Anhydrous Superprotonic Conductivity in the Zirconium Acid Triphosphate ZrH5 (PO4 )3.

The development of solid-state proton conductors with high proton conductivity at low temperatures is crucial for the implementation of hydrogen-based technologies for portable and automotive applications. Here, we report on the discovery of a new crystalline metal acid triphosphate, ZrH5 (PO4 )3 (ZP3), which exhibits record-high proton conductivity of 0.5-3.1×10-2  S cm-1 in the range 25-110 °C in anhydrous conditions. This is the highest anhydrous proton conductivity ever reported in a crystalline solid proton conductor in the range 25-110 °C. Superprotonic conductivity in ZP3 is enabled by extended defective frustrated hydrogen bond chains, where the protons are dynamically disordered over two oxygen centers. The high proton conductivity and stability in anhydrous conditions make ZP3 an excellent candidate for innovative applications in fuel cells without the need for complex water management systems, and in other energy technologies requiring fast proton transfer.

Polydopamine Coated CeO2 as Radical Scavenger Filler for Aquivion Membranes with High Proton Conductivity.

CeO2 nanoparticles were coated with polydopamine (PDA) by dopamine polymerization in water dispersions of CeO2 and characterized by Infrared and Near Edge X-ray Absorption Fine Structure spectroscopy, Transmission Electron Microscopy, Thermogravimetric analysis and X-ray diffraction. The resulting materials (PDAx@CeO2, with x = PDA wt% = 10, 25, 50) were employed as fillers of composite proton exchange membranes with Aquivion 830 as ionomer, to reduce the ionomer chemical degradation due to hydroxyl and hydroperoxyl radicals. Membranes, loaded with 3 and 5 wt% PDAx@CeO2, were prepared by solution casting and characterized by conductivity measurements at 80 and 110 °C, with relative humidity ranging from 50 to 90%, by accelerated ex situ degradation tests with the Fenton reagent, as well as by in situ open circuit voltage stress tests. In comparison with bare CeO2, the PDA coated filler mitigates the conductivity drop occurring at increasing CeO2 loading especially at 110 °C and 50% relative humidity but does not alter the radical scavenger efficiency of bare CeO2 for loadings up to 4 wt%. Fluoride emission rate data arising from the composite membrane degradation are in agreement with the corresponding changes in membrane mass and conductivity.

Ionic and covalent crosslinking in chitosan-succinic acid membranes: Effect on physicochemical properties.

In this work, chitosan-succinic acid membranes were prepared by casting method and the physicochemical and mechanical properties of non-neutralized and neutralized with NaOH films were compared. Mechanical strength, flexibility, thermal stability and water-vapor permeability of chitosan membranes are significantly improved after neutralization. These improvements could be partly ascribed to the use of a dicarboxylic acid, which decreases the spacing between chitosan chains as a consequence of ionic crosslinking. Moreover, the addition of the strong base to the hydrogel promotes the formation of amide bonds, as suggested by FTIR analysis and demonstrated by acid-base titration. The favorable features of chitosan-succinic acid films as well as the possibility to easily incorporate drugs, enzymes, essential oils or other additives in the hydrogel, make such membranes suitable for many applications.

Investigating the effect of positional isomerism on the assembly of zirconium phosphonates based on tritopic linkers.

We report on the use of a novel tritopic phosphonic linker, 2,4,6-tris[3-(phosphonomethyl)phenyl]-1,3,5-triazine, for the synthesis of a layered zirconium phosphonate, named UPG-2. Comparison with the structure of the permanently porous UPG-1, based on the related linker 2,4,6-tris[4-(phosphonomethyl)phenyl]-1,3,5-triazine, reveals that positional isomerism disrupts the porous architecture in UPG-2 by preventing the formation of infinitely extended chains connected through Zr-O-P-O-Zr bonds. The presence of free, acidic P-OH groups and an extended network of hydrogen bonds makes UPG-2 a good proton conductor, reaching values as high as 5.7 × 10-4 S cm-1.

