Proton conductivity of multifunctional metal phosphonate frameworks

Metal phosphonates exhibit attractive characteristics for proton conductivity, such as tunable functionality, chemical and thermal stability and the existence of H-bond networks with acidic protons within their structure.1 In the present work, we examine the relationship between crystal structure and proton conductivity for several metal (mono-, di- and tri-valent) phosphonates containing rigid: (5-(dihydroxyphosphoryl)isophthalic acid, PiPhtA and 2-hydroxyphosphonoacetic acid, HPAA) or flexible: (hexa- or octamethylenediamine-N,N,N′,N′-tetrakis(methylenephosphonic acid, H8HDTMP or H8ODTMP) multifunctional ligands. The crystalline hybrid derivatives prepared show a great structural diversity, from 1D to 3D open-frameworks possessing hydrogen-bonded water molecules and phosphonic and carboxylic acid groups. The rigid 3D framework of Ca-PiPhtA, that exhibits a proton conductivity of 5.7•10-4 S/cm as synthesized, transforms into a layered compound upon exposure to ammonia vapors2 with increased proton conductivity (6.6•10-3 S/cm). The flexible frameworks of magnesium or lanthanide phosphonates, with 1D channels, present conductivities higher than 10-3 S/cm. Their activation energies fall in the range corresponding to a Grotthuss mechanism.3,4 For M(I)-HPAA solids conductivities up to 5.6•10-3 S/cm were measured. References 1. P. Ramaswamy, N.E. Wong, G.K.H. Shimizu, Chem. Soc. Rev. 43 (2014) 5913. 2. M. Bazaga-García, R.M.P. Colodrero, M. Papadaki, P. Garczarek, J. Zoń, P. Olivera-Pastor, E.R. Losilla, L. León-Reina, M.A.G. Aranda, D. Choquesillo-Lazarte, K.D. Demadis, A. Cabeza, J. Amer. Chem. Soc. 136 (2014) 5731. 3. R.M.P. Colodrero, P. Olivera-Pastor, E.R. Losilla, D. Hernández-Alonso, M.A.G. Aranda, L. Leon-Reina, J. Rius, K.D. Demadis, B. Moreau, D. Villemin, M. Palomino, F. Rey, A. Cabeza, Inorg. Chem. 51 (2012) 7689. 4. R.M.P. Colodrero, P. Olivera-Pastor, E.R. Losilla, M.A.G. Aranda, L. Leon-Reina, M. Papadaki, A.C. McKinlay, R.E. Morris, K.D. Demadis, A. Cabeza, Dalton Trans. 41 (2012) 4045.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech. Junta de Andalucía, Proyecto Excelencia FQM-1656. Ministerio de Economía y Competitividad, MAT2013-41836-

Durability and performance of CGO barriers and LSCF cathode deposited by spray-pyrolysis

Ce0.9Gd0.1O1.95 (CGO) protective layers are prepared by two different methods to prevent the reaction between the Zr0.84Y0.16O1.92 (YSZ) electrolyte and the La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) cathode. In the first method, the CGO layers are deposited by an airbrushing technique from an ink containing CGO particles without and with cobalt as sintering aids. The second strategy consists in preparing both a dense CGO barrier layer and a porous LSCF cathode by spray-pyrolysis deposition, in order to further reduce the fabrication temperature and minimize the reaction between the cell components. The samples prepared by spray-pyrolysis exhibit better performance and durability than those obtained by conventional sintering methods. The results suggest that the interfacial reactivity between YSZ and LSCF as well as the Sr-enrichment at the cathode surface can be avoided by using low-temperature fabrication methods and by operating at temperatures lower than 650 °C.Ministerio de Ciencia e Innovación, EC2014-53906-R y MAT2016-77648-

