Biosourced quinones for high-performance environmentally benign electrochemical capacitors via interface engineering

The study addresses the need for sustainable, high-power energy storage compatible with renewable energy integration and applications such as electric vehicles and portable devices. While supercapacitors offer high power density and long cycle life, their performance is often limited by electrode materials and interfaces. Quinone-based biomaterials (e.g., melanins, lignin, tannins) can provide pseudocapacitive charge storage via fast, reversible proton-coupled electron transfers in aqueous electrolytes. However, high contact resistance at quinone/carbon interfaces frequently causes poor rate performance and limited cycling stability. This work investigates whether engineering the carbon electrode surface via chemical treatment can create high-performance interfaces for biosourced quinone electrodes (Sepia melanin and catechin-based) to enhance capacitance, rate capability, and cycling stability in mild aqueous electrolytes.

Prior work has demonstrated quinone-based and bio-derived materials as promising for pseudocapacitive storage. Examples include: polyanthraquinone/carbon in 0.5 M LiClO4 in acetonitrile with up to 650 F g⁻¹ and 88% retention over 1000 cycles; asymmetric devices (with graphene) achieving up to 45.5 Wh kg⁻¹ and 21.4 kW kg⁻¹. Polydopamine on functionalized carbon cloth showed 617 mF cm⁻² (626 F g⁻¹) in PVA–H2SO4 with 81% retention over 10,000 cycles; symmetric devices reached 11.7 Wh kg⁻¹ and 6.4 kW kg⁻¹. Carbonized chitosan–amino acid gels delivered ~478 F g⁻¹ in 6 M KOH with 100% stability after 100,000 cycles, and 30 Wh kg⁻¹, 225 W kg⁻¹. Pyrolyzed benzoquinone–amine electrodes showed ~360 F g⁻¹ in 1 M H2SO4 with 90% stability after 100,000 cycles, and 18.2 Wh kg⁻¹, 300 W kg⁻¹. PHATN-based electrodes reached 689 F g⁻¹ in 6 M KOH; asymmetric devices showed 100% coulombic efficiency and 50% capacity retention after 10,000 cycles at 20 A g⁻¹. Eumelanin has been explored in micro-supercapacitors and Na+/Mg2+ batteries, but earlier eumelanin aqueous supercapacitors showed low capacitance (up to 5.6 mF cm⁻²) due to low conductivity and high contact resistance at the material/current collector interface. Tannins, including tannic acid (TA), are abundant and redox-active (catechol/quinone couple) and have improved electrode performance through metal-phenol interactions and hydrogen-bonding binder effects. Prior work showed ppy–tannin composites increasing capacitance from 100 to 370 F g⁻¹ on carbonized wood via simple aqueous deposition. These studies motivate interface engineering to overcome contact resistance and realize high-rate, stable biosourced-quinone supercapacitors.