A new challenge for nanocrystalline α-zirconium phosphate: reaction with a diepoxyalkane.

New organic derivatives of α-zirconium phosphate (ZrP) were prepared by reaction of a gel of nanocrystalline ZrP with 1,2,7,8-diepoxyoctane (diepoxide), leading to the formation of P-O-C bonds. A series of compounds having composition Zr(O3POH)2-2x(O3POCH2CH(OH)(CH2)4CH(OH)CH2OPO3)x (hereafter indicated as ZrP(dep)x) were obtained by varying the diepoxide/Zr molar ratio in the range 0.25-1. The samples were characterized by elemental, thermal and X-ray powder diffraction analysis. The reaction turned out to be nearly quantitative, the x values of ZrP(dep)x being in the range 0.16 to 1.0. The interlayer distance slightly increased with increasing x, going from 12.7 to 13.2 Å. Interestingly, the materials easily intercalated alkanols at room temperature, both from liquid and vapor phases; it is noteworthy that ZrP(dep)0.30 was able to reversibly take up 42 wt% ethanol from the vapor phase, in agreement with the presence of available free space both in the interlayer and in the intercrystal region. Geometrical structural models in which the α-ZrP layers are connected by the dep chains were proposed to support the obtained results.

Mixed Membrane Matrices Based on Nafion/UiO-66/SO3H-UiO-66 Nano-MOFs: Revealing the Effect of Crystal Size, Sulfonation, and Filler Loading on the Mechanical and Conductivity Properties.

Mixed membrane matrices (MMMs) made up with Nafion and nanocrystals of zirconium metal-organic framework (MOF) UiO-66 or the analogous sulfonated SO3H-UiO-66 were prepared by varying the filler loading and the size of the crystals. The combined effects of size and loading, together with the presence of sulfonic groups covalently linked to the MOFs, were studied with regard to the conductivity and mechanical properties of the obtained composite matrices. A large screening of membranes was preliminarily made and, on the most promising samples, an accurate conductivity study at different relative humidities and temperatures was also carried out. The results showed that membranes containing large crystals (200 nm average size) in low amounts (around 2%) displayed the best results in terms of proton conductivity values, reaching values by 30% higher than those of pure Nafion, while leaving the mechanical properties substantially unchanged. On the contrary, MMMs containing MOFs of small size (20 nm average size) did not show any conductivity improvements if compared to pure Nafion membranes. The effect of MOF sulfonation was negligible at low filler loading whereas it became important at loading values around 10%. Finally, membranes with a high filler loading (up to 60 wt %) of sulfonated UiO-66 showed a slight reduction of conductivity in comparison with membranes loaded at 20% of nonsulfonated ones.

Small is beautiful: the unusual transformation of nanocrystalline layered α-zirconium phosphate into a new 3D structure.

Nanosized α-zirconium phosphate, α-ZrP, undergoes a phase transition at 120 °C, which is not observed with microcrystalline α-ZrP in the same conditions, and which leads to a new 3D phase. The new compound, with formula Zr(HPO4)2 (τ'-ZrP), consists of cubelike nanoparticles and has a tetragonal unit cell (space group P43212, a = 7.955 Å, c = 10.744 Å). The structure of τ'-ZrP is in close relationship with that of the already known τ-ZrP. Both structures are made of packed chains of eight-membered rings, composed of Zr atoms connected to bridging HPO4 groups. The main difference between the two structures concerns the different orientation of the uncoordinated P-OH groups, pointing into the channels. The in situ XRPD analysis on nanosized α-ZrP, performed at 120 °C as a function of time, provided information about the kinetics of the formation of τ'-ZrP, showing that the α-ZrP phase is directly transformed into τ'-ZrP. Moreover, τ'-ZrP is converted into α-ZrP at room temperature in the presence of water vapor. It was proved that the free phosphoric acid, which is originally present in small amounts in nanosized α-ZrP and τ'-ZrP, is necessary for the interconversion between the two phases. As a matter of fact, the removal of phosphoric acid, by washing α-ZrP and τ'-ZrP with anhydrous ethanol, inhibits the above conversion.

