Poly(arylene piperidinium) terpolymer membranes with dual piperidinium cations and semi-fluoroalkyl pendants for anion exchange membrane water electrolyzers

Abstract

To develop high-performance anion exchange membranes (AEMs), the effect of the length of semi-fluoroalkyl pendants on physical and electrochemical properties of the resulting AEMs was investigated. A series of 2,5-dichlorobenzene monomers differing in semi-fluoroalkyl pendant length were synthesized and copolymerized with 2,2-bis(4-chlorophenyl)hexafluoropropane and piperidine functionalized 2,7-dichlorofluorene. The terpolymers provided bendable membranes by solution casting. The membranes with comparable ion exchange capacity (IEC = ca. 1.85 meq g⁻¹) showed similar water uptake, while the hydroxide ion conductivity increased by 24% on increasing the pendant chain from 7 to 11 carbons. The maximum ion conductivity of 112 mS cm⁻¹ (at 80 °C) was achieved for the membrane formulated with the C₁₁ pendant. C_x-QPip-n membranes showed good alkaline stability; in particular, C₁₁-QPip-1.86 retained 75% of the original conductivity after 1056 h under harsh alkaline conditions (8 M KOH at 80 °C). An alkaline water electrolysis cell assembled with the C₁₁-QPip-1.86 membrane and with a PGM-free anode catalyst (Ni_0.8Co_0.2O) showed good performance (1.0 A cm⁻² at 1.64 V) with high voltage efficiency (75%). The cell was durable for 1000 h with minor voltage change (28 μV h⁻¹) under constant current density (1.0 A cm⁻²) operation.