Corrosion of Graphite-Polypropylene Current Collectors during Overcharging in Negative and Positive Vanadium Redox Flow Battery Half-Cell Electrolytes

Abstract

Graphite-polypropylene bipolar plates (BPP) were subjected to galvanostatic treatment in highly charged positive and negative vanadium electrolyte solutions. The tests were performed in an ex-situ three-electrode electrochemical cell in order to simulate aging under harsh overcharging conditions in a vanadium redox flow battery (VRFB). Non-destructive computed microtomography (microCT) technique was employed to study the post aging morphological changes. The investigations revealed that even under massive hydrogen evolution conditions in the negative electrolyte the BPP is stable. However, the BPP suffers from intense corrosion associated with morphological deformations during aging by galvanostatic overcharging in the positive electrolyte. The CO/CO2 gas evolution leads to formation of an open pore network, development of micro-cracks and meso-fractures. These morphological changes cause an expansion of the corroded bulk material. The results show that the corrosion starts at the electrolyte/BPP interface and propagates with time in plane-parallel direction to the back side of the BPP.