Influence of Carbon Material Properties on Activity and Stability of the Negative Electrode in Vanadium Redox Flow Batteries: A Model Electrode Study

The V2+/V3+ reaction occurring at the negative electrode in the all-vanadium redox flow battery has been identified as performance limiting in the system. Given the complexity of the commercial carbon electrodes typically used for this application, a model electrode approach is adopted in this work to study electrode activity and stability. This was done using edge and basal plane pyrolytic graphite electrodes modified by electrochemical oxidation. Differential electrochemical mass spectrometry was used for the first time in this work to investigate the parasitic hydrogen evolution reaction on the oxidized carbon surfaces. The basal plane surface showed a higher faradaic efficiency for V3+ reduction compared to that for the edge plane surface. The oxidized surfaces were subject to extended cycling, after which the basal electrode showed a dramatic loss in activity compared to the edge surface which was relatively stable. This activity loss was related to the poor mechanical stability of the basal plane surface. The electrodes were analyzed before and after cycling by different techniques including X-ray photoelectron spectroscopy, Raman spectroscopy, and scanning electron microscopy. The influences of the type of carbon and its related properties on electrode activity and stability are discussed.