to understand the chemical changes during pyrolysis process, the FTIR spectrum of synthesized biochar at different pressures was shown in Fig. 1 . The peak at 3420 cm − 1 can be related to the stretching vibration of hydroxyl groups in the carboxylic, phenolic and alcoholic functional groups of biochar. The intensity of -OH characteristic peak was decreased by decreasing the pressure during the pyrolysis, means increasing in the elimination of this functional groups as water or CO 2 in the low pressure. Also, presence of N-H bonds associated the amino groups of the proteins in the biomass cell wall, indicated this groups were not eliminated at pressure of 1.5 atm while this characteristic peak has vanished completely at 0.5 atm. The peaks at 1042, 2889 and 1050 cm − 1 represent the vibration of the C–H bond and isomorphic or inhomogeneous stretching vibration) in the aliphatic structures, respectively [36]. These peaks were decreased by decreasing the pressure of the process. This means the conversion of aliphatic hydrocarbons containing at least six carbon atoms to aromatic hydrocarbons can be promoted at low pressure which confirmed by increasing the aromatic C = C stretching vibration intensity. The peaks at 1432 and 1620 cm − 1 can be attributed to the C = C vibration of the aromatic ring that has got more intense by reducing the pressure. The peak at 910 cm − 1 corresponds to the absorption vibration of the epoxy group which is disappeared completely at 0.5 atm. The peak at 1026 cm − 1 can be attributed to the C–O stretch of esters and carboxylic groups which has decreased in intensity by decreasing the pressure meaning removing the carboxylic groups as CO 2 gas by decreasing the procedure pressure [37]. It can be concluded surface of the prepared biochars can decorated by different functional groups, especially carboxyl and hydroxyl groups which may act as active centers for chemisorption. The amount of functional groups on the biochar surface can be manipulated by the pressure. As the pressure increases, the amount of functional groups increases, so it can be said at pressure of 1.5 atm the most functional groups have been survived. This may be because, at moderate pressure (e.g., 1.5 atm), volatile compounds have longer residence time in the reactor, promoting secondary reactions and possible re-condensation of pyrolysis vapors onto the biochar surface. This may contribute to the preservation or formation of additional surface functional groups compared to lower pressure conditions. 3.1. Morphological and structural characterization 3.1.1. SEM Spectra Analysis SEM images of synthesized biochar particles at different pressures of 0.5, 1, and 1.5 atm, were shown in Figure (2 a, b and c). the obtained images indicated the number of holes and porosity on the biochar particles decreases with increasing the pressure. In the other words, the sample produced at 0.5 atm has a highly porous and open structure with numerous uniform and round pores. This indicates under reduced pressure conditions, volatile compounds are released more efficiently, minimizing secondary reactions that could otherwise clog the pores. Additionally, the surface appears relatively smooth and cleaner, with fewer residual particles, suggesting a more effective pyrolysis process. The high porosity of biochar produced under low-pressure pyrolysis conditions can be attributed to the expansion of the biomass matrix during thermal decomposition, as well as to the reduced vapor pressure of volatile compounds within the biomass, which enhances pore development. Indeed, Lower external pressure reduces the boiling points (effective vapor pressures) of volatile compounds formed during pyrolysis. This allows volatiles to escape more easily and rapidly from the biomass matrix, which can promote pore formation and limit secondary tar condensation inside pores. Such a structure is highly desirable for applications like electrochemical energy storage electrodes, where high surface area and pore connectivity are crucial. In Fig. 2 (b), which corresponds to synthesizing pressure of 1 atm, the surface appears denser and shows fewer visible pores. As it can be observed in the SEM image, considerable number of residual particles are present on the surface, likely resulting from prolonged retention of volatile matter and their secondary charring reactions. Consequently, the partial blockage of pores and reduced accessible surface area because of secondary reactions, make this material less suitable for electrochemical applications than the low-pressure sample. The SEM image of produced biochar at 1.5 atm in Fig. 2 (c), exhibited even less porous structure, characterized by only a large, isolated pores. The higher pressure likely hindered the escape of volatile compounds, thereby promoting the formation of a denser carbon matrix. Despite its relatively clean surface, the material exhibits fewer pores and a more limited pore distribution, suggesting reduced permeability and specific surface area that could compromise its electrochemical performance. In summary, reducing the pressure during pyrolysis proves to be an effective strategy for enhancing the porous structure of biochar, thereby increasing its suitability for high-performance electrodes. This is exemplified by the sample produced at 0.5 atm (Fig. 2 (a)), which displays the most favorable morphology for energy storage[38]. 3.1.3. X-ray diffraction Analysis (XRD) XRD analysis was conducted to examine how synthesis pressure (0.5, 1, and 1.5 atm) influences the crystalline structure of biochar, as illustrated in Fig. 3 (a, b, and c). The XRD patterns for all three samples feature a broad peak around 2θ = 20°–30°, which is assigned to the (002) plane of graphitic carbon. The broadness of this peak is indicative of a disordered, turbostratic carbon structure, confirming that the material lacks the long-range order of highly crystalline graphite Among the three samples, the one synthesized at 0.5 atm displays the broadest (002) peak with the lowest intensity, pointing to a highly disordered, amorphous structure. The lower pressure during pyrolysis appears to promote more extensive devolatilization and structural breakdown of the biomass, which in turn inhibits the reorganization of carbon atoms into well-ordered graphitic layers. As a result, this biochar exhibits a predominantly amorphous carbon framework. By increasing the synthesizing pressure to 1 and 1.5 atm, the diffraction peak becomes slightly sharper and more intense compared to the 0.5 atm sample. This indicates an increase in the degree of graphitic ordering, implying that the carbon structure is beginning to exhibit partial alignment or stacking of graphene-like layers. The formation of such ordered domains is also observed under atmospheric pressure conditions. The more increasing of synthesizing pressure to 1.5 atm causes the more sharpest and intense peak. This indicated a higher degree of graphitic structural development, as increased pressure likely enhances the mobility and rearrangement of carbon atoms, thereby promoting the growth of graphitic domains. Consequently, this sample exhibits a relatively more crystalline carbon structure. Overall, as the pyrolysis pressure increased from 0.5 to 1.5 atm, the XRD patterns revealed a clear progression toward enhanced structural order and graphitization in the biochar. Thus, the sample produced at 0.5 atm exhibited a more amorphous structure which may be advantageous for applications requiring high surface area and pore accessibility, such as supercapacitor electrodes. In contrast, the sample prepared at 1.5 atm displayed greater graphitic character, potentially leading to improved electrical conductivity. [39–41]. 3.1.4. Porous Characteristics (BET) The surface area and micropore volume of the biochar samples were determined using nitrogen adsorption–desorption data, applying the multipoint BET method and the Dubinin–Radushkevich (DR) model, respectively and was illustrated in Based on IUPAC classification these curves can be adapted to type IV isotherms, whit hysteresis loop, which can be related to the presence of mesopores[42, 43]. This mesoporous structure of biochar is desirable, as it enhances the material’s adsorption capacity, water retention ability that makes it suitable for supercapacitors and gas storage[44–46]. As can be observed from the obtained curves, increasing the pyrolysis pressure led to a noticeable decrease in both the nitrogen uptake and the surface area of the biochar[44–47].. The BET surface area is indicative of the extent of mesoporosity and the presence of larger micropores within the char structure [48]. It is accepted, during pyrolysis, the release of volatile matter from the biomass typically creates new pore spaces, leading to an increase in the BET surface area. [49–51].Otherwise, at higher temperatures and pressures, the surface area may also decrease as pore walls collapse from the melting and fusion of the biomass cellular structure. [48, 52, 53]. Extensive studies on the temperature dependence of BET surface area show that it generally increases up to around 900°C, after which thermal deactivation becomes dominant. However, the effect of pyrolysis pressure has received far less attention.