A layered mixed zirconium phosphate/phosphonate with exposed carboxylic and phosphonic groups: X-ray powder structure and proton conductivity properties.

A novel mixed zirconium phosphate/phosphonate based on glyphosine, of formula Zr2(PO4)H5(L)2·H2O [L = (O3PCH2)2NCH2COO], was synthesized in mild conditions. The compound has a layered structure that was solved ab initio from laboratory PXRD data. It crystallizes in the monoclinic C2/c space group with the following cell parameters: a = 29.925(3), b = 8.4225(5), c = 9.0985(4) Å, and ß = 98.474(6)°. Phosphate groups are placed inside the sheets and connect the zirconium atoms in a tetradentate fashion, while uncoordinated carboxylate and P-OH phosphonate groups are exposed on the layer surface. Due to the presence of these acidic groups, the compound showed remarkable proton conductivity properties, which were studied in a wide range of temperature and relative humidity (RH). The conductivity is strongly dependent on RH and reaches 1 × 10(-3) S cm(-1) at 140 °C and 95% RH. At this RH, the activation energy of conduction is 0.15 eV in the temperature range 80-140 °C. The similarities of this structure with related structures already reported in the literature were also discussed.

Dynamic nuclear polarisation NMR of nanosized zirconium phosphate polymer fillers.

Surface functionalisation with organic modifiers of multi-layered zirconium phosphate (ZrP) nanoparticles used as polymer fillers can be directly probed by dynamic nuclear polarisation NMR, which provides unambiguous evidence of the presence of P-O-C chemical bonds at the surface of the ZrP layers, thereby confirming successful functionalisation.

Promising aquivion composite membranes based on fluoroalkyl zirconium phosphate for fuel cell applications.

Layered zirconium phosphate (ZP) that bears fluorinated alkyl chains bonded covalently to the layers (ZPR) was used as a nanofiller in membranes based on a short-side-chain perfluorosulfonic acid (PFSA) to mechanically reinforce the PFSA hydrophobic component. Compared to the pristine PFSA, membranes with a ZPR loading up to 30 wt% show enhanced mechanical properties, and the largest improvement of elastic modulus (E) and yield stress (σY ) are observed for the 10 wt% ZPR membrane: ΔE/E up to 90% and ΔσY /σY up 70% at 70°C and 80% relative humidity (RH). In the RH range 50-95%, the in-plane conductivity of the composite membranes reaches 0.43 S cm(-1) for 10 wt% ZPR at 110°C and is on average 30% higher than the conductivity of the pristine PFSA. The 10 wt % ZPR membrane is as hydrated as the neat PFSA membrane at 50% RH but becomes progressively less hydrated with increasing RH both at 80 and 110°C. The fuel cell performance of this membrane, at 80°C and 30% RH, is better than that of the unmodified PFSA.

Water-mediated proton conduction in a robust triazolyl phosphonate metal-organic framework with hydrophilic nanochannels.

The development of water-mediated proton-conducting materials operating above 100 °C remains challenging because the extended structures of existing materials usually deteriorate at high temperatures. A new triazolyl phosphonate metal-organic framework (MOF) [La3L4(H2O)6]Cl⋅x H2O (1, L(2-) = 4-(4H-1,2,4-triazol-4-yl)phenyl phosphonate) with highly hydrophilic 1D channels was synthesized hydrothermally. Compound 1 is an example of a phosphonate MOF with large regular pores with 1.9 nm in diameter. It forms a water-stable, porous structure that can be reversibly hydrated and dehydrated. The proton-conducting properties of 1 were investigated by impedance spectroscopy. Magic-angle spinning (MAS) and pulse field gradient (PFG) NMR spectroscopies confirm the dynamic nature of the incorporated water molecules. The diffusivities, determined by PFG NMR and IR microscopy, were found to be close to that of liquid water. This porous framework accomplishes the challenges of water stability and proton conduction even at 110 °C. The conductivity in 1 is proposed to occur by the vehicle mechanism.

