Piperidinium-functionalized anion exchange membranes and their application in alkaline fuel cells and water electrolysis

Abstract

To produce a stable anion exchange membrane (AEM) for deployable electrochemical devices with a long lifespan, we here present the synthesis and properties of a series of piperidinium-functionalized poly(2,6-dimethyl phenylene oxide)s with different locations of piperidinium groups along the polymer backbones. A distinct phase separated morphology was observed for long side-chain-type AEMs (LSCPi) as confirmed by AFM analysis, which in turn enabled its higher hydroxide conductivity over side-chain-type (SCPi) and standard benzylmethyl piperidinium AEMs (BPi). A hydroxide conductivity of 29.0 mS cm⁻¹ at 20 °C was achieved for the LSCPi membrane with an IEC value of 1.57 meq. g⁻¹. This level of conductivity was lower than that of the corresponding QA-based AEMs (LSCQA) (38.7 mS cm⁻¹ at 20 °C), probably as a result of its low IEC accompanied by low water uptake. The LSCPi membrane displayed excellent alkaline stability with 98% retention in conductivity after 560 h of testing in 1 M NaOH at 80 °C, and no obvious degradation was detected by NMR analysis of the aged sample. To demonstrate the feasibility of piperidinium-functionalized AEMs, both SCPi and LSCPi membranes were fabricated into a membrane electrode assembly for the H₂/O₂ alkaline fuel cell and AEM water electrolyzer applications. The highly conductive LSCPi membrane showed good cell performance with a peak power density of 116 mW cm⁻² at 60 °C in alkaline fuel cells and 300 mA cm⁻² at 1.80 V at 50 °C in AEM water electrolysis working with pure water. Although a gradual drop in performance was observed for both the alkaline fuel cell and water electrolyser durability testing at a constant current during the test of 8.7 h and 35 h respectively, the high durability of AEMs having piperidinium cations was verified by post-mortem analysis of aged AEMs by NMR spectroscopy. The current findings provided fundamental insights into the durability of AEMs under ex situ and in situ operating conditions and demonstrated that the piperidinium-functionalized AEM appears to be a promising material for durable AEM-based devices.