[54, 55]. Studies on heating rate effect on physicochemical properties of the biochar [,53, 56, 57] indicated Fast pyrolysis achieved high biochar yield at 1000°C while Slow pyrolysis produced biochar with a high surface area (763.06 m 2 /g) and superior conductivity (15.94 S/cm).( Effect of pyrolysis temperature and heating rate on the physicochemical properties of alkali lignin-derived biochar: A comparative study of fast and slow pyrolysis). Interestingly, Yang et al. reported that the maximum surface area in coal pyrolysis occurs at an intermediate pressure[58]. This trend was explained by the suppression of volatile release and cracking under these conditions, which is believed to facilitate optimal pore development. In our study, raising the synthesis pressure resulted in a reduction in both specific surface area and pore volume, accompanied by an increase in pore size which is related to the release of volatile matter from the biomass at reduced pressure. Table 1 Porous characteristics of biochar samples Sample Specific Surface Area (m 2 /g) Pore Volume (m 3 /g) Pore Size (nm) 0.5 atm 36.31 0.0151 4.23 1.0 atm 16.04 0.0062 8.25 1.5 atm 10.71 0.0040 12.73 3.1.5. Water Contact Angle Measurements (WCA) To investigate the effect of synthesising pressure on the physicochemical properties of the biochar, the water contact angle (WCA) of the samples was measured and shown in Fig. 5 . As the biochar was obtained as a powder, direct WCA measurement was not feasible. Accordingly, 1 g of the material was pelletized using a 1‑ton press, yielding a smooth‑surfaced cylinder measuring 1 cm in diameter and 0.5 cm in height.The WCA on the surface of biochar produced at a pressure of 1.5 atmospheres, is 4° indicating its highly hydrophilic nature. This hydrophilicity can be due to the presence of polar functional groups on the surface and the reduction of biochar porosity. The contact angles measured for the biochars produced at 1 atm and 0.5 atm were 56° and 109°, respectively. The hydrophobicity observed at the higher pressure is attributed to the development of a porous structure, the elimination of polar functional groups, and the entrapment of air within tubular pores.The observation of hydrophobicity can be expressed by the Cassie-Baxter model [59], where water cannot easily penetrate the micro-nanostructure, due to the trapped air between the water drop and the substrate, so water droplet easily rolls off from the surface. 3.1.6. Elemental Analyser (CHN) To understand the chemistry of synthesized biochar elemental analysis (CHN) was performed and the results of this studypresented in Table 2 . A clear trend correlated with changes in pyrolysis pressure is evident from the data. As the pressure decresed from 1.5 to 0.5 atm, the carbon content of the samples increased significantly from 48.13% to 77.34% to, while the hydrogen and oxygen contents decreasedfrom 5.61% to 1.09% and from 44.28% to 20.56% for hydrogen and oxygen, respectively. These observations correspond to significant alterations in both the chemical structure and surface chemistry of the biochar which was confirmed by FTIR and WCA analysis. The suppression of volatile release at higher pressures leads to the retention of oxygen- and hydrogen-rich species within the biochar matrix. This retention facilitates secondary deposition or repolymerization reactions, resulting in a biochar with lower carbon content and higher relative abundances of hydrogen and oxygen. In summary, CHN analysis confirms that increasing pyrolysis pressure produces biochars with lower carbon content and enhanced surface functionalization. These changes directly impact biochar performance in applications that rely on surface functionality and porosity, such as electrochemistry and adsorption.[ 60, 61]. Table 2 CHN of the different biochar samples. element C (%) H (%) N (%) O (%) 0.5 atm 77.34 1.09 1.01 20.56 1.0 atm 58.48 2.75 1.56 37.21 1.5 atm 48.13 5.61 1.98 44.28 3.2. Electrochemical studies on super-capacitance behavior 3.2.1. Cyclic Voltammetry (CV) The cyclic voltammetry (CV) analyses provide a detailed comparison of the electrochemical performance of electrodes made by synthesized biochar under different pyrolysis pressures. Which were presented in Fig. 6 . Indeed, CV is a convenient tool for obtaining qual. information about electron transfer processes (A Practical Beginner's Guide to Cyclic Voltammetry). As shown in Fig. 6 a, the CV curves recorded at a scan rate of 50 mV s⁻¹ clearly reveal that the sample prepared at 0.5 atm exhibits the largest enclosed area, indicating the highest specific capacitance among the three. The nearly rectangular shape of the CV curve for this sample suggests an ideal electric double-layer capacitor (EDLC) behavior and efficient charge storage. In contrast, the samples produced at 1.0 atm and especially 1.5 atm show smaller CV areas and more distorted shapes, reflecting reduced capacitive performance, likely due to lower porosity and decreased surface accessibility caused by structural densification at higher pressures. Figure 6 b focuses on the CV behavior of the 0.5 atm sample at various scan rates ranging from 10 to 70 mV s⁻¹. The curves retain a quasi-rectangular shape across all scan rates, highlighting the excellent rate capability and fast ion transport within the porous electrode structure. The preservation of curve symmetry and shape with increasing scan rate also suggests stable electrochemical kinetics. The variation of specific capacitance with scan rate for all three samples is presented in Fig. 8 c. The 0.5 atm sample consistently delivers the highest specific capacitance across the entire range of scan rates, further confirming its superior electrochemical properties. While all samples show a typical decrease in capacitance with increasing scan rate—attributable to limited ion diffusion at higher rates—the drop is more significant for the 0.5 atm sample due to its higher reliance on surface-accessible porosity for charge storage. Cycling stability of the electrodes is illustrated in Figs. 6 d to 6 f, where CV curves after multiple cycles are shown for the 1.5 atm, 1.0 atm, and 0.5 atm samples, respectively. The 0.5 atm sample (Fig. 8 f) not only maintains the largest current response but also shows minimal shape distortion over cycles, demonstrating excellent cycling stability. The 1.0 atm sample (Fig. 8 e) exhibits moderate current and acceptable stability, while the 1.5 atm sample (Fig. 8 d) shows the lowest current response, though its CV curves remain consistent in shape. In summary, the CV data underscore that the biochar synthesized at 0.5 atm exhibits the most favorable combination of specific capacitance, rate capability, and long-term stability, making it a highly promising material for supercapacitor applications. Increasing the pyrolysis pressure to 1.0 and 1.5 atm leads to diminished electrochemical performance due to reduced surface area and porosity. 3.2.2. Electrochemical Impedance Spectroscopy (EIS) The Nyquist plot shown in the Fig. 7 illustrates the electrochemical impedance characteristics of biochar electrodes produced under three different pyrolysis pressures. The inset schematic represents the equivalent circuit model used for fitting, consisting of solution resistance (R s ), a constant phase element (CPE dl ) for double-layer capacitance, charge transfer resistance (R ct ), and Warburg impedance (Z w ), which corresponds to ion diffusion [62]. At high frequencies, the intersection of each curve with the real axis (Z′) reflects the solution resistance (R s ). In the mid-frequency region, the semicircle is indicative of the charge transfer process and reflects the charge transfer resistance (R ct ). The 1.5 atm sample shows the largest semicircle, indicating the highest R ct , which implies slower charge transfer kinetics and greater resistance at the electrode–electrolyte interface. On the other hand, the 0.5 atm sample exhibits the smallest semicircle, suggesting the lowest R ct and more efficient electron transfer. This unexpected trend may be attributed to the more open porous network in the low-pressure biochar, which provides a larger electrochemically active surface area, facilitating charge transfer despite its less ordered (amorphous) structure. At low frequencies, the linear portion of the curves corresponds to Warburg impedance, associated with ion diffusion within the porous structure. The slope of the line is steepest for the 0.5 atm sample, reflecting superior ion diffusion and better electrolyte accessibility. Conversely, the 1.5 atm sample shows a flatter line, implying that the denser structure formed under high pressure may hinder ion transport. 