Proof of concept of an innovative barrier layer in vanadium redox flow batteries

The membrane is a key component of Vanadium Redox Flow Batteries (VRFB) because it affects cell performance and it represents a huge share of stack cost. The choice of a suitable membrane for VRFB application depends on the operating condition and results from a compromise between high proton conductivity, to reduce ohmic losses, and low vanadium permeability, to hinder the undesired transport of vanadium ions through the membrane, which results in electrolyte imbalance and in the capacity loss of the battery. Nafion® is widely used in VRFB because of its good conductivity and stability, but, since it is not ideally selective, thicker membrane are employed to mitigate the capacity loss, leading to large ohmic losses and system costs. SPEEK and SPI are alternative cation exchange membranes with reduced vanadium permeability. However, low conductivity and stability hinder their competitiveness. Poor chemical stability and low conductivity limit the application of anion exchange membrane in VRFB, while the promising amphoteric ion exchange membranes are limited by the costs related to the complex preparation. In this work, an additional selective layer, termed as barrier and described in the patent WO 2019/197917, was deposited on a commercial cation exchange membrane. The barrier is a porous component whose morphological properties, such as composition, thickness, pores size and tortuous path, are designed in order to improve the selectivity of the layer towards vanadium ions. The barrier was produced by means of Reactive Spray Deposition Technology (RSDT), a flame-based synthesis process owned by Dr. Radenka Maric’s research group at University of Connecticut. The process consists in one step during which carbon-rich particles are synthesized in the flame and a mixture of ionomer and commercial carbon black particles is sprayed directly onto the membrane. Different barrier morphologies have been designed and tested in this work. The barrier layer was deposited onto a Nafion® 212 and tested in a 25 cm2 cell during cycles of charge and discharge at fixed capacity. The open circuit potential of the battery was measured after each charge and discharge in order to estimate the state of charge (SoC) of the battery and to get insights into the self-discharge. The attached figure shows the evolution of SoC after each charge for both a VRFB with the barrier and a reference VRFB employing Nafion® 115: a significant reduction of the self-discharge is observed. Furthermore, the VRFB with barrier presented a slightly higher efficiency. During further cycles of charge and discharge within the voltage limit of 1.65 V and 1 V and a current density of 50 mA cm-2, the battery with the barrier layer presented a coulombic efficiency higher than 99%. The barrier during these cycles exhibited a good stability. In addition to electrochemical testing, TEM and SEM were performed to characterize the structure of the barrier layer. SEM showed a uniform in thickness and consistent coating from sample to sample, while TEM provided information on pore structure.