Superprotonic Conductivity Research Articles

Rotating-frame nuclear magnetic resonance study of the superprotonic conduction in LiH2PO4

The proton and phosphorus dynamics of the superprotonic conduction in the LiH2PO4 system has been studied by means of 1H and 31P NMR rotating-frame nuclear magnetic resonance (NMR). The temperature-dependent 1H NMR rotating-frame spin–lattice relaxation time measurements revealed in a sensitive manner the evolution of the microscopic environment and proton dynamics of the two distinct hydrogen-bond types in the system, which is quite compatible with the electrical conductivity and the impedance spectroscopy measurements. Besides, the proton dynamics was found to be coupled to the reorientational motion of the PO4 tetrahedra.

Superprotonic conductivity of (NH4)3H(SO4)2 in the high-temperature phase

Abstract Impedance measurements were performed on single crystals of (NH4)3H(SO4)2 in the temperature range of 295–441 K. Anisotropy of protonic conductivity was found in the monoclinic room-temperature phase (space group C2/c) as well as in the rhombohedral high-temperature phase (space group R 3 ¯ m ). Above the high-temperature phase transition at 413 K, conductivity in the hexagonal (001) plane yields a value of ~ 10− 2 S cm− 1, which is typical for superionic conductors. A verification of a sample quality after the impedance measurements, using X-ray powder diffraction, showed minor additional peaks of (NH4)2(SO4). This suggests a beginning of thermal decomposition at high temperature. The possible pathways of proton conduction were discussed according to crystal structure analyses by single-crystal neutron diffraction method.

Superprotonic conductivity in Cs5(HSO4)2(H2PO4)3 single crystal

The conductivity of cesium hydrosulfate-phosphate single crystals obtained for the first time has been studied. It has been shown that these single crystals undergo a phase transition to a state with superprotonic conductivity. It has been found that the state with high proton conductivity is retained during thermal cycling for a long time and has a quasi-reversible nature.

Impedance spectroscopy of the superprotonic conduction in LiH2PO4

Superprotonic conduction in the LiH2PO4 (LDP) system has been studied by means of impedance spectroscopy. A dielectric dispersion associated with room-temperature superionic conduction is manifested, as the temperature-dependent dielectric constant displays a broad peak shifting toward the high temperatures with increasing measuring frequency. In particular, our impedance spectroscopic analysis in distinct temperature ranges has revealed the evolution of the microscopic proton dynamics and inhomogeneous electric environments associated with the dynamics of the two distinct hydrogen-bond types in LDP.

Rule of superprotonic phase transition in CsxRb1 − xH2PO4

We have investigated the relation between the superprotonic phase transition and elastic property in CsH2PO4 (CDP) type compounds. It was found that, in CsxRb1−xH2PO4 (x=0–1), the transition temperature Tc decreases in proportion to the square of spontaneous strain component d. From this result, it is indicated that the spontaneous strain plays an important role in the appearance of superprotonic conductivity in CDP type compounds. In addition, by thermodynamic analyses, proton transfer in superprotonic phase is induced by the release of the strain energy. These results indicate that the superprotonic transition temperature in CDP type compounds is determined by the competition between the kinetic energy of proton, the increase of entropy and the released-strain energy.

Nuclear magnetic resonance study of the superprotonic conduction in LiH2PO4

Superprotonic conduction in the LiH2PO4 system has been studied by means of high-resolution nuclear magnetic resonance measurements, which enabled us to distinguish dynamics of the two different hydrogen bonds in the structure. The protonic motion, primarily associated with the longer hydrogen bond, rather than the Li ionic motion, was revealed to dictate the extraordinarily high electrical conductivity of the system.

Anhydrous proton conductors for use as solid electrolytes

Anhydrous proton conductors in which proton transport proceeds via structure diffusion are interesting alternatives for membrane materials in hydrogen/air fuel cells. Understanding the mechanism of proton diffusion in candidate materials is necessary to improve their performance. We present the results of our studies of two classes of proton conductors in which conductivity does not rely on the presence of water molecules in their structure: crystalline hydrogen sulphates and selenates and new salts of nitrogen-containing heterocyclic molecules combined with a series of dicarboxylic acids. In the former group, superprotonic conductivity results from a high molecular dynamics of S/SeO4 groups, which creates an excess of structurally equivalent positions for protons. Ferroelastic properties of the crystals determine their physical behaviour at high temperatures. In particular, ferroelasticity makes a smooth and reversible character of superionic phase transition possible. In the second group of materials, the heterocycles exhibit extensive specific interactions that result in a fluctuating network of H-bonds. Thus, the proton conduction in these materials depends on the number of protons, which can be involved in the diffusion and on the fluctuation rate of the hydrogen bond network.

