Superprotonic Phase Research Articles

Alternative processing routes on CsH2PO4 proton conductors: Cold sintering and ball-milling routes

Cesium dihydrogen phosphate (CsH2PO4) holds great potential as electrolyte for intermediate-temperature fuel cells and electrochemical devices due to its high proton conductivity (≥10−2 S/cm). This study reports on the variation of the microstructure of CsH2PO4 and its effect on its electrical and thermal properties, with special attention to the low-temperature conductivity behavior. To manipulate morphology, the particle size was decreased by wet ball-milling, while variations in grain growth were achieved by cold sintering. Above the superprotonic phase transition, the electrical conductivity was effectively independent of the grain size. Nonetheless, at lower temperatures, a brick layer analysis on the impedance data revealed that the conductivity of the monoclinic phase is governed by the conduction pathways along the parallel grain boundaries due to the humidified atmosphere. Overall, we propose two conduction mechanisms that could explain the grain boundary conductivity of these samples, revealing critical links between sample morphology and low-temperature electrical behavior.

Enabling Superprotonic Phase Transitions in Solid Acids via Supramolecular Complex Formation: The Case of Crown Ethers and Alkali Hydrogen Sulfates

Enabling Superprotonic Phase Transitions in Solid Acids via Supramolecular Complex Formation: The Case of Crown Ethers and Alkali Hydrogen Sulfates

Implementation of Phase Transitions in Rb3H(SO4)2 under K Substitution

A series of solid acid compounds, representing the large family MmHn(AO4)(m + n)/2·yH2O (where M = K, Rb, Cs, NH4; AO4 = SO4, SeO4, HPO4, HAsO4), is characterized by high values of own proton conductivity, which arises as a result of a phase transition through the formation of a dynamically disordered hydrogen bond network. Such superprotonic phase transitions are observed, however, not for all compounds of the family and Rb3H(SO4)2 is one of them. The occurrence of superprotonic phase transitions has been experimentally demonstrated in the (KxRb1−x)3H(SO4)2 solid solutions through cation substitution. The high-temperature phases are unstable towards decomposition reaction, and their temperature range of existence is about 1–7 °C. The implementation of superprotonic transitions is discussed in terms of hydrogen bond lengths.

An integrated microscopical study of surface properties and surface effects in superprotonic (K1-x(NH4)x)3H(SO4)2 crystals

Surface properties of superprotonic (K1-х(NH4)x)3H(SO4)2 (x ≥ 0.57) single crystals and their evolution under humidity were studied by optical polarization microscopy, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDXS). Traditional method of atomic force microscopy (AFM) and sophisticated methods such as Kelvin probe force microscopy (KPFM) and scanning capacitance force microscopy (SCFM) were also used. Fresh and aged cleavage surfaces perpendicular and parallel to the c axis of the trigonal superprotonic phase were examined. The effect of air humidity on the surface morphology, surface conductivity, surface potential and surface capacitance was studied. The influence of material composition and surface orientation was considered. During ageing for 430 h, the (001) sample surfaces, initially stepped, were smoothened, surface electric potential changed from negative greater than a hundred of millivolts in absolute value to positive of about 80 mV, and a modified layer containing new crystal phases was formed.

Conductivity, its anisotropy and changes as a manifestation of the features of the atomic and real structures of superprotonic [K1-x(NH4)x]3H(SO4)2 crystals.

Single crystals of [K1-x(NH4)x]3H(SO4)2 (x ≥ 0.57) grown in the K3H(SO4)2-(NH4)3H(SO4)2-H2O water-salt system are studied. The atomic structure including H atoms was determined at room temperature using X-ray structural analysis. [K1-x(NH4)x]3H(SO4)2 (x ≥ 0.57) crystals have trigonal symmetry and disordered hydrogen-bond networks at ambient conditions similar to the high-temperature phases of K3H(SO4)2, (NH4)3H(SO4)2 and other superprotonic compounds M3H(AO4)2. Impedance measurements performed on single crystals show high values of conductivity characteristic for superprotonic phases. Using the methods of impedance spectroscopy and atomic force microscopy, a significant anisotropy of the conductivity of crystals has been detected. It was also shown that there is a qualitative correlation of bulk and local conductivity measured for samples of the same composition and orientation at room temperature, which is due to the peculiarities of their crystal structure.

