Effects of Electrode Compressibility in Vanadium Redox Flow Batteries

Vanadium redox flow batteries (VRFBs) have been investigated for their potential utility as large energy storage systems due to their advantageous performances in terms of long cycle life, high energy efficiency, low cost, and flexible design. Carbon materials are typically used as electrodes in redox reactions and as a liquid electrolyte support. The activities, surface areas, and surface morphologies of carbon materials must be optimized to increase the redox flow battery performance. Here, the compressibility of carbon electrode was controlled with the thickness of carbon felts for the same thickness of electrode. The efficiencies with discharging current of VRFB were measured and characterized. The cyclic performances were also characterized with various flow rates. VRFB-2F has the lower compressibility than VRFB-3F. VRFB-2CF has the 5 mm thickness of carbon felt in 5 mm of electrode and VRFB3F has the 8 mm thickness of carbon felt in 5 mm of electrode thickness, which were compressed in VRFB flow cells.

Form the performance measurement with discharging current, VRFB-2CF exhibited 88.7 % of columbic efficiency (CE) and 63 % of energy efficiency (EE) at discharging current of 25mA/cm², which are higher than those of VFRB-3CF (82.5 % of CE, 57.6 % of EE). VRFBs were tested for cyclic performances with 20 times under 50mA/cm² of charging current and 50mA/cm2 of discharging current. VRFB-2CF exhibited over 90% of columbic efficiencies;  VRFB-3CF shows 80% of CE.  Energy efficiencies of VFRB-2CF are 63 %, 61.3 % and 56.8 % at 18cc/min, 36cc/min, and 54cc/min, respectively which are higher values than those of VRFB-3CF (55.3%, 50.7% and 52.3% at 18cc/min, 36cc/min, and 54cc/min, respectively).

The compactness of VRFB-3CF is higher than that if VRFB-2CF, which are concerned with the electrolyte-coupled mass transport. The effects of compactness of electrode will be discussed in terms of the mass and charge transfer polarization.