# Recent developments on multi- versus single-metallic catalytic graphitisation of biocarbon: A review

Shamala Gowri Krishnan, Claire White, Kuo Zeng, Nandakumar Kalarikkal, Yong Sik Ok, Craig Arnold, Sabu Thomas, Ange Nzihou

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Shamala Gowri Krishnan, Claire White, Kuo Zeng, Nandakumar Kalarikkal, Yong Sik Ok, et al. Recent developments on multi- versus single-metallic catalytic graphitisation of biocarbon: A review. Fuel, 2025, 396, pp.135330. 10.1016/j.fuel.2025.135330. hal-05046689

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Review article

# Recent developments on multi- versus single-metallic catalytic graphitisation of biocarbon: A review

Shamala Gowri Krishnan<sup>a</sup>, Claire E. White<sup>b,c</sup>, Kuo Zeng<sup>d</sup>, Nandakumar Kalarikkal<sup>e,f</sup>, Yong Sik Ok<sup>g</sup>, Craig B. Arnold<sup>h</sup>, Sabu Thomas<sup>f,i</sup>, Ange Nzihou<sup>a,c,*</sup>

ARTICLE INFO

Keywords:
Catalysts
Multi-metals
Biocarbon
Graphitisation
Graphene

ABSTRACT

The typical graphitisation process involves non-renewable carbon sources and high temperatures, which lead to increased carbon dioxide emissions and energy consumption in the resulting graphite. Biocarbon derived from biomass acts as a sustainable carbon source that can be graphitised at lower temperatures with the aid of catalysts. This review highlights the significance of both multi- and single-metallic catalytic graphitisation of biocarbon. Introducing a catalyst offers an effective means to modify the graphitisation conditions and the characteristics of graphitic layers formed at the atomic and molecular levels. Multi-metal catalysts demonstrate superior effectiveness in lowering the graphitisation temperature to 800 °C compared to single-metal catalysts (1000–1800 °C) and those without catalysts (>2000 °C), where the synergistic interaction of two distinct metals enhances the transformation of amorphous carbon into graphitic biocarbon, as opposed to single-metal catalysts. This paper establishes a hierarchy of the graphitisation conditions as follows: temperature > carbon precursors > heating rate. Furthermore, this work outlines the existing knowledge gap regarding metallic catalysts and clarifies the roles of transition, alkaline, and alkaline earth metal catalysts in the graphitisation of bioresources.

1. Introduction

Graphite, composed of stacked graphene layers, exhibits remarkable electrical and thermal conductivity properties, leading to widespread applications in batteries, automobiles, solar cells, and more [1]. Graphene is a one-atom-thick planar sheet of sp²-bonded carbon atoms. Since its first discovery in the 1990s and 2004, it has garnered significant interest due to its properties, such as a high specific surface area and excellent electrical and thermal conductivity [2–4]. Most natural graphite powders originate from China and India, accounting for approximately 66% and 14% of the global market, respectively, while natural graphite powders from Europe comprise less than 1% [5,6]. Historically, manufactured graphitic materials have been produced almost exclusively from non-renewable sources, such as petroleum coke.

The graphitisation process has previously required extreme temperatures ranging from 2800 °C to 3500 °C and is also time-consuming [7,8]. Consequently, these processes are energy-intensive and raise concerns regarding availability and sustainability. As the demand for this material increases, it is crucial to consider the impact of production methods on the environment [9]. Therefore, it is essential to develop carbon materials with a desirable graphitic structure and physicochemical performance through efficient methods using carbon precursors from renewable and sustainable sources [10].

A key aspect of producing graphitic materials is the use of high-carbon content material as feedstock, and important considerations in commercial-scale manufacturing include low cost, high carbon content, and availability [11,12]. Biomass, highly lignocellulosic, has been widely utilised as an alternative carbon source with approximately 55 wt% of carbon concentration to obtain graphitic materials [13,14].

<footnote><sup>a</sup> IMT Mines Albi, RAPSODEE CNRS UMR 5302, Université de Toulouse, Campus Jarlard, F.81013 Cedex 09 Albi, France</footnote>

<footnote><sup>b</sup> Department of Civil and Environmental Engineering, Princeton University, Princeton, NJ 08544, United States</footnote>

<footnote><sup>c</sup> Andlinger Center for Energy and the Environment, Princeton University, Princeton, NJ 08544, United States</footnote>

<footnote><sup>d</sup> State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan, Hubei 430074, PR China</footnote>

<footnote><sup>e</sup> School of Pure and Applied Physics, Mahatma Gandhi University, Kottayam 686 560, Kerala, India</footnote>

<footnote><sup>f</sup> International and Inter-University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam 686 560, Kerala, India</footnote>

<footnote><sup>g</sup> Korea Biochar Research Center (KBRC), Division of Environmental Science and Ecological Engineering, Korea University, Seoul 02841, South Korea</footnote>

<footnote><sup>h</sup> Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544, United States</footnote>

<footnote><sup>i</sup> School of Energy Materials, Mahatma Gandhi University, Kottayam 686 560, Kerala, India</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail addresses: ange.nzihou@mines-albi.fr, ange.nzihou@princeton.edu (A. Nzihou).</footnote>
Full name of the abbreviations in the review

CVD Chemical vapour deposition

XRD X-ray diffraction

$L_a$ Lateral size

$L_c$ Layer stacking distance

HTC Hydrothermal carbonisation

HRTEM High-resolution transmission electron microscopy

PDF Pair distribution function

FT-IR Fourier-transform infrared spectroscopy

DTPH Decoupled temperature and pressure hydrothermal

EU European Union

SAED Selected area electron diffraction

DFT Density functional theory

ML Machine learning

MD Molecular dynamics

Numerous researchers have explored various types of biomass as precursors in the synthesis of graphitic carbon [15–19]. Biomass mainly contains cellulose that is aligned and twisted into fibres and fibrils and another component, lignin, a complex polyphenolic molecule [20]. Different synthetic processes, such as the bottom-up approach, have been employed to convert various carbonaceous precursors into graphitic materials [21]. The lignocellulosic material is non-graphitisable carbon, also known as hard carbon, and is difficult to graphitise and requires a high temperature of above 2000 °C. Fromm et al. [22] conducted experiments using various biomass sources, including birch, oak, and bamboo wood, to produce graphitic materials without a catalyst. They varied the temperature between 800 and 2800 °C and discovered that biomass-derived carbon cannot be directly converted into graphitic biocarbon, even at elevated temperatures.

Catalytic graphitisation involves the addition of certain inorganic additives to the carbonaceous matrix, or using feedstock inherently containing species such as alkali and alkali earth metals [23] and subsequent graphitisation at elevated temperatures [24]. Recently, transition-metal-based catalytic graphitisation of biocarbon has gained momentum due to its low catalytic temperature (<1400 °C) and its broad range of applications [25,26]. Several researchers have successfully employed this technique, using a catalyst to lower the graphitisation temperature and reduce the reaction duration (Fig. 1). For instance, iron (Fe) nanoparticles have been shown to convert lignocellulosic

materials into graphitic biocarbon [27,28]. A. Oya et al. [29] introduced metallic elements as catalysts in the graphitisation process, transforming non-graphitisable materials (such as resin) into graphitic substances with good crystalline graphite, thereby confirming the feasibility of catalytic graphitisation. Hoekstra et al. [30] examined the active temperatures of various base metal catalysts (copper, nickel, cobalt, and iron salts) in the graphitisation process, reporting that iron, nickel, and cobalt are effective catalysts at the lowest temperature (above 715 °C), while nickel and cobalt nanoparticles were only activated at temperatures above 800 °C. Biocarbon generated from pine wood sawdust has been catalytically graphitised using a Fe catalyst derived from $Fe(NO_3)_3$ at temperatures ranging from 700 to 1000 °C for a duration of 1 to 4 h [31]. A few researchers have also performed the graphitisation of carbon derived from whey protein using a Ni catalyst sourced from $Ni(NO_3)_2$ at a lower temperature of 700 °C for 1 h [32].

Research has been conducted to further enhance the graphitic crystallinity of bio-based synthetic graphitic materials, focusing particularly on the modification of catalysts, such as the use of multi-metal catalysts. A Ni-Fe bimetallic catalyst, for instance, has been found to improve hydrogen formation and the graphitisation of carbon nanotubes during the catalytic pyrolysis of plastics [33]. Previous studies have indicated that multi-metal catalysts can increase the degree of graphitisation in carbon nanotubes and graphene. Nevertheless, only a limited number of studies have reported on the use of multi-metal Fe-Co and Fe-Ni catalysed graphitisation of biocarbon. Biomass-derived graphitic biocarbon typically comprises carbon allotropes, including two-dimensional (2D) stacked graphene sheets, one-dimensional (1D) graphitic sheets, and zero-dimensional (0D) carbon nanotubes, which contribute to specific functional groups and distinctive properties [34–36].

Multi-metal catalysts can benefit from the synergistic effects between metal contents and carbon precursors to exhibit higher catalytic activity and better efficiency than single-metal catalysts. Several reviews have been published on bio-based graphitic structure synthesis methods. Relative to other reviews on catalytic graphitisation, this review focuses on the limited number of studies involving multi-metal catalysts in biocarbon catalytic graphitisation. Specifically, process parameters are emphasised, including the effect of multi- and single-metal catalysts, types of carbon precursor, graphitisation temperature and heating rate on the degree of graphitisation and carbon aromatisation. In addition, the mechanisms of catalytic graphitisation during pyrolysis are highlighted.

