Due to several advantages (independent power and energy sizing, long cycle life, fast response, and no self-discharge during storage), redox-flow batteries have been recognized as promising electrochemical energy storage devices. A crucial part of the redox flow-cell is the membrane separating the anolyte and catholyte containers (thus providing for charge carriers conductivity and preventing the cross-mixing of the redox-active components). For example, a membrane for vanadium redox-flow battery (VRB) should be stable against oxidation by VO₂⁺ cations as well as to reduction by V²⁺ cations and exhibit high proton conductivity to provide high voltage efficiency of the battery as well as low vanadium permeability to ensure high Coulombic efficiency.

Perfluorinated membranes such as Nafion® are currently the most widely used ion-exchange membranes for VRBs. However, the best-performance membranes recognized so far are generally expensive, and the crossover of vanadium species across the membrane is rather high. Therefore, if a cheaper alternative membrane material is found, the cost efficiency of the battery will be drastically improved.

In this contribution, we demonstrate the applicability of membranes based on polyacrylonitrile (PAN) in redox-flow batteries. Using VBR as a model system, we have shown that microporous PAN membranes provide the desired balance of proton conductivity and vanadium species permeability and are chemically resistant under the VBR operation conditions. The proton conductivity of PAN-based membranes is usually lower than that of Nafion® membranes, but the vanadium ions transport is significantly suppressed. This makes the overall energy efficiency of the VBR cell built using the PAN-based membrane comparable to that determined with Nafion® membranes. It is important to mention that porosity and hydrophilicity of PAN membranes can be easily tuned, enabling facile adjustment of its ion-conducting properties. We have studied the influence of the PAN membranes preparation conditions on their characteristics and elaborated the procedures of the membrane modification by introduction of hydrophilic nanoparticles and by heat treatment.

This work was financially supported by Russian Foundation for Basic Research (project no. 16-33-60185 mol_a_dk) and by Centre for Electrochemical Energy of Skolkovo Institute of Science and Technology.