Production of Graphitic Carbon from Renewable Lignocellulosic Biomass Source

Abstract

Carbon materials derived from lignocellulosic biomass (LCB) precursors have emerged as sustainable and versatile candidates, exhibiting outstanding properties for energy storage applications. This study presents an innovative and cost-efficient approach to produce graphitic carbon from an LCB precursor (pinecone) using an optimized hydrothermal treatment process followed by carbonization and graphitization. The developed pinecone-derived graphitic carbon (PDGC) was analyzed using X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). XRD analysis confirmed the formation of a graphitic phase, indicated by a sharp and intense (002) peak, decreased interplanar spacing (d₀₀₂), increased crystallite size (L_c~20.4 nm), and a high degree of graphitization (g~0.7), closely aligning with the characteristics of pure graphite. Additionally, TEM and SEM micrographs revealed a flake-like morphology with well-defined, continuous, and extended graphitic layers within the PDGC structure. The distinctive structural attributes of the developed material position it as a promising candidate for batteries and capacitors, while also serving as a model for converting LCB into advanced carbon materials.

1. Introduction

The rising demand for efficient energy storage solutions underscores the necessity for advanced electrode materials. Prominent materials in this field include graphene, graphite, carbon nanotubes, silicon–carbon composites, transition metal oxides, MXenes, and hybrid carbon-based materials [1,2,3,4,5,6,7,8,9]. Due to the distinctive structural characteristics and excellent electrochemical behavior, carbon-based materials have become a focal point of research [10,11]. Various forms of carbon, such as carbon nanotubes, porous carbon nanofibers, graphene, hard carbons and graphitic carbons, have been rigorously investigated for their impressive features, including large surface area, tunable pore structures, outstanding conductivity, and excellent physicochemical stability [10,12,13,14]. Despite this progress, the majority of carbon material synthesis methods still rely heavily on unsustainable fossil fuel derivatives, such as byproducts from petroleum refining and coal-tar, which involve energy-intensive and expensive processes [15,16,17,18].

In this context, LCB offers a promising and renewable feedstock for the production of advanced carbon materials. They are typically sourced from plant-based materials, such as byproducts of agriculture, remnants from forestry operations, and specially cultivated energy-rich crops [19], making them abundant and cost-effective precursors for carbon production. LCB, composed of cellulose (C₆H₁₀O₅)_x, hemicelluloses (C₅H₈O₄)_m and lignin [C₉H₁₀O₃·(OCH₃)_0.9–1.7]_n in varying proportions depending on the source, holds great potential for conversion into high-value carbon-based materials [20,21]. Various carbon architectures, including activated carbons [22], carbon spheres [23,24], and carbon molecular sieves [21], have been successfully synthesized from a range of lignocellulosic biomass sources [25,26,27]. The majority of these carbon structures display highly disordered, non-graphitic characteristics, which restrict their utility in energy storage applications. This is primarily due to the intricate composition and resistant nature of LCB, which poses significant challenges for its effective conversion into graphitic carbon materials. In particular, lignin present in LCB, despite being aromatic and carbon-rich, tends to form disordered or turbostratic carbon structures during pyrolysis, making the synthesis of highly ordered graphitic carbon challenging without advanced processing techniques [28]. Despite the promising potential of LCB, attempts to produce highly ordered graphitic carbons have been relatively limited. Graphitic materials have attracted considerable interest due to their exceptional electrical conductivity, thermal stability, and well-defined layered structures, which make them highly suitable for advanced energy storage applications [29,30,31]. These graphitic carbons feature a honeycomb lattice structure composed of sp²-hybridized carbon atoms, closely mirroring the arrangement found in natural graphite [29].

In order to prepare carbon structures from LCB, numerous methods have been used, such as the templating process [32], direct pyrolysis [33], catalytic graphitization [34,35], chemical vapor deposition [36], high-temperature graphitization [27], and hydrothermal carbonization [37,38]. Among these approaches, a combined method involving hydrothermal carbonization followed by high-temperature graphitization has emerged as a particularly promising technique for producing graphitic carbon. This approach effectively integrates mild hydrothermal conditions with high-temperature treatment to produce highly ordered graphitic carbon, offering controlled morphology and enhanced material properties (crystallite size, stacking height, and degree of graphitization) for energy storage applications.

