## 2 Materials and Methods  
The experimental setup consisted of a 27.25 cm\({}^{2}\) single cell H\({}_{2}\)-Br\({}_{2}\) RFB. A typical cell consisted of graphite bipolar plates, with a serpentine (flow-by) flow channel architecture on the negative half-cell and a flow through configuration on the positive side. The bipolar plates were enclosed in ECTFE (ethylene chlorotrifluoroethylene) flow frames with extended flow paths to prevent shunt currents and sealed using FKM (fluorocarbon monomer) gaskets. In order to clearly distinguish and separate the effects of stoichiometry on each half cell, the electrolyte directions are designed to flow perpendicularly with the analyte following the longer end (from left to right) and the catholyte following the shorter end (from bottom to top), as depicted in Figure 1. For the collection of localized current densities, the PCB was placed on the negative half-cell. All of the cells were assembled with F-1075-PK (Fumatech BWT GmbH, Bietigheim-Bissingen, Germany) membranes, coated in house with a Pt/C catalyst achieving a loading of 1.5 mg/cm\({}^{2}\) via spraying technique. F-1075-PK are 80 \(\upmu\)m cation exchange membranes with a 0.9 meq g\({}^{-1}\) ion exchange capacity. A triple layer carbon paper electrode 29 BC (SGL Group GmbH, Wiesbaden, Germany) --as received-- was used on the anode and carbon felt GFA5 (SGL Group GmbH, Wiesbaden, Germany) --compressed to 75% of its original thickness-- on the cathode. The cell was assembled with a clamping force of 3 \(\,\mathrm{Nm}\). Cells were operated with dry hydrogen and a 2.8 M HBr solution at a 50% State of charge (SOC). A high electrolyte volume (3 L) was used, thus the change in state of charge is considered to be negligible. The stoichiometric flow factor \(\lambda\) was calculated at the highest achieved current. Electrolyte was pumped through the cell with a syringe pump (Harvard Apparatus, Holliston, United States). Cell testing was conducted with open hydrogen loop (the outlet gas vented out to the fume hood). A ModuLab 2100 A potentiostat (Ametek GmbH, Meerbusch, Germany) was used to control cell current and record electrochemical impedance spectroscopy (EIS) measurements. The PCB (elbe electronic GmbH, Ettlingen, Germany) has a spatial resolution of 48 segments, distributed to match with the cell active area in a matrix of 4 by 12. Each segment has a dimension of 8.1 by 5.25 mm and 1 mm wide spacing between segments. A single serpentine channel 1 mm wide and 1 mm deep was engraved on the board. Although the board is placed on the negative half cell (gas phase), a certain amount of electrolyte crossover is expected. The acidic nature of this solution can lead to the degradation of the segments. For this reason, the printed circuit board was coated with a protective carbon ink consisting of graphite and phenol resin via screen printing. These materials are typically used in commercial bipolar plates, therefore chemically stable in a wide range of electrolytes. Using the adapted coating for the PCB eliminates the need for significant modifications on the setup configuration (i.e., the utilization of flow plates). Prior to assembly, the resistances of the individual traces between current collector and shunt resistors were measured via EIS using a ModuLab 2100 A potentiostat (Ametek GmbH, Meerbusch, Germany). An exploded view of the experimental setup including the PCB is shown in Figure 2.  
Figure 1: Electrolyte flow direction for anode (**left**) and cathode (**right**). The current was collected at the segment, then passed through a shunt resistor. The voltage drop data is read at this point and ultimately the current from all the segments is combined on the opposite side of the PCB. A data acquisition system NI USB-6225 (National Instruments Germany GmbH, Munchen, Germany) was used to collect data from the PCB in real time with a resolution of 15 bits. The datapoints were acquired and processed with a LabView interface program.