## 4 Conclusions  
The printed circuit board approach for the in-situ measurement of localized current densities of the H\({}_{2}\)-Br\({}_{2}\) RFB was successfully implemented for the first time. This analytical tool was installed in the hydrogen half-cell and used to investigate the conditions limiting the performance of our in-house developed battery.  
Mass-transport limiting current densities and their corresponding current distribution mappings as a function of the flow rate were analyzed in order to define the cell operational limits.  
A strong decrease in H\({}_{2}\) flow rate during charge did not affect the maximum current density, nor did it reflect heavily on the matching current distribution graphs. Such behavior was anticipated, for hydrogen is released as a result of the reduction of protons. Discharge operation, on the other hand, showed a clear correlation to electrolyte supply. At a stoichiometric flow factor of \(\lambda\cong 1\) and upon reaching 300 mA/cm\({}^{2}\), the cell rapidly hit the cutoff voltage while a current gradient decreasing towards the H\({}_{2}\) outlet was identified. Limiting areas that can be attributed to starvation of reaction media could be observed close to the outlet, in agreement with mass transport limitation. It was demonstrated thatthe observed distribution is highly dependent on electrolyte utilization and a defined mass transport limitation was identified at low flow rates and high currents.  
An unexpected outcome was the sharply defined distributions found during the investigation of the limiting factors on the catholyte side, likely due to the enhancement created by the overlapping effects taking place on the positive half-cell.  
While the anolyte composition remains constant throughout the whole cell operation range, catholyte continually reacts from pure HBr and water to incorporate Br\({}_{2}\) (the latter being known as an insulator) during charge and in the opposite direction during discharge, giving the electrolyte a distinct dynamic nature.  
By performing the same discharge experiment on the positive half-cell with a H\({}_{2}\) overflow, mass transport limitations were detected around \(\uplambda\cong 1.5\), characterized by high current density regions around the inlet that gradually fade onto low current spots by the outlet.  
The gradient developed towards the outlet was more prominent in charging conditions. In this case, additional to reactants starvation, the electrolyte evolution must be considered. With bromide ions being converted to Br\({}_{2}\), the proton concentration falls, thus lowering the conductivity. When the bromide concentration in the electrolyte decreases whilst the bromine concentration grows, the amount of free Br\({}^{-}\) ions is no longer sufficient to form polybromides, leading to the formation of non-conductive pure Br\({}_{2}\) in the battery.  
At the high currents reached at cutoff and the low flow rates explored in the current experiments, the conversion rate is considerably high; the reacted bromine cannot be expelled from the cell fast enough. Therefore, the marked rise in overvoltage during charge can be explained as a combination of mass transport and decreasing electrolyte conductivity.  
This study demonstrates the potential of segmented current collectors as a powerful characterizing tool in the research and advancement of H\({}_{2}\)-Br\({}_{2}\) redox flow batteries and highlights a promising improvement area in the operation of such cells.  
Not applicable.  
This data can be found here: [[https://owncloud.fraunhofer.de/index.php/s/CoiwwAQL6](https://owncloud.fraunhofer.de/index.php/s/CoiwwAQL6) RsqcPT]([https://owncloud.fraunhofer.de/index.php/s/CoiwwAQL6](https://owncloud.fraunhofer.de/index.php/s/CoiwwAQL6) RsqcPT) (accessed on 1 August 2021).  
The authors wish to acknowledge the support of elbe electronic GmbH in the development of the printed circuit boards used in this work.  
The authors declare no conflict of interest.