<figure><img src="image_2.png" /><figcaption>Fig. 1. Evolution of metallic catalysts utilisation in the graphitisation process.</figcaption></figure>
## 2. Catalytic graphitisation of biocarbon

Technologies that carbonise biomass into biocarbon products have advanced significantly over the past two decades. Lignocellulosic materials are classified as non-graphitising, generating biocarbon when processed at elevated temperatures (> 2000 °C). The absence of aromatic rings and the prevalence of carbon-oxygen (C-O) and sp³ hybridised carbon-carbon (C-C) bonds in biomass lead to disordered biocarbon material and a lack of high crystallinity [37,38]. During graphitisation, the sp³ hybridisation of carbon atoms in biomass facilitates free rotation during thermal cleavage, resulting in random disorder upon condensation and structural rearrangement [39]. Consequently, direct graphitisation of these materials necessitates high temperatures, which contributes to the elevated energy costs of the graphitisation process and restricts the industrial development of carbon-derived graphitic materials.

The infusion of the catalyst significantly impacts the graphitisation process. Certain non-graphitising materials, such as lignocellulose, will graphitise in the presence of metal catalysts during pyrolysis at relatively low temperatures (<1800 °C). Many transition metals, along with alkaline and alkaline earth metals, notably iron, nickel, potassium, and calcium, are regarded as the most promising catalysts due to their excellent catalytic activity [40–43]. The ability to incorporate and dissolve carbon creates a metal carbide that precipitates graphitic biocarbon in the molten phase (1200–1400 °C). To characterise the average phase evolution, Raman spectroscopy serves as a novel method to quantify the degree of graphitisation and defects (I<sub>D</sub>/I<sub>G</sub>), while X-ray diffraction (XRD) evaluates the crystallinity and interlayer spacing (d<sub>002</sub>). Carbon-metal interaction is a distinguishing trait of specific carbon-metal pairings that affect the properties of graphitic biocarbon. This is attributed to the fact that more carbon atoms gain access to the metal at dissolution-precipitation sites, which facilitates the decomposition of biomass and enhances the degree of graphitisation of graphitic biocarbon [44,45]. Overall, the graphitisation process is significantly influenced by reaction conditions, including the types of catalysts, carbon precursors, temperature, and heating rate. However, research into the catalytic graphitisation of non-graphitising materials for developing nanostructures and defects remains relatively nascent.

### 2.1. Multi-metal vs single-metal catalysts

This section reports previous studies that were done to further improve the graphitic crystalline in the presence of multi-metal catalysts compared to single-metal counterparts. Gopalakrishnan et al. [46] synthesised a few-layer graphitic microporous carbon nanosheet for capacitance using ginger root as a carbon precursor with heteroatoms (potassium, magnesium and phosphorous). The graphitic carbon formation at 800°C had a more intact graphitic structure and lower degree of graphitisation (defects) with microporous and ultrathin nanosheets that exhibited high specific capacitance. Hence, the graphitic biocarbon structure from biomass paved the way for future high-performance energy storage applications. Cai et al. [47] synthesised porous graphitic biocarbon from the Moringa oleifera stem using FeCl<sub>3</sub> and ZnCl<sub>2</sub> as graphitisation catalysts and chemical activators, respectively. Increasing the graphitisation catalyst (FeCl<sub>3</sub>), concentration improved the degree of graphitisation and enhanced the specific surface area where many wrinkles in the nanosheets demonstrate that the porosity, which serves as the active sites for ions to form electric double layers and allows excellent electrochemical performance. Li et al. [48] prepared porous graphitic biocarbon at a temperature of 750 °C using multi-metal nickel and molybdenum (Ni-Mo). It was noted that the degree of graphitisation by multi-metal Ni-Mo increased by 9.4% compared with that of porous carbon prepared without a catalyst. Physical analysis showed that the structure of the porous graphitic biocarbon is controlled by 1–2 nm micropores with few mesopores. This research offers a new method for producing porous graphitic biocarbon without damaging its porous

structure and its benefits as an electrode of supercapacitors with high-rate performance. Besides, Xia et al. [49] synthesised porous graphitic for sustainable hydrogen production by biocarbon graphitisation using a series of single-metal catalysts (Fe, Co, and Ni) and multi-metal catalysts (Fe–Co, Fe–Ni). Among the single-metal catalysts, Fe showed a high degree of graphitisation and the largest surface area. In the presence of the Fe–Co compound and a homogeneous distribution of Fe and Co, the multi-metallic Fe-Co catalyst had a larger pore volume and degree of graphitisation (I<sub>D</sub>/I<sub>G</sub>: 0.432) than the monometallic Fe and Co catalysts, which was attributed to the benefits of Fe-Co distribution. In particular, the specific surface area of biocarbon in the presence of the Fe-Co catalyst increased to a maximum temperature of 850 °C and decreased at subsequent temperatures (850–1300 °C). Similarly, the degree of graphitisation improved with a sharp decrease in the disordered carbon content from 550 to 850 °C but with limited change from 850 to 1300 °C. The optimal pyrolytic temperature was 850 °C as the interaction between Fe and Co was thought to promote C–H bond breakage and deoxygenation reactions. Combining Fe-Co catalysed the carbon dissolution-precipitation process improves the degree of graphitisation and porosity. The outcome demonstrated abundant porous graphitic biocarbon formation and its use as electrocatalysts for the oxygen reduction reaction.

On the other hand, Ouyang et al. [50] used nickel (Ni) nanoparticles as a catalyst for carbon graphitisation and a template for mesopore carbon development from Porphyra (red algae). The Porphyra-derived carbon displayed substantial nitrogen (N)-doping (5.32 %) and degree of graphitisation (4.17). It is noted that the uniform distribution of metal ions within the carbon structure was achieved via the chelation of metal ions with highly available polysaccharides in Porphyra. The Porphyra-derived carbon could be an advanced anode material for high-energy rechargeable lithium-ion batteries [51,52]. Chen et al. [53] used a Ni catalyst for microcrystalline cellulose pyrolysis and graphitisation at temperatures between 1000 °C and 1600 °C. The ideal conditions were determined to be heating at 1400 °C for 3 h with a 3 mmol Ni loading. HRTEM results were quantified and revealed well-defined graphitic structures with more than 30 layers and an interlayer distance of 0.340 nm (Fig. S1). Since cellulose is the main phase of biomass and non-graphitisable, this study fills a gap in the literature on the catalytic graphitisation of biocarbon derived from cellulose and serves as a guide for developing alternative graphitic materials. Zha et al. [54] used mangosteen peel as a carbon source in a catalytic graphitisation process with a Co catalyst derived from cobalt chloride (CoCl<sub>2</sub>), which was then activated with potassium hydroxide, KOH solution before the process. The catalytic graphitisation process was then carried out at temperatures of 600, 700, 800, and 850 °C for 2 h to observe the morphology, degree of crystallinity, and surface area of the graphitic material. The graphitic material had a high degree of graphitisation with an I<sub>D</sub>/I<sub>G</sub> value of 1.26 and a surface area of 1168 m²/g. Yan et al. [55] studied the catalytic graphitisation of Kraft lignin using four transitional metal catalysts (Ni, Cu, Fe, and Mo) through a thermal treatment process under Ar flow at 1000 °C. The formation of multi-layer graphene-like-metal nanoparticles was observed along with some graphitic sheets. The Raman results proved that the graphitisation activity of these four metals is in the order of Fe > Ni > Mo > Cu with I<sub>D</sub>/I<sub>G</sub> values of 1.29, 1.39, 1.46 and 1.50, respectively. Fe with a lower I<sub>D</sub>/I<sub>G</sub> value indicates a higher degree of graphitisation with disorders. Thus, the presence of γ-Fe and iron carbide, Fe<sub>3</sub>C, a good degree of graphitisation with lateral extension, L<sub>a</sub> of 14.87 nm, was achieved with better catalytic activity than the other metals. Metal properties, such as catalytic activity, carbon solubility, and metal carbide formation, were related to the formation of the graphitic structure during the catalytic graphitisation of biocarbon from the Kraft lignin process.

Compared to other transition metals usually employed for catalytic graphitisation, iron is abundant, non-toxic to the environment, and has more significant activity in the graphitisation of biocarbon. It has been established that iron-catalysed graphitisation of biocarbon yields highly
graphitic biocarbon at temperatures lower than 1000 °C. Yang et al. [56] synthesised graphitic shell-like structures containing metal particles by pyrolysing a combination of sucrose and iron (III) nitrate, $Fe(NO_3)_3$, at a temperature exceeding 700 °C. The graphitic porous carbon structure was obtained with a high surface area of 198 m²/g and a high degree of graphitisation close to 1. The metal nanoparticles were removed by acid-washing, leaving a graphitic pod-like structure and several graphitic nanotubes, indicating the potential formation of diverse nanostructures inside the synthesised material. Ng et al. [57] prepared magnetite ($Fe_3O_4$)-graphitic nanocomposite via facile in-situ deposition. According to the FT-IR spectra (Fig. S2), oxygen-containing functional groups on the graphitic sheets may have helped the attachment of $Fe_3O_4$ nanoparticles. The findings demonstrated that the nanohybrids had good electrical and magnetic characteristics. Xia et al. [58] investigated the catalytic graphitisation of biocarbon from cellulose using Fe and studied its effect on pyrolytic behaviour and product properties. The in-situ production of iron carbide, $Fe_3C$ and Fe promoted secondary reactions and activated C-H bonds, increasing hydrogen and solid biocarbon production. Cellulose contains a high concentration of hydroxyl groups, which can chelate Fe ions and prevent Fe atom aggregation, which results in an even Fe distribution on the cellulose. The tiny carbon atoms were then adsorbed and dissolved into the Fe nanoparticles, resulting in liquid metastable $Fe_3C$. Moreover, the Fe addition contributed to the degree of graphitisation and porosity. This study herein guides the application of bioenergy to prepare porous graphite. Thompson et al. [59] worked on transforming raw biomass into nanostructured tubular graphitic carbon in a single step. Catalytic $Fe_3C$ nanoparticles were obtained in situ with a thermal breakdown of absorbed $Fe(NO_3)_3$, followed by carbothermal reduction. The $Fe_3C$ particles etch through the biomass to form entangled graphitic tubules by catalytic graphitisation. The materials were found to be mesoporous, with pore sizes that varied depending on the iron concentration. The conversion of raw biomass into stable graphitic carbon at very low temperatures, such as 800 °C, offers a viable approach towards large-scale and sustainable carbon synthesis for electrode or filtration applications.

