Battery storage systems become increasingly more important to fulfil large demands in peaks of energy consumption due to the increasing supply of intermittent renewable energy. The vanadium redox flow battery systems are attracting attention because of scalability and robustness of these systems make them highly promising. One of the Achilles heels because of its cost is the cell membrane. Exposure of the polymeric membrane to the highly oxi. Battery storage systems become increasingly more important to fulfil large demands in peaks of energy consumption due to the increasing supply of intermittent renewable energy. The vanadium redox flow battery systems are attracting attention because of scalability and robustness of these systems make them highly promising. One of the Achilles heels because of its cost is the cell membrane. Exposure of the polymeric membrane to the highly oxidative and acidic environment of the vanadium electrolyte can result in membrane deterioration. Furthermore, poor membrane selectivity towards vanadium permeability can lead to faster discharge times of the battery. These areas seek room for improvement to increase battery lifetime. The high costs of the currently used membranes substantially contribute to the price of the vanadium redox flow battery systems. Therefore, the reduction of the cost of the membrane by using alternative materials can reduce the overall battery costs substantially, thereby increasing the prospects of the industrial use of these systems. In this report different membrane types are reviewed and the important factors determining membrane performance are analysed. An overview of potential new membranes is presented which could boost the performance of these systems in future and reduce costs substantially.••Battery storage systems are emerging as one of the key solutions to effectively integrate high shares of solar and wind renewables in power systems worldwide. Solar photovoltaics produced 1.8% and wind turbines produced 4.4% of the global electricity production in 2017. The share of renewables in power generation capacity expansion reached 72% in 2019. Most of the new capacities (nearly 90%) came from solar and wind projects. Consequently, more electricity is generated from renewable energy than in the previous year. Energy Information Administration (EIA) projects that renewables will collectively increase to 49% of global electricity generation by 2050. The growing share of variable renewable energy sources (VRE, i.e. solar and wind), calls for a more flexible energy system to ensure that the VRE sources are integrated in an efficient and reliable manner to electricity grid. A wind turbine can peak at night when the demand is low, and its output may vary from GW to MW during the day depending on the wind speed. Similarly, the output of a solar PV plant could vary when clouds pass by. These intermittent gaps in power supply need to be compensated by conventional power plants, which introduce challenges to electricity grid operators.Electrical energy storage (EES) will be a key component in future grid and in a low-carbon society, enabling VRE generation to provide electricity not only for. The all Vanadium Redox Flow Battery (VRB), was developed in the 1980s by the group of Skyllas-Kazacos at the University of New South Wales,,,. The explorative work by the Skyllas-Kazacos group provided new insights for improvements to improve its long-life cycle, flexible design, fast response time, deep-discharge capability and low polluting emissions [1,5].In order to store electrical energy, vanadium species undergo chemical reactions to various oxidation states via reversible redox reactions (Eqs. (1)–(4)). The main constituent in the working medium of this battery is vanadium which is dissolved in a concentration range of 1–3 M in a 1–2 M H2SO4 solution. To avoid mixing of the charged V species separation of the cathode and anode half-cell via a membrane is essential to prevent battery self-discharge. Membranes must be permeable and conductive to enable charge transferring H+ species to move to the two half cells. The standard potential E0 at the cathode is 1.0 V whereas the negative electrode contains a standard potential of −0.26 V. The equilibrium potential is determined using the Nernst equation and depends on the concentrations of the ions present in the cell (Eq. (5)).Cathode reactions(1)VO2+(aq) + H2O(l) → VO2+(aq) + 2H+(aq) + e− (charging)(2)VO2+(aq) + 2H+(aq) + e− → VO2+(aq) + H2O(l) (discharging)3.1. Cationic exchange membranesFrom the mid-80′s large effort has put into developing cation exchange membranes (CEM) which would only transport cations. The early membranes in the late 80 s consisted of pore filled ion exchange membranes (IEMs). In these kinds of membranes, a porous support is filled with an ion-exchange resin or polyelectrolyte together with a cross linking agent. The mixture is than cured to obtain the cross-linking reaction.The early membranes prepared accordingly were sulfonated porous polyethylene and polystyrene materials. A comparison of these materials by Skyllas-Kazacos et al. in VRB showed the influence of the molecular composition of the polymer materials. The polyethylene material revealed a coulombic efficiency of 87% using a current density of 15 mA•cm−2, while the polystyrene material possessed a coulombic efficiency of 90% at a current density of 40 mA•cm−2. These high CE values were obtained due to the low levels of cross-mixing. On the other hand, the polystyrene material outperformed the polyethylene material leading to an overall energy efficiency of 81% over the 10–90% SOC range for the polyvinyl material. The large difference between charge and discharge curves leading to a poor voltage efficiency of the sulfonated polyethylene membrane are likely due to a high membrane resistivity. Valuable screening work by Grossmith et al. in the. An alternative membrane type class is the Anionic Exchange Membrane (AEM). Due to their positively charged functional groups they repulse positively charged V species (Fig. 2) from the membrane. This effect is also described as the Donnan effect. Although, the reduced V permeability of AEM is of high interest, they have drawbacks for appl.
