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.This research was funded by PID2019110249RB-I00 (MICIU/AEI, Spain) and PY20-00416 (Junta de Andalucia, Spain/FEDER) research projects. M.B.-G. thanks PAIDI2020 research grant (DOC_00272 Junta de Andalucia, Spain) and R.M.P.C. thanks University of Malaga under Plan Propio de Investigación for financial support

Cobalt phosphinates as precursors of cobalt phosphide electrocatalysts

https://qies22.icms.us-csic.es/The scarcity and high cost of Pt and Ru/Ir-based noble metal electrocatalysts forces to design alternative low-cost and efficient materials for sustainable energy storage and conversion technologies. Among them, phosphorus-containing coordination polymers, such as phosphinates, have emerged as potential precursors of transition-metal phosphide (TMP) electrocatalysts. The possibility of incorporating two funtionalized organic moieties into the phosphinate ligands makes metal phosphinates highly attractive precursors to obtain core-shell carbon/TMP electrocatalysts. In this research-work, we report the synthesis and crystal structure of two Co2+-phosphinates derived from the (2-carboxyethyl)(phenyl)phosphinic acid (CEPPA), Co2[(O2P(CH2CH2COO)(C6H5)(H2O)]2·2H2O (CoCEPPA-1D) and Co3[O2P(CH2CH2COO)(C6H5)]2(OH)2 (CoCEPPA-2D), synthesized by microwave-assisted method. These solids were used as precursor of cobalt phosphides (Co2P/CoP) by thermal reduction under 5%H2-Ar atmosphere at different temperatures and their electrocatalytic performances were investigated toward Oxygen Evolution Reaction (OER), Oxygen Reduction Reaction (ORR) and Hydrogen Evolution Reaction (HER). The relationship between Co/P molar ratios and/or the Co2+ coordination environment in the precursor structures and the electrocatalytic activity of the prepared cobalt phosphides will be discussed.Proyectos de investigación PID2019-110249RB-I00 (MICIU/AEI, Ministerio de España) y PY20-00416 (Junta de Andalucia, Spain/FEDER). Ayudas para Doctores PAIDI2020 (DOC_00272 Junta de Andalucia, España) y Plan Propio de Investigación de la Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Proton Conduction Properties of Metal phosphonates and Application as Fillers in PEMFCs

Metal phosphonate (MP)-based coordination polymers (CPs) are structurally versatile multifunctional compounds containing different acidic groups such as P-OH; SO3H, COOH, N+-H, etc. These features result in the formation of extended H-bond networks and confer a wide range of proton conduction properties to be used as components of membrane electrode assemblies (MEAs).In this work, the focus is placed on the proton conduction properties of coordination polymers derived from the combination of lanthanide ions with different multifunctional phosphonic ligands, such as phosphonotaurine (2-[bis(phosphonomethyl)amino]-ethanesulfonic acid) or nitrilo-tris (methylenephosphonic acid). High-throughput hydrothermal screening has been used to reach optimal synthesis conditions. Their crystal structures were solved ab initio from X-ray powder or single-crystal diffraction data. Proton conductivities of the MPs and Nafion-mixed membranes were determined by electrochemical impedance spectroscopy (EIS). We report the synthesis and structural features of several members of both families of MPs. The presence of the different functional groups in these ligands such us free sulfonic groups or the incorporating sulphate species into the framework of CPs, lead to a wide variety of metal ligand coordination modes and frameworks, which determine their proton conductivity properties, ranged between 10−3 and 10−2 S·cm−1 at 90 °C and 95% RH. Selected materials were studied as fillers for the preparation of Nafion-mixed membranes, and their responses in proton exchange membrane fuel cells (PEMFCs) were established under operating conditions. The structural role and orientation of the acid groups of the ligand contribute to tune the H-bonding networks and, hence, the proton transfer processes. Preliminary results indicate that these solids perform satisfactorily as fillers of Nafion-based membranes, showing power and current densities higher than those of the pristine Nafion in short times.Esta invetigación fue financiada por los proyectos de investigación PID2019−110249RB-I00 (MICIU/AEI, España) y PY20−00416 (Junta de Andalucía, España/FEDER). M.B.G. agradece la ayuda a los contratatos PAIDI2020 DOC_00272 Junta de Andalucía, España) y R.M.P.C.agradece a la Universidad de Málaga con el Plan Propio de Investigación por la financiación prestada

