## 1 Introduction  
The growth that renewable energy sources have experienced over the past decades has increased the need for cost effective electrical energy storage systems (ESS) that uncouple customer demand from energy generation, making possible a reliable supply to consumers at all times [1, 2]. Large scale grid-connected storage requires durability for large number of charge/discharge cycles, high energy efficiency (at least 70%), and affordable capital costs [3]. Redox flow batteries (RFB) are one of the proposed alternatives due to their particular ability to be scaled separately in terms of energy capacity and power. Redox batteries are electrochemical systems that store energy in the form of oxidized and reduced electroactive species present in flowing media. The redox-active substances are contained in electrolyte, typically stored in external tanks. Therefore, the energy capacity is determined by the solution volume and concentration of the electroactive materials, whilst the power output is determined by the cell active area and number of cells. The vanadium redox flow battery is the most studied and commercially used system to date. This system uses ions of the same element at various oxidation states in both half-cells, minimizing the changes in electrolyte concentration by diffusion through the membrane, an issue that prevailed during the early investigations in pioneer systems [4]. Despite the major advances made in this research field, the all-vanadium RFB is still far from meeting the cost goals [5]. A major cost driver associated with the energy storage capacity is the vanadium electrolyte [6]. Alternative chemistries have been investigated as plausible low-cost solutions. Amongst those, the all-iron stands out due to the ready availability of the storage materials [7]. Just as in all-vanadium RFB, the use of the same redox-active element eliminates cross-contamination problems (although current inefficiency is still to be considered). However, deposition and dissolution are slow, and the hydrogen evolution as side reaction poses additional challenges. The hydrogen-bromine redox flow battery (H\({}_{2}\)-Br\({}_{2}\) RFB) shows promise as a high-power system with considerably low electrolyte costs [8]. Reactants are earth abundant and at least three times cheaper than All-Vanadium RFB (\(\approx\)20 \(\upxi\)/kWh) [9; 10]. The gas phase H\({}_{2}\) electrode also simplifies the separation and recovery of crossover catholyte, which can be returned back to the catholyte tank [11]. Additionally, the high HBr/Br\({}_{2}\) solubility and the fact that no supporting electrolyte is required (HBr works as both reactive material and electrolyte) lead to a high energy density, ranging from 70 Wh/L at 2 M bromine concentration [9] to 200 Wh/L at 7.7 M HBr [12]. Bromine concentrations and operating temperatures are typically kept at a moderate level, due to the high Br\({}_{2}\) vapor pressure and toxic nature [13]. Nonetheless, the incorporation of bromine complexing agents has offered a pathway to making H\({}_{2}\)-Br\({}_{2}\) RFBs safer [14]. Studies on the electrode kinetics of bromine and hydrogen on platinum catalysts have shown that reaction kinetics of both H\({}_{2}\) and Br\({}_{2}\) are very fast and nearly reversible on Pt [15; 16]. During discharge, the hydrogen is oxidized to hydronium ions (H\({}_{3}\)O\({}^{+}\)) and electrons on a Pt catalyst; the hydronium ions pass through the membrane to the positive side. Here, Br\({}_{2}\) is reduced to bromide (Br\({}^{-}\)) thus producing hydrobromic acid with the electrons that pass through the external circuit. The reverse transport processes occur during charge. The electrochemical reactions and cell potentials of the H\({}_{2}\)-Br\({}_{2}\) RFB are described in Equations (1)-(3).