Sustainable co-production of porous graphitic carbon and synthesis gas from biomass resources

Graphite is a critical resource for accelerating the clean energy transition with key applications in battery electrodes¹, fuel cells², solar panel production³, blades and electric brushes of wind turbines³, and nuclear reactors² due to its excellent mechanical, thermal and electrical conductivity properties¹. These properties arise from its crystalline structure in which sp²-hybridized carbon atoms are assembled into a hexagonal ring structure with π − π stacking arrangements enabled by weak van der Waals interactions⁴. The delocalized electrons in graphite contribute to enhanced mechanical, thermal and electrical conductivity properties.

However, conventional means of producing graphite remain unsustainable. Natural graphite is mined from the earth but has been reported as a supply risk material due to rapidly increasing demand and limited reserves¹. As an alternative, synthetic graphite is conventionally produced by the Acheson process⁵ using coal tar pitch and petroleum coke⁶ as the carbon sources. While graphite purity as high as 99.9% is achieved using this process, the need for temperatures as high as ~3000 °C⁵, and uncertainties associated with the generation of volatile constituents and their associated environmental impacts challenge the environmental sustainability of this process. Emissions associated with synthetic graphite production have been reported to be as high as 20 kg of CO₂e per kg of graphite². These challenges motivate advances in the development of energy and resource efficient material conversion pathways that facilitate complete utilization of renewable carbonaceous resources to co-produce Porous Graphitic Carbon (PGC) and synthesis gas (or syngas, which is a mixture of CO and H₂) while lowering the greenhouse gas emissions footprint compared to existing graphite production processes.

In the context of harnessing renewable carbon feedstocks^7,8, several studies have highlighted the potential for producing graphite from biomass^9,10. Lignocellulosic biomass is ubiquitous and comprises a mixture of carbohydrates (~65–75%) and lignin (~18–35%)¹¹. Cellulose and hemicellulose are carbohydrates, consisting of glucose, mannose, galactose, xylose, arabinose, 4-O-methylglucoronic acid and galacturonic acid¹¹. Lignin is a crosslinked polymer of phenolic compounds mainly comprising three precursors: p-cou-maryl alcohol (I), coniferyl alcohol (II), and sinapyl alcohol (III)¹¹. The abundance of lignocellulosic biomass in construction and demolition waste-derived wood and agricultural residues (e.g., coconut shells, rice husks, and sugarcane bagasse) motivates their use for sustainable material and energy conversions^12,13.

Biomass is conventionally considered to be a challenging feedstock for graphitization due to the formation of turbostratic non-graphitic carbon at temperatures as high as 3000 °C as opposed to graphite formation¹⁴. In contrast, ‘graphitizable’ carbons such as petroleum coke form a viscous intermediate at high temperatures and re-arrange to form graphite¹⁵. It has been hypothesized that the carbonization stage (around 300–500 °C) plays a key role in determining the graphitizability of a carbon precursor. Graphitizable carbons form a ‘state of fusion’, whereby a viscous carbon phase enables the re-arrangement of carbon to thermodynamically stable graphite planes. In contrast, non-graphitizable carbon precursors undergo crosslinking reactions which impede fusion state formation and carbon re-arrangement¹⁶. These challenges motivate advances in alternative synthesis pathways to convert biomass to Porous Graphitic Carbon (PGC). Table 1 presents an overview of key parameters in various graphitization processes. As noted in Table 1, the use of potassium – based additives (e.g., KOH, K₂CO₃) lowers the temperatures needed for graphitization.

The transformation of carbon precursors into graphite in the presence of a metal catalyst is termed catalytic graphitization. As shown in Fig. 1, two-step or single-step approaches can be harnessed for graphitization. A two-step process typically starts with carbonization or pyrolysis to produce an amorphous carbon-rich porous intermediate. Coconut shells are an excellent source for this purpose as they form char¹⁵ with high carbon content when carbonized. For example, coconut shell char with carbon content greater than 90 wt.% has been reported under slow pyrolysis conditions¹³. As shown in Table 2, palm shell, sugarcane bagasse, cotton stalk, and coconut fiber may also be suitable, given their high biochar content.^13,17. Process conditions such as the reaction temperature¹⁸ and heating rate¹⁹ influence the composition and yield of biochar. Lignin, a component of lignocellulosic biomass contains an abundance of aromatic groups, and biomass types with high lignin content tend to produce higher char yields and carbon content²⁰. Carbon-rich biochar contains aromatic carbon rings²¹ which are fused and rearranged to form graphitic layers through catalytic graphitization processes²². Alternatively, biomass can be directly mixed with the catalyst and thermochemically treated in a single step to produce graphite²³.

In comparison with the single-step process, the two-step process includes an intermediate carbonization step at ~ 400–600 °C to produce carbon-rich biochar prior to activation or graphitization. In a two-step process, biomass first undergoes carbonization to produce a carbon-rich biochar, which is further heated in the presence of a catalyst. A single-step process involves directly mixing the biomass with a catalyst and heating to the desired reaction temperature.

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There have been developments in catalytic approaches which involve the use of metal catalysts to generate a molten liquid phase at relatively low temperatures²⁴, thus promoting the re-arrangement and formation of graphitic structures in a single step using biomass-derived carbon. Iron-based catalysts have been found to form Fe₃C nanoparticles, which melt and dissolve amorphous carbon, and create carbon nanotube structures²³. Catalytic graphitization of saw dust mixed with Fe or Ni catalysts have been reported at a temperature of 1000 °C resulting in nanosized graphitic structures of high crystallinity. Transition metal nanoparticles promote graphitization at temperatures below 1000 °C¹⁴, forming a variety of ‘nanotube’, ‘nanoribbon’, or ‘nano-onion’ structures, in which graphitic layers are formed locally around the nanoparticle surface, be it stationary or mobile²⁵. However, the use of metals such as Fe or Ni is still hindered by the need for acid washing (typically in 1 M HCl²³) to recover the Fe particles. Excessive use of acid challenges the overall environmental and economic sustainability of graphite production.

Potassium-based catalysts (such as KOH²⁶, K₂CO₃²⁷, and K₂FeO₄²⁸) are also being explored for their capabilities to simultaneously induce chemical activation and graphitization of biomass-derived carbon at temperatures below 1000 °C without excessive acid requirements for separating catalysts from the product. The mechanism for KOH-mediated activation is outlined through a series of Reactions 1–5 below²⁸, resulting in the release of organically bound carbon and creation of porosity. KOH reacts with solid carbon resulting in the formation of K₂CO₃ at 400–600 °C²⁸. At temperatures greater than 700 °C, K₂CO₃ decomposes to K₂O releasing CO₂ which further activates the carbon, releasing CO and metallic potassium²⁹. The Boudouard reaction favors CO production above 680 °C³⁰. Potassium-containing compounds have additional catalytic effects on the pyrolytic decomposition of biomass, promoting dehydration, demethoxylation, decarboxylation, decarbonylation, heterocyclic ring opening and catalytic cracking reactions³¹. These pathways suggest that carbohydrates are primarily decomposed to light oxygenates through ring scission reactions, while lignin forms phenolic compounds. Light oxygenates can undergo secondary cracking to gaseous species or contribute to recombination and char formation with lignin-derived compounds.

