Abstract: Vanadium redox flow battery (VRFB) system is an electrochemical energy device that converts chemical energy into electrical energy. One of the major advantages of this energy device is that power and energy capacities can be independently scaled up. A VRFB consists of positive (VO2 + /VO2+) and negative (V 3+/V2+) electrolyte tanks, a cell/cell-stack and two pumps. The cell is constructed by sandwiching membrane between two porous electrodes, placing a flow channel both sides. The role of a flow channel is to distribute electrolyte in the porous space of electrode which helps in its efficient utilization and in overcoming mass transfer resistance of electroactive species. The non-uniform distribution of electrolyte badly hampers the performance and results in uneven current distribution in electrode.

The focus of current research work is to develop flow channels which will help in achieving better performance of the battery. Motivation came from closely similar systems, such as plate-type heat exchangers and micro reactors, where corrugated flow patterns are employed leading to intensified heat and mass transfer. Two flow patterns, Split Serpentine (SS) and Split-Merged Serpentine (SMS), were designed and developed, and were fabricated on graphite plate; with an active area of ~10 cm². For comparison, a conventional serpentine (CS) channel of same area was also fabricated. The performance of the battery was analyzed through polarization curves – illustrating potential loss stemming from kinetic, ohmic and mass transfer resistance – were obtained by varying flow rates (30, 50, 80, and 120 ml/min) of electrolyte. Electrochemical impedance spectroscopy (EIS) was performed to quantify these resistances. High frequency response of EIS was used to obtain iR-free polarization curves and power density which eliminates the effect of ohmic resistances arising from membrane and electrolyte solutions.

Results show that the peak power densities using CS, SS, and SMS flow patterns increase with increasing flow rate. The highest peak power density was achieved with the SS channel (552 mW/cm2 ) followed by SMS (363 mW/cm2 ) and CS (154 mW/cm2 ) channels at 120 ml/min. Higher flow rates of electrolyte result in increased convective mass transport of vanadium ions to and from the porous space of electrode. The EIS results show that the electrode with CS channel has highest charge transfer resistance, and with SMS, it is lowest. The above results suggests that the SS flow pattern shows highest electrolyte distribution ability in porous electrode followed by SMS and CS channels. Even at as low as 30 ml/min, only 7.5% reduction in power density is observed employing SS channel.