Eutectic CsHSO4-Coordination Polymer Glasses with Superprotonic Conductivity.

Superprotonic phase transition in CsHSO4 allows fast protonic conduction, but only at temperatures above the transition temperature of 141 °C (Tc). Here, we preserve the superprotonic conductivity of CsHSO4 by forming a binary CsHSO4-coordination polymer glass system, showing eutectic melting. Their anhydrous proton conductivities below Tc are at least 3 orders of magnitude higher than CsHSO4 without compromising conductivity at higher temperatures or the need for humidification, reaching 6.3 mS cm-1 at 180 °C. The glass also introduces processability to the conductor, as its viscosity below 103 Pa·s can be achieved at 65 °C. Solid-state NMR and X-ray pair distribution functions reveal the oxyanion exchanges and the origin of the preserved conductivity. Finally, we demonstrate the preparation of a micrometer-scale thin-film proton conductor showing low resistivity with high transparency (transmittance >85% between 380-800 nm).

Stability of Superprotonic CsH2PO4 Hermetically Sealed in Different Environments.

Using powder X-ray diffraction and AC impedance spectroscopy, we have found that the superprotonic CsH2PO4 (CDP) phase is stable at T = 250 °C when sealed in different volumes (15 mL and 50 mL) of dry air or inert gasses. Under these conditions, CDP’s proton conductivity stays constant at 2.5 × 10−2 S·cm−1 for at least 10 h. On the other hand, removing the gas from the chamber leads to a sharp, two-order-of-magnitude drop in the proton conductivity. Our data show no evidence of a self-generated water vapor atmosphere in the chamber, and the gas pressure at T = 250 °C is several orders of magnitude below the pressures previously used to stabilize CDP’s superprotonic phase. These results demonstrate that hermetically sealing CDP in small gas-filled volumes represents a new method to stabilize the superprotonic phase, which opens new paths for large-scale applications of phosphate-based solid acids as fuel cell electrolytes.

Structural, thermal, and transport properties of nanocomposite CsH2PO4/NaH2PO4/TiO2: A novel proton conducting electrolyte for fuel cells

The composites of CsH2PO4/NaH2PO4/TiO2 were prepared by chemical route. The structural, thermal, and conductive properties were investigated by X-ray diffraction analysis, thermal analysis, and conductivity measurements, respectively. CsH2PO4 shows a superprotonic phase transition from the monoclinic to the cubic phase at 230 °C at which the conductivity increases by four orders of magnitude. The initial dehydration event that occurred in CsH2PO4 was observed at 250 °C and the dehydration behavior also decreased at high temperatures due to the additives. The performance of CsH2PO4 was improved based on conductivity and stability by adding TiO2 and NaH2PO4. 5CsH2PO4/3NaH2PO4/2TiO2 and 6CsH2PO4/2NaH2PO4/2TiO2 were found to have the lowest weight loss compared to other composites. The conductivity also increases up to 1.5 orders of magnitude at lower temperatures. The conductivity was found to be highest for the composite 8CsH2PO4/1NaH2PO4/1TiO2 with a value of 1.4 × 10−2 S cm−1.

Correction to: On the tetragonal → monoclinic and superprotonic RbH2PO4 phase transitions: are they physical transformations?

Correction to: On the tetragonal → monoclinic and superprotonic RbH2PO4 phase transitions: are they physical transformations?

On the tetragonal → monoclinic and superprotonic RbH2PO4 phase transitions: are they physical transformations?

