Functionalized graphene nanofiber-based low-cost composite membrane for vanadium redox flow battery applications

Abstract

Nafion has gained widespread recognition as the predominant membrane due to its good proton conductivity, robust chemical resistance, and commendable mechanical stability. However, due to its well-developed water channels, it has poor barrier properties toward vanadium ions. Herein, to reduce vanadium ions permeability across the membranes without compromising the proton conductivity, graphene nanofiber (Herringbone type, GNF-H) as a filler has been incorporated into the Nafion matrix to fabricate the composite membrane. The membranes were subjected to physiochemical characterization, vanadium ion permeability, electrochemical impedance spectroscopy, and galvanostatic charge-discharge at different current densities. Vanadium permeability has significantly reduced in the 0.75% and 1% GNF-H composite membranes. Composite membranes (0.5%, 0.75%, and 1% GNF-H) showed a capacity of ~18.2, ~18.9, and ~16.8 Ah L⁻¹ at 100 mA cm⁻², respectively, whereas Nafion^TM 117 exhibited only ~16.3 Ah L⁻¹ capacity at the same current density. The peak power of the cells consisted of 0.5, 0.75, and 1% GNF-H composite membrane and Nafion^TM 117 is ~538, ~507, ~465 and 388 mW cm⁻², respectively. The present study concludes that applying Nafion/GNF-H in the VRFB system can be a promising strategy to reduce the vanadium ion permeation, cost-cutting and improve the VRFB performance.

GNF-H serves as a physical barrier to vanadium ion movement within the Nafion matrix, potentially lengthening the transport path of vanadium ions through the membrane. This reduces crossover and enhances membrane selectivity while not impeding proton transport, thereby enhancing the performance of VRFB.