Design enhancement in hydroxide ion conductivity of viologen–bakelite organic frameworks for a flexible rechargeable zinc–air battery

Abstract

Quasi-solid-state rechargeable zinc–air batteries (ZABs) are suitable for the generation of portable clean energy due to their high energy and power density, safety, and cost-effectiveness. Compared to the typical alkaline aqueous electrolyte in a ZAB, polymer or gel-based electrolytes can suppress the dissolution of zinc, preventing the precipitation of undesirable irreversible zinc compounds. Their low electronic conductivity minimizes zinc dendrite formation. However, gel electrolytes suffer from capacity fade due to the loss of the volatile solvent, failing to deliver high-energy and high-power ZABs. Consequently, developing polymers with high hydroxide ion conductivity and chemical durability is paramount. We report cationic C–C bonded robust polymers with stoichiometrically controlled mobile hydroxide ions as solid-state hydroxide ion transporters. To boot, we increased the viologen-hydroxide-ion concentration through “by-design” monomers. The polymers constructed with these designer monomers exhibit a commensurate increase in their ionic conductivity. The polymer prepared with 4 OH⁻ ion-containing monomer was superior to the one with 3 OH⁻. The conductivity increases from 7.30 × 10⁻⁴ S cm⁻¹ (30 °C) to 2.96 × 10⁻³ S cm⁻¹ (30 °C) at 95% RH for IISERP-POF12_OH (2_OH) and IISERP-POF13_OH (3_OH), respectively. A rechargeable ZAB (RZAB) constructed using 3_OH@PVA (polyvinyl alcohol) as the electrolyte membrane and Pt/C + RuO₂ catalyst delivers a power density of 158 mW cm⁻². In comparison, RZABs with a PVA interlayer provided only 72 mW cm⁻². Notably, the device suffered an initial charge–discharge voltage gap of merely 0.55 V at 10 mA cm⁻², which increased by only 2 mV after 50 hours of running. The battery operated at 10 mA cm⁻² and worked steadily for 67 hours. We accomplished a flexible and rechargeable zinc–air battery (F-RZAB) exhibiting a maximum power density of 79 mW cm⁻². This demonstration of a cationic viologen–bakelite polymer-based flexible secondary ZAB with versatile stochiometric hydroxide-ion tunability marks an important achievement in hydroxide-ion conducting solid-state electrolyte development.