Exploiting the Multifunctionality of M2+/Imidazole−Etidronates for Proton Conductivity (Zn2+) and Electrocatalysis (Co2+, Ni2+) toward the HER, OER, and ORR

This work deals with the synthesis and characterization of one-dimensional (1D) imidazole-containing etidronates, [M2(ETID)(Im)3]·nH2O (M = Co2+ and Ni2+; n = 0, 1, 3) and Zn2(ETID)2(H2O)2](Im)2, as well as the corresponding Co2+/Ni2+ solid solutions. Depending on the water content, metal ions in the isostructural Co2+ and Ni2+ derivatives are octahedrally coordinated (n = 3) or consist of octahedral together with dimeric trigonal bipyramidal (n = 1) or square pyramidal (n = 0) environments. The imidazole molecule acts as a ligand (Co2+, Ni2+ derivatives) or charge-compensating protonated species (Zn2+ derivative). For the latter, the proton conductivity is determined to be ∼6 × 10−4 S·cm−1 at 80 °C and 95% relative humidity (RH). By pyrolyzing in 5%H2−Ar at 700−850 °C, core−shell electrocatalysts consisting of Co2+-, Ni2+-phosphides or Co2+/Ni2+-phosphide solid solution particles embedded in a N-doped carbon graphitic matrix are obtained, which exhibit improved catalytic performances compared to the non-N-doped carbon materials. Co2+ phosphides consist of CoP and Co2P in variable proportions according to the used precursor and pyrolytic conditions. However, the Ni2+ phosphide is composed of Ni2P exclusively at high temperatures. Exploration of the electrochemical activity of these metal phosphides toward the OER, ORR, and HER reactions reveals that the anhydrous Co2(ETID)(Im)3 pyrolyzed at 800 °C (CoP/Co2P = 80/20 wt %) is the most active trifunctional electrocatalyst, with good integrated capabilities as an anode for overall water splitting (cell voltage of 1.61 V) and potential application in Zn−air batteries. This solid also displays a moderate activity for the HER with an overpotential of 156 mV and a Tafel slope of 79.7 mV·dec−1 in 0.5 M H2SO4. Ni2+- and Co2+/Ni2+-phosphide solid solutions show lower electrochemical performances, which are correlated with the formation of less active crystalline phases.The work at UMA was funded by PID2019-110249RB-I00 (MICIU/AEI, Spain) and PY20-00416 (Junta de Andalucia, Spain/FEDER) research projects. A.V.-C. thanks MICIU for PRE2020-094459 student grant; M.B.-G. thanks PAIDI2020-DOC_00272 research grant (Junta de Andalucia, Spain) and R.M.P.C. thanks University of Malaga under Plan Propio de Investigación for financial support. Funding for open access charge: Universidad de Málaga/CBUA (PMCID# PMC8915163

Guest Molecule-Responsive Functional Calcium Phosphonate Frameworks for Tuned Proton Conductivity

