Composite membrane containing phytic acid-functionalized metal-organic frameworks for enhanced ion selectivity

The application of sulfonated poly (ether ether ketone) (SPEEK) membrane in vanadium redox flow batteries (VRFBs) is hindered by the trade-off between proton conductivity and ion selectivity. To optimize its performance, the metal-organic frameworks loaded with phytic acid (PA-UiO-66-NH2) are introduced into the SPEEK matrix to construct a proton-selective transport channel. Better proton conductivity and ion selectivity are achieved in composite membranes due to the unique porous structure and chemical characteristics of PA-UiO-66-NH2 fillers. When the PA-UiO-66-NH2 content is at 2 wt%, the S/PA-UiO-66-NH2-2 membrane exhibits higher proton conductivity (35.3 mS cm−1) and ion selectivity (41.0 × 103 S min cm−3) compared to other composite membranes, pristine SPEEK and commercial Nafion 212 membranes. As a result, the S/PA-UiO-66-NH2-2 membrane shows excellent energy efficiencies (88.1%-74.0%) at current densities ranging from 60 to 180 mA cm−2. The cell efficiencies maintain stability during the 300 times charge-discharge cycle, proving the outstanding stability and durability of the S/PA-UiO-66-NH2-2 membrane in a strong acidic and oxidizing environment. These results suggest that the combination of phytic acid and metal-organic framework can effectively improve the performance of the membranes, and it also can be further utilized to solve other challenges in the membrane separation field.