3.2.3. Galvanostatic Charge– Discharge (GCD) The galvanostatic charge–discharge (GCD) analyses presented in Fig. 8 provide further insight into the electrochemical performance of the biochar electrodes synthesized at different pressures. In Fig. 8 a, the GCD curves of the samples recorded at a current density of 0.3 A g⁻ 1 clearly demonstrate that the electrode derived at 0.5 atm shows the longest discharge time, indicating the highest specific capacitance among the three. This extended discharge period is indicative of a larger charge storage capacity. In contrast, the samples prepared at 1.0 atm and 1.5 atm display shorter discharge durations, reflecting their comparatively lower capacitance, which can be attributed to reduced porosity and decreased ion accessibility due to structural compaction at higher pyrolysis pressures. Figure 8 b presents the GCD curves of the 0.5 atm sample under varying current densities from 0.3 to 4.8 A g⁻ 1 . As the current density increases, the charge–discharge time gradually decreases, which is a typical behavior due to the limited time available for ion diffusion at higher currents. Nonetheless, the relatively symmetric triangular shapes of the GCD curves across all current densities indicate good electrochemical reversibility and capacitive behavior. The specific capacitance as a function of current density for all three samples is shown in Fig. 8 c. The 0.5 atm sample again demonstrates superior performance, retaining the highest capacitance across all current densities. The capacitance gradually declines with increasing current density, particularly for the 0.5 atm electrode, due to the diminished utilization of active surface sites at higher current loads. The 1.0 atm and 1.5 atm samples show consistently lower capacitance, highlighting the negative impact of higher pyrolysis pressures on the electrochemical properties of the biochar. The long-term cycling stability of the electrodes is assessed in Figs. 8 d, 8 e, and 8 f, which present repetitive GCD curves over multiple cycles for the 1.5 atm, 1.0 atm, and 0.5 atm samples, respectively. In Fig. 8 f, the 0.5 atm sample exhibits excellent cycling performance over 1000 seconds with minimal shape distortion and consistent voltage profiles, indicating strong electrochemical stability and reversibility. Similarly, the 1.0 atm electrode (Fig. 8 e) maintains a stable charge–discharge profile over 300 seconds, though with lower capacity. The 1.5 atm sample (Fig. 8 d) also shows decent stability but has the shortest discharge time and lower current response, confirming its lower charge storage capacity. In summary, the GCD results clearly establish that the electrode synthesized at 0.5 atm exhibits superior specific capacitance, rate capability, and long-term stability, making it the most promising candidate among the studied samples for supercapacitor applications. Increasing the synthesis pressure to 1.0 and 1.5 atm adversely affects electrochemical performance due to structural changes that hinder ion transport and reduce accessible surface area. 4. Conclusion This research confirms that pyrolysis pressure is a decisive factor in engineering biochar for supercapacitor applications. Among the tested pressures, biochar synthesized at 0.5 atm demonstrated the most favorable performance, with a specific surface area of 36.31 m² g − 1 , pore volume of 0.0151 m³ g − 1 , and specific capacitance up to 61 F g − 1 at low scan rates. In contrast, increasing the pressure to 1.5 atm led to structural densification, decreasing the surface area to 10.71 m² g − 1 , pore volume to 0.004 m³ g − 1 , and specific capacitance to only 29 F g − 1 . Additionally, the water contact angle (WCA) reduced to 4° at 1.5 atm, highlighting enhanced hydrophilicity due to increased surface functional groups—confirmed by FTIR and elemental analysis showing higher oxygen content (44.28%). Electrochemical studies including CV, GCD, and EIS revealed superior rate capability, charge–discharge symmetry, and ion diffusion in the 0.5 atm sample. Overall, the results suggest that lower pyrolysis pressures lead to biochar with optimized porosity, higher capacitance, and enhanced electrochemical efficiency, making it an ideal candidate for energy storage applications. Declarations Conflict of interest The authors declares that there is no conflict of interest. Funding sources This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Author contributions The first draft of the manuscript was written by Gholam Reza Allahgholipour. Dr. Somayeh Mohamadi is the corresponding author and takes primary responsibility for communication with the journal and editorial processing. References Y. Zhang, Y. Liang, S. Li, Y. Yuan, D. Zhang, Y. Wu, H. Xie, K. Brindhadevi, A. Pugazhendhi, C. Xia, A review of biomass pyrolysis gas: Forming mechanisms, influencing parameters, and product application upgrades, Fuel , 347 (2023) 128461. D. Lachos-Perez, J.C. Martins-Vieira, J. 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Supplementary Files Graphicalabstract.docx Cite Share Download PDF Status: Under Review Version 1 posted Reviewers agreed at journal 07 May, 2026 Reviewers invited by journal 06 May, 2026 Editor assigned by journal 02 May, 2026 First submitted to journal 27 Apr, 2026 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-9491472","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":635385036,"identity":"2cb0e2de-2271-44d7-ac57-c5d9c9204c4a","order_by":0,"name":"Gholam Reza Allahgholipour","email":"","orcid":"","institution":"Bu-Ali Sina University: Bu Ali Sina University","correspondingAuthor":false,"prefix":"","firstName":"Gholam","middleName":"Reza","lastName":"Allahgholipour","suffix":""},{"id":635385037,"identity":"f5f68d3e-137e-4bf5-837a-4320f48a51ed","order_by":1,"name":"somayeh Mohamadi","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA7klEQVRIiWNgGAWjYHACNhDB2MbAfPjBBxCXnXgtbGmGM0BcZmK1NDDwGEjzgJiEtMhPO3zswc8cO9k+BrYEY5tf2+T5mBkYP3zMwa3F4HZaumHvtmRjoF8OPM7tu23YxszALDlzGx4t0jlmErzbmBPbQLbk9txmBGphY+bFo0V+do6Z5N9t9UAtQL9Y9ty2J6iF4XaOmTTvtsMQLQw/bicS1AL0S5q07Lbjxm3MwEDubbid3MbM2IzXL/Kzk49Jvt1WLTu/vfnwgx9/btsCGQc/fMTnMDgARQcwQhnAcUQC+EOK4lEwCkbBKBgpAAANrEwQCSooqwAAAABJRU5ErkJggg==","orcid":"https://orcid.org/0000-0002-7277-5206","institution":"research center of environment and sustainable development","correspondingAuthor":true,"prefix":"","firstName":"somayeh","middleName":"","lastName":"Mohamadi","suffix":""}],"badges":[],"createdAt":"2026-04-22 06:06:08","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-9491472/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-9491472/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":109293924,"identity":"3a9bac79-49d2-4fee-842c-318b6da89106","added_by":"auto","created_at":"2026-05-15 08:12:45","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":816511,"visible":true,"origin":"","legend":"\u003cp\u003eFE-SEM image of (a) 0.5 atm, (b)1 atm, and (c) 1.5 atm\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-9491472/v1/e6aa315ca978bab8817948c8.png"},{"id":109293925,"identity":"b04cb2fc-9368-457d-803c-6de2ca8d840d","added_by":"auto","created_at":"2026-05-15 08:12:45","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":74132,"visible":true,"origin":"","legend":"\u003cp\u003eXRD patterns of the different biochar samples.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-9491472/v1/653446f4fcb5d35450f7ed5b.png"},{"id":109293930,"identity":"c82625b5-767a-4ff0-a1be-c2e997e875ea","added_by":"auto","created_at":"2026-05-15 08:12:46","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":39833,"visible":true,"origin":"","legend":"\u003cp\u003eFTIR of the different biochar samples.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-9491472/v1/dfafbd8d5a66c71f762c1c1b.png"},{"id":109293926,"identity":"d7f7175a-b666-45d2-ab97-2e106ad11cf3","added_by":"auto","created_at":"2026-05-15 08:12:45","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":53575,"visible":true,"origin":"","legend":"\u003cp\u003eBET of the different biochar samples.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-9491472/v1/5304ea1739b53efab5b0e152.png"},{"id":109293932,"identity":"1a8a7de7-6841-4e96-ad7c-5695472e11ab","added_by":"auto","created_at":"2026-05-15 08:12:46","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":32000,"visible":true,"origin":"","legend":"\u003cp\u003eThe WCA of the different biochar samples.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-9491472/v1/b0a6c2a2941d490553692c06.png"},{"id":109296449,"identity":"bd59ca63-8086-442f-a47a-9676e48d744b","added_by":"auto","created_at":"2026-05-15 08:47:03","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":496863,"visible":true,"origin":"","legend":"\u003cp\u003e\u0026nbsp;(a) GCD curves at 0.3 A g\u003csup\u003e-1\u003c/sup\u003e for sampeles, (b) GCD curves at different current density for 0.5 atm sample, (c) Specific capacitance relationship with current density, (d, e, f, and g) Cycling stability of sampeles\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-9491472/v1/0afd7263811f1a835d2cdeb7.png"},{"id":109296648,"identity":"2233b385-9f41-4aef-b5a2-a430105f51b4","added_by":"auto","created_at":"2026-05-15 08:48:56","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1537745,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-9491472/v1/3483fcf8-f912-4114-b0eb-9f84307cca54.pdf"},{"id":109296439,"identity":"1d6e2170-db5f-40cf-b91f-d8fbf7fb3923","added_by":"auto","created_at":"2026-05-15 08:47:01","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":664931,"visible":true,"origin":"","legend":"","description":"","filename":"Graphicalabstract.docx","url":"https://assets-eu.researchsquare.com/files/rs-9491472/v1/b001519f323d91c31912f2b6.docx"}],"financialInterests":"","formattedTitle":"Investigating the Effect of Pyrolysis Process Pressure on the Supercapacitive Properties of Biochar","fulltext":[{"header":"Highlights","content":"\u003cp\u003e\u0026bull; Low-pressure pyrolysis (0.5 atm) yields biochar with higher surface area, porosity, and superior supercapacitive performance.