The proton conduction behavior of two 1D open-framework metal phosphates with similar crystal structures and different hydrogen bond networks.

Two open-framework zinc phosphates [C 3 N 2 H 12 ][Zn(HPO 4 ) 2 ] (1) and [C 6 N 4 H 22 ] 0.5 [Zn(HPO 4 ) 2 ] (2) were synthesized via hydrothermal reaction and characterized by powder X-ray diffraction, thermogravimetric analysis and scanning electron microscopy. Both compounds have a similar crystal structure and macroscopic morphology. However, the difference in equilibrium cations, in which the propylene diamine is for 1 and the triethylenetetramine is for 2, results in a significant distinction in the dense hydrogen grid. The diprotonated propylene diamine molecule in 1 is more favorable for forming a hydrogen-bond network in three dimensions than in 2, in which the twisted triethylenetetramine forms a hydrogen bond grid with the inorganic framework only in two dimensions owing to its large steric effect. This distinction further leads to a disparity in the proton conductivity of both compounds. The proton conductivity of 1 can reach 1.00 × 10 -3 S cm -1 under ambient conditions (303 K and 75% RH) and then increase to 1.11 × 10 -2 S cm -1 at 333 K and 99% RH, which is the highest value among the open-framework metal phosphate proton conductors operated in the same conduction. In contrast, the proton conductivity of 2 is four orders of magnitude smaller than 1 at 303 K and 75% RH and two orders smaller than 1 at 333 K and 99% RH.