Nanocomposite bio-sponge air-electrode biogenically derived from Plantago ovata for applications in wearable and biodegradable zinc-air batteries

Abstract

Flexible zinc-air batteries (ZABs) are emerging as sustainable alternatives for next-generation wearable devices. This study introduces an innovative cost-effective and eco-friendly strategy for fabricating a conductive 3D nanocomposite bio-sponge derived from Plantago ovata (psyllium) husk via biogenic synthesis, bypassing conventional pyrolytic carbonization. The resulting bio-sponge features a mesoporous structure characterized by a type-IV adsorption/desorption isotherm, with an average pore diameter of 19.9 nm, a BET surface area of 48.5 m²·g⁻¹, and a predominantly amorphous framework exhibiting low crystallinity (12.9 %). Structural, compositional, and thermal analyses using XRD, Raman spectroscopy, XPS, and TGA confirmed the incorporation of diverse phytochemicals and functional groups within the matrix, along with notable thermal stability, evidenced by a mass loss of only 7.6 % at 266.1 °C. As a proof-of-concept, flexible primary ZABs were fabricated using green-synthesized MnO₂ nanoparticles as the oxygen reduction reaction (ORR) catalyst, with an optimized catalyst loading of 0.2 mg·cm⁻², and a Plantago ovata-derived alkaline hydrogel serving as the electrolyte. The batteries delivered promising performance, with an open-circuit voltage of ∼1.4 V, a discharge time of ∼8.8 h, a peak power density of 51 mW·cm⁻², and a specific capacity of 737 mAh·g⁻¹. The ZABs maintained robust performance under mechanical deformation, successfully powering LEDs and small electronic gadgets even under bending conditions. Furthermore, biodegradation studies revealed over 95 % decomposition of the spent ZABs within 64 days, demonstrating their environmentally benign end-of-life profile. This innovative approach underscores the potential of biogenic materials for developing sustainable, flexible, and disposable energy solutions for wearable technology.