Carbon paper (CP) was cut (5 cm × 0.5 cm), solvent-cleaned (ethanol, acetone, 40 kHz ultrasound), and vacuum-dried (60 °C). A two-step oxidative/functionalization treatment produced treated carbon paper (TCP): (1) sonication 2 h in mixed acids (H2SO4:HNO3 3:1 v/v), followed by 20 min autoclave at 120 °C; rinse with DI water. (2) Autoclave 24 h at 180 °C in 7 M (NH4)2HPO4 saturated solution; rinse and vacuum-dry 6 h at 60 °C. Sepia melanin (extracted/purified from Sepia officinalis ink) and catechin (Ctn hydrate)/tannic acid (TA) (commercial) were used as actives; r-GO and Super P were used only in comparative electrodes. Sepia, sepia/r-GO, sepia/SP, and Ctn/r-GO slurries were prepared in DMSO with mass ratios 8:2, 7:3, 6:4, 5:5, stirred overnight, and brush-coated onto TCP (1 cm × 0.5 cm area). Ctn/TA and Ctn/TA/SP were prepared in water/DI mixtures (2:1 v/v) with mass ratios 7:1 and 7:1:2, respectively; 63 µL solution drop-cast on TCP. All electrodes were vacuum-dried (60 °C, 20 min). Active loading was ~3.0 ± 0.2 mg cm⁻² on TCP; bare TCP mass for the covered area was 4.74 ± 0.20 mg cm⁻². Electrolyte: 0.5 M Na2SO4 (pH ~5) in DI water. Characterization: SEM/EDX (JEOL JSM-7600F, 5 kV), including Ag-staining (0.5 M AgNO3, 48 h) to visualize Ctn; XRD (Bruker D2-Phaser, Cu Kα, 30 kV); XPS (VG ESCALAB 2250, Al Kα, 1 W, <1e−9 mbar); Raman (532 nm, 100–3200 cm⁻¹, 3–5 cm⁻¹ resolution, 20 mW). BET/BJH (Micromeritics TriStar 3000): degas 120 °C overnight; N2 adsorption at −196 °C; surface area and pore distributions via BET/BJH. Contact angle via sessile/captive drop (2 µL water). Electrochemistry: Three-electrode CV (−1 to 1 V vs Ag/AgCl) at 5–100 mV s⁻¹ and EIS (10⁵ to 10⁻¹ Hz, 10 mV AC) with CP or TCP as working electrode, Pt mesh counter, Ag/AgCl reference (Biologic SP-300). Symmetric two-electrode cells used two identical TCP-based electrodes separated by filter paper; Ag/AgCl monitored individual electrode potentials. GCD at 0.5, 2, 4, 8, 10 A g⁻¹ within 0–1.6 V; long-term cycling: 50,000 (Sepia) and 10,000 (Ctn/TA) cycles at 10 A g⁻¹. Data analysis: capacitance from CV and GCD integrals; coulombic efficiency η = ∫Idis dt / ∫Icha dt; ESR from ΔV/I; energy E = ½CV² or ∫V dt / m; power P = E/t; Pmax = Vmax²/(4 ESR). At least five electrodes and three devices per configuration were tested; performance variation ±5%.

• Surface engineering of carbon paper drastically increases surface area and improves wettability and pore structure: BET area from 0.4 to 43.0 m² g⁻¹; pore volume from 6×10⁻⁴ to 2.0×10⁻³ cm³ g⁻¹; abundant micro/mesopores (~1–4 nm); TCP becomes hydrophilic (water droplet rapidly absorbed) vs CP contact angle ~133°. Raman/XRD indicate increased disorder and slightly larger interplanar spacing after treatment (ID/IG: TCP 0.88 vs CP 0.39; d002: TCP ~3.49 Å vs CP ~3.45 Å). EDX/XPS confirm incorporation of O, N, S, P on TCP.
• Three-electrode electrochemistry in 0.5 M Na2SO4 shows a wide ~2 V stability window and large capacitive currents for TCP: areal capacitance at 5 mV s⁻¹ ~500 mF cm⁻² for TCP vs ~1.2 mF cm⁻² for CP. EIS indicates Faradaic features and low charge-transfer resistance on TCP (semicircle with Rct ~0.3 Ω; improved vertical low-frequency line and lower imaginary impedance).
• Depositing biosourced quinones on TCP yields major performance boosts versus CP: • Sepia on TCP: capacitance increases from 38 to 1355 mF cm⁻² (13 to 452 F g⁻¹); Rct decreases from 4 to 0.15 Ω cm⁻²; distinct redox features at ~0.16/0.09 V vs Ag/AgCl. • Ctn/TA on TCP: capacitance increases from 21 to 898 mF cm⁻² (7 to 300 F g⁻¹); Rct decreases from 10.8 to 1.4 Ω cm⁻²; redox features at ~0.50/0.45 V vs Ag/AgCl. Mixing catechin with TA mitigates catechin solubility without significantly altering its redox signature. • High-rate capability: at 100 mV s⁻¹, sepia and Ctn/TA deliver ~670 and ~680 mF cm⁻² (~223 and ~227 F g⁻¹), respectively.
• Charge storage contributions: On CP, biosourced quinones show ~95% Faradaic contribution. On TCP, non-Faradaic (EDL) contributions become dominant at ~73% (sepia) and ~85% (Ctn/TA), indicating hybrid charge storage.
• Symmetric supercapacitors (two-electrode) in 0.5 M Na2SO4, operated at 0–1.6 V for longevity, show: • Near-ideal triangular GCD, ~100% coulombic efficiency at high current densities (up to 10 A g⁻¹, ~150 mA cm⁻²). • Excellent cycling stability: ~100% capacitance retention and ~100% η over 50,000 cycles (Sepia) and 10,000 cycles (Ctn/TA) at 10 A g⁻¹. • Low ESR inferred from small IR drops (ΔV ~21 mV for Sepia, ~29 mV for Ctn/TA at 0.5 A g⁻¹), corresponding to ~1 and ~2 Ω cm⁻². • Ragone performance: maximum energy densities ~0.56 and 0.65 mWh cm⁻² (~20 and 23 Wh kg⁻¹) and maximum power densities ~1274 and 727 mW cm⁻² (~46 and 26 kW kg⁻¹) for Sepia and Ctn/TA, respectively (normalized to total device mass: current collector + active material).