\u003c/p\u003e\u003cp\u003e\u0026bull; Increasing pyrolysis pressure enhances surface functional groups and hydrophilicity but decreases pore volume and surface area.\u003c/p\u003e\u003cp\u003e\u0026bull; Biochar synthesized at 0.5 atm demonstrates highest specific capacitance and best charge-discharge stability.\u003c/p\u003e\u003cp\u003e\u0026bull; SEM, XRD, BET, FTIR, and CHN analyses reveal pressure-dependent structural and chemical evolution of biochar.\u003c/p\u003e\u003cp\u003e\u0026bull; Optimizing pyrolysis pressure is key to engineering biochar for efficient energy storage applications.\u003c/p\u003e"},{"header":"1. Introduction","content":"\u003cp\u003eConverting biomass into other products is highly significant, as it enables waste recycling and increases the added value of the overall industrial chain. [1, 2]. The term biomass generally denotes organic materials originating from agricultural and forestry waste. While these resources are plentiful, they are often characterized by a high water content [3, 4]. Due to the vast volume of biomass worldwide, if it is burned or left to decompose in the environment, it will oxidize and release large amounts of CO\u003csub\u003e2\u003c/sub\u003e into the atmosphere. [5, 6]. Therefore, it can be said stabilizing the carbon contained in biomass is essential to prevent CO\u003csub\u003e2\u003c/sub\u003e emissions. Among current biomass recycling and reprocessing techniques, pyrolysis is often the preferred approach, as it generates minimal pollution while yielding valuable by-products[7, 8]. In addition to fixing carbon and generating valuable biochar, pyrolysis serves as the initial step in gasification and other thermochemical conversion processes used to harness energy from biomass.[9, 10]. Accordingly, understanding how operating parameters influence the thermal conversion of biomass during pyrolysis is key to optimizing the overall process. [11, 12].\u003c/p\u003e \u003cp\u003eBiomass pyrolysis involves the thermal decomposition of macromolecular chemical bonds within raw biomass under an oxygen-free atmosphere, yielding biogas, bio-oil, and biochar\u0026mdash;products characterized by compounds of significantly lower molecular weight than those present in the original material. [13, 14]. In contrast to conventional pyrolysis, fast pyrolysis utilizes a higher heating rate and a shorter residence time for hot vapors. When carried out at a moderate temperature, this process maximizes the production of liquid biofuels. [15, 16].\u003c/p\u003e \u003cp\u003eWith its versatile properties, biochar is widely used as a carbon template for net-shaped materials and as a catalyst support. It is also a key component in electrodes for electrochemical energy storage systems [17, 18]. In particular, there has been growing interest in the application of biochar for electrodes in electrochemical capacitors.[19\u0026ndash;23]. Furthermore, electrochemical performance is affected by several factors, including microporosity, surface chemistry, and electrical conductivity. [24]. The properties of biochar can vary significantly, as they are strongly influenced by both the biomass feedstock and the pyrolysis conditions. [22, 25]. Key parameters in the pyrolysis process include time, heating rate, temperature, and pressure (i.e., atmospheric conditions)[26]. Consequently, understanding how to optimize these parameters is essential for obtaining the desired biochar properties for specific applications[27].\u003c/p\u003e \u003cp\u003eWith respect to pressure, three primary thermal decomposition techniques\u0026mdash;low pressure pyrolysis, atmospheric pyrolysis and higy pressure \u0026mdash;are commonly employed in industry [28]. Pyrolysis in low pressure is conducted in an oxygen-free environment at pressures significantly below atmospheric levels, enabling biofuel extraction. This method has comparable effects to atmospheric pyrolysis, which requires a rapid heating rate and finely sized biomass feedstocks [28, 29]. Furthermore, compared to atmospheric pyrolysis, pyrolysis in low pressure offers several advantages: (1) reduced residence time of volatile compounds, preventing pore blockage from secondary charring reactions; (2) greater sensitivity of the biochar surface to oxidation; and (3) higher carbon yield [30\u0026ndash;33]. As a result, biochar produced under low pressure conditions is expected to possess a more open pore structure and be more suitable as a precursor for activated carbon. Nevertheless, there is currently limited research reporting on the physicochemical characteristics of biochar monoliths prepared under low pressure conditions, especially regarding their use as electrodes in electrochemical energy storage applications.\u003c/p\u003e \u003cp\u003eK. M. Subramaniam et al. investigated feedstock variability, process parameters, and energy efficiency in biomass pyrolysis. This research assesses the pyrolysis characteristics of five types of agricultural wastes\u0026mdash;rice husk, corn stalk, palm kernel shell, coconut shell, and wheat straw\u0026mdash;using a fixed-bed reactor. The study examined temperature (300 to 700\u0026deg;C), heating rate (5\u0026ndash;20\u0026deg;C/min), and residence time (30 to 120 min) as key process variables. The biochar and bio-oil yields from the pyrolysis of different biomass feedstocks were analyzed. It was observed that biochar yield increased with rising pyrolysis temperature, with the magnitude of this effect being feedstock-dependent. Furthermore, an increase in pyrolysis temperature altered the bio-oil composition, resulting in elevated ketone (7.8%) and ester (5.1%) concentrations and a reduction in phenolic content from 15.2% to 13.0%[34].\u003c/p\u003e \u003cp\u003eThis study aims to investigate the influence of pyrolysis pressure on the microstructure and consequently supercapacitive properties (specifically, electrical double layer capacitance) of biochar derived from plantain leaves. Biochar samples were obtained from pyrolysis prosecc under three different pressures\u0026mdash;0.5, 1, and 1.5 atmospheres\u0026mdash; with fixed other parameters to assess the resulting changes in material properties and electrochemical performance.\u003c/p\u003e"},{"header":"2. Experimental","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\n \u003ch2\u003e2.1 Materials:\u003c/h2\u003e\n \u003cp\u003eThe biomass used in this research was plane tree leaves, which were collected in autumn and dried in the atmosphere for 4 months. PVDF pelletlet (Kynar\u0026reg;1000HD) was provided by Atofina Co. N-Methyl-2-pyrrolidone and concentrated Sulfuric acid were obtained from Merck Co.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\n \u003ch2\u003e2.2 Feedstock and Biochar Preparation:\u003c/h2\u003e\n \u003cp\u003eThe collected leaves were powdered in the same conditions with a butterfly mill. Finally, these powders entered the reactor and were dried at 90\u0026deg;C for 4 hours. Then 3 kg of it was weighed and entered into the reactor and the main pyrolysis operation started. The pyrolysis temperature was 300\u0026deg;C and continued for 3 hours after the reactor temperature reached 300\u0026deg;C. In this research, three biochar preparation experiments were conducted at three different pressures of 0.5, 0.1, and 1.5 atmospheres and the same conditions. All production processes were carried out in a stainless steel reactor that had no openings and only contained one outlet and was designed for this purpose. A vacuum pump was used for a pressure of 0.5 atm, a cheek valve was used for a pressure of 1.0 atm to prevent the entry of oxygen, and a pressure relief valve was used for a pressure of 1.5 atm that was adjusted for this pressure.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\n \u003ch2\u003e2.3. Electrochemical studies:\u003c/h2\u003e\n \u003cp\u003eTo evaluate the electrochemical performance of the synthesized samples, cyclic voltammetry (CV), galvanostatic charge\u0026ndash;discharge (GCD) and electrochemical impedance spectroscopy (EIS) were carried out at room temperature in a three-electrode system by utilizing an Ivium-Vertex PGSTAT. The working electrode was prepared by containing 1 mg of active material consisting of 90% biochar and 10% polyvinylidene fluoride (PVDF) dispersed in N-Methyl-2-pyrrolidone (NMP) solvent. Then, the mixture was sonicated for 45 minutes, and three drops from the resulting slurry were coated on a 2 mm glassy carbon electrode. Finally, the specimens were dried for 30 minutes at room temperature and deployed as working electrodes. A saturated calomel electrode (SCE) with a potential of 0.242 V was used as a reference electrode, and a platinum foil (4 cm\u003csup\u003e2\u003c/sup\u003e) was exploited as the counter electrode. Cyclic voltammetry tests were performed with scan rates ranging from 10 to 70 mV s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, and GCD curves were obtained through various current densities (0.3, 0.6, 1.2, 2.4 and 4.8 A g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) in a range of 0\u0026ndash;1 V potential window. The EIS tests were performed from 1mHz to 100 kHz frequency range at 0 V vs. SCE. All experiments were conducted in 0.5 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e as the aqueous electrolyte. The specific capacitance values for CV of the electrodes are obtained using the following Eq.