Redox active polymers for non-aqueous redox flow batteries: Validation of the size-exclusion approach

Non-aqueous redox flow batteries (NRFBs) are emerging technologies that promise higher energy densities than aqueous counterparts. Unfortunately, cell resistance and redox component crossover observed when using ion-exchange membranes (IEMs) hinders NRFB development. The size exclusion approach for polymer-based NRFBs addresses these issues by using macromolecular design to mitigate crossover. Here, we highlight the benefits of this approach using inexpensive nano-porous separators (PS) (Celgard and Daramic). We evaluated these along with an IEM (Fumasep) in a flow cell configuration using a classical redox couple of viologen and ferrocene, both in monomer and polymer forms. High Coulombic efficiencies above 98% and access up to 80% of capacity were observed for the polymer cells. These displayed better performance with PS than with the IEM, which exhibited lower energy efficiencies from higher overpotentials. The monomer equivalent cells with PS resulted in lower efficiencies and rapid decrease in depth of discharge. Post-cycling analysis by ultramicroelectrode voltammetry showed that the small molecules freely crossed PS and to a lesser degree through the IEM. Therefore, here we demonstrate that the combination of redox active polymers and simple PS enables a potential next-generation NRFB system that provides a competitive alternative to the use of IEMs in NRFBs.