Navigating harsh ageing

Bipolar plates made of carbon-polymer composites with different conductive carbon fillers, viz. graphite and carbon black, were galvanostatically corroded in a high-valence vanadium electrolyte. The harsh ageing conditions led to surface oxidation and roughening of the graphite with the formation of defective graphitic regions as revealed by X-ray photoelectron spectroscopy, scanning electron microscopy (SEM) and Raman spectroscopy. Further insight into the structural changes from the corroded surface to the bulk was gained by combining synchrotron computed tomography (CT) techniques based on X-ray diffraction (XRD) and phase contrasts. The extent of corrosion increases as the relative amount of carbon black to graphite increases. Furthermore, corrosion progresses through significant structural and chemical changes in the graphite, manifested by the expansion of graphite flakes, their exfoliation, the development of disordered graphite and the formation of graphite oxide. The bulk of the polymer remains essentially unchanged, as determined by XRD-CT. SEM studies of the cross-section and nano-CT showed decreased carbon black content in the corroded region. It can be concluded that graphite delamination and oxidation, in combination with carbon black depletion, are the main causes of corrosion of the bipolar plates investigated.