In a study by Nakayasu et al., [31] high-crystalline graphitic biocarbon was synthesised from wood sawdust by using Fe and partially carbonised in an HTC at 250 °C, then carbonised again in an $N_2$ environment followed by acid-washing. At a low temperature of 850 °C, graphite-like carbon was synthesised with an iron/sawdust weight ratio of 4:10. The average interlayer distance and crystallite size of this carbon ($d_{002}$: 0.337 nm, $L_q$: 35.8 nm, and $L_c$: 56.1 nm) are equivalent to those of commercial graphite. The large size of the Fe crystal particles allowed for the easier formation of a layered structure of carbon sheets. Similarly, Gomez-Martin et al. [60] explained that understanding the mechanism of transition metals graphitisation is essential for developing graphitic materials. They confirmed the formation of $Fe_3C$ through the graphitisation of non-graphitisable carbon at a lower temperature of 750 °C by in-situ XRD and ex-situ total scattering experiments. The findings exhibited the influence of graphitisation temperature, the amorphous carbon's structure, and the Fe particles' size on the formation of graphitic-like carbon materials. However, Fe particles' graphitisation or growth mechanism for graphitic carbon formation has not yet been fully explored. Recently, Ghogia et al. [61] presented the mechanism of Fe-derived catalytic graphitisation of cellulose-derived biocarbon. The mechanism was determined by analytical approaches to evaluate the graphitic structure, Fe crystallinity and metal reduction at various graphitisation temperatures (1000, 1400 and 1800 °C). It was noted that an increase in the pyrolysis temperature increased iron particle size and degree of Fe reduction, as well as the development of larger graphitic carbon crystallite sizes. Besides, the variation in particle sizes and reduction of iron have influenced the magnetic characteristics. Indeed, the magnetisation value of biocarbon increases with graphitisation temperatures at 1000, 1400, and 1800 °C with 87, 90, and 100 emu/g, respectively. This work was supported by Romero et al. in the investigation of the carbon nanostructure by evaluating the bulk,

molecular, and atomic levels [62]. The combination of Raman spectroscopy and XRD demonstrated that the influence of Fe impregnation in carbon graphitisation is substantially stronger than temperature. Moreover, HRTEM and X-ray PDF analysis (Fig. S3) of Fe-doped cellulose biocarbon showed the presence of highly crystalline rhombohedral graphene-like with a disordered alignment of short-range assembled carbon layers. The experimental findings revealed that increasing the pyrolysis temperature and the presence of Fe favoured the graphitisation process. In association with the graphitisation degree, an average reduction of 72 % in the $I_D/I_G$ ratio was observed, suggesting the reduction of amorphous structure and formation of graphitic layers. This research demonstrates how multiple experimental approaches are needed to fully comprehend the nanostructure features of complex carbon materials. Based on the studies described above, Fe has the highest catalytic activity and is a more prominent metal for graphitisation than other transition metals.

Apart from the transition metals, alkaline and alkaline earth metals such as potassium, calcium, and magnesium have been extensively used as biomass pyro-gasification promoters to produce biocarbon and influence carbon structuration [63]. Xia et al. [64] designed a low-temperature graphitisation process at 900 °C using potassium carbonate ($K_2CO_3$) as a molten salt medium and catalyst to efficiently transform the complex carbon structure of coconut shell biocarbon into three-dimensional porous graphitic-like sheets. Firstly, $K_2CO_3$ chemically dissolved the carbon atoms in non-graphitisable carbon that consists of $sp^3$ cross-links. Afterwards, the graphene microsheets combined and expanded to create the graphitic lamellar structure with the aid of alkali metal atoms, potassium, K. The material developed a porous structure due to the alkali metal carbonate pore-forming properties and three-dimensional porous graphitic sheets with a large specific surface area. Pan et al. [65] synthesised porous graphitic biocarbon using Ni-Co doped activated carbon with KOH. The porous graphitic biocarbon possessed a surface area of more than 1000 m²/g. The molten KOH can disperse the catalysts to boost the formation of graphitic sheets and can disperse the graphitic sheets into thin flakes. It showed molten KOH can promote metal-catalysed graphitisation of carbon precursors, forming thin graphitised flakes that create highly porous structures. Meanwhile, calcium (Ca), which is non-toxic and inexpensive, is one of the most abundant elements found in biomass. Previously, calcium was used for the catalytic graphitisation of charcoal, and it was revealed that it could be a potential catalyst for the graphitisation. A new breakthrough was reported by Beguerie et al. [66], who demonstrated the graphitisation of calcium-doped cellulose at 1800 °C. It was noted that doping at 2 wt% calcium leads to well-organised and less-defective stacked graphene fringes with crystalline size ~ 20 nm and an average interlayer spacing closer to 0.345 nm, like commercial graphite. The increase in the calcium concentration did not significantly improve the crystallite sizes of the graphitic materials but drastically improved their activity rate. Recently, Beguerie et al. [67] presented new insight into the mechanism governing the synthesis of ordered graphitic biocarbon from cellulose and lignin using Ca as a catalyst. The formation of metastable calcium carbide ($CaC_2$) surrounded by graphitic layers at temperatures between 1000 and 1200 °C indicated that a mechanism of calcium-catalysed graphitisation occurred. The HRTEM and XRD analyses (Fig. S4) confirmed this, and at higher temperatures > 1400 °C, the $CaC_2$ decomposed and disordered graphitic structures were observed. Moreover, it was noted that lignin-derived graphitic biocarbon has a more organised structure than cellulose-derived material, with crystallite size > 10 nm and an average interlayer spacing closer to 0.340 nm. Thus, this study opens a new path on the use of alkaline metals such as Ca for the graphitisation process and the catalytic mechanism that will help to expand research prospects in the field of green synthesis of revolutionary graphitic materials employing inexpensive, abundant, and renewable resources [68].

Although numerous characterisation approaches, theoretical models, and catalytic findings have established the existence of metal-
support interactions, the nature and the significance of multi-metals remain unknown. According to the literature, the use of transition-alkaline earth metals as multi-metals in the graphitisation process has yet to be explored. Through these interactions, the carbon can boost the native catalytic activity of metal by improving electron transport to the metal or by encouraging improved metal dispersion and narrow particle size distribution, resulting in excellent catalytic performance. Table 1 summarises the catalytic graphitisation of biomass using multi-metal and single-metal catalysts and their effect on the degree of graphitisation, specific surface area, crystallite size and interlayer spacing.

### 2.1.1. Parametric effect on catalytic graphitisation

In addition to the catalyst's effect, the catalytic graphitisation process is also influenced by various parameters, including the type of carbon precursor, graphitisation temperature, and heating rate. The effects of these parameters lead to different crystallography, degrees of graphitisation, atomic arrangements, and the quality of graphitic biocarbon. However, there are only a limited number of reports regarding the impact of multi-metallic catalysis on the degree of graphitisation, carbon arrangement, and the mechanism of catalytic graphitisation. Therefore, the influence of these parameters is discussed here in the context of single-metal catalytic graphitisation.

#### 2.1.1.1. Carbon precursor.
Recently, efforts have been made to develop graphitic biocarbon materials from lignocellulosic (non-graphitisable), specifically lignin and cellulose, through catalytic graphitisation. The complexity of these non-graphitisable carbons with high microporosity, consisting of combined sp² and sp³ hybridised carbon atoms and short-range ordered regions, makes them challenging to graphitise [71]. Whether synthetic or biological, heating any organic substance in a low- or no-oxygen environment produces a carbon-rich solid with the structure of the carbon depending on the temperature and the nature of the precursor [72]. Most of these carbon compounds do not have a regular structure like diamond or graphite but rather a combination of characteristics with significant variability and disorder. Depending on the synthesis technique, the surface of amorphous carbon often consists of numerous reactive and dangling bonds [73]. Deng et al. [74] revealed

that hemicellulose disintegrated more quickly with heat treatment at 900 °C than cellulose and lignin, with 9% and 39% yield, respectively. The XRD result for lignin exhibited a larger peak at 2θ diffraction angles of 24° and 44° attributed to the graphitic reflection peak, indicating that lignin had a higher degree of graphitisation than cellulose (Fig. S5). Shi et al. [75] developed a multi-metallic catalyst (Fe:Ni:Mn) to convert biocarbon from sawdust into graphitic biocarbon with a degree of graphitisation (in percentage) of 89.28 %. It was explained that during the graphitisation process, the biocarbon matrix degraded into carbon atoms, dissolved into the liquid-state catalyst particles and precipitated as graphitic sheets on the surfaces. However, Raman studies showed the existence of amorphous carbon despite a longer graphitisation time, 3 h at 1300°C, which indicates that more research is required to improve the heating conditions, degree of graphitisation and performance of biocarbon derived from sawdust. On the other hand, Dyjak et al. [76] studied the effect of heteroatom (nitrogen-, sulfur-, and oxygen-doped) in Fe-assisted graphitisation of porous synthetic polymers (non-graphitisable carbon). The presence of sulfur affected the transformation of non-graphitisable carbon and had less effect using N-doped carbon. However, the HRTEM results revealed that the carbon materials of the three materials were non-graphitic and consisted of turbostrically heterogeneous materials. Similarly, Hunter et al. [77] investigated the effect of carbon precursors on the structure and porosity of the carbon material for the graphitisation process. They used glucose, starch, and cellulose by mixing with iron (III) nitrate, followed by heating in a N₂ environment to form Fe₃C nanoparticles. The nanoparticles catalyse the conversion of carbon materials to graphitic nanostructures via the graphitisation process. Under the same reaction conditions, cellulose and glucose formed mesoporous carbons, but starch developed a mixture of micro and mesoporous carbon by a slow graphitisation process, resulting in a mixture of graphitic nanostructures and turbostratic carbons. It was understood that the chemical composition of carbon sources is an essential aspect influencing the degree of graphitisation. Teawon Kim et al. [78] effectively converted activated carbon with poor crystallinity to highly graphitic biocarbon using nickel nitrate, Ni(NO₃)₂, as a catalyst. They found that the characteristics of synthesised graphitic biocarbon are substantially equal to commercial graphite. The XRD

