Insight into the High Proton Conductivity of One-/Two-Dimensional Cadmium Phosphites.

The study describes the synthesis and structural attributes of two new cadmium phosphites, [Cd{OP(O)(OH)H} 2 (4,4'-bipy)] ( 1 ) and [H 2 pip][Cd(HPO 3 ) 2 (H 2 O)]·H 2 O ( 2 ). The structure of 1 adopts a two-dimensional motif featuring alternate [Cd-μ 2 -O] 2 and [Cd-O-P-O] 2 -cyclic rings, while the inorganic chains are held together by 4,4'-bipyridine. The presence of strong hydrogen bonding interactions between the appended H 2 PO 3 groups (O---O = 2.55 Å) provides a facile proton conduction pathway and results in a proton conductivity of 3.2 × 10 -3 S cm -1 at 75 °C under 77% relative humidity (RH). Compound 2 comprises an anionic framework formed by vertex-shared [Cd-O-P-O] 2 -cyclic rings, while the [H 2 pip] cations between the adjacent chains assist a well-directed O-H---O hydrogen-bonded network between coordinated water, lattice water, and phospite groups. The bulk proton conductivity value under conditions as in 1 reaches 4.3 × 10 -1 S cm -1 . For both 1 and 2 , the proton conductivity remains practically unchanged under ambient temperatures (25-35 °C), suggesting their potential in low-temperature fuel cells.