Development of Hierarchically Porous Ionomer Membranes for Versatile and Fast Metal Ion Conduction

Innovative design concepts can play a key role in the realization of high-performance ionomer membranes that are capable of exclusive metal ion conduction and potentially applicable in electrochemical devices including sensors, fuel cells, and high-energy batteries. Herein, we report on the development of new ionomers, based on sulfonated poly(ether ether ketone) (SPEEK), engineered to conduct a variety of ions, namely, Li⁺, Na⁺, K⁺, Zn²⁺, and Mg²⁺, when soaked with nonaqueous solvents. Application of a facile phase-inversion method results in M-SPEEK (M = Li/Na/K/Zn/Mg) membranes with a hierarchical porous network, facilitating organic solvent infusion that is necessary to promote dissociation and rapid transport of cations between anionic sulfonate groups on the polymer chains. This strategy leads to membranes with alkali ion conductivities approaching 10^-4 S cm^-1 at room temperature, and near unity cation transference numbers (t _M ≥ 0.9). Furthermore, an exceptionally high Zn-ion conductivity of 10^-2 S cm^-1 is obtained for the water-infused Zn-SPEEK membrane. In comparison, the dense membranes demonstrate 2-3 orders of magnitude lower conductivities because of insufficient solvent infusion. Preliminary electrochemical studies with solvent-infused ionomer membranes as the electrolyte look promising.