<table><caption>**Table 1**
Summary of biomass catalytic graphitisation using multi-metal and single-metal catalysts.</caption><thead><tr><th rowspan="2">Carbon source</th><th rowspan="2">Catalyst</th><th colspan="2">Reaction conditions</th><th rowspan="2">Surface area, m²/g</th><th rowspan="2">I<sub>D</sub>/I<sub>G</sub></th><th colspan="2">Crystallite size, nm</th><th rowspan="2">Interlayer distance (d<sub>200</sub>), nm</th><th rowspan="2">Ref.</th></tr><tr><th>Temperature, °C</th><th>Time, h</th><th>L<sub>a</sub></th><th>L<sub>c</sub></th></tr></thead><tbody><tr><td colspan="10"><strong>Multi-metal catalysts</strong></td></tr><tr><td>Ginger root</td><td>Pt/Al</td><td>800</td><td>1</td><td>320.67</td><td>1.10</td><td>—</td><td>—</td><td>0.363</td><td>[46]</td></tr><tr><td>Moringa oleifera stem</td><td>FeCl<sub>3</sub>;ZnCl<sub>2</sub></td><td>800</td><td>2</td><td>2250</td><td>1.08</td><td>—</td><td>—</td><td>—</td><td>[47]</td></tr><tr><td>Sawdust</td><td>Ni-Mo</td><td>750</td><td>2</td><td>1366.54</td><td>1.05</td><td>—</td><td>—</td><td>—</td><td>[48]</td></tr><tr><td>Bamboo</td><td>Fe-Co</td><td>850</td><td>1</td><td>317</td><td>1.78</td><td>—</td><td>—</td><td>0.34</td><td>[49]</td></tr><tr><td colspan="10"><strong>Single-metal catalysts</strong></td></tr><tr><td>Coconut shell</td><td>K<sub>2</sub>CO<sub>3</sub></td><td>900</td><td>2</td><td>1506.2</td><td>0.088</td><td>—</td><td>—</td><td>0.346</td><td>[64]</td></tr><tr><td>Porphyra</td><td>Ni(NO<sub>3</sub>)<sub>2</sub></td><td>800</td><td>1</td><td>811.2</td><td>4.17</td><td>—</td><td>—</td><td>0.345</td><td>[50]</td></tr><tr><td>Microcrystalline cellulose</td><td>NiCl<sub>2</sub></td><td>1400</td><td>3</td><td>n.a</td><td>0.50</td><td>—</td><td>—</td><td>0.340</td><td>[53]</td></tr><tr><td>Mangosteen peel</td><td>CoCl<sub>2</sub></td><td>800</td><td>2</td><td>1168.0</td><td>1.26</td><td>—</td><td>—</td><td>0.340</td><td>[54]</td></tr><tr><td>Kraft lignin</td><td>Cu, Ni, Mo, Fe</td><td>1000</td><td>1</td><td>Fe – 108</td><td>Fe – 1.29</td><td>14.9</td><td>—</td><td>Fe > Ni > Mo > Cu</td><td>[55]</td></tr><tr><td>Sucrose</td><td>Fe(NO<sub>3</sub>)<sub>3</sub></td><td>700</td><td>1</td><td>198.0</td><td>1.0</td><td>—</td><td>—</td><td>0.337</td><td>[56]</td></tr><tr><td>Cellulose</td><td>Fe(NO<sub>3</sub>)<sub>3</sub></td><td>850</td><td>0.5</td><td>185.23</td><td>1.65</td><td>—</td><td>—</td><td>—</td><td>[58]</td></tr><tr><td>Softwood sawdust</td><td>Fe(NO<sub>3</sub>)<sub>3</sub></td><td>800</td><td>1</td><td>220.0</td><td>—</td><td>—</td><td>—</td><td>—</td><td>[59]</td></tr><tr><td>Wood sawdust</td><td>Fe(NO<sub>3</sub>)<sub>3</sub></td><td>850</td><td>1</td><td>—</td><td>—</td><td>35.8</td><td>56.1</td><td>0.337</td><td>[31]</td></tr><tr><td>Microcrystalline cellulose</td><td>Fe(NO<sub>3</sub>)<sub>3</sub></td><td>1800</td><td>1</td><td>—</td><td>~0.1</td><td>7.2</td><td>6.4</td><td>0.204</td><td>[62,69]</td></tr><tr><td>Microcrystalline cellulose</td><td>Ca(NO<sub>3</sub>)<sub>2</sub></td><td>1400</td><td>1</td><td>—</td><td>0.59</td><td>> 6.0</td><td>7.69</td><td>0.345</td><td>[70]</td></tr><tr><td>Microcrystalline cellulose</td><td>Ca(NO<sub>3</sub>)<sub>2</sub></td><td>1400</td><td>1</td><td>—</td><td>—</td><td>—</td><td>7.69</td><td>0.341</td><td>[66,67]</td></tr><tr><td>Lignin</td><td>Ca(NO<sub>3</sub>)<sub>2</sub></td><td>1400</td><td>1</td><td>—</td><td>—</td><td>—</td><td>17.41</td><td>0.340</td><td></td></tr></tbody></table>

Note: "-": not available.
pattern confirmed the change to ordered carbon by the appearance of a peak at an angle of 26.5° with a d-spacing of 0.337 nm, almost similar to the commercial graphite at 0.336 nm (Fig. 86). This finding supports the transition from amorphous to graphitic material. Furthermore, the Raman spectra of the synthesised graphitic biocarbon and commercial graphite, $I_D/I_G$ values are 3.34 and 3.29, respectively, showing that poorly crystalline carbon can be converted into ordered carbon that possesses a good degree of graphitisation using the catalyst.

Meanwhile, the preparation conditions, notably the pyrolysis temperature, have a considerable impact on the physicochemical features of biocarbon, such as pore structure, chemical composition, surface area, and stability [79]. The H:C (hydrogen to carbon) molar ratio is an essential indicator of the aromatisation degree, which refers to the biocarbon's graphitisation degree [80]. It has been discovered that dehydrogenation and dehydration dominate the early pyrolysis process until temperatures exceed 300 °C. At that point, polymers in biomass, including cellulose, hemicellulose, and lignin, begin to depolymerise. As the pyrolysis temperature increases, the depolymerisation and dehydrogenation processes take control, resulting in increased aromatisation, decreased structural heterogeneity, and enhanced chemical stability of biocarbon. As the temperature rises over 700 °C, the highly stable lignin macromolecules and the volatiles content begin to degrade. The H:C and O:C ratios of biocarbon decrease, resulting in a disordered graphitic microcrystalline structure [81]. Typically, as the pyrolysis temperature rises, biocarbon yield, fixed carbon, and H:O concentration drop due to the aromatisation. In a study by Zhang et al., [82] Kraft lignin was thermally processed in an inert environment at temperatures ranging from 500 to 1000 °C. The findings of the experiments showed that raising the temperature reduced amorphous carbon while increasing aromaticity in thermally treated Kraft lignin. As the temperature rose from 500 to 1000 °C, the $L_a$ value increased from 0.7 to 1.4 nm. The rise in $L_a$ was mainly caused by the breakdown of lignin side chains and the aromatisation of free radical-containing aromatic groups. Yan et al. [55] reported that Kraft lignin was catalytically graphitised to graphitic nanostructures under an Ar environment using four Fe catalysts such as iron (III)nitrate ($Fe(NO_3)_3$), iron (II)chloride ($FeCl_2$), iron (III) chloride ($FeCl_3$), and iron (II)sulfate ($FeSO_4$). The catalytic breakdown process of iron-promoted lignin materials to biocarbon showed that $Fe-NO_3$ has been the most active Fe salt for the graphitisation process. The FT-IR analysis results demonstrated that as the graphitisation temperature increased, the functional groups of Kraft lignin continued to degrade, resulting in the aromatisation of the Kraft lignin biocarbon matrix. Debarma et al. [83] used pyrolysis at low temperatures ranging from 250 to 450 °C to synthesise graphene oxide from sugarcane bagasse. The pyrolysed material was filtered, washed with warm deionised water, and dried for 24 h. It was discovered that 350 °C was adequate for large-scale condensation and aromatisation of glucose monomers to generate graphene oxide nanosheets. Yu et al. [84] developed a DTPH reaction system to lower the HTC temperature. The results indicated that the HTC reaction of lignocellulosic biomass such as rice straw and poplar leaves could be achieved via the DTPH process at a low temperature of 200°C. The DTPH process breaks the barrier of the cellulose crystalline structure in lignocellulosic biomass with a high cellulose content, allowing the carbonisation of cellulose and hemicellulose by dehydration, unsaturated bond formation, and aromatisation. The resulting hydrobiocarbon resembled carbon microspheres, with high calorific values, an abundance of oxygen-containing functional groups, a degree of graphitisation, and strong thermal stability. Therefore, biocarbon formation via pyrolysis is a solid-to-solid conversion process in which biomass exists as either powder or in small to large pieces, thereby transforming into biocarbon under optimised processing conditions. Furthermore, the carbonisation of biomass is complex due to its nonhomogeneity in composition. For lignocellulosic biomass, this complexity depends on the source and the ratios of cellulose, hemicellulose, and lignin. The composition of lignocellulosic biomass reflects the unique structure of its biopolymers. The thermal degradation

temperatures and mechanisms of these three constituents are entirely different, which can result in various degradation products.

