## 1 Introduction
The original all-vanadium redox flow battery, also known as Generation 1 (G1) VRB uses a solution of vanadium in sulphuric acid in both half cells with V\({}^{2+}\)/V\({}^{3+}\) redox couple operating in the negative half-cell and the VO\({}^{2+}\)/VO\({}_{2}\)\({}^{+}\) redox couple in the positive half-cell. During charging, V\({}^{3+}\) is reduced to V\({}^{2+}\) at the negative electrode while VO\({}^{2+}\) is oxidized to VO\({}_{2}\)\({}^{+}\) at the positive electrode. These reactions are reversed during cell discharge. The maximum vanadium ion concentration that can be employed for wide temperature range operation is typically 2 M or less [5, 6, 7, 8, 9]. This concentration is equivalent to an energy density of around 25 Wh/kg and represents the solubility limit of the V(II) and/or V(III) ions in sulphuric acid supporting electrolyte at temperatures below 5 \({}^{\circ}\)C and the stability of the V(V) ions at temperatures above 40 \({}^{\circ}\)C [10]. Furthermore, significantly lower electrolyte concentrations are needed in many geographic locations where the climate is more extreme and temperatures go below zero degrees in winter. In such climates, vanadium sulphate concentrations as low as 1 M may be needed to avoid precipitation and this leads to further reduction in overall energy capacity. Recent studies showed the use of a halide supporting electrolyte that allows the preparation of vanadium electrolyte with a concentration of up to 4 M leading to the development of Generation 2 (G2) VRB or the vanadium bromide redox flow battery (V/Br) [4]. The G2 VRB employs a vanadium bromide/chloride mixed electrolyte in both half-cells. Since the bromide/polyhalide couple has a less positive potential than the V (IV)/V (V) couple, the bromide ions will preferentially oxidize at the positive electrode during charging. The positive half-cell thus utilizes the Br\({}^{-}\)/ClBr\({}_{2}\) or Cl\({}^{-}\)/BrCl\({}_{2}\)\({}^{-}\) redox couple while the negative half-cell utilizes the same V\({}^{2+}\)/V\({}^{3+}\) redox couple reaction, similar to the G1 VRB. Since the same electrolyte are used in both half cells, the G2 V/Br shares all the benefits of the G1 VRB technology, particularly with the fact that cross contamination is eliminated, resulting in solutions having an indefinite life [3]. The additional benefit of G2 VRB is the ability to use electrolytes up to a concentration of 4 M and hence having the potential to double the energy density of the G1 VRB and thus extending its energy storage usage for mobile applications. The G2 VRB can also operate at higher temperature ranges (0-50 \({}^{\circ}\)C), thus eliminating the thermal precipitation reaction for V (V) and increasing the solubility limits for the other vanadium ions. More recently, researchers at the Pacific Northwest Laboratories demonstrated a significant increase in energy density and a stable temperature range by utilizing a mixed H\({}_{2}\)SO\({}_{\text{u}}\)/ HCl supporting electrolyte that can optimize the solubilities of each of the vanadium oxidation states, allowing up to 2.7 M Vanadium solutions to remain stable over the temperature range of 0-50 \({}^{\circ}\)C [11]. Both electrolyte improvements will enable installations in extreme climates such as Northern China, Canada and Scandinavian countries that may not be suitable for the G1 VRB electrolyte. In both G1 and G2 VRB system, the key material is the membrane as it defines the performance and economic viability of the batteries. In fact, membrane is identified as the stumbling block towards commercialization of many redox flow batteries as it can contribute up to 20% to the overall battery system cost. Finding and developing a suitable membrane to provide the right balance between performance and cost has now become a main focus for redox flow cell developers around the world.