Properties and Applications of Metal Phosphates and Pyrophosphates as Proton Conductors

We review the progress in metal phosphate structural chemistry focused on proton conductivity properties and applications. Attention is paid to structure-property relationships, which ultimately determine the potential use of metal phosphates and derivatives in devices relying on proton conduction. The origin of their conducting properties, including both intrinsic and extrinsic conductivity, is rationalized in terms of distinctive structural features and the presence of specific proton carriers or the factors involved in the formation of extended hydrogen-bond networks. To make the exposition of this large class of proton conductor materials more comprehensive, we group/combine metal phosphates by their metal oxidation state, starting with metal (IV) phosphates and pyrophosphates, considering historical rationales and taking into account the accumulated body of knowledge of these compounds. We highlight the main characteristics of super protonic CsH2PO4, its applicability, as well as the affordance of its composite derivatives. We finish by discussing relevant structure-conducting property correlations for divalent and trivalent metal phosphates. Overall, emphasis is placed on materials exhibiting outstanding properties for applications as electrolyte components or single electrolytes in Polymer Electrolyte Membrane Fuel Cells and Intermediate Temperature Fuel Cells.

The embedding of fluorescent N-methyl-9-acridone into a topological new layered aluminophosphate SYSU-2 by one-pot synthesis

A layered aluminophosphate |C14H11NO|2[Al4(HPO4)4F4(H2O)2] (denoted as SYSU-2) with a new topology has been hydrothermally synthesized with N-methyl-9-acridone (NMA) as the organic structure-directing agent. Single-crystal X-ray diffraction analysis reveals that SYSU-2 crystallizes in a triclinic space group P1[combining macron], with the inorganic sheets stacked in an AA sequence. Hydrogen bonds are responsible for the neutral inorganic-organic layer connection. The layer structure of SYSU-2 is constructed by alternating AlO4F2 octahedra and PO4 tetrahedra. The topological analysis of SYSU-2 indicates an independent topology. The NMA layers are self-assembled with π-π interaction. SYSU-2 crystals show interesting dual-band emission fluorescence properties compared with NMA crystals. Under 406 nm UV irradiation, SYSU-2 crystals emit yellow light with two emission bands at 477 and 566 nm, while NMA crystals emit blue light with only one band at 473 nm. The differences may be derived from the difference of stacking orders and distance of NMA molecule layers between the two crystals.

Two amino acid-templated metal phosphates: surfactant-thermal synthesis, water stability, and proton conduction

Two new metal phosphates, namely, Zn(HPO₄)(C₆H₁₁NO₂) (1) and (C₅H₁₀NO₂)Ga₄(PO₄)₄F·3H₂O (2), were prepared under surfactant-thermal conditions. Single-crystal X-ray diffraction analyses reveal that compound 1 has a zigzag-chain structure decorated with homoproline and compound 2 has a three-dimensional zeolite-like structure templated by proline. Notably, compound 2 remains stable in both boiling water and aqueous solutions with a pH range of 2-12. It shows a proton conductivity of 8.89 × 10^-4 S cm^-1 at 85 °C and 95% relative humidity.

Cluster–oxalate frameworks with extra-large channels: solvent-free synthesis, chemical stability, and proton conduction

Two cluster–oxalate frameworks (denoted as SCU-62 and SCU-65) were prepared under solvent-free conditions.

High proton conductivity behavior in a 2D metal sulfite constructed from a histidine ligand

In the presence of the amino acid histidine, an inorganic–organic hybrid metal sulfite, Zn₂(SO₃)₂(C₆N₃O₂H₉)₂·H₂O (1), has been prepared under hydrothermal conditions. Single-crystal X-ray diffraction analysis reveals that 1 shows a 2D layer framework built up from a classical second building unit (S4R), and bridged histidine molecule. Notably, it is the first report of a metal sulfite in the presence of an amino acid molecule. A 1D H-bonding array can be constructed by the H-bonding interaction between histidine molecules and sulfite groups. Moreover, a new function of metal sulfite for proton conduction was investigated by alternating-current impedance analysis. The results demonstrate that compound 1 shows a high proton conductivity of approximately 10⁻³ S cm⁻¹ at 348 K and 98% relative humidity.

In the presence of the amino acid histidine, an inorganic–organic hybrid metal sulfite (compound 1), has been prepared under hydrothermal conditions. Compound 1 shows a high proton conductivity of, approximately 10⁻³ S cm⁻¹ at 348 K and 98% RH.

Water stable oxalate-based coordination polymers with in situ generated cyclic dipeptides showing high proton conductivity

Presented here are two water stable oxalate-based coordination polymers with in situ generated cyclic dipeptides, which show high proton conductivities on the order of 10⁻³ S cm⁻¹ at 85 °C under 98% relative humidity.