Besides the aforementioned lignocellulosic materials, algae as a carbon feedstock can be an alternative option for biocarbon production and its benefits in energy applications [50,85]. The fast growth rate, global distribution, and strong carbon sequestration ability of algae, rich in carbon and nitrogen, have been the driving forces in selecting algae as a carbon source for biocarbon production. A recent study by J. Li et al. showed the prospect of solar pyrolysis using algae with molten carbonates for capacitive graphitic carbon production [86]. The physicochemical characteristics and electrochemical performance of the synthesised biocarbon were conducted to determine the potential of solar pyrolysis of algae. The synthesised biocarbon has carbon-, hydroxyl- and nitrogen-containing functional groups such as carbonyl ($C=O$), hydroxyl (-OH), pyrrolic-N and quaternary-N. The presence of nitrogen improves the carbon capacitive as the current density increases, with the capacitance retention at 75.5 %. It showed that the electrode with biocarbon pyrolysed at 900°C using molten carbonates had a good capacitance between 173.9–230.2F/g and stability with 90.4 % retention after 5000 cycles. Algae pyrolysis exhibited its efficiency in capacitive performance and suitable carbon materials for supercapacitors. As the EU approaches a sustainable future, the EU aims to cut greenhouse gas emissions by at least 55 % by 2030 and achieve climate neutrality by 2050 [87]. An estimated 26–50 million tonnes of lignocellulosic crops are expected in Europe by 2030, and algae production in Europe is still limited, with 287 390 tonnes in 2019 [88,89]. Yet, this report emphasises European algae's potential for sustainable development and provides an overview of the potential availability of feedstocks in Europe for sustainable development.

Although limited in number, the aforementioned studies have successfully demonstrated the catalytic graphitisation of lignocellulosic materials at relatively low temperatures (< 1800 °C). Gaps in research persist regarding the optimal catalysts and the types of lignocellulosic feedstocks. Future studies should investigate the use of lignocellulosic materials rich in Fe and Ca, as these are established catalysts for lowering the graphitisation temperature and producing long-stacked graphitic biocarbon structures. Additionally, there exists intriguing potential for the creation of nanostructural defects from lignocellulosic materials and their impact on the properties of graphitic biocarbon compared to commercial graphite or graphene.

2.1.1.2. *Graphitisation temperature.* Heat treatment is a key aspect of graphitisation since it supplies energy for the macromolecule rearrangement process. Chen et al. [90] developed lignin and cellulose-derived biocarbons by direct graphitisation under ultra-high temperatures. It was discovered that cellulose-derived biocarbon was mainly constituted of carbon microcrystalline, but lignin-derived biocarbon retained part of its skeleton structures, and this structural difference was critical in the morphology of generated graphitic layers. The results showed that at 2800 °C, the graphitised lignin-derived carbon had an 89.53 % graphitic degree and an interlayer spacing of 0.336 nm. In contrast, the cellulose-derived biocarbon had a graphitic degree of 76.74 % and an interlayer distance of 0.337 nm. The stable and aromatic ring in lignin was beneficial to ring-enlarging rebuilding and the development of large regions of continuous graphitic layers during the graphitising process. This research is anticipated to give theoretical guidelines for synthesising high-performance functional carbon compounds from lignocellulosic biomass. High-temperature graphitisation has been examined as a sustainable process of creating these technologically essential materials [91]. Still, transition metals are known to accelerate the synthesis of graphitic carbon, reducing the temperature and potentially influencing the pore structure. Major et al. [92] emphasised the catalytic impact of Fe(III) and Co(II) nitrates on the pyrolysis of *Miscanthus* grass and demonstrated the employment of multi-metallic to be more efficient in producing highly ordered graphitic
biocarbon than using Fe and Co as individual catalysts. It has been suggested that a degree of graphitisation of 0.77 has been achieved due to the formation of Fe-Co alloy nanoparticles. The presence of amorphous carbon at a graphitisation temperature < 900°C indicates an incomplete graphitisation process, and the yield of graphitic biocarbon could be improved by increasing the graphitisation temperature (>1000°C).

Sun et al. [93] described using an iron nitrate solution for catalysed graphitisation of biocarbon from sawdust. This study reveals that temperature significantly impacts the morphology of Fe and, as a result, the graphitisation of biocarbon. The sawdust was pyrolysed in a furnace at a rate of 5°C/min under a stream of N₂ (50 mL/min) at temperatures ranging from 600 to 800°C. The degree of graphitisation rose as the graphitisation temperature increased from 600 to 800°C, but the solid content of the biocarbon dropped. The degree of graphitisation of the biocarbon increased by 17.4% when the amount of catalyst applied was raised thrice. Increasing the temperature to 800°C might increase Fe reduction, accelerate the dissolution and precipitation of Fe-C, and accelerate the development of the graphitic structure with an enhanced graphitisation degree of 76.2%. Liu et al. [94] prepared porous Fe₃O₄/carbon fibre composites from bagasse waste via in-situ growth and graphitisation. The micromorphology of Fe₃O₄/carbon fibre with the ratio of 1:1 graphitised at 600 and 800 °C indicates that the graphitisation temperature significantly impacts the microstructure of carbon fibre and the development of Fe₃O₄ nanocrystals. The morphology analysis showed the composite consisted of a rough surface and porous structure, creating many interfaces between the Fe₃O₄ nanocrystals and the carbon fibre, which resulted in multiple scattering. This shows that the graphitisation temperature dominates the defect and graphitisation degree. Guizani et al. [95] evaluated the effect of entrained flow pyrolysis on the characteristics of biomass-based carbon with temperatures ranging from 500 to 1400 °C. The results showed that macropore and organised biocarbon structures were achieved at higher temperatures. Destyorini et al. [96] prepared graphitic carbon from waste biomass, coconut coir, via pyrolysis. Amorphous biocarbon was developed by carbonising coconut coir and converting it into graphitic biocarbon using a Ni-based catalyst at 1300°C for 3 h. The results indicated well-defined graphitic layers with an interplanar distance of 0.3367 nm around Ni particles. The graphitisation degree of synthesised graphitic carbon is 84.88%, lower than commercial graphite (90.23%). This study paved the way for the carbonisation of coconut waste at 1300°C to produce graphitic and conductive carbon.

Based on Marsh Griffith's model for carbonisation, the aromatic carbon content increases with temperature, as illustrated in Fig. S7 [97]. Above 700 °C, carbon exists as amorphous carbon with sp¹, sp², and sp³ bonds that do not exhibit any stacking. As the temperature rises from 700 °C to 1200 °C, the carbon bonding transitions into the sp² hybridised state. A further increase in temperature from 1400 °C to 1700 °C results in the formation of non-graphitic carbon. Beyond 1700 °C, the non-graphitic carbon transforms into graphitic sheets, which are organised in ordered stacks. This can vary depending on the feedstock type, owing to differences in bonding, networking abilities, number of hetero-atoms (nitrogen, sulfur, silica) and the defects present in the non-graphitic and graphitic phases [98]. The aforementioned characteristic transformation of the various carbon phases can be examined using diffraction and spectroscopic analysis.

#### 2.1.1.3. Heating rate.
The heating rate is proportional to the development of pores and cracks, followed by the production of volatile gases. The slower heating rate promotes the uniform carbonisation of biomass. However, the fast-heating rate speeds up the release of condensable gases and inhibits secondary pyrolysis reactions. More precisely, changes in biocarbon properties such as porosity and graphitisation degree were detected due to heating rate variations under the standard heating parameter of slow and rapid pyrolysis [18,99]. The effect of

heating rate on the graphitisation process has only been reported in a few studies. A study by Jabarullah et al. [100] demonstrated the conversion of palm waste into bio-based graphitic biocarbon using a multi-metallic catalyst, Fe-Ni. The structural modification of carbon was discovered to be significant by using the multi-metallic catalyst at a 5°/min heating rate. A high degree of graphitisation for bio-based graphitic biocarbon was observed at 1000°C with a heating rate of 5°/min, proving that the combination of Fe-Ni significantly enhanced the degree of graphitisation and reduced graphitisation temperature with a moderate heating rate. This result was supported by the structural analysis reflecting the presence of highly ordered graphitic biocarbon structures. According to Tarelho et al. [101], the carbon content of the biocarbon generated rose with an increasing heating rate. This is because many lignocellulosic structures are degraded and transformed into carbon. Karim et al. [102] synthesised graphitic biocarbon at a higher temperature of around 2500°C via pyrolysis. The group studied the effect of heating rates ranging from 5°C/min, 10°C/min, and 20°C/min. The graphitisation process at 800 °C with a heating rate of 20 °C/min caused an incomplete formation of graphitic structures. Graphitisation occurs when limited movement and rearrangement of carbon atoms undergo a reconstructive transformation during the heat treatment process. According to Lee et al. [103], at a higher heating rate of 20 °C/min, the shrinkage rate was around 32.3% lower than when it was 2°C/min, knowing all of the graphitic blocks tend to contract after the graphitisation process. It was attributed that the processing time was shorter when the heating rate was higher, which may reduce the crystallinity. At 2 °C/min heating rate, the degree of graphitisation was 83.7%, whereas, at 20 °C/min, a decrease in crystallinity was observed with a degree of graphitisation of 80.5% where the temperature reached 2800 °C in around 14.2 h at a heating rate of 2 °C/min while it took roughly 2.3 h at 20°C/min. Therefore, a six-fold reduction in the graphitisation time is the reason for the variation in the graphitisation degree. The graphitic blocks' physical characteristics were good at a modest 2 °C/min heating rate, and the flexural strength measurement of graphitic blocks showed a significant variation at a heating rate of 20°C/min. This phenomenon was supported by Lee et al. [104], stating that at a higher heating rate, such as 9°C/min, the pore size of graphitic blocks increased, and rapid gas release caused irregular graphitic structures.

Based on the heating rate of graphitisation, there are two types: slow (2—10 °C/min) and fast (100—1000 °C/s) pyrolysis. The primary products of slow pyrolysis are biocarbon, along with smaller quantities of water and non-condensable gases, including CO, CO₂, and H₂ [105]. A higher heating rate enhances bio-oil yield, while a lower heating rate results in a greater biocarbon yield. According to nanostructural characterisation, reducing the heating rate can enhance the yield of graphitic biocarbon, the degree of graphitisation, and porosity, all of which contribute to the properties of biocarbon. Table 2 summarises the parametric effects of catalytic graphitisation, such as carbon precursor, graphitisation temperature, and heating rate.