Building on the potential of LCB-derived carbons, this study focuses on the synthesis of graphitic carbon from pinecones, a renewable and widely available biomass resource. The high lignin content in pinecones may facilitate the formation of graphitic domains under controlled hydrothermal and thermal treatment conditions, highlighting their potential for generating high-quality carbon and providing valuable insights for optimizing other lignocellulosic feedstocks in advanced material development [39,40]. Despite this promise, their use in fabricating highly ordered graphitic carbons has been largely unexplored, highlighting the novelty and importance of this work in advancing sustainable approaches to graphitic carbon synthesis. Therefore, in this study, high-quality graphitic carbon from pinecone biomass was produced through an optimized hydrothermal pretreatment process followed by carbonization and graphitization. The crystallite parameters and morphology of the resulting graphitic carbon were examined using XRD, TEM, and SEM and were compared with natural graphite. This study contributes to materials science and engineering by highlighting the potential of sustainable feedstocks to produce eco-friendly graphitic carbons, which hold promise for energy storage applications.

2. Materials and Methods

2.1. Materials

Pinecones from American pine tree (Pinus strobus) were collected locally from State College (Pennsylvania, PA, USA). Hydrochloric acid (37%, ACS reagent) was purchased from Sigma Aldrich, Inc., St. Louis, MO, USA. The pinecone biomass was then utilized to synthesize the graphitic carbon.

2.2. Methods

2.2.1. Development of Pinecone-Derived Graphitic Carbon

To prepare highly ordered graphitic carbon, the pinecones were first thoroughly washed with deionized water to remove impurities, followed by drying at 110 °C for 24 h. The dried material was then ground into fine powder and sieved (with 850 μm sieve size) to achieve a uniform particle size. The powdered biomass was mixed with deionized water at a 1:10 (w/v) ratio and transferred to a Teflon-lined hydrothermal reactor. The reactor was then heated to 200 °C for 24 h under a pressure of 35 bar [41]. After cooling, the hydrochar was collected through filtration, rinsed thoroughly with deionized water and ethanol, and subsequently dried at 100 °C. Further, we enclosed 10 g of hydrochar in brass-foil and inserted into the sealed tubing reactor. For the carbonization at 500 °C for 4 h in inert environment, we placed the reactor into the air-fluidized sand bath. Following carbonization, the sample underwent graphitization, where it was heated to 2500 °C for 1 h in an argon atmosphere. The graphitization process was carried out using a Series 45 Centorr Vacuum Industries furnace, where graphite served as the heating element. Prior to heating, the furnace underwent three stages of vacuum evacuation, vacuuming to below 100 mTorr, and subsequently argon backfilling to purge residual air. To uphold inert conditions, argon gas was continuously purged throughout the graphitization stage. The final step involved washing the product with hydrochloric acid to remove remaining impurities, followed by thorough rinsing with deionized water until it reached a neutral pH. The material was then dried to obtain highly ordered graphitic carbon. A visual schematic of the PDGC preparation method is provided in Figure 1.

2.2.2. Characterization of Graphitic Carbon

The bulk graphitic quality of the graphitized carbon sample was assessed using Powder XRD, while TEM and SEM were employed to analyze its morphology and lamellar structure.

X-Ray Diffraction Analysis

A Malvern PANalytical Empyrean diffractometer employed with Cu radiation source of λ = 1.54 Å, paraxial focus optics, PIXcel 3D X-ray detector was utilized for XRD analysis. The angular range of 2θ = 5° to 80° was used to collect XRD spectra, which was further deconvoluted and quantified using MDI JADE software, selecting best fit with the minimal residual values for accurate analysis of data. To ensure accuracy, we accounted for instrumental broadening by measuring the diffraction pattern of a well-crystallized standard material (such as silicon) under the same experimental conditions. The observed peak broadening was then corrected using the following equation:

$$B_{true}=\sqrt{B_{measured}^2-B_{instrumental}^2}$$

(1)

where $B_{measured}^2$ is the experimentally observed FWHM, and $B_{instrumental}^2$ is the FWHM obtained from the instrumental standard. The corrected peak width $B_{true}$ was then used to estimate the crystallite size using the Scherrer formula:

$$L={\displaystyle \frac{K\lambda }{B_{true} cos\theta }}$$

(2)

where L is the crystallite size, K is the shape factor, λ is the X-ray wavelength, and θ is the Bragg angle of the peak. To determine the in-plane crystallite diameter (L_a) and stacking height (L_c), the Scherrer equation was applied using K_a and K_c values of 1.84 and 0.89, respectively [42].

