Monoammonium salts of multiprotic acids as dopants for proton-conductive hydrogel membranes: the effects of anions

Abstract

Monoammonium salts of diprotic acid (NH₄HSO₄) and triprotic acid (NH₄H₂PO₄), whose cations and anions are both potential proton carriers, were employed as dopants for proton-conductive hydrogel membranes to explore the effects of anions on thermal, mechanical, and electrochemical performance. Robust hydrogel membranes were obtained by radical copolymerization of acrylamide and ethylene glycol dimethacrylate dissolved in aqueous solutions of NH₄HSO₄ and NH₄H₂PO₄. By virtue of the protonated ammonium cation, the ionic conductivities of the hydrogels doped with NH₄HSO₄ and NH₄H₂PO₄ were superior to those doped with the corresponding inorganic acids (H₂SO₄ and H₃PO₄). The hydrogel doped with NH₄HSO₄ exhibited a higher ionic conductivity but lower mechanical strength and thermostability than that with NH₄H₂PO₄. Ionic conduction in the doped hydrogels was dominated by the vehicle mechanism and NH₄HSO₄ resulted in lower activation energy for the conduction than NH₄H₂PO₄. In addition, the fuel cell performances of the hydrogel membranes at room temperature were evaluated.