The remaining biochar primarily consists of carbon, in the form of sp² graphite microcrystals connected by sp³ hybridized carbons³². K₂CO₃ promotes the formation of graphitic structures by two main pathways²⁷. The first pathway is a preferential reaction with sp³ hybridized carbons over sp² hybridized carbons during its activation mechanism, removing the crosslinks which were previously separating graphite microcrystals. Furthermore, as the temperature reaches around 900 °C and exceeds the melting point of K₂CO₃ (891 °C), the molten salt promotes re-arrangement and combination of graphite microcrystals through the formation of charge transfer complexes and enables the crystallization of amorphous carbon. We have noted the multiple benefits of alkali metal-catalyzed graphitization of biomass including enhanced decomposition and volatilization, catalytic cracking to produce higher gas yields, combined with its beneficial impacts on the quality of char produced such as the creation of micropores and the formation of graphitic structures. However, gas selectivity and the need for separate carbonization and graphitization stages can be improved. The formation of tar in a single-step scheme has been reported to reduce the graphitizing ability of KOH leading to the formation of activated carbon rather than PGC³³ compared to a two-step process, where the volatile tars are released in the carbonization stage prior to graphitization²⁷. Moreover, the resulting volatile products of catalytic graphitization processes are generally not studied. There remains scope to tune the composition of the gaseous products for maximum utilization of the biomass resource.

Amorphous carbon can also be graphitized by cathodic polarization in a molten CaCl₂ electrolyte. High-quality graphitic carbon product has been reported at temperatures of 850 °C³⁴. One advantage of the electrochemical method is the absence of a catalyst separation step compared with catalytic graphitization. This approach involves electro-deoxygenation from the surface of carbon precursors which is followed by long-range rearrangement of carbon atoms to transform an amorphous structure to a highly directional nanoflake architecture³⁴. Like catalytic graphitization, the electrochemical method has been applied to carbon precursors such as carbon black³⁴ and coconut char³⁵. Significant knowledge gaps remain in our understanding of the underlying reactions, detailed mechanisms, electrolyte stability, the influence of various carbon precursors, and the presence of impurities on electrochemical graphitization³⁶.

Microwave radiation can potentially increase the degree of graphitization and required reaction times compared to conventional heating methods³⁷. In a conventional furnace, heat travels inwards from the surface of the material through heat conduction. In this way, a thermal gradient is formed between the material surface and internal particles, which results in uneven heating of the material contributing to incomplete pore formation and a low specific surface area³⁷. In contrast, microwave heating provides rapid heat transfer resulting in complete biomass conversion in 15 minutes³⁷. Rapid fluctuations around the temperature set point are hypothesized to promote an improved degree of graphitization by the precipitation-dissolution mechanism³⁷.

As an alternative to single step graphitization using alkaline catalysts, Alkaline Thermal Graphitization (ATG) that utilizes both alkaline catalyst as well as a metal catalyst such as nickel to promote gas phase reforming reactions can be tuned to produce highly porous graphitic carbon (PGC), and high purity synthesis gas (H₂ and CO) from lignocellulosic biomass in a single step. ATG is analogous to Alkaline Thermal Treatment (ATT)^38,39,40, where biomass mixed with alkaline hydroxide results in the production of high purity H₂ by promoting gas phase reforming reactions using a Ni/ZrO₂ (10 wt.%) catalyst at 500 °C⁴¹. In the proposed ATG process, the Ni/ZrO₂ catalyst alleviates the issue of tar-mediated potassium volatilization⁴² in a single-step reaction by promoting catalytic tar cracking reactions. After thermochemical treatment, the produced PGC is washed in a relatively dilute 0.1 M HCl to remove potassium and other impurities. Table 3 shows the gas yields generated by various catalytic thermochemical treatment processes. Factors including the biomass type, reaction temperature and choice of catalyst influence the quantities of gas generated⁴³. Evidently, the use of Ni-based catalysts enhances gas production due to catalytic tar cracking. Figure 2 shows the mechanisms associated with ATG. The underlying reactions are summarized in Fig. 3. The overall reaction is as follows:

Schematic representation of the mechanisms involved in the Alkaline Thermal Graphitization (ATG) process.

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Schematic representation of the integrated, single-step alkaline thermal graphitization (ATG) approach to co-produce porous graphitic carbon and syn gas from woody biomass.

