Integrating renewable energy sources such as wind and solar into the electric grid will require energy storage systems to handle their intermittent nature. One promising possibility is the vanadium redox flow battery (VRB), based on active species of vanadium dissolved in electrolyte. To date, VRBs have been limited by their use of perfluorinated-polymer ion-exchange membranes, which are expensive and have low selectivity between vanadium and other ions. Now a group from the Dalian Institute of Chemical Physics in China has demonstrated a VRB based on a much cheaper nanofiltration membrane. H. Zhang and colleagues reported their findings in a recent issue of Energy & Environmental Science (DOI: 10.1039/c1ee01117k; p. 1676).
VRBs were first proposed and demonstrated 25 years ago, and have a number of advantages for grid-scale storage, including fast response times and the ability to scale to essentially unlimited storage capacity. They function by flowing solutions of vanadium ions in a sulfuric acid electrolyte by either side of an ion-exchange membrane. The researchers hypothesized that it would be possible to eliminate the use of the expensive ion-exchange membrane by using a nanofiltration (NF) membrane, which conducts ions mechanically through nanometer-scale pores, and costs (by their estimate) roughly 1/20th as much.
To test this idea, the researchers prepared samples of polyacrylonitrile NF membranes using a phase-inversion method, varying the polymer concentration and the addition of volatile co-solvents in order to control the distribution of pore sizes. They next tested the ionic selectivity of the membranes by measuring the permeation rate of VO2+ and H+ in a 3 M H2SO4 solution across the samples into deionized water, finding that the rate for H+ exceeded that of VO2+ by factors ranging from 6.9 to 14.9, with the largest ratio occurring for the sample with the smallest membrane pore sizes. The researchers then constructed single VRB cells using the samples, finding a coulombic efficiency (CE) of 95% and energy efficiency (EE) of 76% at a current density of 80 mA/cm2 (comparable to the performance of standard VRB cells), which remained stable over 200 cycles.
These results are particularly noteworthy in light of a recent result from a team of researchers at Pacific Northwest National Laboratory in Richland, WA, who found that adding HCl to the electrolyte increased the energy density of VRBs by up to 70% and improved their operating temperature range. Taken in combination, these advances suggest that vanadium redox flow batteries may someday be the technology of choice for grid-scale energy storage.
