Impact of Varying Bipolar Plates Having Flow-Fields on the Performance of Redox Flow Battery at High Current Density
Authors: Mushtaq Kashif , Monteiro Ricardo , Mendes Adelio 
Published: December 2020

Redox flow Cell (RFC) stores electricity in the form of chemical energy into the electrolyte. The interface of the electrode surface and electrolyte is responsible for redox reaction and resultantly increase in the state-of-charge (SOC) during charging of the cell. Dispersion of the electrolyte into the electrode (porous carbon felt) and flowrate are important parameters for the performance of the cell. A bipolar plate having flow-field is responsible to disperse electrolyte. Two different configurations of interdigitated flow-field (IFF) and serpentine flow-fields (SFF) were fabricated on the impregnated graphite bipolar plates. The performance was compared with RFC having a conventional bipolar plate without any flow-field.
The as-received Fumatech FAP-450 anion exchange membrane was used. All membranes were cleaned in 1M H2SO4 solution at 80oC and stored in DI water. The as-received commercial electrolyte consisted of vanadium solution (1.6 M) was used. Graphite felt electrode (GFE) having a thickness of 4.6 mm was used with a compression ratio of 25%. GFE was boiled in 1M H2SO4 for 12 hours and kept in the same solution for 12 hours. These were washed with DI water and dried at 100oC. Afterward, GFE was thermally treated at 390oC for 12 hrs inside the furnace injected by air and then cooled down in the furnace up to room temperature. Membranes and GFE were used from the same batch for each experiment.
The performance of RFC was characterized using charge-discharge cycling (CDC), impedance curves (EIS), and polarization (iV-curve). EIS performed in a range of 100mHz to 50kHz having AC perturbation of 10mV. RFC was charged from SOC of 25% to 75%. It was ensured by recording open-circuit-potential (OCP) after each cycle and the measurement of OCP was co-related with SOC values computed based on concentrations. This allows us to set the cut-off potential from 0.8V to 2.0V and it helps to remain in the safe SOC limits to avoid hydrogen evolution.
EIS results of conventional (CFF), IFF, and SFF at the flowrate of 40ml.min-1 were shown in Figure 1(a). The introduction of the new bipolar plates having flow-fields has a significant effect on the area-specific resistance (ASR) which was decreased and charge transfer resistance (CTR) was almost similar. The equivalent circuit analysis of the impedance curve was shown in Figure 1(b). Ohmic & charge transfer resistance of the cell having IFF and SFF is less than conventional whereas capacitance highlighted by CPE is almost double. The effect of varying flowrate on EIS results was tested on the IFF as shown in Figure 1(c) and have a negligible effect. CDC was performed using each flow-field to assess the performance of RFC. Impact of flow-field on the capacity as a function of potential at 120mA.cm-2 having 40ml. min-1 is shown in Figure 1(d) and inset showed the average efficiency of 10 cycles. RFC having IFF reached to higher capacity and overall energy efficiency (EE) of this arrangement is also higher than SFF. However, it was observed that columbic efficiency (CE) is low, but it was the effect of membrane aging after repeated cycles. This observation was eliminated in future studies by using a fresh membrane. The impact of varying flowrate was on the capacity and EE of the RFC having IFF at 80 mA.cm-2 was shown in Figure 1(e). Best capacity and EE results were obtained by using IFF at 20 ml.min-1.
Therefore, CDC was performed to check the impact of varying current density having IFF at 20ml.min-1 as shown in Figure 1(f). The current density ranging from 40 to 155mA.cm-2 was tested and EE was decreased from 85% to 60% after increasing the current density up to four times. The CE remains the same which means that no side reactions and least crossover. The voltaic efficiency (VE) was kept on decreasing gradually as the current density was increasing. To further investigate this effect polarization analysis was performed to get limiting current density and peak power as shown in Figure 1(g). It highlights that this arrangement can reach up to a higher current density of 410mA.cm-2 and peak power of 56.25W.l-1 for an RFC having 80ml of electrolyte. Overpotentials were shown in Figure 1(h). It signifies that activation overpotential is negligible, ohmic overpotential is minimum at 10ml.min-1, and mass transport overpotential is minimum at 20ml.min-1.
This study highlights that optimization of bipolar plate is significant and has a considerable impact on the performance. It is not a straight-forward isolated process and future researchers need to decide about many trade-offs while optimizing even a single component of RFC.

Figure 1