Stability and electrochemical performance of nanostructured La2CuO4þd cathodes

La2CuO4þd cathode layers are prepared by spray-pyrolysis deposition and their structural, microstructural and electrical properties are compared with those of submicrometric powders obtained from freeze-dried precursors. In order to improve the cathode performance, three different electrode architectures have been proposed: (i) powder cathodes obtained by conventional screen-printing and sintering, and cathodes deposited by spray-pyrolysis on: (ii) as-prepared electrolyte surfaces and (iii) porous electrolyte backbones. The cathode activity for the oxygen reduction reaction has been investigated as a function of the microstructure and the sintering temperature. The microstructural optimization of the cathodes and the low fabrication temperature minimize the instability problems between the electrolyte and the cathode materials, leading to polarization resistances as low as 0.14 U cm2 at 600 C.MINECO (Ministerio de Economía y Competitividad), MAT2016-77648-R y EC2014-53906-

Effect of Zn addition on the structure and electrochemical properties of codoped BaCe0.6Zr0.2Ln0.2O3-δ (Ln=Y, Gd, Yb) proton conductors

In this work, BaCe0.6Zr0.2Y0.2-xYbxO3-δ and BaCe0.6Zr0.2Gd0.2-xYbxO3-δ (x=0–0.20), proton conducting materials are prepared by the freeze-drying precursor method. The sintering conditions were optimized by adding Zn (NO3)2·6H2O as sintering additive. The materials are thoroughly characterized by different structural and microstructural techniques, including X-ray diffraction, scanning and transmission electron microscopy, and thermogravimetric-differential thermal analysis. The addition of Zn favours the phase formation and densification at lower sintering temperatures; however, it leads to the segregation of a Zn-rich secondary phase, with general formula BaLn2ZnO5 (Ln˭Y, Gd and Yb), which is identified and quantified for the first time. All samples with Zn as sintering aid exhibit cubic structure; however, the samples without Zn crystallize with orthorhombic or cubic structure, depending on the composition and thermal treatment. The electrical properties are studied by impedance spectroscopy. A deep analysis of the bulk and grain boundary contributions to the conductivity has revealed that the bulk conductivity remains almost unchanged along both series over Yb-doping; however, the grain boundary resistance decreases. The highest conductivity values are found for the intermediate members of both series, BaCe0.6Zr0.2Y0.1Yb0.1O3-δ and BaCe0.6Zr0.2Gd0.1Yb1O3-δ, with 33 and 28 mS cm−1 at 750 °C, respectively.Ministerio de Economía y Competitividad), MAT2016-77648-

Highly efficient La0.8Sr0.2MnO3-δ - Ce0.9Gd0.1O1.95 nanocomposite cathodes for solid oxide fuel cells

La0.8Sr0.2MnO3-δ-Ce0.9Gd0.1O1.95 (LSM-CGO) nanostructured cathodes are successfully prepared in a single process by a chemical spray-pyrolysis deposition method. The cathode is composed of nanometric particles of approximately 15 nm of diameter, providing high triple-phase boundary sites for the oxygen reduction reactions. A low polarization resistance of 0.046 Ω cm2 is obtained at 700 °C, which is comparable to the most efficient cobaltite-based perovskite cathodes. A NiO-YSZ anode supported fuel cell with the nanostructured cathode generates a power output of 1.4 W cm−2 at 800 °C, significantly higher than 0.75 W cm−2 for a cell with conventional LSM-CGO cathode. The results suggest that this is a promising strategy to achieve high efficiency electrodes for Solid Oxide Fuel Cells in a single preparation step, simplifying notably the fabrication process compared to traditional methods.Ministerio de Ciencia e Innovació

Relationship between the Structure and Transport Properties in the Ce1−xLaxO2−x/2 System

La-doped CeO2 materials have been widely investigated for potential applications in different high-temperature electrochemical devices, such as fuel cells and ceramic membranes for hydrogen production. However, the crystal structure is still controversial, and different models based on fluorite, pyrochlore, and/or type-C structures have been considered, depending on the lanthanum content and synthesis method used. In this work, an exhaustive structural analysis of the Ce1−xLaxO2−x/2 system (0.2 < x ≤ 0.7) is performed with different techniques. The average crystal structure, studied by conventional X-ray diffraction, could be considered to be a disordered fluorite; however, the local structure, examined by electron diffraction and Raman spectroscopy, reveals a biphasic mixture of fluorite and C-type phases. The thermal and electrical properties demonstrate that the materials with x ≥ 0.4 are oxide ion proton conductors in an oxidizing atmosphere and mixed ionic electronic conductors in a reducing atmosphere. The water uptake and proton conductivity increase gradually with the increase in La content, suggesting that the formation of the C-type phase is responsible for the proton conduction in these materials.MINECO (RTI2018-093735-B-I00 y MAT2016-77648-R