During the last quarter century, the high-temperature phase transition (HTPT) at $${\text{72 < T}}_{{\text{p}}} { < 130 }$$ °C on $${\text{RbH}}_{{2}} {\text{PO}}_{{4}}$$ has become a controversial issue; while most researchers state that this phenomenon corresponds to a physical transformation (tetragonal → monoclinic structural phase transition), others support its chemical nature. On the other hand, it has been established that if the salt is heated above $${\text{T}}_{{\text{p}}}$$ , a superprotonic phase transition at $${\text{72 < T}}_{{\text{p}}} {\text{ < 130}}$$ °C takes place. By using thermal analyses (conventional and modulated differential scanning calorimetry, thermogravimetric analysis, simultaneous thermogravimetric analysis and differential scanning calorimetry) and temperature evolution of X-ray diffraction measurements, we found solid evidence that none of these transformations is physical in nature; instead, they correspond to chemical thermal decompositions. Therefore, according to these results, when $${\text{RbH}}_{{2}} {\text{PO}}_{{4}}$$ is submitted to a heating process, it keeps its room-temperature tetragonal-phase until the sample reaches its total thermal decomposition. Consequently, we believe that the tetragonal → monoclinic phase transition and the transition to the superprotonic phase do not take place.

Microscopic studies of the surface layer of (NH4)3H(SeO4)2 crystals subject to phase transformations

Surface of (NH4)3H(SeO4)2 crystals and its evolution in the course of the transition from the ferroelastic to the superprotonic phase were studied by optical polarization microscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), and energy-dispersive X-ray spectroscopy (EDXS). The morphology of fresh cleavage surfaces was examined. The composition and local electrical properties of the surface crystal layers were investigated. Local measurements of current-voltage curves and piezoresponse hysteresis loops were carried out. A transition to the phase with a superprotonic conductivity was proved to exist at T ≈ 308 K. A thin crystal layer of varying composition was found to form owing to topochemical reactions. Phase transformations and solid-state reactions at the crystal surface were analyzed based on the data obtained.

A humidity-controlled precipitation technique enabling discovery of Rb3(H1.5PO4)2

The previously unknown compound Rb3(H1.5PO4)2 is successfully synthesized here using a newly developed variant of aqueous precipitation crystal growth. The approach exploits the phenomenon of boiling point elevation in concentrated solutions. Crystals of the title compound were obtained upon heating a stoichiometric solution from 110 to 150 ​°C under a high steam partial pressure of 0.83 ​atm. Single crystal X-ray diffraction studies revealed Rb3(H1.5PO4)2 crystallizes in space group C2/m and is isostructural to Cs3(H1.5PO4)2. As evidenced by thermal analysis, Rb3(H1.5PO4)2 does not undergo a phase transition to a trigonal superprotonic phase upon heating. Even under a steam partial pressure of 0.82 ​atm, under which dehydration is suppressed to a temperature of 263 ​°C, no polymorphic transition is detected. The behavior parallels that of Cs3(H1.5PO4)2 and contrasts that of several structurally and chemically similar selenate compounds. The crystal growth approach developed here may prove particularly useful for obtaining water soluble compounds which are thermodynamically or kinetically disfavored at temperatures close to ambient.

Proton transport mechanism and pathways in the superprotonic phase of M3H(AO4)2 solid acids from ab initio molecular dynamics simulations.

The proton transport mechanism in superprotonic phases of solid acids has been a subject of experimental and theoretical studies for a number of years. Despite this, details of the mechanism still need further clarification. In particular in the M3H(AO4)2 family of crystals, where M = NH4, K, Rb, Cs, and A = S, Se, the proton diffusion is mostly considered in the (001) plane, whereas it is relatively high in the [001] direction as well. In this paper, we report the results of our ab initio molecular dynamics simulations of the Cs3H(SeO4)2 superprotonic phase and propose an atomic-level mechanism of proton transport and pathways both in the (001) plane and along the [001] direction. It turned out that structural configurations formed by hydrogen-bonded tetrahedral anions during the proton diffusion are more complicated and diverse than those considered so far in the literature. Our predicted values of the proton conductivity and activation energy agree well with available experimental data. This validates the reliability of the computational results obtained.