\u0026nbsp;(\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e)[35]:\u003c/p\u003e\n \u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\n \u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\u003cimg src=\"https://myfiles.space/user_files/58893_b39df98f09c4a4bb/58893_custom_files/img1778831557.png\" width=\"541\" height=\"97\"\u003e\u003c/div\u003e\n \u003c/div\u003e\n \u003cp\u003ewhere Csp (F g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) is the specific capacitance, I (A) is the voltametric current, m (g) is the mass of the electrode material, ʋ (V s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) is the scan rate, ∆V (V) is the potential window, and the integral shows the area under the curves. The specific capacitance values for GCD of the electrodes are calculated as following Eq.\u0026nbsp;(\u003cspan refid=\"Equ2\" class=\"InternalRef\"\u003e2\u003c/span\u003e):\u003c/p\u003e\n \u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\n \u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e$$\\:{C}_{sp}=\\:\\frac{I\\times\\:t}{m\\:\\varDelta\\:V}$$\u003c/div\u003e\n \u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\n \u003c/div\u003e\n \u003cp\u003ewhere Cs (F g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), I (A), t (s), m (g) and ∆V (V) are the specific capacity, discharge current, discharge time, the mass of the active material and potential window, respectively.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\n \u003ch2\u003e2.4. Characterization\u003c/h2\u003e\n \u003cp\u003eScanning Electron Microscopy (SEM) analysis was performed on FT- SRM, model MIRA3-TESCAN instrument to examine the surface structure of the biochar particles. The microscope was adjusted to an acceleration voltage of 15 kV and a magnification range of 500\u0026ndash;1000 times. FTIR (FT/IR-4600; JASCO Co., Japan) was utilized to identify the functional groups and chemistry of synthesized biochar. The elemental (Carbon, hydrogen, nitrogen, and oxygen) analyzer (CHN) (Vario CHNEL-2) was employed to determine the elemental compositions of the biochar. X-ray diffraction analysis (XRD), and Branueur\u0026ndash;Emmett\u0026ndash;Teller (BET) analyses. XRD pattern was obtained using an X-ray diffraction (XRD) spectroscopy (model: APD 2000, G.N.R. srl, Novara, Italy) with Cu K\u0026alpha; radiation (\u0026lambda;\u0026thinsp;=\u0026thinsp;1.5405 \u0026Aring;) over the angular range 15\u0026deg; \u0026le; 2\u0026theta;\u0026thinsp;\u0026le;\u0026thinsp;60\u0026deg;, operating at 30 kV and 10 mA. The surface area and pore width of the biochar were measured using a BET analyzer (Micromeritics Gemini 2375 V4). The biochar samples\u0026apos; surface area was determined from the nitrogen adsorption-desorption isotherm using the BET theory. Also, the total pore volume and pore diameter by using the Barrett\u0026ndash;Joyner\u0026ndash;Halenda (BJH) method was employed to calculate them in the range of mesopores and small macropores from experimental N2 isotherms (desorption branch) using the Kelvin model of pore filling. The water contact angle measurement was conducted using the images of 10 \u0026micro;L water droplets on four different points of the biochar surface using a digital microscope.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"3. Results and Discussion","content":"\u003cdiv id=\"Sec8\" class=\"Section3\"\u003e \u003cdiv class=\"Heading\"\u003e3.1.3. Fourier Transform-Infrared (FT-IR) Spectra:\u003c/div\u003e \u003cp\u003eIn order to understand the chemical changes during pyrolysis process, the FTIR spectrum of synthesized biochar at different pressures was shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The peak at 3420 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e can be related to the stretching vibration of hydroxyl groups in the carboxylic, phenolic and alcoholic functional groups of biochar. The intensity of -OH characteristic peak was decreased by decreasing the pressure during the pyrolysis, means increasing in the elimination of this functional groups as water or CO\u003csub\u003e2\u003c/sub\u003e in the low pressure. Also, presence of N-H bonds associated the amino groups of the proteins in the biomass cell wall, indicated this groups were not eliminated at pressure of 1.5 atm while this characteristic peak has vanished completely at 0.5 atm. The peaks at 1042, 2889 and 1050 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e represent the vibration of the C\u0026ndash;H bond and isomorphic or inhomogeneous stretching vibration) in the aliphatic structures, respectively [36]. These peaks were decreased by decreasing the pressure of the process. This means the conversion of aliphatic hydrocarbons containing at least six carbon atoms to aromatic hydrocarbons can be promoted at low pressure which confirmed by increasing the aromatic C\u0026thinsp;=\u0026thinsp;C stretching vibration intensity. The peaks at 1432 and 1620 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e can be attributed to the C\u0026thinsp;=\u0026thinsp;C vibration of the aromatic ring that has got more intense by reducing the pressure. The peak at 910 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e corresponds to the absorption vibration of the epoxy group which is disappeared completely at 0.5 atm.\u003c/p\u003e \u003cp\u003eThe peak at 1026 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e can be attributed to the C\u0026ndash;O stretch of esters and carboxylic groups which has decreased in intensity by decreasing the pressure meaning removing the carboxylic groups as CO\u003csub\u003e2\u003c/sub\u003e gas by decreasing the procedure pressure [37].\u003c/p\u003e \u003cp\u003eIt can be concluded surface of the prepared biochars can decorated by different functional groups, especially carboxyl and hydroxyl groups which may act as active centers for chemisorption. The amount of functional groups on the biochar surface can be manipulated by the pressure. As the pressure increases, the amount of functional groups increases, so it can be said at pressure of 1.5 atm the most functional groups have been survived. This may be because, at moderate pressure (e.g., 1.5 atm), volatile compounds have longer residence time in the reactor, promoting secondary reactions and possible re-condensation of pyrolysis vapors onto the biochar surface. This may contribute to the preservation or formation of additional surface functional groups compared to lower pressure conditions.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.1. Morphological and structural characterization\u003c/h2\u003e \u003cdiv id=\"Sec10\" class=\"Section3\"\u003e \u003ch2\u003e3.1.1. SEM Spectra Analysis\u003c/h2\u003e \u003cp\u003eSEM images of synthesized biochar particles at different pressures of 0.5, 1, and 1.5 atm, were shown in Figure (2 a, b and c). the obtained images indicated the number of holes and porosity on the biochar particles decreases with increasing the pressure. In the other words, the sample produced at 0.5 atm has a highly porous and open structure with numerous uniform and round pores. This indicates under reduced pressure conditions, volatile compounds are released more efficiently, minimizing secondary reactions that could otherwise clog the pores. Additionally, the surface appears relatively smooth and cleaner, with fewer residual particles, suggesting a more effective pyrolysis process.\u003c/p\u003e \u003cp\u003eThe high porosity of biochar produced under low-pressure pyrolysis conditions can be attributed to the expansion of the biomass matrix during thermal decomposition, as well as to the reduced vapor pressure of volatile compounds within the biomass, which enhances pore development. Indeed, Lower external pressure reduces the boiling points (effective vapor pressures) of volatile compounds formed during pyrolysis. This allows volatiles to escape more easily and rapidly from the biomass matrix, which can promote pore formation and limit secondary tar condensation inside pores. Such a structure is highly desirable for applications like electrochemical energy storage electrodes, where high surface area and pore connectivity are crucial. In Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e2\u003c/span\u003e(b), which corresponds to synthesizing pressure of 1 atm, the surface appears denser and shows fewer visible pores.\u003c/p\u003e \u003cp\u003eAs it can be observed in the SEM image, considerable number of residual particles are present on the surface, likely resulting from prolonged retention of volatile matter and their secondary charring reactions. Consequently, the partial blockage of pores and reduced accessible surface area because of secondary reactions, make this material less suitable for electrochemical applications than the low-pressure sample.\u003c/p\u003e \u003cp\u003eThe SEM image of produced biochar at 1.5 atm in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e2\u003c/span\u003e(c), exhibited even less porous structure, characterized by only a large, isolated pores. The higher pressure likely hindered the escape of volatile compounds, thereby promoting the formation of a denser carbon matrix. Despite its relatively clean surface, the material exhibits fewer pores and a more limited pore distribution, suggesting reduced permeability and specific surface area that could compromise its electrochemical performance.