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To demonstrate the feasibility of ATG, wood material from construction and demolition waste obtained from a processing facility in Cortland, NY, USA was used as a lignocellulosic biomass precursor. Wood was pre-processed by milling, passing through a 1 mm sieve, and drying at 105 °C for 24 h. The composition of the wood is shown in Supplementary Table 1. 1.5 g of wood was then mixed with KOH at a 1:1 mass ratio, and 15 mL water was added to make a slurry, followed by drying at 105 °C for 4 h. Reactants were loaded in a zirconia ceramic crucible and covered by a zirconia ceramic foam filter (VUKOPOR^® HT). This mixture was reacted in the presence of 250 mg Ni/ZrO₂ catalyst (10 wt.%) in a separate crucible. The Ni/ZrO₂ catalyst was prepared by a wet impregnation method⁴¹. The crucibles were loaded into a horizontal tubular furnace, purged with N₂ gas, and heated at a ramp rate of 4 °C/min up to 600 °C and 2 °C/min thereafter until reaching 900 °C and held for 2 h. A constant N₂ flow of 50 cm³/min was maintained, and the outlet gas flowrate and composition were measured by a mass flowmeter followed by a micro-GC analyzer (Inficon Micro GC Fusion®). The solid product, porous graphitic carbon (PGC) was washed in deionized water and 0.1 M HCl (2 h stirring in each) and dried at 105 °C for 12 h. For comparison, 1.5 g of dried wood was pyrolyzed under a constant N₂ flow to 900 °C under the same furnace settings. Two-step graphitization was performed by carbonizing 1.5 g of dried wood at 400 °C for 3 h (ramp rate of 4 °C/min). The resulting biochar was loaded with KOH in a 1:2 mass ratio as before and heated under a constant N₂ flow to 900 °C under the same furnace settings. PGC was analyzed by Raman Spectroscopy (WITec Alpha 300 R, 785 nm), Scanning/Transmission Electron Microscopy (S/TEM) imaging (Thermo Fisher Spectra 300) and X-ray Photoelectron Spectroscopy (XPS) (Thermo Fisher Nexsa G2). Potassium recovery was measured by ICP-AES (Thermo Fisher ICAP Pro). All experiments were carried out in duplicate.

Based on the generated gas yields shown in Fig. 4a, it is evident that H₂, CO and CO₂ are the main components in the gaseous phase. Alkaline Thermal Graphitization (ATG) yields 21.1 mmol H₂/g biomass, corresponding to a conversion of 55% (relative to the combined H content of biomass and KOH). Significant CO₂ and CO gas yields are due to the decomposition of K₂CO₃ as per its activation mechanism (Reactions 2–5). A yield of 23.5 mmol CO/g biomass was obtained. The H₂/CO ratio is 0.90. As seen in Fig. 4a, there is elevated H₂, CO and CO₂ production (2.1, 2.4 and 1.3 times respectively) compared to pyrolysis of biomass at 900 °C, indicating the enhanced activation of biomass and decomposition of volatiles in the presence of KOH and Ni/ZrO₂ catalyst. Evidently, there is an increase in H₂ and CO evolution with ATG compared to a ‘more typical’ two-step graphitization with KOH and biomass only. It is primarily a result of the catalytic tar cracking reactions promoted by the Ni/ZrO₂ catalyst in ATG. According to Fig. 4b, the Raman spectrum of the produced PGC shows prominent ‘D’ and ‘G’ peaks located at 1340 cm⁻¹ and 1584 cm⁻¹, respectively. The ‘G’ peak corresponds to sp² hybridized carbon in an ideal graphitic lattice, whereas the ‘D’ peak is due to defects in graphitic lattices and the presence of sp³ hybridized amorphous carbon⁴⁴. The apparent broadening and overlap of these peaks are explained by the presence of two additional peaks, termed ‘D3’ and ‘D4’ peaks at 1477 cm⁻¹ and 1200 cm⁻¹ respectively, corresponding to amorphous carbon and graphitic defects⁴⁵. By fitting these four peaks, the integrated intensity ratio, I_D/I_G, was estimated to be 1.75. Similarly, an I_D/I_G ratio of 1.70 was estimated for the PGC product of two-step graphitization.

a gas yields, b evidence of PGC from Raman spectra, and c TEM image of PGC.