Layered metal(IV) phosphonates with rigid pendant groups: new synthetic approaches to nanosized zirconium phosphate phenylphosphonates.

Single phase mixed zirconium phosphate phenylphosphonates, ZrP(PP)x, were prepared by two different synthetic approaches: reaction of gels of nanosized α-zirconium phosphate in propanol with solutions of phenylphosphonic acid (H2PP), leading to the topotactic exchange of monohydrogen phosphate groups with phenylphosphonate groups, and precipitation from propanol solutions of H2PP, phosphoric acid, and zirconyl propionate. In both cases, propanol intercalated compounds were obtained. The x values of the ZrP(PP)x materials prepared by topotactic anion exchange ranged from 0.37 to 0.56 for (H2PP/Zr) molar ratios in the range 0.52-4.16 and [H2PP] = 0.1 M, while a maximum x value of 0.73 was only reached at 60 °C, with (H2PP/Zr) = 4.16 and [H2PP] = 0.31 M. Direct precipitation of ZrP(PP)x provided samples with 0.13 ≤ x ≤ 1.54, for H2PP molar fractions in the range 0.05-0.5 and (P/Zr) molar ratio = 6. At 90% relative humidity, the (H2O/Zr) molar ratio for the precipitated ZrP(PP)x powder samples increased in the range 1.3-3.0 with increasing x and resulted in being higher than that of nanosized ZrP (0.8). The analysis of the X-ray diffraction patterns of the gel and powder samples, together with the hydration data of the powder samples, suggested a structural model in which the random distribution of the phosphate and phenylphosphonate groups creates cavities which can accommodate propanol molecules in the gel samples and water molecules in the hydrated powder samples.

Synthesis, crystal structure, and proton conductivity of one-dimensional, two-dimensional, and three-dimensional zirconium phosphonates based on glyphosate and glyphosine.

The reaction of two small phosphono-amino acids based on glycine (glyphosine and glyphosate) with zirconium under mild conditions led to the attainment of three related zirconium derivatives with 1D, 2D, and 3D structures of formulas ZrF[H3(O3PCH2NHCH2COO)2] (1), Zr3H8[(O3PCH2)2NCH2COO]4·2H2O (2), and Zr[(O3PCH2)(HO3PCH2)NHCH2COOH]2·2H2O (3), respectively, whose structures were solved by X-ray powder and single-crystal diffraction data. The glyphosate derivative has 1D ribbon-type structure whereas the dimensionality of the glyphosine-derived materials (2D and 3D) can be tuned by changing the synthesis conditions. The low-dimensional compounds (1 and 2) can be directly produced in the form of nanoparticles with different size and morphology whereas the 3D compound (3) has a higher crystallinity and can be obtained as single crystals with a prismatic shape. The different structural dimensionality reflects the shape and size of the crystals and also differently affects the proton conductivity properties, measured over a wide range of temperature at 95% relative humidity. Their high thermal and chemical stability together with the small size may promote their use as fillers for polymeric electrolyte membranes for fuel cells applications.

Looking for new hybrid polymer fillers: synthesis of nanosized α-type Zr(IV) organophosphonates through an unconventional topotactic anion exchange reaction.