### 2.1.2. Parametric interrelationship effect on graphitic biocarbon

Commercial graphite is a suitable alternative to natural graphite; however, the production cost is high. To reduce the costs associated with energy-intensive processes, catalytic graphitisation has emerged as a potential solution. Catalytic graphitisation facilitates the transformation of non-graphitisable carbon into graphitic carbon materials without requiring carbon activation. In the patent presented by Acheson [106,107], heating amorphous or non-graphitisable carbon with iron, aluminium, or calcium aids in forming carbides from the respective compounds, followed by thermal decomposition to produce graphitic carbon. Transition metals with specific d-orbital vacancies contribute to the dissolution of carbon at elevated temperatures and precipitation into graphitic carbon upon cooling. The heating process is significantly important in catalytic graphitisation, and not all final products are classified as graphite, as this is dictated by the maximum graphitisation temperature. Consequently, catalytic graphitisation has garnered
<table><caption>**Table 2**
Summary of parametric effects of catalytic graphitisation.</caption><thead><tr><th rowspan="2">Samples</th><th rowspan="2">Catalysts</th><th colspan="2">Parameters</th><th rowspan="2">Notes</th><th rowspan="2">Ref.</th></tr><tr><th>Temperature, °C</th><th>Heating rate, °C/min</th></tr></thead><tbody><tr><td colspan="6"><strong>Carbon precursor</strong></td></tr><tr><td>Sawdust</td><td>Fe:Ni:Mn (Trimetallic)</td><td>1300</td><td>20</td><td>Existence of amorphous carbon despite a longer graphitisation time with I<sub>D</sub>/I<sub>G</sub> (in percentage) of 89.28 %</td><td>[75]</td></tr><tr><td>Synthetic polymers (nitrogen; sulfur; oxygen)</td><td>Fe</td><td>750—1150</td><td>5</td><td>Presence of sulfur affected the transformation of non-graphitisable carbon at ~ 1130 °C and had less effect using N-doped carbon, which resulted in turbostrically heterogeneous biocarbon</td><td>[76]</td></tr><tr><td>Glucose; starch; cellulose</td><td>Fe</td><td>800</td><td>5</td><td>The Fe<sub>3</sub>C nanoparticles catalysed the carbon materials, resulting mixture of graphitic nanostructures and turbostratic carbons. Composition of carbon sources is an essential aspect influencing the degree of graphitisation</td><td>[77]</td></tr><tr><td>Activated carbon (G60, KB)</td><td>Ni</td><td>1000</td><td>33.3</td><td>Appearance of a peak at an angle of 26.5° with a d-spacing of 0.337 nm and I<sub>D</sub>/I<sub>G</sub> values are 3.34</td><td>[78]</td></tr><tr><td>Kraft lignin</td><td>—</td><td>500—1000</td><td>20</td><td>As the temperature rose from 500 to 1000 °C, the L<sub>a</sub> value increased from 0.7 to 1.4 nm, caused by the breakdown of lignin side chains</td><td>[82]</td></tr><tr><td>Kraft lignin</td><td>Fe; Ni; Cu; Mo</td><td>1000</td><td>10</td><td>Fe-NO<sub>3</sub> has been the most active Fe showed the prominent catalyst to graphitise lignin at 1000 °C</td><td>[55]</td></tr><tr><td>Sugarcane bagasse</td><td>—</td><td>350</td><td>4.2—45</td><td>Pyrolysis of glucose monomers at low temperatures without catalyst generated graphene oxide nanosheets</td><td>[83]</td></tr><tr><td>Rice straw; poplar leaves</td><td>—</td><td>200</td><td>—</td><td>The DTPH process disrupts the crystalline structure of cellulose in lignocellulosic biomass that has a high cellulose content, facilitating the carbonisation of both cellulose and hemicellulose to produce graphene oxide with a favourable I<sub>D</sub>/I<sub>G</sub> ratio and strong thermal stability</td><td>[84]</td></tr><tr><td>Algae</td><td>—</td><td>800—900</td><td>15</td><td>The synthesised biocarbon at 900 °C contains functional groups including carbonyl (C=O), hydroxyl (-OH), pyrrolic-N, and quaternary-N, indicating its effectiveness in capacitive performance and its suitability as a carbon material for supercapacitors</td><td>[86]</td></tr><tr><td colspan="6"><strong>Graphitisation temperature</strong></td></tr><tr><td>Lignin; Cellulose</td><td>—</td><td>1600—2800</td><td>10</td><td>At 2800 °C, the graphitised lignin-derived biocarbon had an 89.53 % I<sub>D</sub>/I<sub>G</sub> and an interlayer spacing of 0.336 nm. In contrast, the cellulose-derived biocarbon had I<sub>D</sub>/I<sub>G</sub> of 76.74 % and an interlayer distance of 0.337 nm.</td><td>[90]</td></tr><tr><td>Miscanthus grass</td><td>Fe; Co</td><td>500—900</td><td>10</td><td>I<sub>D</sub>/I<sub>G</sub> ratio of 0.77 has been achieved owing to the formation of Fe-Co (multi-metal) alloy nanoparticles, and the yield of graphitic biocarbon may be enhanced by raising the graphitisation temperature (>1000 °C).</td><td>[92]</td></tr><tr><td>Pine sawdust</td><td>Fe</td><td>600—800</td><td>5</td><td>Increasing the temperature to 800 °C enhances the reduction of Fe, accelerates the dissolution and precipitation of Fe-C, and promotes the development of the graphitic structure with an improved I<sub>D</sub>/I<sub>G</sub> of 76.2 %.</td><td>[93]</td></tr><tr><td>Bagasse waste</td><td>Fe</td><td>700</td><td>5</td><td>The materials consisted of a rough surface and porous structure, creating many interfaces between the Fe<sub>3</sub>O<sub>4</sub> nanocrystals and the carbon fibre, which resulted in multiple scattering. This shows that the graphitisation temperature dominates the defect and I<sub>D</sub>/I<sub>G</sub></td><td>[94]</td></tr><tr><td>Coconut coir</td><td>Ni</td><td>1300</td><td>7.2</td><td>Well-defined graphitic layers with an interplanar distance of 0.3367 nm around Ni particles and I<sub>D</sub>/I<sub>G</sub> of synthesised graphitic carbon is 84.88 %</td><td>[96]</td></tr><tr><td>Pitches; Resins</td><td>—</td><td>3000</td><td>20 (120 °C)<br>0.1 – 2<br>(>120 °C)</td><td>Four-staged transformation: from a molecule-like precursor via amorphous (1) and nanoparticular carbon (2) to non-graphitic carbon (3) or even graphitic carbon (4). In stages 3 and 4, the 2D Raman signal emerges, and its spectral shape allows to distinguish whether non-graphitic or graphitic carbon is formed</td><td>[97]</td></tr><tr><td colspan="6"><strong>Heating rate</strong></td></tr><tr><td>Palm waste</td><td>Fe-Ni</td><td>800; 1000</td><td>5</td><td>A high degree of graphitisation for bio-based graphitic biocarbon was observed at 1000 °C with a heating rate of 5°/min, proving that the combination of Fe-Ni significantly enhanced the I<sub>D</sub>/I<sub>G</sub> and reduced graphitisation temperature with a moderate heating rate</td><td>[100]</td></tr><tr><td>Oil palm trunk waste</td><td>—</td><td>500—1000</td><td>5; 10; 20</td><td>The graphitisation process at 800 °C with a heating rate of 20 °C/min caused an incomplete formation of graphitic structures and required a lower heating rate, 5 °C/min.</td><td>[102]</td></tr><tr><td>Isotropic coke</td><td>—</td><td>2800</td><td>2</td><td>At 2 °C/min heating rate, the I<sub>D</sub>/I<sub>G</sub> was 83.7 %, whereas, at 20 °C/min, a decrease in crystallinity was observed with I<sub>D</sub>/I<sub>G</sub> of 80.5 % when the temperature reached 2800 °C</td><td>[103]</td></tr></tbody></table>

Note: "-": not available.

interest over the last two decades, observing its effects on essential conditions such as graphitisation temperature, the use of non-graphitising materials, and heating rate. This review section presents the impact of multi- and single-metallic catalysts on biocarbon graphitisation. Fig. 2 illustrates the effect of multi-metal (M–M), single-metal (S-M), and without-metal (W-M) catalysts on graphitisation conditions, such as graphitisation temperature, carbon precursors, and heating rate. The impact has been classified by level of intensity, with the highest value (4) indicating a more robust effect and the lowest value (1) as least

robust. The intensity of multi-metallic catalysts proves to be more robust regarding temperature and carbon precursors, suggesting that multi-metallic catalysts can lower the graphitisation temperature and enhance metallic interactions in converting amorphous or non-graphitisable carbon into graphitic biocarbon. Meanwhile, single-metallic catalysts display moderate robustness concerning graphitisation temperature, as the carbon materials are partially graphitised and remain turbostratic, despite being graphitised at elevated temperatures. Conversely, it has been observed that the conventional method (W-M) of graphitisation at
<figure><img src="image_3.png" /><figcaption>Fig. 2. The hierarchy of influential graphitisation conditions</figcaption></figure>

high temperatures yields negative feedback, as the robustness fluctuates due to the necessity for a higher graphitisation temperature, which significantly impacts the remaining parameters.

