(201d) Selection of Ion-Exchange Membrane for Vanadium Redox Flow Battery

The increasing share of renewable energy sources in the total energy production emphasizes
the need for a reliable and cheap stationary energy storage in order to compensate for hardly predictable non-stabilities in output power of photovoltaics or wind turbines on the power transmission level. Vanadium redox flow battery (VRFB) appears as a promising solution for the stationary energy storage as it offers high efficiency (80% DC-DC), versatile arrangement of decoupled power (kW) to capacity (kWh), extended durability and fast demand response. However, the broader commercialization of the technology is still obstructed by relatively high investment costs.

Ion-exchange membrane is a key component of VRFB cell as it directly influences the power and the efficiency of the stack - the resistance of the typically used cation-exchange membrane counts for up to 50 % of the total internal resistance of the battery stack. In the same time, price of the membrane may exceed 25 % of the overall costs. Thus, the identification of highly conductive, low-cost membrane with minimal permeability for vanadium ions and sufficient durability in acidic electrolytes is vitally needed for VRFB developers.

In our contribution we present the results of a broad systematic study focused on the effect
of membrane properties (charge, ion-exchange capacity, thickness, method of preparation and preconditioning) on VRFB operation. The various types of commercially available ion-exchange membranes were characterized with the respect to the properties relevant for VRFB operation such as: i) through-plane ionic conductivity in the environment of VRFB cell, ii) permeability for vanadium ions of different oxidation states, iii) performance in VRFB single-cell and iv) durability in the presence of oxidizing V^Velectrolyte.

From the results obtained from permeation measurement it is evident that the effective diffusion coefficient of V³⁺ ions is significantly lower than of VO²⁺ions for all the studied membranes, which corresponds with the differences in the solvation numbers of the ions. Generally higher conductivities were found for cation-exchange membranes when compared to anion-exchange ones, which is caused by the different mobility of the transported ions (hydronium vs. sulfate ions). According to our assumptions, the conductivity and selectivity of membrane significantly depend on the concentration of ion-exchange groups in the membrane. However, these properties are also influenced by the specific inner arrangement of hydrophobic and hydrophilic domains in the material. The observed trends were also confirmed by the characterization of membranes in VRFB single-cell. Based on these results we identified optimum membranes for VRFB with the respect to the energy efficiency of charging discharging cycle and maximal power output. Perfluorinated cation-exchange membranes are the solution when high power and long durability is required. Anion-exchange membranes show lower vanadium crossover and thus appears more suitable for the application when low maintenance is required.