Engineering the carbon/quinone interface by chemically treating carbon paper (TCP) substantially enhances electrode–electrolyte interactions and charge transport. The treatment increases surface area and provides hierarchical porosity, facilitating ion adsorption (micropores), transport (mesopores), and buffering (macropores). It also introduces heteroatoms (O, N, S, P) that improve wettability and induce additional Faradaic activity, reducing charge-transfer resistance and improving capacitive behavior. These synergistic effects enable biosourced quinone electrodes (Sepia melanin and Ctn/TA) to achieve high areal and gravimetric capacitance with strong rate performance, while maintaining remarkable stability and nearly 100% coulombic efficiency over extensive cycling. The observed shift toward higher electric double-layer contributions on TCP, alongside preserved pseudocapacitance, underscores a hybrid storage mechanism that benefits both power and durability. In device form, careful limitation of the voltage window to 1.6 V ensures reversible operation and mitigates overoxidation/overreduction, enabling long-term stability at high specific currents. Differences between Sepia and Ctn/TA rate behavior likely arise from morphology (Sepia nano-aggregates may hinder pore access), while small-molecule Ctn/TA conforms better to the porous TCP surface, supporting sustained rate capability. Overall, interface engineering directly addresses the prior bottleneck of high contact resistance in biosourced quinone pseudocapacitors, advancing sustainable, high-performance aqueous supercapacitors without conductive additives or fluorinated binders.

The work demonstrates that biosourced, redox-active quinone materials (Sepia melanin and catechin/tannic acid) deposited on chemically treated carbon paper enable environmentally benign supercapacitors with high capacitance, outstanding rate capability, and exceptional cycling stability in mild aqueous electrolytes. Chemical treatment of carbon enhances surface area, porosity, wettability, and introduces heteroatoms that add Faradaic activity, collectively improving interfacial charge transfer and ion access. Symmetric devices achieved up to 1355 mF cm⁻² (452 F g⁻¹) for Sepia and 898 mF cm⁻² (300 F g⁻¹) for Ctn/TA, with ~100% coulombic efficiency and ~100% capacitance retention over 50,000 and 10,000 cycles, respectively, and notable energy/power densities. These results validate interface engineering as a key strategy to unlock high performance from biodegradable, biosourced electrodes in aqueous systems.

• Catechin’s high solubility in aqueous electrolytes limits cycling stability; incorporation of tannic acid as a hydrogen-bonding binder was necessary to stabilize Ctn electrodes.
• To ensure long-term device stability, the operating voltage was reduced from the ~2 V stability seen in three-electrode CV to 1.6 V, indicating potential limitations near the extremes of the aqueous window (risk of overoxidation/overreduction).
• Sepia morphology (spherical nano-aggregates) may impede ion access to TCP pores, leading to a somewhat faster capacitance decrease with increasing current density vs Ctn/TA.
• Despite improved rate capability, both Sepia and Ctn/TA devices exhibit the typical decrease in capacitance at higher current densities due to ion diffusion limitations.
• Performance enhancements are strongly tied to the specific chemical treatment and resulting heteroatom functionalization and porosity; variations in treatment could affect reproducibility or scalability.