Dynamics of Caged Hydronium Ions and Super-protonic Conduction in (H3O)SbTeO6

Abstract The defect pyrochlores have become the focus of attention for the development and optimization of novel solid-state proton conductors as alternatives to costly polymer-based materials. A promising candidate is (H3O)SbTeO6, a super-proton conductor characterized by an outstanding conductivity of 10–1 S cm–1 at ambient conditions. The crystal structure of this compound has been known for some time yet, to date, the physical mechanism underpinning proton conduction has remained elusive. Structurally, (H3O)SbTeO6 consists of corner-shared (Sb/Te)O6 octahedra forming a cubic framework of three-dimensional interconnected cages partially occupied with hydronium H3O+ ions. These ions are connected to the inorganic framework by weak hydrogen bonds. In this work, we present the first characterization of this compound by means of quasielastic and inelastic neutron spectroscopy over a wide temperature range (T = 1.5–500 K), as well as by heat capacity measurements. Our results show that the onset of low-frequency stochastic motions of the hydronium moiety takes place around room temperature and persists up to the highest temperatures investigated. Unlike other inorganic proton conductors, these motions are effectively decoupled from vibrational motions at higher frequencies, as evidenced by the temperature dependence of the vibrational density of states. These findings have important implications for the exploitation of the defect pyrochlores as proton-conducting media for fuel-cell and gas-sensing applications.

Long Range Proton Diffusive Motion of CsHSO4and CsHSeO4: High Energy Resolution Quasielastic Neutron Scattering of Superprotonic Conductors

In order to clarify the mechanism of superprotonic conductivity (SPC) of Phase I for CsHSO 4 and CsHSeO 4 , we have performed quasielastic neutron scattering measurements using high resolution back scattering spectrometer at NIST. As a result, we have found a new diffusive motion of proton with the diffusion length λ H = 6.8 Å, which corresponds to the distance of the seventh nearest neighboring proton in the lattice. SPC is not only caused by the short-range self-diffusion detected by the past neutron experiments but also caused by the long range self-diffusion in the lattice.

Investigations of Proton Conduction in the Monoclinic Phase of RbH2PO4 Using Multinuclear Solid-State NMR

To understand the nature of the high-temperature phase transition and the origin of proton conductivity in rubidium dihydrogen phosphate RbH2PO4, (RDP), we utilized variable-temperature 1H, 31P, and 87Rb MAS NMR at 11.7 and 21.1 T. The variable-temperature 1H MAS NMR spectra do not show water peaks in the temperature range of 300−400 K, which argues against a partial polymerization and/or decomposition. The 1H DQ MAS NMR spectra acquired at high field (21.1 T) reveal three different proton sites, and dipolar recoupling experiments give estimates of distances. This evidence supports the formation of a superstructure (doubling of the unit cell along the a-axis) in the high-temperature monoclinic phase. The variable-temperature proton spin−lattice relaxation (T1) reveals significant proton dynamics above 340 K, coinciding with the temperature at which an increase of bulk proton conductivity is reported in monoclinic RDP, but well below the macroscopic phase transition. These precursor dynamics foreshadow the onset of superprotonic conductivity. From variable- temperature 31P and 87Rb MAS NMR spectra, it is deduced that significant proton dynamics above 340 K are due to the proton transfer between hydrogen bonds accompanied by reorientation of PO4 tetrahedra.

Interrelationship between superprotonic conductivity and strain in CsHXO 4

In order to clarify the mechanism of superprotonic conductivity (SPC) for the hydrogen-bonded compound CsHXO 4 ( X = S , Se), we have investigated the elastic properties of CsHXO 4 with various transition temperatures ( T c ), at which CsHXO 4 exhibits a structural phase transition from a low-temperature monoclinic phase with low protonic conductivity (phase II) to a high-temperature SPC phase (phase I). It was found that, in CsHS x Se 1 − x O 4 ( x = 0 – 1 ), the transition temperature T c decreases in proportion to the square of spontaneous strain b in the a II – b II plane of phase II. This result indicates that the spontaneous strain in the a II – b II plane plays an important role in the appearance of SPC in CsHXO 4. Moreover, thermodynamic analyses suggest that proton migration in phase I is induced by the release of the strain energy. From these results, it is deduced that the phase transition temperature in CsHXO 4 is determined by the competition between the kinetic energy of proton and the released-strain energy.

