Fabrication and Characterization of Composite Membranes for Vanadium Redox Flow Batteries

Abstract

Vanadium redox flow battery (VRFB) is a form of rechargeable battery which converts chemical energy directly to electrical energy. VRFB have been investigated for their potential utility as large energy storage systems due to their advantageous performances in terms of long cycle life, high energy efficiency, low cost, and flexible design. Catholyte and anolyte, in the negative and positive half-cells respectively, containing dissolved electro-active species flows through an electrochemical cell and generate cell potential. During operation of VRFB, ion-exchange membrane separate the catholyte and anolyte for preventing cross contamination which is induced by diffusion of different ions across from one half-cell to the other, will result in self-discharge and thus the loss of the chemical energy. Therefore, the ion-exchange is one of the most important components in the VRFB system and it should have low permeability and high ionic conductivity. A perfluorosulfonic acid (PSFA) membrane, such as Nafion (DuPont) for example, is the most commonly used membrane due to its high chemical-mechanical stability and high ionic conductivity. However, Nafion is permeable to electrolyte and expensive. The major focus is to modify the water-filled channels of the Nafion membrane by introducing non-conducting metal oxides. For example, SiO2 and TiO2 have been used to prepare Nafion-metal oxide composite membranes. The inorganic fillers, such as SiO2, act as a physical barrier for diffusing vanadium cation through the membrane channels, while allowing proton transport. In this study, composite membranes based on Nafion and nanoclay were fabricated and characterized in terms of ionic conductivity and permeability. The ionic conductivity was measured with four point prove method and the permeability was measured with UV spectroscopy. For fabrication of the composite membranes, nanoclay solution was blended with Nafion solution according to the nanoclay composition in the dry membrane (5, 3 and 1 wt.% : identified as NF-NC 5, 3 or 1 wt.%, respectively). Ionic conductivity and the permeability were decreased with the increasing of the nanoclay composition in the composite membrane. With performances of VRFBs, the energy efficiencies of VRFBs and optimum composition of the composite membrane will be discussed.