PXRD and PDF analysis of multifunctional lanthanide nitrilotris-methylphosphonate-based proton conductors

Metal phosphonates are multifunctional solids which possess tunable properties, such as H-bond networks, while exhibiting high chemical and thermal stability. Depending on the protonation of the ligand, different crystalline phases can be obtained. Here, we report three different families of proton conductors based on lanthanide nitrilotrismethylphosphonates. Compounds having cationic layers compensate by chloride or sulfate anions were isolated: [Ln(H4NMP)(H2O)2]Cl·2H2O and Ln(H5NMP)]·SO4·4H2O [H6NMP = nitrilotris(methylphosphonic acid)]. The crystal structure of Gd-(H5NMP)]·SO4·4H2O was solved ab initio from synchrotron powder diffraction data (λ=0.4124 Å, beamline BL04-MSPD ALBA) and refined by the Rietveld method. Chloride containing phases show two irreversible solid state transformations take place: (1) a crystalline-to-crystalline phase transition, {Ln-H4NMP → [Ln2(H3NMP)2(H2O)4]·4.5H2O for Ln= La, Pr}, and (2) crystalline-to-amorphous phase transition, {LnH4NMP → [Ln(H3NMP)]·1.5H2O for Ln= Gd - Ho}, both implies the loss of HCl and structural rearrangements of the frameworks. Variations in average and local structure have been monitored by high resolution powder diffraction and PDF analysis, upon exposure the samples at high relative humidity and temperature (95% RH and 80 ºC), in order to understand their behavior as proton conductors.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Proton conductivity of lanthanide nitrilotris-methylphosphonates

Multifunctional metal phosphonates are acidic coordination polymers (CPs) with remarkable stability and proton conducting properties owing to their structure is usually composed of extended hydrogen-bond networks that favor proton transfer pathways [1]. In this communication, three different families of proton conductors based on lanthanide nitrilotris-methylphosphonates are examined. Compounds were isolated by crystallization at room temperature at pH <0.8 in the presence of. When chloride is presented in solution two families of compounds were isolated, depending on the concentration of chloride in solution: free-chloride 1D solids with formula Ln2(H3NMP)2(H2O)4]·4.5H2O [Ln= La3+] [2] or layered chloride-containing Ln(H4NMP)(H2O)2]Cl·2H2O [Ln= La3+ - Ho3+] materials [3]. In absence of chloride, a third series of compounds was obtained. This structural versatility leads to a wide range of proton conductivity varying between 3 × 10−4 S·cm−1 and 2 × 10−3 S·cm−1 as measured at 80 °C and 95% RH.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Structural study of the local order in ammonia-modulated FE(II) hydroxyphosphonoacetate proton conductors

Layered Fe(II) carboxiphosphonate, Fe-HPAA·2H2O, is a crystalline multifunctional coordination polymer exhibiting properties as photocatalyst and proton conductor. Postsynthesis modification by ammonia/water adsorption strongly enhances its proton conductivity. However, this process entails a progressive amorphization but in no case intercalation of the guest species was detected. Understanding the mechanism involved in this increased conductivity is crucial to develop novel high performance proton conductors for PEMFCs. Thus, total scattering and PDF study has been carried out to explore the mechanism of ammonia adsorption and subsequent amorphization. Different lenght scales have been investigated to characterize the average and local structure at variable ammonia loaded in order to ascertain posible structural modifications after gas/solid reactions. While significant short range order (from 1.4 to 10 Å) variations were observed even for low loadings, the average structure seems to be basically preserved except for the highest ammonia/water contents.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Tuning Proton Conductivity Properties of Lanthanide Amino-Sulfophosphonates-Loaded Nafion Composite Membranes

Polymer-based electrolytes in proton exchange membrane fuel cells (PEMFCs) utilize acidic groups as proton carriers and hydrogen bonding networks as proton-conducting pathways to facilitate proton transport. Crystalline acid-functionalized metal phosphonates are potential proton conductors while maintaining a high hydration degree below 100 °C. This property may be combined with Nafion-like polymers which tend to dehydrate at the operating conditions of PEMFCs [1,2]. In this work, preliminary results of the preparation of lanthanide amino-sulfophosphonates-loaded Nafion composites membranes and the corresponding electrical properties are reported. Synthesis conditions of lanthanide derivatives were optimized following a hightrough-put screening at 140 °C. Their crystal structures, solved from synchrotron X-ray powder diffraction data, corresponds to layered frameworks where the acidic groups, -CPO3H or -SO3H, point toward the interlamellar region interacting by H-bond with the lattice water. The composites were prepared by mixing the metal phosphonates with Nafion solution at different loadings. The membranes were characterized by SEM, XRD and FT-IR. A study of the proton conductivity as a function of the composite membranes was carried out at 90 °C and 95% RH. Referencias [1] Y. Gao, R. Broersen, W. Hageman, N. Yan, M. C. Mittelmeijer-Hazeleger, G. Rothenberg, S. Tanase. J. Mater. Chem. A, 2015, 3, 22347–22352. [2] A. Cabeza, P. Olivera-Pastor, R. M. P. Colodrero. Tailored Organic-Inorganic Materials, Brunet, E., Colón, J.L., Clearfield, A., Eds.; John Wiley & Sons, Inc. 2015; Ch. 4, 137−191.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech