Vanadium redox flow batteries (VRBs) are one of the most practical candidates for large-scale energy storage. Its electrolyte as one key component can intensively influence its electrochemical performance. Recently, much significant research has been carried out to improve the properties of the electrolytes.
In the case of Vanadium redox flow batteries (VRFBs), the electrolyte solution containing different valences of vanadium in the anolyte and catholyte is separated by a membrane. Due to their independent power output and energy capacity, VRFBs are easily scalable and therefore suitable for large-scale energy storage applications.
Because protons and vanadium ions are both cations, a trade-off relation is typically observed , , , . A commercially available standard membrane material is Nafion, a perfluorosulfonic acid polymer, which shows excellent chemical stability in highly oxidative vanadium electrolytes and high proton conductivity .
Another effect that should necessarily be considered is the proton conductivity of the membrane in the presence of vanadium ions. As previously reported, the proton conductivity of membranes decreases in vanadium electrolytes.
The VRFB using an optimized phosphoric acid pre-treated PBI membrane demonstrates coulombic, voltage, and energy efficiencies of 99.7, 90.3, and 90.0%, respectively, at 40 mA cm while achieving ∼40% higher discharge capacity compared to Nafion-212.
To study the effects of phosphoric acid doping, we used three different concentrations (5, 10 and 15 M) for doping of the PBI membranes. In addition, membranes two different thicknesses (25 and 65 μm) were pre-treated with phosphoric acids to optimize conductivity and crossover in VRFBs.