\u003c/p\u003e \u003cp\u003eIn summary, reducing the pressure during pyrolysis proves to be an effective strategy for enhancing the porous structure of biochar, thereby increasing its suitability for high-performance electrodes. This is exemplified by the sample produced at 0.5 atm (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e2\u003c/span\u003e (a)), which displays the most favorable morphology for energy storage[38].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section3\"\u003e \u003ch2\u003e3.1.3. X-ray diffraction Analysis (XRD)\u003c/h2\u003e \u003cp\u003eXRD analysis was conducted to examine how synthesis pressure (0.5, 1, and 1.5 atm) influences the crystalline structure of biochar, as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e3\u003c/span\u003e(a, b, and c). The XRD patterns for all three samples feature a broad peak around 2θ\u0026thinsp;=\u0026thinsp;20\u0026deg;\u0026ndash;30\u0026deg;, which is assigned to the (002) plane of graphitic carbon. The broadness of this peak is indicative of a disordered, turbostratic carbon structure, confirming that the material lacks the long-range order of highly crystalline graphite Among the three samples, the one synthesized at 0.5 atm displays the broadest (002) peak with the lowest intensity, pointing to a highly disordered, amorphous structure. The lower pressure during pyrolysis appears to promote more extensive devolatilization and structural breakdown of the biomass, which in turn inhibits the reorganization of carbon atoms into well-ordered graphitic layers. As a result, this biochar exhibits a predominantly amorphous carbon framework.\u003c/p\u003e \u003cp\u003eBy increasing the synthesizing pressure to 1 and 1.5 atm, the diffraction peak becomes slightly sharper and more intense compared to the 0.5 atm sample. This indicates an increase in the degree of graphitic ordering, implying that the carbon structure is beginning to exhibit partial alignment or stacking of graphene-like layers. The formation of such ordered domains is also observed under atmospheric pressure conditions. The more increasing of synthesizing pressure to 1.5 atm causes the more sharpest and intense peak.\u003c/p\u003e \u003cp\u003eThis indicated a higher degree of graphitic structural development, as increased pressure likely enhances the mobility and rearrangement of carbon atoms, thereby promoting the growth of graphitic domains. Consequently, this sample exhibits a relatively more crystalline carbon structure.\u003c/p\u003e \u003cp\u003eOverall, as the pyrolysis pressure increased from 0.5 to 1.5 atm, the XRD patterns revealed a clear progression toward enhanced structural order and graphitization in the biochar. Thus, the sample produced at 0.5 atm exhibited a more amorphous structure which may be advantageous for applications requiring high surface area and pore accessibility, such as supercapacitor electrodes. In contrast, the sample prepared at 1.5 atm displayed greater graphitic character, potentially leading to improved electrical conductivity. [39\u0026ndash;41].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section3\"\u003e \u003ch2\u003e3.1.4. Porous Characteristics (BET)\u003c/h2\u003e \u003cp\u003eThe surface area and micropore volume of the biochar samples were determined using nitrogen adsorption\u0026ndash;desorption data, applying the multipoint BET method and the Dubinin\u0026ndash;Radushkevich (DR) model, respectively and was illustrated in Based on IUPAC classification these curves can be adapted to type IV isotherms, whit hysteresis loop, which can be related to the presence of mesopores[42, 43]. This mesoporous structure of biochar is desirable, as it enhances the material\u0026rsquo;s adsorption capacity, water retention ability that makes it suitable for supercapacitors and gas storage[44\u0026ndash;46]. As can be observed from the obtained curves, increasing the pyrolysis pressure led to a noticeable decrease in both the nitrogen uptake and the surface area of the biochar[44\u0026ndash;47]..\u003c/p\u003e \u003cp\u003eThe BET surface area is indicative of the extent of mesoporosity and the presence of larger micropores within the char structure [48]. It is accepted, during pyrolysis, the release of volatile matter from the biomass typically creates new pore spaces, leading to an increase in the BET surface area. [49\u0026ndash;51].Otherwise, at higher temperatures and pressures, the surface area may also decrease as pore walls collapse from the melting and fusion of the biomass cellular structure. [48, 52, 53]. Extensive studies on the temperature dependence of BET surface area show that it generally increases up to around 900\u0026deg;C, after which thermal deactivation becomes dominant. However, the effect of pyrolysis pressure has received far less attention.[54, 55].\u003c/p\u003e \u003cp\u003eStudies on heating rate effect on physicochemical properties of the biochar [,53, 56, 57] indicated Fast pyrolysis achieved high biochar yield at 1000\u0026deg;C while Slow pyrolysis produced biochar with a high surface area (763.06 m\u003csup\u003e2\u003c/sup\u003e/g) and superior conductivity (15.94 S/cm).( Effect of pyrolysis temperature and heating rate on the physicochemical properties of alkali lignin-derived biochar: A comparative study of fast and slow pyrolysis). Interestingly, Yang et al. reported that the maximum surface area in coal pyrolysis occurs at an intermediate pressure[58]. This trend was explained by the suppression of volatile release and cracking under these conditions, which is believed to facilitate optimal pore development. In our study, raising the synthesis pressure resulted in a reduction in both specific surface area and pore volume, accompanied by an increase in pore size which is related to the release of volatile matter from the biomass at reduced pressure.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003ePorous characteristics of biochar samples\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSample\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSpecific Surface Area\u003c/p\u003e \u003cp\u003e(m\u003csup\u003e2\u003c/sup\u003e/g)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003ePore Volume\u003c/p\u003e \u003cp\u003e(m\u003csup\u003e3\u003c/sup\u003e/g)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003ePore Size\u003c/p\u003e \u003cp\u003e(nm)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e0.5 atm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e36.31\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.0151\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e4.23\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1.0 atm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e16.04\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.0062\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e8.25\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1.5 atm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e10.71\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.0040\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e12.73\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section3\"\u003e \u003ch2\u003e3.1.5. Water Contact Angle Measurements (WCA)\u003c/h2\u003e \u003cp\u003eTo investigate the effect of synthesising pressure on the physicochemical properties of the biochar, the water contact angle (WCA) of the samples was measured and shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. As the biochar was obtained as a powder, direct WCA measurement was not feasible. Accordingly, 1 g of the material was pelletized using a 1‑ton press, yielding a smooth‑surfaced cylinder measuring 1 cm in diameter and 0.5 cm in height.The WCA on the surface of biochar produced at a pressure of 1.5 atmospheres, is 4\u0026deg; indicating its highly hydrophilic nature. This hydrophilicity can be due to the presence of polar functional groups on the surface and the reduction of biochar porosity.\u003c/p\u003e \u003cp\u003eThe contact angles measured for the biochars produced at 1 atm and 0.5 atm were 56\u0026deg; and 109\u0026deg;, respectively. The hydrophobicity observed at the higher pressure is attributed to the development of a porous structure, the elimination of polar functional groups, and the entrapment of air within tubular pores.The observation of hydrophobicity can be expressed by the Cassie-Baxter model [59], where water cannot easily penetrate the micro-nanostructure, due to the trapped air between the water drop and the substrate, so water droplet easily rolls off from the surface.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section3\"\u003e \u003ch2\u003e3.1.6. Elemental Analyser (CHN)\u003c/h2\u003e \u003cp\u003eTo understand the chemistry of synthesized biochar elemental analysis (CHN) was performed and the results of this studypresented in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. A clear trend correlated with changes in pyrolysis pressure is evident from the data. As the pressure decresed from 1.5 to 0.5 atm, the carbon content of the samples increased significantly from 48.13% to 77.34% to, while the hydrogen and oxygen contents decreasedfrom 5.61% to 1.09% and from 44.28% to 20.56% for hydrogen and oxygen, respectively. These observations correspond to significant alterations in both the chemical structure and surface chemistry of the biochar which was confirmed by FTIR and WCA analysis.