We report the synthesis, structural characterization, and functionality of an open-framework hybrid that combines Ca2+ ions and the rigid polyfunctional ligand 5-(dihydroxyphosphoryl) isophthalic acid (PiPhtA). Ca-PiPhtA-I is obtained by slow crystallization at ambient conditions from acidic (pH≈3) aqueous solutions. It possesses a high water content (both Ca coordinated and in the lattice), and importantly, it exhibits water-filled 1D channels. At 75 °C, Ca-PiPhtA-I is partially dehydrated and exhibits a crystalline diffraction pattern that can be indexed in a monoclinic cell with parameters close to the pristine phase. Rietveld refinement was carried out for the sample heated at 75 °C, Ca-PiPhtA-II, using synchrotron powder X-ray diffraction data.All connectivity modes of the “parent” Ca-PiPhtA-I framework are retained in Ca-PiPhtA-II. Upon Ca-PiPhtA-I exposure to ammonia vapors (28% aqueous NH3) a new derivative is obtained (Ca-PiPhtA-NH3) containing 7 NH3 and 16 H2O molecules according to elemental and thermal analyses. Ca-PiPhtA-NH3 exhibits a complex X-ray diffraction pattern with peaks at 15.3 and 13.0 Å that suggest partial breaking and transformation of the parent pillared structure. Although detailed structural identification of Ca-PiPhtA-NH3 was not possible, due in part to nonequilibrium adsorption conditions and the lack of crystallinity, FT-IR spectra and DTA-TG analysis indicate profound structural changes compared to the pristine Ca-PiPhtA-I. At 98% RH and T = 24 °C, proton conductivity, σ, for Ca PiPhtA-I is 5.7 ×10−4 S·cm−1. It increases to 1.3 × 10−3 S·cm−1 upon activation by preheating the sample at 40 °C for 2 h followed by water equilibration at room temperature under controlled conditions. Ca-PiPhtA-NH3 exhibits the highest proton conductivity, 6.6 × 10−3 S·cm−1, measured at 98% RH and T = 24 °C. Ea for proton transfer in the above-mentioned frameworks range between 0.23 and 0.4 eV, typical of a Grothuss mechanism of proton conduction.Proyecto nacional MAT2010-1517

Photodegradation of Phenol over a Hybrid Organo-Inorganic Material: Iron(II) Hydroxyphosphonoacetate

Water treatment is a hot topic, and it will become much more important in the decades ahead. Advanced oxidation processes are being increasingly used for organic contaminant removal, for example using photo-Fenton reactions. Here we report the use of an organo-inorganic hybrid, Fe[HO3PCH(OH)COO]·2H2O, as Fenton photocatalyst for phenol oxidation with H2O2 under UVA radiation. Preactivation, catalyst content, and particle size parameters have been studied/optimized for increasing phenol mineralization. Upon reaction, iron species are leached from the catalyst making a homogeneous catalysis contribution to the overall phenol photo-oxidation. Under optimized conditions, the mineralization degree was slightly larger than 90% after 80 min of irradiation. Analysis by X-ray photoelectron spectroscopy revealed important chemical modifications occurring on the surface of the catalyst after activation and phenol photodegradation. The sustained slow delivery of iron species upon phenol photoreaction is advantageous as the mixed heterogeneous−homogeneous catalytic processes result in very high phenol mineralization.Proyecto nacional MAT2010-1517

Divalent Metal Vinylphosphonate Layered Materials: Compositional Variability, Structural Peculiarities, Dehydration Behavior, and Photoluminescent Properties

A family of M-VP (M = Ni, Co, Cd, Mn, Zn, Fe, Cu, Pb; VP = vinylphosphonate) and M-PVP (M = Co, Cd; PVP = phenylvinylphosphonate) materials have been synthesized by hydrothermal methods and characterized by FTIR, elemental analysis, and thermogravimetric analysis (TGA). Their structures were determined either by single crystal X-ray crystallography or from laboratory X-ray powder diffraction data. The crystal structure of some M-VP and M-PVP materials is two-dimensional (2D) layered, with the organic groups (vinyl or phenylvinyl) protruding into the interlamellar space. However, the Pb-VP and Cu-VP materials show dramatically different structural features. The porous, three-dimensional (3D) structure of Pb-VP contains the Pb center in a pentagonal pyramid. A Cu-VP variant of the common 2D layered structure shows a very peculiar structure. The structure of the material is 2D with the layers based upon three crystallographically distinct Cu atoms; an octahedrally coordinated Cu2+ atom, a square planar Cu2+ atom and a Cu+ atom. The latter has an unusual co-ordination environment as it is 3-coordinated to two oxygen atoms with the third bond across the double bond of the vinyl group. Metal-coordinated water loss was studied by TGA and thermodiffractometry. The rehydration of the anhydrous phases to give the initial phase takes place rapidly for Cd-PVP but it takes several days for Co-PVP. The M-VP materials exhibit variable dehydration-rehydration behavior, with most of them losing crystallinity during the process.Proyecto nacional MAT2010-15175 (MICINN, España