(28c) Commercial Perfluorosulfonic Acid Membranes for Vanadium Redox Flow Battery

Vanadium redox flow batteries (VRFB) are nowadays successfully integrated as energy storage onto grid networks in dozens of demonstrational projects. High efficiency (80% DC-DC), versatile arrangement of decoupled power (kW) to capacity (kWh), extended durability and fast demand response predetermine VRFB as a great, cheap and competitive stationary energy storage solution allowing compensation of hardly predictable non-stabilities in output power of photovoltaics or wind turbines on the power transmission level. 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 efficiency of the stack – the resistance of the typically used cation-exchange membrane counts for up to 50 % of total internal resistance of the battery stack. At 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 (IEC), thickness) 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 and iii) performance in VRFB single-cell.

Results obtained from permeation measurement showed that the permeation of V²⁺ ions contributes most to the self-discharge losses of VRFB from all vanadium cations. 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. Small- and wide-angle X-ray scattering of fully hydrated membranes revealed the effect of polymer molecular architecture on the size of hydrophilic domains. The size of hydrophilic domains is a consequence of different side-chain lengths in tested polymers.

The observed trends were also confirmed by the characterization of membranes in VRFB single-cell. It was found that optimum membrane properties depend strongly on the VRFB target application. Energy efficiency evaluation showed that membranes with lower IEC are suitable for batteries operating at lower current densities when reduction of vanadium cross-over is important. Oppositely, membranes with higher IEC are more suitable for VRFB operating at higher current densities. Membrane inner morphology must be always considered. For some VRFB applications, the suppressed operating costs (related to electrolytes rebalancing) with less permeable membrane can, in the long-term perspective, compensate higher investment costs due to lower power densities at given efficiency.