\u003c/p\u003e \u003cp\u003eThe suppression of volatile release at higher pressures leads to the retention of oxygen- and hydrogen-rich species within the biochar matrix. This retention facilitates secondary deposition or repolymerization reactions, resulting in a biochar with lower carbon content and higher relative abundances of hydrogen and oxygen.\u003c/p\u003e \u003cp\u003eIn summary, CHN analysis confirms that increasing pyrolysis pressure produces biochars with lower carbon content and enhanced surface functionalization. These changes directly impact biochar performance in applications that rely on surface functionality and porosity, such as electrochemistry and adsorption.[ 60, 61].\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eCHN of the different biochar samples.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eelement\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eC (%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eH (%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eN (%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eO (%)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e0.5 atm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e77.34\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1.09\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e20.56\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1.0 atm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e58.48\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2.75\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.56\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e37.21\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1.5 atm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e48.13\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e5.61\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.98\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e44.28\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section2\"\u003e \u003ch2\u003e3.2. Electrochemical studies on super-capacitance behavior\u003c/h2\u003e \u003cdiv id=\"Sec16\" class=\"Section3\"\u003e \u003ch2\u003e3.2.1. Cyclic Voltammetry (CV)\u003c/h2\u003e \u003cp\u003eThe cyclic voltammetry (CV) analyses provide a detailed comparison of the electrochemical performance of electrodes made by synthesized biochar under different pyrolysis pressures. Which were presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e. Indeed, CV is a convenient tool for obtaining qual. information about electron transfer processes (A Practical Beginner's Guide to Cyclic Voltammetry). As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea, the CV curves recorded at a scan rate of 50 mV s⁻\u0026sup1; clearly reveal that the sample prepared at 0.5 atm exhibits the largest enclosed area, indicating the highest specific capacitance among the three. The nearly rectangular shape of the CV curve for this sample suggests an ideal electric double-layer capacitor (EDLC) behavior and efficient charge storage. In contrast, the samples produced at 1.0 atm and especially 1.5 atm show smaller CV areas and more distorted shapes, reflecting reduced capacitive performance, likely due to lower porosity and decreased surface accessibility caused by structural densification at higher pressures.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb focuses on the CV behavior of the 0.5 atm sample at various scan rates ranging from 10 to 70 mV s⁻\u0026sup1;. The curves retain a quasi-rectangular shape across all scan rates, highlighting the excellent rate capability and fast ion transport within the porous electrode structure. The preservation of curve symmetry and shape with increasing scan rate also suggests stable electrochemical kinetics.\u003c/p\u003e \u003cp\u003eThe variation of specific capacitance with scan rate for all three samples is presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ec. The 0.5 atm sample consistently delivers the highest specific capacitance across the entire range of scan rates, further confirming its superior electrochemical properties. While all samples show a typical decrease in capacitance with increasing scan rate\u0026mdash;attributable to limited ion diffusion at higher rates\u0026mdash;the drop is more significant for the 0.5 atm sample due to its higher reliance on surface-accessible porosity for charge storage.\u003c/p\u003e \u003cp\u003eCycling stability of the electrodes is illustrated in Figs.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ed to \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ef, where CV curves after multiple cycles are shown for the 1.5 atm, 1.0 atm, and 0.5 atm samples, respectively. The 0.5 atm sample (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ef) not only maintains the largest current response but also shows minimal shape distortion over cycles, demonstrating excellent cycling stability. The 1.0 atm sample (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ee) exhibits moderate current and acceptable stability, while the 1.5 atm sample (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ed) shows the lowest current response, though its CV curves remain consistent in shape.\u003c/p\u003e \u003cp\u003eIn summary, the CV data underscore that the biochar synthesized at 0.5 atm exhibits the most favorable combination of specific capacitance, rate capability, and long-term stability, making it a highly promising material for supercapacitor applications. Increasing the pyrolysis pressure to 1.0 and 1.5 atm leads to diminished electrochemical performance due to reduced surface area and porosity.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec17\" class=\"Section3\"\u003e \u003ch2\u003e3.2.2. Electrochemical Impedance Spectroscopy (EIS)\u003c/h2\u003e \u003cp\u003eThe Nyquist plot shown in the Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e illustrates the electrochemical impedance characteristics of biochar electrodes produced under three different pyrolysis pressures. The inset schematic represents the equivalent circuit model used for fitting, consisting of solution resistance (R\u003csub\u003es\u003c/sub\u003e), a constant phase element (CPE\u003csub\u003edl\u003c/sub\u003e) for double-layer capacitance, charge transfer resistance (R\u003csub\u003ect\u003c/sub\u003e), and Warburg impedance (Z\u003csub\u003ew\u003c/sub\u003e), which corresponds to ion diffusion [62].\u003c/p\u003e \u003cp\u003eAt high frequencies, the intersection of each curve with the real axis (Z\u0026prime;) reflects the solution resistance (R\u003csub\u003es\u003c/sub\u003e). In the mid-frequency region, the semicircle is indicative of the charge transfer process and reflects the charge transfer resistance (R\u003csub\u003ect\u003c/sub\u003e). The 1.5 atm sample shows the largest semicircle, indicating the highest R\u003csub\u003ect\u003c/sub\u003e, which implies slower charge transfer kinetics and greater resistance at the electrode\u0026ndash;electrolyte interface. On the other hand, the 0.5 atm sample exhibits the smallest semicircle, suggesting the lowest R\u003csub\u003ect\u003c/sub\u003e and more efficient electron transfer. This unexpected trend may be attributed to the more open porous network in the low-pressure biochar, which provides a larger electrochemically active surface area, facilitating charge transfer despite its less ordered (amorphous) structure. At low frequencies, the linear portion of the curves corresponds to Warburg impedance, associated with ion diffusion within the porous structure. The slope of the line is steepest for the 0.5 atm sample, reflecting superior ion diffusion and better electrolyte accessibility. Conversely, the 1.5 atm sample shows a flatter line, implying that the denser structure formed under high pressure may hinder ion transport.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec18\" class=\"Section3\"\u003e \u003ch2\u003e3.2.3. Galvanostatic Charge\u0026ndash; Discharge (GCD)\u003c/h2\u003e \u003cp\u003eThe galvanostatic charge\u0026ndash;discharge (GCD) analyses presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e provide further insight into the electrochemical performance of the biochar electrodes synthesized at different pressures.\u003c/p\u003e \u003cp\u003eIn Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ea, the GCD curves of the samples recorded at a current density of 0.3 A g⁻\u003csup\u003e1\u003c/sup\u003e clearly demonstrate that the electrode derived at 0.5 atm shows the longest discharge time, indicating the highest specific capacitance among the three. This extended discharge period is indicative of a larger charge storage capacity. In contrast, the samples prepared at 1.0 atm and 1.5 atm display shorter discharge durations, reflecting their comparatively lower capacitance, which can be attributed to reduced porosity and decreased ion accessibility due to structural compaction at higher pyrolysis pressures.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003eb presents the GCD curves of the 0.5 atm sample under varying current densities from 0.3 to 4.8 A g⁻\u003csup\u003e1\u003c/sup\u003e. As the current density increases, the charge\u0026ndash;discharge time gradually decreases, which is a typical behavior due to the limited time available for ion diffusion at higher currents. Nonetheless, the relatively symmetric triangular shapes of the GCD curves across all current densities indicate good electrochemical reversibility and capacitive behavior.