Overall, the quality of graphitic biocarbon is determined by the interaction of the catalysts with the carbon precursor at an optimal graphitisation temperature and specific heating rate. Based on the review, it was found that employing multi-metal could effectively enhance the degree of graphitisation and carbon aromatisation at a lower temperature and shorter duration compared to single-metal. Thus, the hierarchy of the influential catalytic graphitisation conditions is as follows: graphitisation temperature > carbon precursors > heating rate. A hypothetical idea suggests that the graphitisation of non-graphitisable carbon using multi-metal could exhibit high crystalline graphitic domains, promoting the growth of long-range stacked carbon layers at temperatures below 1800 °C. The outcome could contribute to the formation of metal-carbide initiated at a lower temperature, 1200 °C, and the visibility of graphitic biocarbon structures at 1800 °C. The interaction of two different metals could aid in the dissolution of amorphous carbon and the rearrangement of graphitic carbon individually, which commences at a lower graphitisation temperature [108]. This finding corresponds with the literature, where the electron configuration of a metal with full d-shell energy is scarcely altered when accepting electrons from carbon. Hence, this metal can react with carbon to form covalent bonds and facilitate carbon dissolution. This demonstrates a practical approach to the catalytic graphitisation of amorphous carbon via a solid-state transformation process at a lower heating rate, ranging between 2 – 5 °C/min, and for a shorter duration. Studies have shown that the physical properties, such as $d_{002}$, L$_{a}$, L$_{c}$, I$_{D}$/I$_{G}$, and specific surface area, are comparatively enhanced at a lower heating rate (< 10 °C/min) due to the uneven formation of the graphitic structure at high heating rates [109]. Furthermore, the characteristics of multi-metal

have attracted new attention towards catalytic graphitisation by harnessing the synergistic benefits of these metals in developing crystalline graphitic biocarbon with fewer nanostructure defects and their potential impact on properties such as electrical conductivity, thermal conductivity, and light absorption.

### 2.1.3. Limitation of the catalytic graphitisation

While significant progress has been made in the graphitisation conversion, challenges remain in the graphitisation process of amorphous carbon, as well as in the preparation and properties of graphitic biocarbon [110–112]. These challenges relate to the complex graphitisation mechanism at atomic and molecular scales, the unclear relationship between graphitisation and multi-metal doping, and the structure–activity relationship involving nanostructure evolution and properties [113]. Lignocellulosic materials generally contain a variety of volatiles and impurities, which can disrupt the synthesis process and compromise the quality of the produced graphitic biocarbon. Additionally, optimising the processing conditions necessary for synthesising graphitic biocarbon from lignocellulosic materials is notably challenging. Factors such as temperature, catalyst selection, and various other parameters must be precisely controlled to ensure the desired product is achieved.

Moreover, introducing computational approaches can significantly contribute to studying the interaction between multi-metal and non-graphitising materials. However, the studies are still relatively new, as they mainly focus on coal or black carbon rather than lignocellulosic materials due to their structural complexity and primarily emphasise absorption studies [114–117]. Recently, a study was conducted on the homogeneous mixture of ionic liquid ([Bmim] $H_2PO_4$) and biocarbon precursor (pine sawdust) for co-pyrolysis, which not only facilitated the graphitisation of the precursor but also enabled N/P hetero-atom doping [118]. The computational analysis based on DFT indicated that
pyridinic N exhibits a strong affinity for both Li⁺ and Na⁺, whilst P
atoms embedded within the graphitic layers significantly augment the
interlayer spacing. This study has paved the way for new insights into
determining the synergetic effect of multi-metal on catalytic graphiti-
sation by computational. Additionally, Huang et al. [119] combined
DFT with ML methods to investigate the incorporation of alkali metal
(Li, Na, K) atoms in model carbon systems within specific density and
disorder ranges. This study provided atomic-scale insights into the
insertion of three alkali metals relevant to batteries, marking a signifi-
cant advancement towards energy material modelling driven by DFT/
ML. Liu et al. [120] integrated ML with a structural-property database of
non-graphitising carbon materials, designed suitable models to effec-
tively screen appropriate structural parameters, and proposed a stand-
ardised process for the programmed preparation of high-performance
electrode materials. These approaches allow the continuously updated
database to effectively guide computational calculations regarding the
characteristics of graphitic biocarbon and the engineering applications
of non-graphitising carbon materials, which remain limited.

3. Mechanism of catalytic graphitisation

Pyrolysis has recently been used to synthesise graphene or graphitic carbon-based materials from various types of biomass. Understanding the mechanism of graphitic biocarbon synthesis during biomass pyrolysis requires understanding the mechanism of biocarbon development, as the two are closely connected [121]. Carbonisation is the process of removing light-molecular-weight compounds by heating, whereas graphitisation is the process of arranging the remaining carbon structures into a graphitic biocarbon. Metal catalysts for graphitisation reduce the activation energy required to transform disordered carbon into a layered structure, allowing graphitisation processes to occur at a lower temperature. Studies showed catalytic graphitisation follows two primary mechanisms: (1) dissolution-precipitation and (2) carbide formation-decomposition. In this section, physical analyses, including XRD, Raman spectroscopy, and HRTEM, elucidated the catalytic mechanism. XRD provides a foundational description of the various carbon structures present in the biocarbon and determines the crystallite size. Raman spectroscopy conveys information regarding amorphousness and defective graphitic structures, while HRTEM examines graphitic structures at the nanoscale, complementing the insights from XRD and Raman spectroscopy.

A few researchers have described the utilisation of multi-metallic

catalysts Fe-Co and Fe-Ni catalysed graphitisation of biocarbon. Jabar-
ullah et al. [100] suggested that using metallic catalysts (Fe-Ni) has a
considerable and noticeable effect on decreasing the graphitisation
temperature. Carbon atoms react with the catalyst during pyrolysis to
generate multiple carbides, then decompose to form graphitic carbon.
This process involves moving, assembling, and disintegrating carbon
atoms to form graphitic carbon. Dissolution-precipitation processes
have been postulated to produce graphitic carbon from amorphous
carbon. At a particular temperature, disordered carbon diffuses and
dissolves into metal or metal carbide, resulting in saturated carbon
solubility in an equilibrium state. With a lower temperature, the metal
saturated with disordered carbon becomes super-saturated with carbon.
As a result, carbon precipitates in the form of graphitic materials, which
are highly organised carbons with the lowest free energy levels.

Gökçeli et al. [122] developed graphitic carbon from hazelnut shell waste at a lower graphitisation temperature at 800 °C using multi-metallic, Co-Mo catalysts. The addition of Mo can result in significant improvements since it helps to stabilise the growth of Co particles during the reaction by reducing unwanted Co aggregation. As a result, it may be concluded that this interaction is critical for sustaining the catalyst’s efficacy throughout the process, as increasing the amount of Mo can remain unused. As shown in the HRTEM (Fig. 3), the stacked structures of graphitic carbons are surrounded by catalyst particles, which means catalyst particles promote growth sites for graphitic layers.

Furthermore, the catalytic mechanism of multi-metallic catalysts in biocarbon graphitisation remains unclear and necessitates comprehensive investigations using various analytical and computational techniques.

Enough studies have reported on the mechanisms involving single-
metallic catalysts. Thus, this section highlights recent investigations
using transition metals (Ni and Fe) and an alkaline metal (Ca) in
graphitisation. Khoshk Rish et al. reported that their work followed the
cycles of dissolution, saturation, and precipitation, along with metal-
induced graphitisation for graphitic biocarbon formation. The catalyst
particles melted and agglomerated to form larger Ni particles during
heating. Moreover, graphitisation occurred after several dissolution-
precipitation cycles, leading to the formation of multiple graphitic
layers around the catalyst particles, which were geometrically compat-
ible with the corresponding catalyst particles. Each layer is produced by
one graphitisation cycle (dissolution-precipitation). Specifically, the
molten catalyst initially dissolved the disordered carbon before precip-
itating graphitic carbon due to graphitic's negative Gibbs free energy

<figure><img src="image_4.png" /><figcaption>Fig. 3. HRTEM images of a-d) Multi-metal Co-Mo [1:4] and e-h) Multi-metal Co-Mo [1:5] [122].</figcaption></figure>
upon saturation. Fig. 4 illustrates the HRTEM analysis results of graphitised activated carbon using Ni particles at 1400 °C.

In another study by Sun et al., pyrolysis of Fe(NO₃)₃-modified sawdust was conducted, in which Fe(NO₃)₃ initially degraded into hematite (Fe₂O₃). As the temperature reached 600 °C, Fe₂O₃ was converted into magnetite (Fe₃O₄) in the presence of reducing gases such as H₂ and CO. Consequently, the transition from Fe(III) to Fe(II) occurred. When the temperature was raised to 700 °C, a reduction of Fe from Fe(II) was observed, along with the transition of Fe(III) to Fe(II). At 800 °C, more Fe(III) was converted to Fe(II) and Fe nanoparticles, while the presence of Fe nanoparticles is essential for developing the graphite carbon structure. At this stage, the amorphous carbon began to melt and release Fe nanoparticles to produce graphite carbon. As the temperature increased, a greater number of Fe nanoparticles appeared, suggesting that Fe and C had increased potential for interaction. The penetration of Fe into the carbon surface facilitates the creation of iron carbide (Fe₃C), which continually forms and decomposes, demonstrating the dissolution of amorphous carbon and the precipitation of ordered graphite carbon. Following the reaction, the graphite structure and unreacted amorphous carbon remained in the active site. Fig. 5 illustrates the schematic pathway of sawdust graphitisation by pyrolysis in the presence of Fe nanoparticles.

Recently, Ghogia et al. [61] determined the graphitisation mechanism by evaluating iron reduction, particle size distribution, and magnetism. It was observed that an increase in temperature significantly affects the degree of iron reduction and particle size by decomposing cellulose. This resulted in an altered atomic arrangement and increased graphitic layers. The morphological analysis indicated the presence of

highly crystalline stacked graphitic layers with enhanced crystalline sizes upon increasing the temperature (Fig. 6). A high degree of iron reduction improves catalytic activity by increasing the Fe-metallic phase and magnetisation value (as mentioned in Section 2.1). It has been reported that carbon atoms dissolve in the presence of Fe-metallic, creating carbon precipitates that lead to the formation of graphitic structures with larger layers at a higher temperature of 1800 °C. The catalytic graphitisation mechanism of cellulose-derived biocarbon is explained in six steps, as illustrated in Fig. 7: (1) the impregnation of Fe (NO₃)₃ with cellulose (amorphous carbon), (2) the decomposition and dehydration of Fe and biocarbon via carbon gasification at varying pyrolysis temperatures to produce iron oxide (Fe-O), reducing gases (H₂ and CO), and water molecules, (3) the reduction of Fe-O into Fe-metallic through an exothermic reaction at lower temperatures, (4) rearrangement of particles by thermal sintering to produce Fe particles, where larger Fe particles are formed at higher temperatures and smaller Fe particles at lower temperatures, (5) the decomposition of biocarbon by dissolving carbon atoms into metastable liquid carbon, and finally (6) the precipitation process forming graphitic layers on the surface of Fe-metallic. Therefore, changes in graphitisation temperatures have significantly impacted the degree of iron reduction and particle sizes in forming graphitic layers [125].

