Abstract
Commercially available anion exchange membranes were retrieved from VRFB field tests and their degradation due to the various operation conditions is analyzed by in-situ and ex-situ measurements. Ion exchange capacity, permeability and swelling power are used as direct criteria for irreversible changes. Small-angle X-ray scattering (SAXS) and Differential scanning calorimetry (DSC) analyses are used as fingerprint methods and provide information about the morphology and change of the structural properties. A decrease in crystallinity can be detected due to membrane degradation, and, in addition, an indication of reduced polymer chain length is found. While the proton diffusion either increase or decline significantly, the ion exchange capacity and swelling power both are reduced. The observed extent of changes was in good agreement with in-situ measurements in a test cell, where the coulombic and voltage efficiencies are reduced compared to a pristine reference material due to the degradation process.
Highlights
Vanadium redox flow batteries (VRFBs) are an attractive option for today’s stationary energy storage systems
anion-exchange membranes (AEM) from battery field tests with very different operation conditions and from different locations in the stacks were analysed
Membranes that had faced excessive stress by operation in high SOC were compared with ones subjected to only mild operation conditions or oil contamination
Summary
Vanadium redox flow batteries (VRFBs) are an attractive option for today’s stationary energy storage systems. Due to many advantages, such as high storage capacity, quick response, individual scalability of power and capacity (amount of energy), and relatively long cycle life, they show a great potential to improve the reliability and efficiency of electricity distribution networks. A full commercialization of VRFB systems is currently limited by several hurdles, such as a relatively low energy density, a poor stability of the utilized electrolyte solutions, high investment costs, and a performance loss due to degradation of battery components [5,6,7]. The cell stack system of the battery involves two half-cells, with redox reactions occurring on commercially available carbon cloth (flow-by) or carbon felt electrodes (flow through)
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