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.

Counterion Mobility in Ion-Exchange Membranes: Spatial Effect and Valency-Dependent Electrostatic Interaction

“…11 Because membrane swelling is affected by the counterion in the membrane matrix, 66 the membranes were soaked in the same electrolyte solutions as the conductivity measurement experiments (1.0 equiv/L). For each IEM, SDs for different counterions were calculated from the wet and dry masses of the membranes, 73 correcting for the contribution from sorbed counterions. Polymer densities of the dry membranes, ρ p , were characterized with a pycnometer using DI water.…”

“…Ion-exchange capacity, IEC, is defined as the number of fixed charges per unit weight of the dry membrane . The IECs of CEMs were determined using the acid titration method, , while the AEM was characterized by the ion elution method. , SD is defined as the mass of sorbed water per unit mass of the dry polymer . Because membrane swelling is affected by the counterion in the membrane matrix, the membranes were soaked in the same electrolyte solutions as the conductivity measurement experiments (1.0 equiv/L).…”

“…Ion-exchange capacity, IEC, is defined as the number of fixed charges per unit weight of the dry membrane. 11 The IECs of CEMs were determined using the acid titration method, 73,74 while the AEM was characterized by the ion elution method. 75,76 SD is defined as the mass of sorbed water per unit mass of the dry polymer.…”

“…Diffusivities of different counterions in each membrane were characterized using direct current chronopotentiometry in a four-electrode cell with an electrochemical workstation (Inter-face 1010E, Gamry, Warminster, PA). 73 Counterions investigated for the three CEMs are Na + , K + , Mg 2+ , Ca 2+ , Al 3+ , and La 3+ , whereas Cl − , Br − , NO 3 − , CO 3 2− , SO 4 2− , and PO 4 3− were examined for the AEM. Co-ions for the CEMs and AEM are Cl − and Na + , respectively.…”

Reformulating the permselectivity‐conductivity tradeoff relation in ion‐exchange membranes

“…Permselectivity-conductivity plots are commonly used to evaluate the performance of new IEMs relative to that of existing materials. [20][21][22][23][24][25][26] Permselectivity is not an intrinsic membrane property since it depends on the properties of the solution contacting the membrane. Moreover, ionic conductivity depends on the transport of both counter-ions and coions, so high ionic conductivity does not necessarily mean that the electrical current is carried entirely by the counter-ions, which is the optimal scenario for applications that use IEMs.…”