Rational Designs for Highly Proton-Conductive Metal−Organic Frameworks

A novel metal-organic framework (MOF), (NH(4))(2)(adp)[Zn(2)(ox)(3)] x 3 H(2)O (1) was synthesized and its structure was determined. We propose three types of rational design to introduce proton carriers into MOFs. The simplest method is to introduce them directly as counterions such as NH(4)(+), H(3)O(+), and HSO(4)(-) into the pores of frameworks (type I). The second is to put acid groups on frameworks, the protons being provided from them (type II). The third is to incorporate acidic molecules into voids (type III). 1 demonstrated a combination of two of the concepts by introducing NH(4)(+) ions using the anionic framework (type I) and putting carboxyl end groups of adipic acid in a honeycomb-shaped void (type III). 1 showed a superprotonic conductivity of 10(-2) S cm(-1) at ambient temperature, comparable to organic polymers such as Nafion, which is in practical use in fuel cells. This is the first example of an MOF to exhibit a superprotonic conductivity of 10(-2) S cm(-1) at ambient temperature.

Ferroelastic Domain Structure in some Superprotonic Conductors

The paper reviews the results of our studies concerning the ferroelastic domain structure in the crystals exhibiting superprotonic conductivity. We would like to show the ferroelastic domain structure and its temperature behaviour in solid acids which belong to the family of crystals with a general formula M x Li y H z (XO 4 )(x + y + z)/2 (M = Rb,K,Cs,NH 4 ; X = S and Se). For the majority of crystals, which belong to that family, the transition to the phase with high proton conductivity is simultaneously the ferroelastic-paraelastic one. However, this does not apply to all crystals of the family. Therefore, a question arises about the role of the phenomenon of ferroelasticity in the superprotonic phase transition. Depending on x,y and z, there are different groups of crystals. Our paper presents the results of temperature evolution of ferroelastic domain pattern at ferroelastic-superprotonic phase transitions as well as at other structural phase transitions, which appear in different crystal groups.

Superprotonic conductivity in MH(PO 3H) (M = Li +, Na +, K +, Rb +, Cs +, NH 4+)

Electrical conductivity studies have been performed of solid acid phosphites MH(PO 3H) with M = Li +, Na +, K +, Rb +, Cs +, NH 4 +. Superprotonic conductivity following a phase transition in the temperature range from 120 to 190 °C is observed for the monoclinic forms, including Na +, K +, Rb +, Cs + and NH 4 + derivatives. At temperatures beyond the superprotonic transition temperature, thermal decomposition occurs in the case of the Na +, Rb +, and NH 4 + salts. No superprotonic phase transition is observed for orthorhombic LiH(PO 3H). The magnitude of the superprotonic conductivity of KH(PO 3H) and CsH(PO 3H), which is known to be stable under hydrogen atmospheres from literature, amounts to 4.2 · 10 − 3 Ω − 1 cm − 1 (at 140 °C) and 3 · 10 − 3 Ω − 1 cm − 1 (at 160 °C), respectively, which values are comparable to that of well known CsHSO 4.

Quasi-reversible solid-phase reaction in the superprotonic conductor Cs5H3(SO4)4·xH2O

The thermal effect and heat capacity in a single crystal of nonstoichiometric cesium sulfate hydrate were measured. It was established that superprotonic conduction in this material is caused by an isothermal solid-phase transformation in the bulk of the single crystal. The results of thermal measurements were compared with experimental data on the electrical conductivity.

Superprotonic Conductors of Gypsums Containing H[sub 3]PO[sub 4] for Fuel Cells below 200°C

Powders of hemihydrate gypsum (CaSO 4 .0.5H 2 O) are set and hardened at room temperature by mixing with a phosphoric acid solution to be formed into the dihydrate gypsum (CaSO 4 .2H 2 O) membrane. These gypsum-derived membranes exhibit superprotonic conductivities of approximately 10 -2 S cm -1 in the entire temperature range from -10 to 200°C. A H 2 /air configuration cell using 0.4 mm thick electrolyte membranes is operated in nonhumidified conditions at 20°C (max. power density ∼44 mW cm -2 ) to 80°C (max. power density ∼65 mW cm -2 ).

