## 3 Results and Discussion  
A polarization curve at a constant flow rate was performed to observe changes in the current distribution as the cell voltage changed. Cell operating conditions were varied significantly to determine what effect the flow rate would have on the current distribution of the cell, thus finding the optimum \(\lambda\). Initially, the flow rate of H\({}_{2}\) was stepwise adjusted, whilst the catholyte flow rate was set to V\({}_{\text{B}2}\) = 30 mL min\({}^{-1}\) (equivalent to a stoichiometric flow factor of \(\lambda\cong\) 20 at 10 A and 100% SOC utilization). During charge, Br\({}^{-}\) is converted to Br\({}_{2}\) on the positive electrode, which forms additional polybromide ions with additional Br\({}^{-}\) ions. To equilibrate the charges, hydronium ions migrate across the membrane and recombine in the anode, effectively releasing H\({}_{2}\). Consequently, a decrease in stoichiometry on the H\({}_{2}\) supply is not expected to have a pronounced limiting influence on the cell performance or current distributions on charging operation. Figure 3 shows a comparison of performance and limiting current distribution as a function of anolyte flow rate. Figure 3a shows polarization curves (charging) at three different flow rate values. Figure 3b-d show the corresponding distributions under limiting current conditions at the end of the polarization curves. During discharge, hydrogen is oxidized with water to form hydronium ions (H\({}_{3}\)O\({}^{+}\)) at the negative electrode and the polybromides react to bromide ions on the cathode. The hydronium ions again migrate to the cathode to equilibrate the charges. Different from charging operation, a sufficient hydrogen flow is necessary to sustain the reactions. At a flow rate of V\({}_{2}\) = 170 mL min\({}^{-1}\) the polarization curve shows a nearly linear behavior, indicating that at high currents the cell is dominated by ohmic losses. The corresponding current distribution is rather homogeneous (Figure 4b). A small decrease (V\({}_{\text{H}2}\) = 110 mL min\({}^{-1}\)) barely limits the polarization curve but is already visible in the current density distribution in Figure 4c. At V\({}_{\text{H}2}\) = 65 mL min\({}^{-1}\) (equivalent to a stoichiometric flow factor of \(\lambda\cong\) 1) the polarization curve breaks in after 300 mA/cm\({}^{2}\), indicating considerable mass transport overpotential. This is clearly depicted in the limiting current distribution in Figure 4d, with the lowest local current density near the H\({}_{2}\) outlet. These large gradients indicate a significant mass transport limitation because the left side of  
Figure 2: Exploded view of the H\({}_{2}\)-Br\({}_{2}\) cell with PCB on the anode. Figure 5 shows a current density distribution under the same operating conditions as Figure 4d but with reversed H\({}_{2}\) flow (from right to left). The mapping shows that the high and low current density areas alternate, with the distribution uniformly decreasing towards the outlet. The same approach was then applied to analyze the corresponding correlation for the catholyte. Therefore, the flow rate of Br\({}_{2}\) was stepwise adjusted, whilst the anolyte flow rate was set to V\({}_{\rm{H2}}\) = 170 mL min\({}^{-1}\) (equivalent to a stoichiometric flow factor of \(\lambda\cong 2.5\) at 10 A). A characteristic contrast between electrolytes must be highlighted; while the composition of the anolyte remains constant throughout the whole battery operation range, the catholyte properties strongly depend on the state of charge. During charge, a HBr aqueous solution is fed onto the cathode half-cell and converted to Br\({}_{2}\) and polybromides, respectively, at the positive electrode. During discharge, a solution of Br\({}_{2}\) and polybromides in HBr reacts to bromide ions. The supply of Br\({}_{2}\)/polybromide solution is thus essential both during charge and discharge operation.