Transmission Electron Microscopy Imaging

The FEI Talos™ model F200X Scanning/TEM, featuring a field-emission gun source with a 0.12 nm resolution, was utilized for TEM analysis. For TEM sample preparation, a small amount of the powdered material was sonicated in ethanol and drop-cast onto a 300-mesh C/Cu lacey TEM grid. The microscope operated at an acceleration voltage of 200 kV, capturing images at magnifications ranging from 10 to 500 kX. Moreover, the local crystallinity of the sample was analyzed using Fast-Fourier Transform (FFT) analysis.

Scanning Electron Microscopy Imaging

SEM imaging was carried out using an Apreo system. For imaging, a small amount of the sample was mounted on a pin stub holder with carbon tape attached to it. SEM images were acquired at an acceleration voltage of 7 kV, with the working distance adjusted between 11 mm and 7 mm.

3. Results and Discussion

The synthesis route, as illustrated in Figure 1, details the gradual conversion of pinecone biomass into graphitic carbon via a series of steps, including grinding, sieving, hydrothermal treatment, carbonization, and graphitization. The process begins with the preparation of biomass to achieve a uniform particle size, followed by hydrothermal treatment at 200 °C for 24 h, which enhances the structural properties and preconditions the material for further carbonization. The hydrothermal pretreatment enhances carbon yield and graphitic domain formation by partially decomposing cellulose, hemicellulose and restructuring lignin before carbonization. The optimized conditions (200 °C, 24 h, 35 bar) facilitate hydrolysis, dehydration, and decarboxylation, improving aromatization and reducing oxygen content [43]. Additionally, the autogenous pressure prevents excessive porosity, ensuring a dense and uniform carbon structure, making this approach more effective than conventional methods. Moreover, subjecting the hydrochar to carbonization (500 °C, 4 h) in an inert environment results in the formation of an amorphous carbon-dense structure, while subsequent graphitization at 2500 °C for 1 h achieves a highly ordered graphitic structure. The produced PDGC sample underwent characterization to assess its structural features, such as crystallite dimensions, lattice characteristics, and morphological attributes, which are elaborated in the following sections.

3.1. Bulk Study of PDGC Material

The XRD pattern and deconvoluted diffractogram of PDGC and natural graphite are illustrated in Figure 2. A sharp and intense (002) peak is observed at 26.3° (Figure 2a,b), resembling that of pure graphite [44,45]. This indicates small d₀₀₂ and improved graphitic quality following the graphitization process, as compared to other graphitic carbon materials derived from LCB sources, as reported in Refs. [46,47,48]. Here, the graphitic quality refers to the degree of structural order and crystallinity in graphitic carbon materials. Furthermore, the deconvoluted XRD pattern of PDGC (Figure 2d) in the 40–50° range reveals three distinct peaks similar to natural graphite. The two prominent peaks correspond to the (100) and (101) reflection planes of graphitic carbon, while the third (Figure 2c and inset of Figure 2d), which is typically weak and less intense, arises from the (102) reflection, likely due to in-plane stacking variations or partial turbostratic disorder, influencing the diffraction pattern [49]. A comparison of lattice parameters between PDGC and natural graphite, derived from the deconvoluted XRD diffractogram (Figure 2d), is provided in

Table 1. Lattice parameters obtained from deconvoluted XRD spectra of PDGC.

Lattice Parameter|Sample	PDGC	Natural Graphite
d002 [Å]	3.37	3.354
FWHM (deg)	0.36	~0.1
Lc (using 002) [nm]	20.4	24.6
La (using100) [nm]	28.4	28.6
La (using 110) [nm]	44.4	~100
g (degree of graphitization)	0.7	1

. The d₀₀₂ for PDGC was determined to be 3.37 Å, slightly larger than that of natural graphite (3.35 Å) (as reported in Ref. [50]), suggesting the presence of minor structural imperfections or turbostratic arrangements within its graphitic planes. Additionally, the L_c value calculated from the (002) peak was 20.4 nm for PDGC, slightly lower than the 24.6 nm observed for natural graphite, reflecting smaller and less ordered stack sizes. Moreover, the Seeha and Pavloic relation was applied to calculate the graphitization degree (g) (Equation (3)) [51].

$$\mathrm{Degree} \mathrm{of} \mathrm{graphitization} \left(g\right)={\displaystyle \frac{3.440-d_{002}}{3.440-3.345}}$$

(3)

As a critical parameter, the g-value in carbon materials quantifies the graphitization level, providing insight into its resemblance to pure graphite. For PDGC, the value of g, calculated from the d₀₀₂, was approximately 0.7, slightly lower than the ideal value of 1 for natural graphite, indicating a marginally lower but still substantial level of graphitic ordering. Despite these differences, the high L_c and g values confirm the superior graphitic characteristics of PDGC.

