The vanadium redox flow battery (VRFB) has emerged as a promising large-scale energy storage device for natural, renewable energy technologies such as wind and solar power. Nafion, the most widely used polymer membrane in VRFBs, exhibits excellent chemical and thermal stability as well as high proton conductivity, but suffers from high crossover of vanadium ions, which reduces energy efficiency. To combat this issue, loading Nafion membranes with SiO2 nanoparticles has been proposed as a means of mitigating vanadium ion crossover by disrupting and blocking the water/ion transporting channels.
In this study, the crossover of VO2+ ions in two series of Nafion-SiO2 hybrid membranes was measured using ultraviolet-visible (UV-vis) spectroscopy. One series of Nafion-SiO2 membranes was created by solution casting Nafion dispersions loaded with SiO2 nanoparticles, while the other series was fabricated using a previously established in situ silica sol-gel approach. For both series, the crossover of vanadium ions was lower than that observed for unannealed, neat Nafion membranes. Surprisingly, the crossover of vanadium ions was also observed to be lower in neat Nafion membranes subjected to thermal annealing. To gain further insight into the structure of these Nafion-SiO2 hybrid membranes, contrast matching small-angle neutron scattering (SANS) experiments were performed on both the solution-cast and sol-gel synthesized membranes. While the scattering profiles of the solution-cast membranes did not differ drastically from their neat Nafion analogue, drastic changes were observed in the SANS profiles of the sol-gel synthesized membranes. Through modeling the SANS data, the diameter of the nanoparticles was determined to be on the order of 10s of nanometers, which was corroborated by real-space electron microscopy imaging, indicating that these particles are much too large to solely reside inside a single ionic domain of Nafion. Results of this investigation bring into question the current hypothesis of how these nanocomposite membranes function to reduce vanadium crossover.
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