Research into alkaline earth metals as catalytic materials has increased, and scientists are keen to investigate these metals for graphitisation, although they face limitations. Accordingly, Beguerie et al. [67] explored the formation of cellulose and lignin-derived graphitic biocarbon using calcium (Ca). It was observed that both cellulose and lignin-derived biocarbon exhibit markedly distinct graphitic

<figure><img src="image_5.png" /><figcaption>Fig. 4. HRTEM analysis of graphitised activated carbon using Ni particles at 1400 °C [123].</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Fig. 5. Schematic pathway of sawdust graphitisation [124].</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 6. HRTEM images and SAED of graphitic biocarbon graphitisation at (a-c) 1000 °C, (d-f) 1400 °C, and (g-j) 1800 °C [61].</figcaption></figure>

structures. Based on the XRD and TEM results, temperature emerges as the critical factor driving the catalytic graphitisation of biocarbon. Calcium exerts minimal catalytic influence below 1200 °C, with the calcium primarily existing as CaO particles embedded within the amorphous carbon matrix. At 1300 °C, calcium oxide reacts with carbon to produce calcium carbide (CaC₂, Eq. (1)). Given the poor stability of CaC₂, it decomposes into Ca vapour and graphitic layers at higher graphitisation temperatures (Eq. (2)). The formation of calcium vapour at temperatures exceeding 1200 °C indicates the removal of calcium. According to the literature, this process is predominantly driven by the diffusion of Ca vapour through the graphitic shell, concluding when a thick graphitic layer encases the CaC₂ particle. This suggests that CaC₂

forms in biocarbon between 1000 and 1200 °C, which is slightly lower than the typical production temperature cited in the literature (1300 °C). Consequently, calcium facilitates biocarbon graphitisation by forming a metastable CaC₂, which subsequently decomposes into Ca vapour and graphitic layers or calcium cyanamide (CaCN₂) alongside graphitic layers in a nitrogen environment. Above 1400 °C, unreacted CaC₂ and CaCN₂ particles may disengage from their graphitic shells and be expelled from the biocarbon. The graphite shells agglomerate, imparting a crumpled-paper appearance to the carbon matrix. Based on these findings, a schematic representation of the calcium-catalysed pathway is proposed in Fig. 8.
<figure><img src="image_8.png" /><figcaption>Fig. 7. The mechanism of Fe-catalysed graphitisation of cellulose-derived biocarbon [61].</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 8. Schematic description of the calcium-catalysed graphitisation [67].</figcaption></figure>

$$CaO + 3C_{\text{amorphous}} \rightarrow CaC_{2} + CO \quad (1)$$

$$CaC_{2} \rightarrow Ca + 2C_{\text{graphene}} \quad (2)$$

On the other hand, Hunter et al. [126] have discovered a new type of mechanism referred to as homogeneous solid-state catalysis, employing Fe. This was achieved using a Dark-field in situ transmission electron microscopy technique, MD simulation, and in situ synchrotron XRD to report the rapid diffusion of a crystalline catalyst particle (Fe) through a solid amorphous carbon matrix in developing multi-walled graphitic nanotubes. It was observed that in Fe or Fe₃C, the carbon atoms are presented in the close-packed Fe lattice at interstitial sites and diffuse through the lattice by moving between interstices. According to Cermak and Kral [127], the diffusion of carbon in the Fe lattice depends on graphitisation temperature and carbon concentration but is markedly faster in α-Fe (Dc ≈ 10⁻⁶ cm² s⁻¹) at 800 °C than in γ-Fe (Dc ≈ 10⁻⁸ cm² s⁻¹) at the same temperature due to the lower density of the body-centred cubic (BCC) structure. Meanwhile, Schneider and Inden observed that the diffusion of carbon is slowest in the Fe₃C phase (Dc ≈ 10⁻¹⁰ cm² s⁻¹) at 800 °C [128]. The diffusion of Fe atoms in Fe through self-diffusion occurs via the movement of atoms between lattice vacancies (structural defects). This process is slower than interstitial carbon diffusion (Dc ≈ 10⁻¹² cm² s⁻¹) in α-Fe at 800 °C, but faster at grain boundaries or on surfaces (structural defects) [129]. Thus, the diffusion of atoms through nanoscale solids can induce rapid structural

transformations and may provide a plausible mechanism for the movement of the catalyst particle during graphitisation (Fig. 9). This study also offers insight into the influence of defects on the movement of atoms during graphitisation, which is proportional to the quality of graphitic biocarbon. However, comprehensive computational studies are necessary to investigate the development of well-structured graphitic biocarbon at optimal graphitisation temperatures (1000 – 1800 °C) with more robust characterisations.

Converting lignocellulosic biomass into graphitic biocarbon requires increasing the carbon content and organising the carbon structures in a graphitic form at a specific temperature [121]. After reviewing the mechanisms outlined above, it is possible to infer that the graphitisation process includes molecule breaking, recombination, transformation, catalyst movement and structural defects.

## 4. Future Perspectives

An increasing body of knowledge on the benefits of the single-metallic catalysts graphitisation process is proposed in the literature, as discussed in the above sections. The inclusion of another metal to form multi-metallic catalysts improves graphitisation while lowering the reaction temperature and increasing the catalytic interaction of metallic catalysts. Despite the number of multi-metallic catalysts in this
<figure><img src="image_10.png" /><figcaption>Fig. 9. Schematic of proposed homogeneous solid-state catalysis mechanism (diffusion and precipitation) [126].</figcaption></figure>

field, various limitations still need attention:

The advantage of multi-metallic catalysts is that they facilitate better metal dispersion, particle sizes of metal catalysts, and synergetic effects between two or more metal particles, which may not be seen when using single-metal catalysts. Hence, further research on the metallic ratio could determine the synergistic effect of multi-metallic catalysts on the graphitisation conditions, as limited studies have been reported on metallic interaction and stability during the graphitisation process.

Carbon phase transformation may occur on the surface of multi-metallic catalysts during the graphitisation process, resulting in new catalytic sites. The interaction between two metals may initiate a new metallic combination that could enhance or hinder the catalytic and carbon “dissolution-precipitation” processes. However, to our knowledge, the catalytic process of multi-metallic on amorphous carbon is still unknown. Therefore, advanced in situ detection methods, robust characterisation and detailed DFT studies, for instance, should be conducted to determine the active sites and the structural defects, which will help researchers understand the reaction mechanism in these materials.

Carbon graphitisation offers a promising route to convert non-graphitisable carbon into functional carbon for specific applications, including energy storage, light absorption, photocatalysis, biomedical, environmental, etc. The challenge of using this material in these applications is controlling the graphitic biocarbon structural formation; thus, detailed studies are required to determine the carbon structure and its effect on the applications.

## 5. Conclusion

In summary, utilising biomass waste to synthesise graphitic bio-carbon can considerably reduce high manufacturing costs and pollution issues. Typically, direct graphitisation is associated with elevated temperatures (over 2000 °C) and significant energy consumption. Incorporating catalysts before the carbonisation and graphitisation processes will enhance the structures of graphitic biocarbon and modify the experimental conditions. Single-metal catalysts have long been investigated and continue to represent an evolving area of research due to the requirement for high graphitisation temperatures (1800–1000 °C) and the challenge of understanding the variability in carbon structures when using different carbon precursors. In contrast, the benefits of multi-

metal catalysts may address the limitations of single-metal catalysts regarding the synergistic effects between metal contents and carbon precursors. Furthermore, the interaction of two metals could promote the dissolution of amorphous carbon and the individual rearrangement of graphitic biocarbon, thereby lowering the graphitisation temperature to 800 °C and enhancing carbon aromatisation, which can be examined using the NMR technique. Based on a critical analysis of the literature, this review proposes a hierarchy of the most significant catalytic graphitisation conditions as follows: graphitisation temperature > carbon precursors > heating rate and a hypothetical approach. Although the reaction mechanism in the single-metal catalyst approach has been elucidated, the reaction pathways and mechanisms involving multi-metal catalysts remain unclear and necessitate advanced characterisation along with modelling approaches to comprehend metal interactions on biocarbon and the formation of active sites. This underscores the need for further investigation and presents promising research opportunities involving various biomass feedstocks and metal catalysts.

### CRediT authorship contribution statement

**Shamala Gowri Krishnan:** Writing – review & editing, Writing – original draft, Visualization, Investigation, Conceptualization. **Claire E. White:** Writing – review & editing. **Kuo Zeng:** Writing – review & editing. **Nandakumar Kalarikkal:** Writing – review & editing. **Yong Sik Ok:** Writing – review & editing. **Craig B. Arnold:** Writing – review & editing. **Sabu Thomas:** Writing – review & editing. **Ange Nzihou:** Writing – review & editing, Visualization, Supervision, Resources, Conceptualization.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgement

The authors thank A. C. Ghogia, L. M. Romero Millán, T. Béguerie and E. Weiss-Hortala for their contribution to the iron- and calcium-catalysed graphitisation research works. The support from the Fulbright Commission, which granted Professor A. Nzihou a research scholarship at Princeton University, United States, is acknowledged.
# Appendix A. Supplementary material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.fuel.2025.135330.

## Data availability

Data will be made available on request.

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