Biomass-Derived Hard Carbon

• Biomass-derived hard carbon is a renewable, porous material produced via pyrolysis or hydrothermal techniques that generates both graphenic and amorphous structures.
• It exhibits hierarchical porosity, heteroatom doping, and tunable microstructure, which enhance ion transport and intercalation in sodium-ion and lithium-ion battery anodes.
• Scalable synthesis processes, supported by machine learning optimization, improve structural properties, charge storage, cycling stability, and commercial potential.

Biomass-derived hard carbon refers to graphenic or amorphous carbonaceous materials synthesized from renewable biological precursors, commonly employed in advanced energy devices such as batteries and supercapacitors. These carbons exhibit characteristic hierarchical porosity, disordered graphitic domains, tunable microstructures, and optional heteroatom doping, conferring high capacity, excellent rate capability, and robust cycling stability, especially as sodium-ion and lithium-ion battery anodes. Recent developments have focused on the mechanistic understanding of their structure–property relationships through machine learning and molecular modeling, as well as scalable synthesis via chemical activation and hydrothermal liquefaction methods.

1. Synthesis Routes and Process Optimizations

Biomass-derived hard carbon is typically synthesized by pyrolyzing various biological feedstocks—lignocellulosic biomasses (e.g., bamboo, Miscanthus), starches (e.g., corn starch), and floral detritus (e.g., bougainvillea petals). Two prominent routes with distinct characteristics are:

• Hydrothermal Liquefaction (HTL) Co-Processing of Synthetic and Biomass Feedstocks:

Combined HTL (e.g., with polyurethane (PUR) and Miscanthus) utilizes subcritical water (300–350°C, >10–20 bar) in bomb-type batch reactors or continuous flow pilot plants. The blending of synthetic PUR residues with lignocellulosic biomass is essential—biomass enables slurry pumpability and introduces synergistic reactions. Maillard-type reactivity between PUR-derived amines and biomass sugars/ketones enhances oil phase aromatization, yielding highly condensed hydrocarbon and nitrogen heteroaromatic species (Passos et al., 2021).

• Chemical Activation and Controlled Carbonization:

Bougainvillea petals are carbonized at 800°C under argon in presence of ZnCl₂, which disrupts cellulose hydrogen bonds and forms a zinc–cellulose complex that, upon heating, yields porous, graphenic carbon. Post-carbonization acid washing removes residual Zn²⁺ ions (Yadav et al., 2021). Nitrogen doping is achieved by co-pyrolyzing pre-treated corn starch with melamine or triethylenetetramine at 1000°C under nitrogen, integrating graphitic, pyridinic, and pyrrolic nitrogen into the carbon lattice. This produces microstructural ordering and introduces new porosity (5 μm pores) (Huang et al., 21 Jul 2024).

2. Structural Features, Micro-and Nanoscale Control

Structural and chemical analyses—TEM, XRD, Raman, FTIR, XPS—document features central to energy storage performance:

• Porosity and Graphenic Domains:

Morphologies comprise nanoplatelets (5–250 nm), nanorods, nanodots, and nanoribbons embedded in interconnected meso/microporous networks. XRD (002) peaks indicate turbostratic-to-amorphous transitions, and the introduction of activating agents or heteroatoms modulates pore size and lattice ordering (Yadav et al., 2021, Huang et al., 21 Jul 2024).

• Heteroatom Doping and Defect Engineering:

Nitrogen is successfully embedded as graphitic, pyridinic, and pyrrolic types, confirmed by XPS. Such doping reduces disorder (quantified by decreasing Raman D/G intensity ratios), lowers charge transfer resistance, and increases ion-accessible sites. Pore formation from chemical etching and oligomer retention from HTL (polyol/aromatic species) further tune diffusivity (Huang et al., 21 Jul 2024, Passos et al., 2021).

3. Charge-Storage and Electrochemical Performance

Hard carbons derived from biomass display high reversibility, capacity, and stable cycling, making them suitable anodes for alkali-ion batteries. Key findings include:

• Lithium-ion Systems:

Nitrogen-doped corn starch carbon (NCSH1, NCSH2, NCSH3) achieves first discharge capacities above 426 mAh g⁻¹ (compared to ~373 mAh g⁻¹ for undoped). After 100 cycles at 0.2 C, NCSH1 retains ~274 mAh g⁻¹ and 100% coulombic efficiency. At 2 C, >120 mAh g⁻¹ reversible capacity is maintained (Huang et al., 21 Jul 2024). Enhanced capacity is attributed to added active sites from graphitic and edge nitrogen and improved electronic conductivity.