3.2. Morphological and Microstructural Analysis

The morphology of the synthesized PDGC was analyzed using TEM imaging. Figure 3 illustrates both low-resolution and high-resolution TEM (HRTEM) images, revealing a layered and flaky structure. The images depict thin, overlapping sheets and layers characteristic of graphitic structures with well-aligned planes. The extracted d₀₀₂ of 0.337 nm closely matches the theoretical interlayer spacing of natural graphite (002) planes, indicating a high degree of graphitization (as reported in Ref. [44]). Additionally, the presence of slightly curved and wrinkled regions suggests partially ordered graphitic domains, likely formed during the graphitization process. High-resolution images, particularly in Figure 3b,c, highlight well-ordered graphitic layers, confirming the material’s high crystallinity. However, a minor presence of non-graphitic content was also found (refer to Figure S1). In Figure 3d, the FFT pattern exhibited bright, well-defined spots along with concentric rings around the central region, signifying a polycrystalline nature formed by the multi-crystallite diffraction. These findings align with the XRD results detailed in Section 3.1, reinforcing the confirmation of the graphitic phase in the PDGC sample.

Furthermore, SEM analysis was performed to study the microstructural features of the PDGC material. As shown in Figure 4, the material exhibits a flaky, layered morphology, resembling that of natural graphite (as described in Ref. [52]). This layered structure may facilitate efficient ion intercalation and deintercalation during charge and discharge cycles, which could enhance cycle life and stability. The structural features of the developed carbon, such as high crystallinity, well-defined graphitic layers, and a layered morphology, show its great viability as an electrode material for next-generation batteries and capacitors, making it a promising candidate for advanced energy storage systems. Furthermore, it also demonstrates that pinecone biomass serves as a model for other LCBs, showcasing an efficient synthetic route for producing high-quality carbons.

3.3. Graphitization Mechanism for the Formation of Pinecone-Derived Graphitic Carbon

The possible graphitization mechanism of LCB into graphitic carbon follows a multi-step process, involving dehydration, decarboxylation, carbonization, and graphitization, as shown in Figure 5. Initially, lignocellulosic biomass, composed of cellulose, hemicellulose, and lignin, undergoes thermal decomposition, leading to the removal of hydroxyl (-OH) and carboxyl (-COOH) functional groups through dehydration and decarboxylation reactions. These reactions eliminate H₂O and CO₂, reducing the oxygen content and increasing the carbon concentration [43,53]. As the temperature rises, the breakdown of biopolymers results in the formation of amorphous carbon structures with randomly oriented aromatic clusters [43]. With further heating at 500 °C, these clusters start to align, forming small graphitic-like domains. At high temperatures (~2500 °C), the amorphous carbon undergoes significant structural reorganization, leading to the development of well-ordered graphitic layers with enhanced crystallinity.

4. Conclusions

This research extensively analyzed the process of graphitic carbon formation from sustainable pinecone biomass using a hydrothermal pretreatment followed by carbonization and graphitization. The structural parameters, morphology, and microstructure of the developed PDGC were examined using XRD, TEM and SEM. The XRD results confirmed the formation of a graphitic phase, evidenced by a sharp and intense (002) peak, reduced d₀₀₂ value, a larger crystallite size (L_c~20.4 nm), and a high degree of graphitization (g~0.7) compared to other graphitic carbon materials derived from LCB sources, and closely resembling pure graphite. In addition, TEM and HRTEM analysis corroborated these findings, revealing a flake-like morphology with well-defined, long, and continuous graphitic lamellae within the PDGC structure. Similarly, SEM analysis revealed a layered, lamellar structure comparable to natural graphite. These characteristics establish PDGC as a highly promising electrode material for batteries and capacitors, showcasing its strong potential for energy storage applications. Moreover, this study paves the way for leveraging pinecone biomass as a model system to convert lignocellulosic feedstocks into advanced carbon materials optimized for energy solutions.