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Table 1 provides a comparison of the obtained I_D/I_G ratio to those reported in prior publications. Previously, K₂CO₃ activation using coconut shell biomass produced PGC with I_D/I_G ratio as low as 0.088²⁷, and 0.84 using KOH activation of enzymatic hydrolysis lignin⁴⁶. Here, the higher I_D/I_G ratio indicates a relatively lower graphitic content and higher amorphous C content, primarily due to the difference in the choice of biomass type and activating agent. As shown in Table 2, coconut shells can produce higher graphitic content than wood because they carbonize to form a char with a higher carbon content (and likely greater aromaticity), whereas the presence of heteroatoms in wood char may impede the graphitization process. Significant oxygen content was detected by XPS analysis (Supplementary Fig. 1). The use of KOH as an activating agent has been thought to reduce the graphitic quality compared to direct use of K₂CO₃, due to greater activation and porosity development, leading to formation of more defects⁴⁶. The I_D/I_G ratios of 1.75 and 1.70 (for ATG and two-step graphitization respectively) are significantly higher than some others in literature^28,37 due to the choice of calculation method. Here, I_D/I_G is reported as a ratio of integrated peak intensities whereas it is often reported as a ratio of peak intensities. If we were to consider the peak intensity ratio, the PGC product of ATG and two-step graphitization would have an I_D/I_G of 1.06 and 0.86 respectively. However, the ratio of integrated intensities is more representative of samples with high defect densities⁴⁷. Figure 4c is a TEM image of a porous graphitic carbon particle. The presence of graphitic planes is evident and the interplanar spacing is found to be 0.50 nm. This observed spacing is larger than the ideal graphite interplanar spacing of 0.335 nm⁴⁸, likely due to the intercalation of potassium species⁴⁹. Significant scientific opportunities exist to tune the compositions of PGC to increase the content of graphite, as discussed later.

Sustainability analysis

Alkaline Thermal Graphitization aims to be an environmentally sustainable process for co-producing porous graphitic carbon (PGC) and syngas; thus, an outlook is presented on its potential environmental impacts. A major concern of any thermochemical treatment process is the global warming impact, primarily due to energy consumption of maintaining the high reaction temperatures required⁵⁰. ATG is preferable in this regard due to the relatively low reaction temperature of 900 °C, as compared to the Acheson process at ~3000 °C⁵. To put this in quantitative context, this is approximately equivalent to an emissions reduction of 840 kg CO₂ per metric ton of carbon produced (based on the heating requirement of graphite^51,52). CO₂ emissions associated with heating to the required reaction temperature during ATG can be further reduced via integration with solar thermal energy. Direct or indirectly heated solar concentrator systems can be used to heat chemical reactors up to 1100 °C⁵³. As biomass acts as a store for atmospheric carbon, combining thermochemical treatment with reduced CO₂ emissions of solar thermal heating can potentially lead to a carbon-negative process. For example, a recent Lifecycle Assessment (LCA) study predicted a solar thermal biomass gasification process to have net negative global warming potential impact⁵⁴. Another consideration is material utilization efficiency. Figure 5 shows the carbon balance of the processes, comparing the carbon conversion of ATG with two-step graphitization and pyrolysis processes. Carbon content in the solid phase was calculated using XPS measurements (Supplementary Figure 1, Supplementary Table 2). ATG has the highest rate of carbon utilization, with a combined solid and gas conversion of 75.3%, compared to 46.5% for two-step graphitization and 56.8% for pyrolysis. Figure 5 also shows that the conversion of carbon to the solid phase decreases from 17.4% for pyrolysis to 1.3% for ATG. It is believed that the presence of KOH in ATG promotes volatilization of carbon (reactions 1–5) and as a result, more carbon is converted to the gas phase. The solid carbon yield of two-step graphitization is intermediate, at 6.6% as KOH is added after the char formation step. ATG leads to lower solid carbon yields as Ni/ZrO₂-promoted catalytic tar cracking of volatiles is favored over recombination and secondary char forming reactions²¹.

