Lignin, a natural biopolymer abundant in peanut shells, has emerged as a promising precursor for carbon nanofiber composites used in supercapacitor electrodes. This renewable feedstock offers a sustainable alternative to fossil-derived carbons, aligning with green energy goals while maintaining competitive electrochemical performance. The conversion of peanut shell lignin into high-performance electrode materials involves precise carbonization techniques, pore structure engineering, and composite optimization to enhance charge storage capabilities in asymmetric supercapacitors.

The carbonization of peanut shell lignin begins with pretreatment to isolate lignin from cellulose and hemicellulose. Acid or alkaline hydrolysis separates lignin, which is then dissolved in solvents for electrospinning. During stabilization at 200-300°C in air, the lignin nanofibers undergo oxidative crosslinking to prevent melting. Carbonization follows at 600-1000°C under inert atmosphere, where heteroatoms like oxygen and hydrogen are eliminated, leaving a turbostratic carbon structure. Higher temperatures increase graphitization but reduce yield, requiring optimization. Peanut shell lignin's high aromatic content and crosslinked structure yield carbon nanofibers with 60-75% carbon retention, superior to many agricultural wastes.

Pore structure optimization critically determines performance in supercapacitors. Peanut shell lignin-derived carbons naturally develop a hierarchical pore network during carbonization due to volatile release. Micropores below 2 nm provide high surface area for electric double-layer capacitance, while mesopores (2-50 nm) facilitate ion transport. Chemical activation with KOH at 700-800°C enhances porosity, creating surfaces areas exceeding 1500 m²/g. The KOH ratio controls pore distribution, with 3:1 KOH:carbon producing optimal mesopore volume of 0.8-1.2 cm³/g. Steam activation at 800°C offers an alternative, generating more uniform mesopores but lower surface areas around 1000 m²/g. The natural potassium content in peanut shells further self-activates pores during carbonization, reducing chemical usage.

In asymmetric supercapacitors, peanut shell lignin-derived carbon nanofibers serve as the negative electrode paired with metal oxide or conductive polymer positives. The carbon's wide pore distribution accommodates various electrolytes, with ionic liquids utilizing micropores and aqueous electrolytes favoring mesopores. In 1M H₂SO₄, these carbons deliver 180-220 F/g capacitance at 1 A/g, retaining 85% after 10,000 cycles. Their nitrogen self-doping (1-3 at%) from peanut shell proteins enhances wettability and pseudocapacitance. When assembled in asymmetric devices with MnO₂ positives, energy densities reach 25-30 Wh/kg at power densities of 500-1000 W/kg, outperforming many biomass-derived carbons.

Comparisons with fossil-derived carbons reveal distinct advantages and tradeoffs. Petroleum pitch-based carbons exhibit higher graphitization (85-90% carbon yield) but require energy-intensive processing. Their narrower pore distribution limits rate capability, showing 20-30% lower capacitance at high current densities. Coal tar derivatives offer higher surface areas (2000-2500 m²/g) but contain sulfur impurities that degrade electrolyte stability. Peanut shell lignin carbons demonstrate superior sustainability metrics, with 80% lower embodied energy and 90% reduced CO₂ emissions during production. Their natural heteroatom doping eliminates post-processing steps needed for fossil carbons, though absolute conductivity remains 10-15% lower due to less ordered graphene domains.

Composite formation enhances performance further. Incorporating 5-10 wt% carbon nanotubes creates conductive pathways, reducing equivalent series resistance to 2-3 Ω/cm². Graphene oxide additions during electrospinning yield mechanically robust fibers with 50% higher tensile strength, enabling binder-free electrodes. Transition metal doping, such as iron from peanut shell minerals, generates additional redox-active sites without separate activation steps. These composites maintain 95% capacitance retention after mechanical bending, suitable for flexible devices.

The economic viability of peanut shell lignin-derived carbons benefits from waste utilization. Peanut processing generates 3-5 million tons of shells annually as agricultural waste. Converting this to carbon nanofibers at 30% yield could produce 900,000 tons of electrode material, sufficient for 180 million supercapacitors yearly. Production costs estimate $5-8/kg compared to $15-20/kg for synthetic graphite, with potential reductions through scale-up. Life cycle analyses show 70% lower environmental impact than activated carbons from coconut shells.

Performance limitations persist in three areas. First, the natural variability in peanut shell composition requires stringent feedstock preprocessing to ensure batch consistency. Second, the moderate electrical conductivity (50-100 S/cm) restricts ultra-high power applications exceeding 10 kW/kg. Third, the oxygen functional groups that aid wettability can also promote side reactions in organic electrolytes at voltages above 3V. Ongoing research addresses these through controlled pyrolysis atmospheres and conductive polymer coatings.

Future developments will likely focus on multifunctional composites that combine lignin-derived carbons with other nanomaterials to create tailored pore architectures. The integration of machine learning for process optimization could further enhance yield and performance reproducibility. As supercapacitor markets expand into renewable energy storage and electric vehicles, peanut shell lignin-derived carbons offer a scalable, sustainable solution that balances electrochemical performance with environmental responsibility. Their development exemplifies the potential of agricultural waste valorization in advancing energy storage technologies without competing with food resources or relying on fossil inputs.

The transition from fossil-derived carbons to biomass alternatives like peanut shell lignin represents more than material substitution—it demonstrates how nanotechnology can transform low-value agricultural residues into high-performance energy materials. This approach closes resource loops while meeting the increasing demand for efficient, sustainable energy storage solutions in a decarbonizing economy.