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.

Organic template-free synthesis of an open framework silicoaluminophosphate (SAPO) with high thermal stability and high ionic conductivity

This article describes the organic-template free synthesis, structural elucidation, thermal stability and conductivity property investigation of the silicon-substituted AlPO-CJ19.

Organic-Free Synthesis of Silicoaluminophosphate Molecular Sieves

Silicoaluminophosphate (SAPO) molecular sieves are an important class of microporous materials and are useful for industrial catalysis and separations. They have been synthesized exclusively by the use of expensive and environmentally unfriendly organic structure-directing agents. Now the synthesis of SAPO molecular sieves is reported with MER, EDI, GIS, and ANA topologies under wholly inorganic conditions. Multinuclear MAS NMR analyses demonstrate the presence of Si, Al, and P atoms in their frameworks. These SAPO materials all have unusually high framework charge densities (0.25-0.46), owing to the small size of alkali metal cations used as an inorganic structure-directing agent. A continuous Si increase in the synthesis gel for MER-type SAPO molecular sieves led to the formation of framework Si(0Al) units, decreasing the number of extra-framework cations per unit cell and thus making the resulting solid useful for CO₂ adsorption.

Solvent-free synthesis and room temperature proton conductivity of new cobalt phosphite–oxalates

Two new cobalt phosphite–oxalates (1 and 2) have been synthesized under solvent-free conditions. Compound 2 possesses room temperature proton conductivity.

Hydrogen positions in single nanocrystals revealed by electron diffraction

The localization of hydrogen atoms is an essential part of crystal structure analysis, but it is difficult because of their small scattering power. We report the direct localization of hydrogen atoms in nanocrystalline materials, achieved using the recently developed approach of dynamical refinement of precession electron diffraction tomography data. We used this method to locate hydrogen atoms in both an organic (paracetamol) and an inorganic (framework cobalt aluminophosphate) material. The results demonstrate that the technique can reliably reveal fine structural details, including the positions of hydrogen atoms in single crystals with micro- to nanosized dimensions.

Computational evaluation of aluminophosphate zeotypes for CO2/N2 separation

Zeolites and structurally related materials (zeotypes) have received considerable attention as potential adsorbents for selective carbon dioxide adsorption. Within this group, zeotypes with aluminophosphate composition (AlPOs) could be an interesting alternative to the more frequently studied aluminosilicate zeolites. So far, however, only a few AlPOs have been characterised experimentally in terms of their CO₂ adsorption properties. In this study, force-field based grand-canonical Monte Carlo (GCMC) simulations were used to evaluate the potential of AlPOs for CO₂/N₂ separation, a binary mixture that constitutes a suitable model system for the removal of carbon dioxide from flue gases. A total of 51 frameworks were considered, all of which have been reported either as pure AlPOs or as heteroatom-containing AlPO derivatives. Prior to the GCMC simulations, all structures were optimised using dispersion-corrected density-functional theory calculations. The potential of these 51 systems for CO₂/N₂ separation was assessed in preliminary calculations (Henry constants and CO₂ uptake at selected pressures). On the basis of these calculations, 21 AlPOs of particular interest were selected, for which 15 : 85 CO₂/N₂ mixture adsorption isotherms were calculated up to 10 bar. For adsorption-based separations using an adsorption pressure of 1 bar (vacuum-swing adsorption), AlPOs with GIS, ATN, ATT, and SIV topologies were predicted to be most attractive, as they combine high CO₂/N₂ selectivities (75 to 140) and reasonable CO₂ working capacities (1 to 1.7 mmol g^-1). Under pressure-swing adsorption conditions, there is a tradeoff between selectivity and working capacity: while highly selective AlPOs like GIS reach only moderate working capacities, the frameworks with the highest working capacities above 2 mmol g^-1, AFY, KFI, and SAV, have lower selectivities between 25 and 35. To gain atomic-level insights into the host-guest interactions, interaction energy maps were computed for selected AlPOs. The computational assessment presented here can guide future experimental efforts in the development and optimisation of AlPO-based adsorbents for selective CO₂ adsorption.

Synthesis of new cobalt aluminophosphate framework by opening a cobalt methylphosphonate layered material

A highly stable, cobalt rich, template free Co–AlPO material was prepared by opening the structure of CoMeP under hydrothermal treatment in the presence of aluminium.

High proton conduction in a new alkali metal-templated open-framework aluminophosphate

High proton conduction has been found in a new alkali metal-templated open-framework aluminophosphate Na₆[(AlPO₄)₈(OH)₆]·8H₂O with 2D intersecting 8-ring channels.

Organotemplate-free synthesis of two open-framework metal borophosphates

Two new isostructural metal borophosphates, |K₂(H₂O)|[CoB₂P₃O₁₂(OH)] (1) and |K₂(H₂O)|[NiB₂P₃O₁₂(OH)] (2), have been hydrothermally synthesized by the “green” organotemplate-free route. Notably, the introduction of potassium ions in our synthetic system endows 1 and 2 with the K⁺ ion conductivity.

Organotemplate-free synthesis of an open-framework magnesium aluminophosphate with proton conduction properties

A facile organotemplate-free route has been developed to synthesize a new open-framework magnesium aluminophosphate with proton conduction properties.