Biopolymeric hydrogel electrolytes obtained by using natural polysaccharide–poly(itaconic acid-co-2-hydroxyethyl methacrylate) in deep eutectic solvents for rechargeable Zn–air batteries

Abstract

Rechargeable Zn–air batteries (ZABs) are safe energy storage systems, making them attractive for different applications, while their attractiveness can be boosted by adding natural constituents for eco-friendly disposal. In this work, biopolymeric hydrogels (biohydrogels) were synthesized using deep eutectic solvents (DESs), poly(itaconic acid) (PIA), and polysaccharides cross-linked with 2-hydroxyethyl methacrylate (HEMA). Three biohydrogels were studied, one based on alginate from Sargassum (PIA–ALG), another on inulin from Dahlia tubers (PIA–INU), and the third on dextran from Leuconostoc spp. (PIA–DEX), chosen due to their eco-friendly, cost-effective, and excellent water retention properties. Among these, PIA–INU presents pH-dependent water swelling absorption and an amorphous structure, both beneficial properties in applying biohydrogels as bioelectrolytes. These biohydrogels were then tested in a ZAB by mixing them with 6 M KOH solutions, displaying low internal resistances (below 7 Ω cm²). The ZAB assembled with 3 wt% of PIA–INU showed the best performance. It presented an open circuit voltage of 1.44 V, a current density of 112 mA cm⁻²@0.6 V, and a maximum power density of 62 mW cm⁻². Additionally, it presented high rechargeability, being functional for more than 1600 cycles. According to post-mortem analyses, the long life of the battery can be attributed to the ability of this biohydrogel to decrease zinc issues such as shape changes and passivation.