Electrodes for Vanadium Flow Batteries (VFBs)

Electrodes for vanadium flow batteries (VFBs) are typically fabricated from carbon felt or carbon paper, although other novel materials are also being investigated. For optimum performance, electrodes are usually thermally pretreated in air, although a variety of chemical and electrochemical treatments are also being investigated. The electrode kinetics of both the V^II/V^IIIand V^IV/V^Vreactions have been examined in detail both in experimental flow cells and in conventional three-electrode cells and they depend strongly on the pretreatment of the electrode. Typically, cycling an electrode between positive and negative potentials increases its roughness and activates it for both the V^II/V^IIIand V^IV/V^Vreactions. Furthermore, the activities for the V^II/V^IIIand V^IV/V^Vredox reactions are affected oppositely by the potential at which the electrode is held immediately before measurement. The reaction mechanisms have been the subject of several studies but are still not well understood: we briefly review some proposed mechanisms. Side-reactions such as hydrogen evolution at the negative electrode can contribute to imbalance of the state of charge (SoC) between the negative and positive sides of a VFB. Although carbon is stable against corrosion under normal conditions, certain situations such as SoC imbalance or high shunt currents can lead to corrosion at the positive electrode. Electrode degradation in VFBs can lead to increased overpotential over time, particularly at the negative electrode, with a resulting loss of performance.