An Alkali Metal Ion-Exchanged Metal-Phosphate (C2H10N2)xNa1–x[Mn2(PO4)2] with High Proton Conductivity of 10–2 S·cm–1

A two-dimensional layered inorganic–organic hybrid metal hydrogenophosphate (1) was treated with 0.1 M NaOH–ethanol solution, which resulted in a Na+-ion substitution product that exhibits excellent thermal and aqueous stability with 1, as well as much higher proton conductivity (σ = 10–2 S·cm–1) even at low temperature (283 K). This is because Na+ ions in aqueous solution make a more dense and extensive H-bonding network of water molecules, which enables protons to more easily transfer along the network.