Carbon conversion (wt.%) to solid, liquid and gas phase resulting from pyrolysis, two-step graphitization and Alkaline Thermal Graphitization of construction and demolition waste wood at 900 °C.

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KOH utilization is also another major consideration for the environmental sustainability of ATG. Washing in 30% HCl has been predicted to recover potassium and thus can bring down the ecotoxicity impacts by up to 60% compared to activation without the recovery step⁵⁵. By lowering the acid consumption required with ATG, we can further reduce the associated ecotoxicity impacts. Potassium-catalyzed graphitization results in formation of metallic K (reaction 5). This can intercalate into the graphite and can be converted back to KOH by washing, accompanied by evolution of H₂ as per the following reaction⁵⁶:

There is potential to harness the additional H₂ generated during the recovery of KOH. However, reaction with water alone does not remove all potassium from the solid phase⁵⁷. 2 M HCl has been used in prior works^26,37 for potassium removal. In this study, PGC was washed in a more dilute 0.1 M HCl after an initial water wash with 2 h of stirring in each step. The amount of potassium recovered during washing, relative to the initial KOH loading, is shown in Fig. 6. Potassium recovered by two-step graphitization (8.1%) is higher than that of ATG (4.9%), corresponding to the greater solid carbon yield, in which slightly more potassium was trapped. In both cases, the volatilization of potassium above 800 °C⁵⁸ is limiting potassium recovery, with potassium being released in the vapor phase. Condensed vapors remaining on the walls of the tubular furnace after ATG were analyzed by XPS (Supplementary Figure 1) and show elevated potassium levels. Volatilization of potassium is also a practical issue, leading to reactor corrosion. Minor amounts of potassium may remain in the PGC product after washing, as seen in the XPS spectrum of ATG char (Supplementary Fig. 1), although it was only observed in a single sample. The concentration of HCl and time required for complete potassium removal requires further optimization. While the sustainability analysis has been done by assuming the final application of the PGC is in replacing graphite in battery electrodes, it is important to note that there remains scope for further process optimization of ATG to achieve a higher degree of graphitization for several other applications. Improving PGC yields will be crucial to maximize the economic viability of ATG since the current value of graphite (>$70,000 per metric ton²) is higher than that of syngas (~$700 per metric ton⁵⁹).

Potassium recovered from deionized water and 0.1 M HCl washing steps in Alkaline Thermal Graphitization and two-step graphitization.

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The Alkaline Thermal Graphitization (ATG) of low value biomass residues has promising potential for sustainable, carbon-negative production of porous graphitic carbon (PGC), a versatile material for energy storage and conversion. The combination of porous and graphitic structures in PGC make it well suited for supercapacitor applications²². Amorphous carbon with a porous structure contributes to elevated electrochemical active surface area and improves the overall capacity. Hence, biomass-derived activated carbon is an effective supercapacitor material⁶⁰. A hierarchical porous structure facilitates rapid ion transport⁶¹ due to micropores contributing to a large surface area, while mesopores provide ion transport channels and macropores act as electrolyte reservoirs. Graphitic carbon further improves performance by enhancing electrical conductivity²⁴. PGC generated by ATG has comparable graphitic quality to prior studies which used similar carbon materials as electrodes in lithium-ion batteries⁴⁶ and supercapacitors²⁸ and found significant performance improvements. Gong et al. ²⁸ produced PGCs with I_D/I_G ratio 0.86 from bamboo and obtained energy densities ranging from 3.3 to 16.7 W h kg⁻¹, compared to 4 to 5 W h kg⁻¹ of commercial activated carbon-based supercapacitors. Similarly, Xi et al. ⁴⁶ produced lignin-derived PGC has an I_D/I_G ratio of 0.70 and maintained a specific capacity of 520 mA h g⁻¹, higher than other carbon-based electrodes. Thus, it is predicted that ATG-derived PGC will show promise as an electrode material for supercapacitors or lithium-ion batteries. ATG offers several transformative benefits over conventional synthetic graphite. First, ATG reduces the detrimental environmental impacts from conventional graphite production, limits emissions of greenhouse gases and other harmful volatile gases and reduces tar generation. Second, the co-production of PGC and syngas leads to comprehensive utilization of low value and locally available biomass residues and generates additional revenue for farmers. Third, the relatively lower temperature at which ATG occurs, which is 1000 °C or lower compared to 3000 °C required for thermal graphitization, enables ease of integration with renewable solar thermal energy. Fourth, lignin which has limited uses is ideally suited for ATG. Approaches that can effectively separate cellulosic and hemicellulosic biomass components for alternate applications, while directing lignin for ATG can unlock the complete value of heterogeneous biomass resources.