Structural Variability in Multifunctional Metal Xylenediaminetetraphosphonate Hybrids

The two cornerstones in the field of MOFs are the Secondary Building Units (SBU’s or “bricks”) and the organic linkers. The structural tunability of crystalline MOFs coupled, when possible, with high chemical and thermal robustness, makes them suitable materials to correlate structure with function [1]. Hence, the research focus has been set recently to the potential applications of some of these compounds, including phosphonate-based MOFs [2]. We report two families of isostructural divalent (Ca, Mg, Co, Zn) hybrid phosphonate MOFs based on the tetraphosphonate ligands 1,4- and 1,3-bis(aminomethyl)benzene-N,N’-bis(methylenephosphonic acid), (H2O3PCH2)2-N- CH2C6H4CH2-N(CH2PO3H2)2, (designated as p-C12H20O12N2P4•or m-C12H20O12N2P4, respectively). The use of these two functionalized ligands, otherwise chemically and structurally quite similar, represents a good example of how small stereochemical changes in the organic linker may dramatically affect structural features and dimensionality of the resulting solids and, hence, their final properties. The crystal structures of representative compounds of each family were solved ab initio from synchrotron powder diffraction data (λ=0.2998 Å, beamline ID31-ESRF) and were used as starting models for the Rietveld refinements of the remaining components. M-p-C12H20O12N2P4 (M = Mg, Co and Zn) present low dimensionality (1D) within a wide range of experimental conditions (figure 1). In contrast, solids containing the linker m-H8L, M-m-C12H20O12N2P4 (M = Mg, Ca and Zn) tend to acquire a 3D pillared open-framework for a wide range of metal ion sizes (figure 2). Two representative members, Mg-p-C12H20O12N2P4 and Zn-m-C12H20O12N2P4, were studied for characterizing their proton-conducting behavior. At 98 % RH and T = 297 K, σ are close to 9.4x10-5 Sxcm-1 for both compounds [3]. Their crystal structures, thermal stability and proton conductivity properties will be reported and discussed.Proyecto MAT2010-15175, grupo PAIDI FQM-11

In situ high pressure powder diffraction study of proton conductors based on metal phosphonates

Soft Porous Metal Organic frameworks (MOFs) are referred to as a class of coordination polymers that exhibit structural flexibility in response to guest interactions or physical stimuli [1]. By combining softness and regularity, the responsive crystalline frameworks show, for instance, unique mechanisms of separation and storage of gases. Here we report the effects of high pressures of CO2 on the frameworks of two types of coordination polymers based on multifunctional metal phosphonates, which exhibit proton conductivity at high relative humidity in addition to porous properties. The first one, Ni2(H2O)2(O3PCH2N(C4H8)NCH2PO3)⋅8H2O (Ni-STA-12) is a well-known MOF material structural featured by 1D channels build from MO5N octahedra linked by the piperazinyl moieties [2]. The second solid, Mg[(HO3PCH2)2NHCH2C6H4CH2NH-(CH2PO3H)2]·2H2O, (MgHDTMP·2H2O), is a pillared layer metal phosphonate containing flexible alkyldiaminetetraphosphonate as linker of the inorganic layers. For both solids, in situ synchrotron powder diffraction data were collected on BL04-MSPD under different pressures of CO2 (up to ~10 bar) and temperatures at ALBA (Barcelona, Spain). The resulting structural changes observed on their frameworks as well as their proton conductivities will be discussed.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech. Junta de Andalucía,Proyecto de Excelencia P12-FQM-1656 MINECO: MAT2013-41836-R