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 [H2pip][Cd(HPO3)2(H2O)]·H2O (2). The structure of 1 adopts a two-dimensional motif featuring alternate [Cd-μ2-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 H2PO3 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 [H2pip] 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.