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.³The 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 cm² 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 density on 10AA generally surpasses that of X-2 at elevated overpotential. This trend should be attributed to better mass transport property of 10AA, though the detailed mechanism is not clear yet. 

We will present a thorough study of the mass transport properties of carbon paper electrode in VRFB cell. A systematic study of the physical properties and their influence on the convective and diffusive transport of redox active species within the electrode will be discussed. The transport study will be correlated to electrochemical test to develop a protocol to evaluate the mass transport properties’ influence on battery’s performance.

This work is supported by Dr. Imre Gyuk, Office of Electricity Delivery and Energy Reliability, US Department of Energy.

Reference:

1. M. H. Chakrabarti, N. P. Brandon, S. A. Hajimolana, F. Tariq, V. Yufit, M. A. Hashim, M. A. Hussain, C. T. J. Low, and P. V. Aravind, J. Power Sources, 253, 150–166 (2014)

2. A. M. Pezeshki, J. T. Clement, G. M. Veith, T. A. Zawodzinski, and M. M. Mench, J. Power Sources, 294, 333–338 (2015)

3. D. S. Aaron, Z. Tang, A. B. Papandrew, and T. A. Zawodzinski, J. Appl. Electrochem., 41, 1175–1182 (2011).

Figure 1.  IR corrected discharging polarization curves of negative and positive redox couple on SGL 10AA and X-2 in 5 cm² FCT flow battery cell of varying flow rates with 1.7 M V^2/3+/5 M bisulfate/sulfate electrolyte at 55% SoC.