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

Monoammonium salts of diprotic acid (NH 4 HSO 4 ) and triprotic acid (NH 4 H 2 PO 4 ), 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 NH 4 HSO 4 and NH 4 H 2 PO 4 . By virtue of the protonated ammonium cation, the ionic conductivities of the hydrogels doped with NH 4 HSO 4 and NH 4 H 2 PO 4 were superior to those doped with the corresponding inorganic acids (H 2 SO 4 and H 3 PO 4 ). The hydrogel doped with NH 4 HSO 4 exhibited a higher ionic conductivity but lower mechanical strength and thermostability than that with NH 4 H 2 PO 4 . Ionic conduction in the doped hydrogels was dominated by the vehicle mechanism and NH 4 HSO 4 resulted in lower activation energy for the conduction than NH 4 H 2 PO 4 . In addition, the fuel cell performances of the hydrogel membranes at room temperature were evaluated.