Molecularly engineered bacterial biopolymer as multifunctional interfacial regulators for dendrite-free and stable aqueous zinc-ion batteries

Aqueous zinc-ion batteries (ZIBs) are promising candidates for next-generation energy storage owing to their intrinsic safety, low cost, and high theoretical capacity. However, their practical application remains hindered by dendrite formation and parasitic side reactions such as hydrogen evolution reaction (HER), anode corrosion, and passivation. Herein, we introduce a sustainable and scalable strategy based on a bacterial eumelanin biopolymer (S2–20), synthesized via the oxidative metabolism of Pseudomonas aeruginosa with indole derivatives, and directly deposited onto Zn foil via spin coating. The S2–20 biopolymer provides a conformal, ion-permeable interface that simultaneously guides uniform Zn nucleation, enhances electrolyte wettability, and suppresses parasitic side reactions. Symmetric cells demonstrate significantly increased Zn2+ transference number (0.908), and prolonged cycling stability exceeding 1200 h at 1.0 mA cm−2 and 200 h under harsh conditions up to 20 mA cm−2. Full cells with V2O5 cathodes further confirm remarkable specific capacity (374 mAh g−1 at 1.0 A g−1) and capacity retention, maintaining 150.5 mAh g−1 after 2200 cycles at 1.0 A g−1. This work presents the first demonstration of a bacterial eumelanin biopolymer as a multifunctional interfacial stabilizer for Zn metal, offering a truly green, cost-effective, and scalable pathway toward safe and sustainable aqueous ZIBs.