An optimized Zn-air battery using a polymer-blend anion transfer membrane and a biochar electrocatalyst

Abstract

Alkaline stable blend membranes based on polyisatin copolymers and poly(2,2′-(m-phenylene)-5,5′-bibenzimidazole) (m-PBI) were fabricated and tested as ion solvating membranes (ISMs) in an optimized quasi-solid state Zn-air battery. In specific, the prepared blend membranes with two different compositions (80 and 70 wt % with respect to m-PBI) displayed increased water uptake (ranging between 93 and 102 %) due to the presence of hydrophilic polyethylene oxide (PEO) groups compared to m-PBI and high KOH uptakes (105 %) even at low temperature of 5 °C. They also exhibited much lower electrolyte swelling than m-PBI and high ionic conductivity values up to 109 mS cm⁻¹ in 20 wt % KOH at 80 °C. Long-term alkaline stability test of both blends revealed their excellent alkaline resilience after immersion in 20 wt % KOH at 80 °C for 6 months as evidenced by conductivity measurements, tensile strength and ATR-FTIR data. The PBI80/P(IB-PEO) membrane, i.e. the membrane containing 80 wt % m-PBI, was chosen for the construction of an optimized quasi-solid-state-electrolyte Zn-air battery due to its optimal characteristics. To that end, a membrane-gas-diffusion electrode assembly has been constructed using a PBI80/P(IB-PEO) membrane and a gas-diffusion electrode containing activated biochar as pure carbonaceous electrocatalyst. Activated biochar has been prepared by pyrolysis of luffa, the vegetable sponge, followed by mixing of the obtained powder with KOH and a second pyrolysis step. This choice offered a satisfactory battery performance peaking at a power density value of 126 mW cm⁻² at ambient temperature, which is comparable with that of optimized liquid electrolyte cells. Cell characteristics were improved with increasing temperature following the same trend with conductivity. As a result, the power density reached a peak value of 182 mW cm⁻² at 45 °C.