Deciphering the role of 2D graphene oxide nanofillers in polymer membranes for vanadium redox flow batteries

Abstract

The Vanadium Redox Flow Battery (VRFB) offers improved capacity and increased safety, making it a prospective route for large-scale energy storage. Nonetheless, the limitations of ion-exchange membranes (IEMs) are a major hindrance to VRFB's widespread application. Modifying IEMs with nanofillers has emerged as a potential solution, with an emphasis on their compatibility with the polymer matrix in membranes to facilitate proton transport via both vehicle (diffusion) and Grotthuss (hopping) mechanisms while restricting vanadium ion migration. The sheet-like structure of GO, distinguished by its nanoscale thickness, has a multitude of oxygen-centric functional groups, which enhance the functions of IEMs. Notably, while GO has been extensively studied, there has been limited assessment of GO-integrated hybrid membranes for VRFB. This comprehensive article explains the various GO-polymer membrane modifications, which range from cation and anion exchange to amphoteric and zwitterion membranes. We have also explored the role of GO across many categories: (i) surface-based coating of GO and the GO framework; altering the pore size of GO effectively controls the migration of hydrated multivalent vanadium metal ions via the size exclusion effect and does not affect proton transportation. (ii) GO-infused nanocomposite IEMs with hydrogen bonds between GO and polymers improve mechanical resilience while limiting vanadium ion migration via tortuosity and sieving processes. (iii) Sulfonated GO increases the SO₃H group concentration in the IEM by including sGO, facilitating the formation of more interconnected channels for higher proton conductivity, and layer-structured sGO influences the vanadium ion permeation by decreasing free volume and prolonging/bending the diffusion path. (iv) Amine-functionalized GO–acid–base pairs occur between the sulfonation (–SO₃H) functional group in polymer- and amine-containing GO by electrostatic interaction, which helps to generate well-interconnected proton transport channels, and vanadium permeability controlled by the size exclusion effect of the narrow transport channels and Donnan exclusion effect. (v) Amphoteric and zwitterion GO-modification of the internal structure of the membranes limit the mobility of the polymer chain, thus controlling water uptake and enhancing its stability. We have also highlighted potential research avenues and roadblocks in the effort to optimize GO-reinforced IEMs for VRFB applications.