A 3D open-framework iron hydrogenophosphate showing high proton conductance under water and aqua-ammonia vapor

Herein we report the first example of the proton conductivity of an open-framework metal phosphate (NH3(CH2)3NH3)2–[Fe4(OH)3(HPO4)2(PO4)3]·4H2O under aqua-ammonia vapor. Its optimized proton conductivity is 5 × 10−2 S cm−1 at 313 K and aqua-ammonium vapor from 1 M NH3·H2O solution. That is approximately two orders of magnitude greater than the maximum value under water vapor (8.0 × 10−4 S cm−1 at 317 K and 99% RH). The proton transfer mechanism has been proposed in terms of the structural analyses, activation energy calculations, and PXRD determinations.