In order to increase the chemical stability of polybenzimidazole (PBI) membrane against the highly oxidizing environment of a vanadium redox flow battery (VRFB), PBI/Nafion hybrid membrane was developed by spray coating a Nafion ionomer onto one surface of the PBI membrane. The acid–base interaction between the sulfonic acid of the Nafion and the benzimidazole of the PBI created a stable interfacial adhesion between the Nafion layer and the PBI layer. The hybrid membrane showed an area resistance of 0.269 Ω cm² and a very low vanadium permeability of 1.95 × 10⁻⁹ cm² min⁻¹. The Nafion layer protected the PBI from chemical degradation under accelerated oxidizing conditions of 1 M VO₂ ⁺/5 M H₂SO₄, and this was subsequently examined in spectroscopic analysis. In the VRFB single cell performance test, the cell with the hybrid membrane showed better energy efficiency than the Nafion cell with 92.66% at 40 mA cm⁻² and 78.1% at 100 mA cm⁻² with no delamination observed between the Nafion layer and the PBI layer after the test was completed.

Poly(p-phenylene)-based ionomers with remarkable durability and rate capability for use in vanadium redox flow batteries (VRFBs) are reported. The family of synthesized ionomers, sPBPSP-z, exhibited not only well-developed phase separation between hydrophilic domains and hydrophobic domains but also well-connected hydrophilic channels, resulting in enhanced proton conductivities and excellent dimensional stabilities. sPBPSP-8, which has an ion exchange capacity of 1.83 meq g⁻¹, showed high discharge capacity retention and superior efficiencies over 100 cycles at a current density of 50 mA cm⁻². In addition, the sPBPSP-8 ionomer exhibited stable performance at various current densities (50–180 mA cm⁻²) and retained high efficiencies at high current densities. Furthermore, this material exhibited superior chemical stability under oxidizing conditions, excellent capacity retention, and high efficiencies during long-term VRFB operation (1000 cycles). These results indicate that the sPBPSP-8 membrane is a superb material for VRFB applications.