Application and degradation mechanism of polyoxadiazole based membrane for vanadium flow batteries

A new type of membrane composed of poly (4,4'-diphenylether-1,3,4-oxadiazole) (POD) was prepared and first investigated in a vanadium flow battery (VFB). In contrast to traditional hydrocarbon ion exchange membranes, the proton transport of POD was driven by the interaction between the acid in the electrolytes and the heterocyclic atoms in the POD backbone. A VFB single cell assembled with a POD membrane exhibits higher columbic efficiency (96.87%) and energy efficiency (83.29%) than does a Nafion 115 membrane (CE 94.6%, EE 82.1%) under the same operating conditions. However, the cell performance of the assembled POD membrane suddenly drops after continuously running for 60 cycles. The degradation mechanism of POD under strongly acidic and oxidized medium was investigated by UV-vis spectrometry, nuclear magnetic resonance (NMR) and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). The chemical structure of the degradation product was clarified, and the degradation mechanism was proposed, indicating that the oxadiazole ring in POD membrane becomes a strong electrophilic center due to the protonated ethereal oxygen atom and highly electronegative nitrogen atoms under strong acidic conditions. Next, the electrophilic center was attacked by the lone electron pair of the vanadium oxygen species. This work will provide valuable information to further improve the stability of polyoxadiazole under VFB conditions. (C) 2015 Elsevier B.V. All rights reserved.