Advancing the conductivity-permselectivity tradeoff of electrodialysis ion-exchange membranes with sulfonated CNT nanocomposites

Abstract

Ion exchange membranes, IEMs, are widely applied in water and energy technologies, such as, electrodialysis for desalination and reverse electrodialysis for sustainable power generation. However, a tradeoff between conductivity and permselectivity constrains the efficiency of IEM-based technologies. The incorporation of rationally functionalized 1-dimensional nanomaterials as fillers into the polymer matrix offers opportunities to depart from this tradeoff. In this study, we develop nanocomposite cation exchange membranes by incorporating sulfonic acid-functionalized carbon nanotubes, sCNTs, in sulfonated poly(2,6-dimethyl-1,4-phenyleneoxide) polymer matrix. The fabricated nanocomposite IEMs exhibit improved conductivity while maintaining permselectivity. Intrinsic resistivity, the reciprocal of conductivity, is lowered with greater blending of sCNTs fillers, decreasing by approximately 25% with 20 w/w% incorporation of sCNTs, while permselectivity is effectively unchanged across the different degrees of sCNT incorporation (within 2% variation). Compared with pristine membranes, the conductivity-permselectivity tradeoff line of the fabricated nanocomposite membranes is advantageously advanced, thus improving overall performance. Further characterization and analysis show a percolating network of carbon nanotubes is achieved in the polymer matrix with 10 w/w% sCNTs. We posit that the improved effective ionic conductivity is attributed to the interconnected sCNT network reducing the tortuosity of the ion transport path. This study demonstrates the promise of percolating 1D nanomaterial networks to potentially advance the conductivity-permselectivity tradeoff governing conventional IEMs.