Hybrid membranes for fuel cells based on nanometer YSZ and polyacrylonitrile matrix

A great deal of effort is required to design polymer electrolyte fuel cells (PEFCs) using new polymers and hybrid organic/inorganic compounds that can work at higher temperatures. These materials must have lasting thermal stability as well as improved ionic conductivity. Operation at elevated temperatures is desirable for PEFCs systems since they draw high power density in fast electrode kinetics and also for improvement of CO-tolerance, etc. In the higher range of temperatures (150–200 °C), new materials such as solid acids with phase transition to superprotonic conduction or strong solid acids supported on metal oxide systems seem to be an appropriate alternative to polybenzimidazoles (PBI) and other composite membranes. In this context, we evaluate a new hybrid matrix resistant at high temperatures, made of pyridine polymer obtained from polyacrylonitrile (PAN) and nanometric oxides (e.g., zirconium(IV) oxide-yttria stabilized, YSZ) by thermo-oxidative process in a centrifugal field. These hybrid matrixes aim further developments either to embed solid acids nanoparticles or to design strong solid acids on nanooxides, making them appropriate for proton exchange membranes. SEM, TEM, XRD and Raman spectroscopy were used to establish the structure and morphology and to characterize the composite membranes. The dependence of the electrical conductivity on temperature, water uptake, and methanol permeability are evaluated.

Superprotonic conducting phosphate glasses containing water

Fuel cells using H 2–O 2 offer the potential to minimize atmospheric pollution. Newly developed baria-phosphate glasses containing an appreciable amount of mobile hydrogen ions (protons) as well as molecular water exhibit super proton-conductivity. The new BaO–La 2O 3–Al 2O 3–P 2O 5 glasses exhibit high proton conductivities of 10 −2 S/cm at 200 °C and 10 −3 S/cm at 25 °C with a low activation energy of 0.17 eV. A H 2–O 2 fuel cell using this superprotonic glass electrolyte is operable at temperatures from 25 °C to 200 °C even under non-humidified conditions. The protons in oxide glasses have been often considered for a long time to be almost immobile. However, here we show that these superprotonic conductors of phosphate glasses are a good candidate material for viable electrolytes of fuel cells.

Dynamics of the superprotonic conductorK9H7(SO4)8·H2O studied by means of nuclear magnetic resonance

The dynamics of a recently discovered superprotonic conductorK9H7(SO4)8·H2O has been studied between 40 and 425 K by techniques based on the NMR spectrumshape, spin–lattice and spin–spin relaxation. At low temperatures (such as 40 K),proton intra-H-bond hopping is already intensive. At higher temperatures, water180° reorientations become observable in the NMR experiments, whereas above 250 K, proton interbondjumps—a precursor of the superprotonic conductivity above the transition temperatureTsp = 398 K—becomefrequent. Above Tsp, a large increase in the proton spin–spin relaxation timeT2 indicates that proton long-range diffusion becomes significant.Proton interbond jumps are assisted by reorientations of theSO4 tetrahedra, which also cause breaking of the water bonds, so water molecules become freeand consequently diffuse out of the crystal. The loss of water allows rearrangement of thelattice, so the number of structurally equivalent proton sites in the superprotonic phase isincreased, resulting in a very open structure for the hydrogen interbond transfer.

Phase transition behavior and proton conduction mechanism in cesium hydrogen sulfate/silica composite

Proton conduction and crystal structure in CsHSO4/SiO2 composite composed of polycrystalline CsHSO4 and mesoporous silica particles were investigated based on conductivity measurement and characterizations using Raman spectroscopy, XRD, and differential thermal analysis. The conductivity of pure CsHSO4 abruptly changes at around 414K (superprotonic phase transition), being accompanied with the structural transformation from a monoclinic phase to a tetragonal phase, while the conductivity of CsHSO4/SiO2 composite is significantly larger by over three orders of magnitude than that of pure CsHSO4 below the critical temperature of the superprotonic phase transition (353–414K). Raman spectroscopy and XRD indicate that this remarkable conductivity-enhancement in the composite is not due to the stabilization of the tetragonal phase (superprotonic phase) below its critical temperature. The line-broadening of the internal modes in the Raman spectra suggests that the rapid reorientational motion of the HSO4− ion, which leads to superprotonic conduction, is induced in the composite even below the critical temperature. The reorientational motion of the HSO4− ion below the critical temperature will occur at the interfacial phase which is structurally disordered and forms between CsHSO4 and SiO2 in the mesopores and/or on the surfaces of silica particles. Proton transfer will be accelerated via the interfacial conduction-pathway in the composite.