Agarose-based Gel Electrolytes for Sustainable Primary and Secondary Zinc-Air Batteries

Present Zn-air batteries (ZABs) are based on concentrated alkaline liquid electrolytes, with high ionic conductivity, but suffer from leakage, evaporation, and carbonate precipitation due to the semi-open characteristic of these systems. To overcome these issues, gel polymer electrolytes (GPEs), based on naturally occurring biopolymers, arise as a green option to overcome the above-mentioned limitations. In this work, a novel GPE based on pure agarose from seaweed is presented as a smart alternative to liquid (adsorbed on a separator) and gel electrolytes (based on synthetic polymers). The innovative synthesis method described can directly encapsulate concentrated KOH liquid electrolytes into an agarose matrix in one-pot; the process requiring approx. 10 min. The unique gel developed in this work, with 2 wt% agarose and 8 M KOH electrolyte, presents the best compromise between physicochemical and electrochemical properties, at lab scale. The characterization results revealed an outstanding ionic conductivity of 0.45 ± 0.05 S cm−1, ≈100% water retention up to 200–250 h, retarded Zn self-corrosion up to 30 days (symmetric cell under open circuit), average Zn utilization > 70–80% in primary ZABs in the range 1–20 mA cm−2 with peaks of ≈96%. In secondary ZABs the gel electrolyte presents high round-trip efficiency and improved cyclability at high areal capacities, under soft and severe cycling conditions, never tested before. This agarose gel represents a potential benchmark for future development of GPE-based ZABs for stationary applications.