Gels of α-type zirconium(IV) phosphate alkylphosphonates, ZP(Cn)x, were prepared by reacting, at room temperature, propanol intercalated nanosized α-zirconium phosphate (α-ZrP) with propanol solutions of alkylphosphonic acids (H2Cn, n = number of carbon atoms in the alkyl chain = 4, 5, 6), with (H2Cn/Zr) molar ratios in the range 0.4-4.0. (31)P MAS NMR showed the presence of resonances due to the phosphate and phosphonate groups bonded to the Zr atoms mainly by three oxygen atoms, as in the α-type layer. The composition of the ZP(Cn)x materials, obtained by thermogravimetric analysis, ranges from x ≈ 0.2 to x ≈ 1.1. On the basis of the NMR data and of the analysis of the X-ray patterns of gels and powders, it is inferred that the ZP(Cn)x compounds have an α-type layered structure and that the reaction between α-ZrP and H2Cn is a topotactic anion exchange process. The evolution of the X-ray patterns during propanol deintercalation is consistent with a random distribution of the alkylphosphonate groups on the α-type layers which gives rise to porous pathways in the interlayer region. To test the possibility of using ZP(Cn)x as mechanical reinforcement of a polymer matrix, a starch membrane filled with 5 wt % ZP(C6)0.54 was prepared and characterized by stress-strain mechanical tests. Besides an excellent flexibility, this membrane exhibited a proportional increase of the Young's modulus by 230% in comparison with neat starch.

Aminoalcohol functionalized zirconium phosphate as versatile filler for starch-based composite membranes.

Microcrystalline zirconium phosphate was exfoliated by treatment with aqueous solutions of α,ω-alkylaminoalcohols and employed for the fabrication of potato starch composite membranes. Glycerol-based and glycerol-free composite membranes, containing 5 wt% of filler, were prepared from gelatinized starch and characterized for their physico-chemical properties. Despite of a partial filler reaggregation, as revealed by XRD and SEM analysis, all the composites exhibited a significant increase in the Young's modulus with respect to the glycerol-starch membrane, up to 80% and 190% for the glycerol-based and the glycerol-free composites, respectively. For both kinds of membranes the filler delays to a large extent the starch decomposition above about 300°C. A significant reduction in the water uptake of the composites was also observed with respect to the neat glycerol-based membrane, up to about 70% for the glycerol-free composites.

Survey on the phase transitions and their effect on the ion-exchange and on the proton-conduction properties of a flexible and robust Zr phosphonate coordination polymer.

The flexible zirconium tetraphosphonate coordination polymer with formula Zr(O(3)PCH(2))(2)N-C(6)H(10)-N(O(3)CH(2)P)(2)X(2-x)H(2+x)·nH(2)O (X = H, Li, Na, K, 0 < x < 1, 4 < n < 7.5) (1) possesses an open framework structure with 1D cavities decorated with polar and acids P═O and P-OH groups. 1 has been fully protonated by adding HCl and then subjected to several acid-base ion-exchange reactions with alkaline metals hydroxides. 1 is a very robust coordination polymer because it can be regenerated in H- form using strong acid solutions and ri-exchanged several times without hydrolysis and loss of crystallinity. The flexibility of 1 has been also studied by means of TDXD (temperature dependent X-ray diffraction) evidencing remarkable phase transformations that lead to a different disposition of the water molecules. These transformations also influence the accessibility of the cations on the P-OH groups placed inside the channels and thus the ion-exchange properties. The dependence of the proton conductivity properties on these phase transitions has been also investigated and discussed.

Advances in the chemistry of nanosized zirconium phosphates: a new mild and quick route to the synthesis of nanocrystals.

Simple addition of zirconyl propionate to phosphoric acid in alcoholic media surprisingly led to the formation, in few minutes, of transparent gels containing solvent intercalated zirconium phosphate (ZrP) nanoparticles with hexagonal shape and a planar size of about 40 nm. With the help of elemental analysis, inductively coupled plasma-optical emission spectrometry (ICP-OES), and (31)P magic angle spinning (MAS) NMR, the nanoparticle composition was formulated as Zr(R)(w)(HPO(4))(x)(H(2)PO(4))(y), in which R can be an hydroxyl or a propionate group. The stoichiometric coefficients for propanol intercalated ZrP are x = 1.43, y = 0.83, and w = 0.32. Solvent elimination at 60 °C gave rise to an increase in the x value and a decrease in the y and w values. X-ray powder diffraction analysis and transmission electron microscopy (TEM) observations showed a concomitant increase in the particle size: planar size and thickness ranged from 90 to 200 nm and from 20 to 85 nm, respectively, depending on the nature of the solvent. A possible mechanism explaining the change in the x, y, and w values, the growth of nanoparticles, and the role of the solvent is proposed. Finally, the possibility of using these gels to disperse the ZrP nanoparticles within the polymer matrix of Nafion117 is shown.