• Sodium-ion Systems:

Biomass-derived hard carbon exhibits capacity >320 mAh g⁻¹ and initial coulombic efficiency (ICE) ~80% with optimized synthesis (e.g., bamboo-derived, high-temperature processing). Machine learning models (XGBoost, R² = 0.854, RMSE = 23.3 mAh g⁻¹ for capacity) demonstrate that carbonization temperature (Temperature_2) is the dominant parameter, with a threshold effect observed above 1000 °C (Chen et al., 13 Oct 2025).

• Supercapacitor Electrodes:

Bougainvillea carbon electrodes in 1 M ZnCl₂ electrolyte, when subjected to external magnetic fields (1000 G), show increased current density and improved double-layer formation. Magnetohydrodynamic effects, induced by Lorentz forces on Zn²⁺ ions, facilitate more uniform charge transport, especially across large graphenic domains (Yadav et al., 2021).

4. Structure–Transport Relationships and Mechanistic Insights

Ion mobility within biomass-derived hard carbon is dictated by local microstructural heterogeneity, rather than global parameters.

• Descriptor-Based Analysis (Physics-Informed ML):

Per-ion descriptors (Na–Na, Na–C coordination, tortuosity, accessible volume) govern sodium diffusion. Diffusion coefficients are calculated via the Einstein relation:

$D = \frac{1}{2d} \frac{d⟨r(t)^2⟩}{dt}$

Lower tortuosity and maximized accessible volume favor fast ion transport; high Na–Na coordination leads to clustering and trapping, reducing effective mobility (Rampal et al., 20 Aug 2025).

• Diffusion Modes and Correlation Mapping:

Eight migration regimes are reported: single-ion hopping, defect-assisted transport, interlayer migration, cavity hopping, void diffusion, cluster-based mechanisms, and trapping in dense or tortuous environments. Bulk density and sodium content are mapped against these regimes; intermediate carbon density and balanced Na loading optimize transport and avoid detrimental crowding (Rampal et al., 20 Aug 2025). Machine learning identifies carbonization temperature as the most predictive feature for both capacity and ICE, with nonlinear response above threshold values (Chen et al., 13 Oct 2025).

5. Scalability, Process Efficiency, and Commercial Potential

Biomass-derived hard carbon synthesis is increasingly evaluated at pilot and industrial scale:

• Pilot Plant HTL of PUR/Miscanthus:

Continuous processing (316.4 ± 6.2°C, 55.1 kg/h, residence time 18.5 min) achieves carbon recovery into oil of 71.2%, chemical energy recovery of 75.0%, total process efficiency of 60.7%, and energy return over investment (EROI) of 3.2 (Passos et al., 2021). Slurry pumpability—enabled by biomass—is crucial for scale-up. Composite feedstocks and synergistic recombination (aromatics/nitrogen compounds) increase both yield and product stability.

• Machine Learning Acceleration:

ML models (e.g., XGBoost) integrated with data augmentation (TabPFN) reliably predict performance metrics and guide parameter selection, streamlining experimental workflows (Chen et al., 13 Oct 2025). This approach reduces resource expenditure, informs precursor choice, and enables rapid optimization of processing conditions for high-performance anode development.

6. Applications and Outlook

Biomass-derived hard carbon oils or powders are used in:

• Refinery Feedstocks:

High-energy, aromatics-rich fractions serve as precursors for fuel and chemical synthesis.

• Battery Anodes:

Their hierarchical porosity, disorder tuning, and dopability meet profile requirements of sodium- and lithium-ion battery anodes, providing necessary interlayer spacing and active sites for ion intercalation/storage (Huang et al., 21 Jul 2024, Chen et al., 13 Oct 2025, Rampal et al., 20 Aug 2025).

• Supercapacitor Electrodes and Magnetic Sensors:

The graphenic porous structure and magnetic field sensitivity (via MHD effects) enable dual applications in energy storage and field sensing (Yadav et al., 2021).

Potential limitations include precursor and process variability, challenges in slurry preparation for pumpability, and the necessity of product upgrading to meet end-use specifications. However, the combination of scalable process efficiencies, mechanistic insights from ML/modeling, and the alignment with circular economy objectives positions biomass-derived hard carbon as a critical material in sustainable energy technologies.