Several key opportunities can be further explored to tune ATG. First, approaches to lower the temperature for ATG need to be developed to improve potassium recovery. The addition of other salts can lower the melting point by eutectic formation and promote graphitization at lower temperatures. For example, it has been found that a mixture of K₂CO₃ and Ca(OH)₂ with coal forms a eutectic mixture around 525 °C⁶². Alternatively, Na₂CO₃ and K₂CO₃ can be combined to make a eutectic mixture with a melting point of 710 °C⁶³. Second, the role of steam in ATG needs to be resolved. It is hypothesized that the addition of steam can enhance tar cracking reactions in the presence of the Ni/ZrO₂ catalyst and further improve the graphitic quality of the carbon product by reducing the concentration of tar in the system. However, it is also important to note that steam-enhanced cracking reactions could also reduce solid carbon yields further. Another concern is the steam promoted volatilization of K₂CO₃ by conversion to KOH at temperatures of 800 °C or greater⁶⁴. Third, MgCl₂ can also be considered as a useful reagent in promoting the conversion of phenols to polycyclic aromatic hydrocarbons (PAHs) and oligomers in the solid product⁶⁵, thereby improving char yields and aromaticity. K₂CO₃, Na₂CO₃ and MgCl₂ can all be later recovered by water washing. Fourth, tuning the reaction temperature for ATG can have unintended consequences that need to be resolved. For example, CO and CO₂ evolution reactions can potentially be suppressed by limiting the maximum temperature to 700 °C⁵⁸. However, this temperature is not sufficient for molten salt formation and graphitization with K₂CO₃ alone. In theory, if K₂CO₃ remains stable in the solid phase, it can be trivially separated post-reaction by water washing and recovered, either to be stored as a stable carbon sink or otherwise utilized. Fifth, there is potential to explore synthesis routes which produce increased graphitic quality PGC. The combination of transition metal-catalyzed and potassium-catalyzed graphitization has been found to induce more graphitic carbon formation than potassium-based methods alone^28,66. Such methods involve the use of reagents which contain both iron and potassium, e.g. K₂FeO₄²⁸ and K₃[Fe(C₂O₄)₃]⁶⁶, releasing the metals upon decomposition. However, the multi-step synthesis of such salts^67,68 adds complexity and likely increases the environmental footprint of the process. As a more sustainable alternative, there remains scope to harness plant materials with elevated metal content derived from phytomining or agromining of previously mined or low productivity agricultural lands⁶⁹. More than 520 nickel-hyperaccumulating plant species⁷⁰ can potentially provide a source of Ni-rich biomass for improved ATG without the need for additional synthetic metal-bearing reagents, while also remediating poor quality or polluted soils. Lastly, opportunities for whole system integration including lignin separation from low value biomass resources, co-production of porous graphitic carbon and syngas, and syngas utilization for electricity generation in a fuel cell⁷¹, or conversion into liquid hydrocarbon fuels by Fischer-Tropsch Synthesis or commodity chemicals such as methanol⁷², need to be evaluated in more detail for life cycle and economic impacts. Exploration of these opportunities will enable successful scalable and transformative advances in the co-production of porous graphitic carbon and syngas from renewable and low value biomass resources with inherent suppression of CO₂ emissions. These pathways are crucial for closing material cycles and creating a resilient supply chain of critical materials for a sustainable future.