\u003c/p\u003e \u003cp\u003eThe specific capacitance as a function of current density for all three samples is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ec. The 0.5 atm sample again demonstrates superior performance, retaining the highest capacitance across all current densities. The capacitance gradually declines with increasing current density, particularly for the 0.5 atm electrode, due to the diminished utilization of active surface sites at higher current loads. The 1.0 atm and 1.5 atm samples show consistently lower capacitance, highlighting the negative impact of higher pyrolysis pressures on the electrochemical properties of the biochar.\u003c/p\u003e \u003cp\u003eThe long-term cycling stability of the electrodes is assessed in Figs.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ed, \u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ee, and \u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ef, which present repetitive GCD curves over multiple cycles for the 1.5 atm, 1.0 atm, and 0.5 atm samples, respectively. In Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ef, the 0.5 atm sample exhibits excellent cycling performance over 1000 seconds with minimal shape distortion and consistent voltage profiles, indicating strong electrochemical stability and reversibility. Similarly, the 1.0 atm electrode (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ee) maintains a stable charge\u0026ndash;discharge profile over 300 seconds, though with lower capacity. The 1.5 atm sample (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ed) also shows decent stability but has the shortest discharge time and lower current response, confirming its lower charge storage capacity.\u003c/p\u003e \u003cp\u003eIn summary, the GCD results clearly establish that the electrode synthesized at 0.5 atm exhibits superior specific capacitance, rate capability, and long-term stability, making it the most promising candidate among the studied samples for supercapacitor applications. Increasing the synthesis pressure to 1.0 and 1.5 atm adversely affects electrochemical performance due to structural changes that hinder ion transport and reduce accessible surface area.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003c/div\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eThis research confirms that pyrolysis pressure is a decisive factor in engineering biochar for supercapacitor applications. Among the tested pressures, biochar synthesized at 0.5 atm demonstrated the most favorable performance, with a specific surface area of 36.31 m\u0026sup2; g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, pore volume of 0.0151 m\u0026sup3; g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, and specific capacitance up to 61 F g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at low scan rates. In contrast, increasing the pressure to 1.5 atm led to structural densification, decreasing the surface area to 10.71 m\u0026sup2; g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, pore volume to 0.004 m\u0026sup3; g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, and specific capacitance to only 29 F g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Additionally, the water contact angle (WCA) reduced to 4\u0026deg; at 1.5 atm, highlighting enhanced hydrophilicity due to increased surface functional groups\u0026mdash;confirmed by FTIR and elemental analysis showing higher oxygen content (44.28%). Electrochemical studies including CV, GCD, and EIS revealed superior rate capability, charge\u0026ndash;discharge symmetry, and ion diffusion in the 0.5 atm sample. Overall, the results suggest that lower pyrolysis pressures lead to biochar with optimized porosity, higher capacitance, and enhanced electrochemical efficiency, making it an ideal candidate for energy storage applications.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eConflict of interest\u003c/h2\u003e \u003cp\u003eThe authors declares that there is no conflict of interest.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eFunding sources\u003c/h2\u003e \u003cp\u003eThis research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.\u003c/p\u003e\u003ch2\u003eAuthor contributions\u003c/h2\u003e \u003cp\u003eThe first draft of the manuscript was written by Gholam Reza Allahgholipour. Dr. Somayeh Mohamadi is the corresponding author and takes primary responsibility for communication with the journal and editorial processing.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eY. Zhang, Y. Liang, S. Li, Y. Yuan, D. Zhang, Y. Wu, H. Xie, K. Brindhadevi, A. Pugazhendhi, C. Xia, A review of biomass pyrolysis gas: Forming mechanisms, influencing parameters, and product application upgrades, \u003cem\u003eFuel\u003c/em\u003e, \u003cstrong\u003e347\u003c/strong\u003e (2023) 128461.\u003c/li\u003e\n\u003cli\u003eD. Lachos-Perez, J.C. Martins-Vieira, J. Missau, K. Anshu, O.K. Siakpebru, S.K. Thengane, A.R.C. Morais, E.H. Tanabe, D.A. Bertuol, Review on biomass pyrolysis with a focus on bio-oil upgrading techniques, \u003cem\u003eAnalytica\u003c/em\u003e, \u003cstrong\u003e4\u003c/strong\u003e (2023) 182-205.\u003c/li\u003e\n\u003cli\u003eA. Shafizadeh, H. Rastegari, H. Shahbeik, H. Mobli, J. Pan, W. Peng, G. Li, M. Tabatabaei, M. 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Chem.\u003c/em\u003e, \u003cstrong\u003e46\u003c/strong\u003e (2022) 20932-20939.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"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":"korean-journal-of-chemical-engineering","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"kjce","sideBox":"Learn more about [Korean Journal of Chemical Engineering](http://link.springer.com/journal/11814)","snPcode":"11814","submissionUrl":"https://www.editorialmanager.com/kjce/default2.aspx","title":"Korean Journal of Chemical Engineering","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Subscription","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Biochar, Pyrolysis, Pressure, supercapacitor, Biochar structure","lastPublishedDoi":"10.21203/rs.3.rs-9491472/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-9491472/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThis study investigated- the influence of pyrolysis pressure on the supercapacitive properties of biochar derived from plane tree leaves. Biochar samples were prepared under three different pressures: 0.5, 1.0, and 1.5 atm, with constant temperature and duration. Morphological characterization of synthesized biochar was studied via SEM and BET analyses which revealed greater porosity and higher surface area at low-pressure pyrolysis. FTIR and CHN elemental analysis indicated an increase in surface functional groups at higher pressures, resulting in enhanced hydrophilicity. XRD analysis showed a progressive increase in graphitic ordering with pressure. Electrochemical evaluations using cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS) demonstrated that biochar synthesized at 0.5 atm exhibited superior specific capacitance, rate capability, and cycling stability due to its porous architecture and efficient ion transport. In contrast, samples prepared at higher pressures showed diminished electrochemical performance. The results suggest that optimizing pyrolysis pressure is crucial for tailoring biochar properties for energy storage applications, specifically for use in supercapacitor electrodes.\u003c/p\u003e","manuscriptTitle":"Investigating the Effect of Pyrolysis Process Pressure on the Supercapacitive Properties of Biochar","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-05-15 08:12:40","doi":"10.21203/rs.3.rs-9491472/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewerAgreed","content":"","date":"2026-05-07T11:37:34+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-05-06T09:08:13+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-05-02T10:07:02+00:00","index":"","fulltext":""},{"type":"submitted","content":"Korean Journal of Chemical Engineering","date":"2026-04-28T02:12:58+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"korean-journal-of-chemical-engineering","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"kjce","sideBox":"Learn more about [Korean Journal of Chemical Engineering](http://link.springer.com/journal/11814)","snPcode":"11814","submissionUrl":"https://www.editorialmanager.com/kjce/default2.aspx","title":"Korean Journal of Chemical Engineering","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Subscription","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"2b0b4b02-52f5-4c27-a2f6-1e4295d9174c","owner":[],"postedDate":"May 15th, 2026","published":true,"recentEditorialEvents":[{"type":"reviewerAgreed","content":"","date":"2026-05-07T11:37:34+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-05-06T09:08:13+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-05-02T10:07:02+00:00","index":"","fulltext":""}],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-05-15T08:12:41+00:00","versionOfRecord":[],"versionCreatedAt":"2026-05-15 08:12:40","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-9491472","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-9491472","identity":"rs-9491472","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}