High yield precipitation of crystalline α-zirconium phosphate from oxalic acid solutions.

Microcrystalline zirconium phosphate (ZrP) has been synthesized by precipitating a Zr(IV) salt (i.e., zirconium propionate, chloride, or oxide chloride) with H(3)PO(4) from aqueous solutions of oxalic acid (H(2)C(2)O(4)) at 80 °C. Independent of the Zr(IV) salt, crystalline materials have been obtained with reaction yields >90% and reaction time of one day for the following molar ratios: H(3)PO(4)/Zr = 6 and H(2)C(2)O(4)/Zr = 10. The material prepared from Zr propionate (ZrP(prop)) has been further investigated by scanning electron microscopy, thermogravimetry, and ion exchange titrations. Structural characterization has been performed by X-ray powder diffraction and solid state (1)H-(31)P 2D correlation NMR experiments. Structural parameters obtained by Rietveld analysis of powder diffraction data agree with those reported in the literature for single crystal determinations. Moreover, NMR data show that the closest proton environment of the phosphorus atom in ZrP(prop) is the same as in ZrP samples of similar crystallinity prepared according to literature methods.

Organically modified zirconium phosphate by reaction with 1,2-epoxydodecane as host material for polymer intercalation: synthesis and physicochemical characterization.

Organically modified alpha-layered zirconium phosphate samples (ZrP(C(12))(x)) containing dodecyl groups bonded to the alpha-layers through P-O-C bonds have been prepared by reaction of 1,2-epoxydodecane solutions in tetrahydrofuran (THF) with gels of partially exfoliated zirconium phosphate in THF. Two dimensional correlation solid state NMR experiments for (1)H-(13)C and (1)H-(31)P nuclei have been used to prove the formation of P-O-C bonds arising from nucleophilic attack of POH mainly to carbon 1 and, to a lesser extent, to carbon 2 of epoxydodecane. ZrP(C(12))(x) samples with x in the range from approximately 0.5 to approximately 2.0 are thermally stable up to at least 200 degrees C, and their interlayer distance increases continuously with x from approximately 20 to approximately 35 A. On the basis of structural considerations, it has been suggested that samples with low x values could intercalate aliphatic polymers. Accordingly, preliminary results have shown that molten polyethylene is intercalated in ZrP(C(12))(0.49) and ZrP(C(12))(0.73). These materials can therefore be regarded as filler candidates for polymer matrixes.

Preparation of nano-structured polymeric proton conducting membranes for use in fuel cells.

We briefly discuss the state of the art of polymer electrolyte membrane fuel cells and suggest that the main obstacles to the commercial development of these fuel cells are essentially the high costs and poor characteristics of present proton conducting membranes. A strategy for the preparation of improved nanocomposite membranes based on the introduction of proton conducting lamell? in the polymeric matrix of present ionomeric membranes is then discussed. Due to their high proton conductivity (in some cases even higher than 10(-1) S cm(-1)), tailor made lamellae obtained by exfoliation of superacid metal (IV) phosphonates are particularly suitable for the preparation of these hybrid membranes. The expected positive influence of the dispersed lamellae on important properties of proton conducting membranes, such as swelling, mechanical resistance, proton transport, and diffusion of methanol, are also discussed. The methods used to obtain good lamellar dispersions into ionomeric polymers and the preparation and main characteristics of some hybrid membranes are also briefly described. The presence of nanoparticles of metal phosphonates in the electrodic interfaces Nafion/Pt already considerably improves the electrochemical characteristics of fuel cells in the temperature range 80-130 degrees C. The increased working temperature of the fuel cell considerably reduces CO poisoning of the platinum electrodes and allows better control of the cooling system, thus overcoming important obstacles to the development of medium temperature PEM fuel cells.