## 1 Introduction
\[\text{Cathode}:\ \text{Br}_{2}(\text{aq})+2\text{e}^{-}+2\text{H}^{+}\ \leftrightarrows\ 2\text{HBr}\ (\text{aq}) \tag{1}\]  
\[\text{Anode}:\ \text{H}_{2}(\text{g})\ \leftrightarrows\ 2\text{H}^{+}\ (\text{aq})+2\text{e}^{-} \tag{2}\]  
\[\text{Overall}:\ \text{H}_{2}(\text{g})\ +\ \text{Br}_{2}(\text{aq})\ \leftrightarrows\ 2\text{HBr} \tag{3}\]  
H\({}_{2}\)-Br\({}_{2}\) RFBs have been reported to show limiting current densities of approximately 2.5 A/cm\({}^{2}\) and peak power densities of 1.4 W/cm\({}^{2}\) at 80 \({}^{\circ}\)C [17]. At a component level, the membrane has been found to be one of the crucial elements. For this chemistry, both proton exchange (PEM) and nanoporous (NP-PCM) membranes are applicable, the latter being a low-cost alternative to state-of-the-art Nafion membranes. The unique property of NP-PCM (its high-water permeability under hydraulic pressure) allows the excess water at the cathode to return back to the anode, which eliminates the need to humidity the hydrogen feed. However, it may cause extra electrolyte crossover [18]. Low ionic selectivity results in bromide crossover to the anode compartment, leading to self-discharge and catalyst degradation [19]. Life testing on the system has already demonstrated operational lifetimes of about 10,000 h [20]. In regular operations, the limiting conditions of cells are difficult to probe. For instance, a study investigating the cyclic performance of H\({}_{2}\)-Br\({}_{2}\) cells and the effects of operating conditions on cell efficiency indicated that a maximum in energy efficiency is met from 150 to 300 mA/cm\({}^{2}\)[21]. This limitation was mainly attributed to the large overpotentials that arise at higher current densities, even though transport losses could also have played a role. The geometrical limitations of a cell responsible for these losses are hard to resolve with conventional characterization techniques. In order to better understand these processes, advanced diagnostic techniques must be developed, particularly those helping the study of the operation performance (namely in situ characterization). One of these techniques is the measurement of localized current across the active area with segmented current collectors, often referred to as \"segmented electrodes\" in the literature [22; 23]. Current distribution measurements have been extensively studied in fuel cells [24; 25; 26] and have more recently been translated onto vanadium redox flow batteries [23; 27; 28]. This was possible due to the similarities in cell configuration, and allowed the assessment of transport phenomena within the devices. Different methods of measuring current distribution in fuel cells have been reported, these include a printed circuit board (PCB), resistors network, and Hall effect sensors. The resistor network approach consists of shunt resistors hard-wired on a segmented contact plate [25]. The voltage drop at the shunt is measured to calculate the current density. This approach has been found to induce added contact resistance between the segments and the measurement system [29]. Hall sensors operate by measuring the magnetic induction of the field surrounding an electric current [30]. In this technique, one Hall effect sensor is linked to every single electrically isolated segment of the cell. This approach can be used to measure high currents in a wide range of temperatures. The PCB technique for electrochemical characterization of fuel cells was introduced by [PERSON] et al. [31]. The concept is based on replacing one of the current collectors with a PCB. Conventional PCB manufacturing techniques provide electrically insulated paths for individual current/voltage collection embedded on a flat surface, simplifying the assembly into the cell. This work focused on the implementation of a printed circuit board (PCB) with built-in shunt resistors, given the repeatability of their fabrication method and assembly simplicity.