Transport Properties Study on Carbon Electrodes for Vanadium Redox Flow Batteries

The electrode is a key component to optimize in the vanadium redox flow battery (VRFB) to achieve high battery performance and market competitiveness. Generally, carbon based materials are the most prevalent type of electrode utilized in commercial and experimental VRFBs. 1 Fibrous carbon materials and their derivatives are widely used as electrodes in VRFB system because they are porous to allow electrolyte penetration, resistant to the electrolyte chemical environment and relatively low-cost. Recently, significant performance improvement of VRFB has been achieved by utilization of carbon paper materials with enhanced surface properties. 2 Mass transport is a major factor in the efficiency loss in VRFB, particularly at low states of charge. Ohmic, activation and mass transport overpotential losses cause efficiency loss in an electrochemical system. 3 The electrode loss is mainly comprised of activation and mass transport losses, while the ohmic loss is mainly due to separator resistance. The activation overpotential is caused by limited reaction rate of redox couple and limited surface area. The mass transport overpotential is caused by reaction active species depletion due to insufficient active species supply on electrode surface. Because of the slow motion of cation in solution, it is critical to understand the relationship between the electrode properties and mass transport in electrode. Future material development and system optimization can benefit from this work. The properties of the carbon electrode can have a strong influence on the active species mass transport inside the cell. In Figure 1, the IR corrected polarization curves of positive and negative redox reactions on SGL 10AA and X-2 in VRFB are shown at varying electrolyte flow rates. The electrode materials were housed in a 5 cm 2 battery hardware (Fuel Cell Technologies Inc.) with serpentine flow field. The electrolyte was composed of 1.7 M vanadium ions with 5 M total sulfate/bisulfate. The state of charge was maintained at 55% during the entire polarization curve measurement. In both reactions, the polarization curves show a typical mass transport limiting current density ( i MT ) at high overpotential. In both reactions, higher i MT on 10AA indicates the better mass transport towards the surface of 10AA in the experimental scenario. X-2 is advantageous in reaction kinetics, since the activation overpotential is always lower on X-2 than 10AA in both reactions. However, the current den