Proton dynamics in superprotonic Rb3H(SeO4)2 crystal by broadband dielectric spectroscopy

Broadband dielectric and AC conductivity spectra (1 Hz to 1 THz) of the superprotonic single crystal Rb3H(SeO4)2 (RHSe) along the c axis were studied in a wide temperature range 10 K < T < 475 K that covers the ferroelastic (T < 453 K) and superprotonic (T > 453 K) phases. A contribution of the interfacial electrode polarization layers was separated from the bulk electrical properties and the bulk DC conductivity was evaluated above room temperature. The phase transition to the superprotonic phase was shown to be connected with the steep but almost continuous increase in bulk DC conductivity, and with giant permittivity effects due to the enhanced bulk proton hopping and interfacial electrode polarization layers. The AC conductivity scaling analysis confirms validity of the first universality above room temperature. At low temperatures, although the conductivity was low, the frequency dependence of dielectric loss indicates no clear evidence of the nearly constant loss effect, so-called second universality. The bulk (intrinsic) dielectric properties, AC and DC conductivity of the RHSe crystal at frequencies up to 1 GHz are shown to be caused by the thermally activated proton hopping. The increase of the AC conductivity above 100 GHz could be assigned to the low-frequency wing of proton vibrational modes.

Optical second harmonic generation imaging and x-ray diffraction of Cs1 − xRbxH2PO4 proton conductor series

The solid acid compound CsH2PO4 (CDP) is a superprotonic conductor for intermediate temperature range fuel cell applications. Doping CDP with rubidium can alter its transition temperature from normal phase to superprotonic phase. Powder samples of Cs1 − xRbxH2PO4 (0 ≤ x ≤ 1) were synthesized with 10 at. % intervals of x and analyzed at room temperature. A powder x-ray diffraction study showed three major structures between the various samples: monoclinic P21/m for x ≤ 0.5 samples, monoclinic P21/c for x = 0.8 samples, tetragonal I-42d for x = 0.9 and 1 samples, and a mixture of these structures in the x = 0.6 and 0.7 samples. Optical second harmonic generation (SHG) microscopy was performed on these samples, and a unique quadrupolar behavior in an SHG polar plot was found and correlated with the monoclinic P21/c structure in the x = 0.8 sample and some part of the x = 0.6 and 0.7 samples. Although in principle this P21/c structure is centrosymmetric with no SHG signal, the loss of local symmetry while maintaining the overall super structure can possibly explain this phenomenon. The tetragonal I-42d structure is noncentrosymmetric and shows dipolar behavior in the SHG polar plot from x = 0.9 and 1 samples and part of the x = 0.6 and 0.7 samples. The monoclinic P21/m structure is centrosymmetric with no SHG signal from the x ≤ 0.5 samples. Our discovery confirms the early finding of phases of Cs1 − xRbxH2PO4 (0 ≤ x ≤ 1) superprotonic conductor series at room temperature and indicates the potential of developing SHG microscopy to study the phases of these compounds at an intermediate working temperature in situ.

Crystal structure, conductivity, and phase stability of Cs3(H1.5PO4)2 under controlled humidity

Upon heating, a number of solid acids of structure type M3H(XO4)2 (M = NH4, K, Rb, Cs; X = S, Se) transform from a low conductivity monoclinic phase into a high conductivity (superprotonic) phase. Here, a new member of this material class, Cs3(H1.5PO4)2, is studied using a combination of single crystal X-ray diffraction, in-situ powder diffraction, thermogravimetric analysis, and A.C. impedance spectroscopy. The room temperature crystal structure of Cs3(H1.5PO4)2 was solved in the C2/m space group and found to be essentially isostructural to Cs3H(SeO4)2. On heating, no transition to a superprotonic phase was observed. Instead, Cs3(H1.5PO4)2 exsolves a small quantity of a CsH2PO4-like cubic phase at an onset temperature of ~192 °C prior to decomposition. The absence of a superprotonic phase transition is discussed in the context of other M3H(XO4)2 solid acids and the crystal-chemical features that define this class of materials.

Hydrogen Motion of CsH2PO4 Electrolyte Using 1H and 31P High-Resolution Nuclear Magnetic Resonance (NMR) Spectroscopy

This study investigated hydrogen-bonded CsH2PO4 (CDP) solid acid using 1H and 31P high-resolution nuclear magnetic resonance (NMR). Below the superprotonic phase transition temperature, the temperature dependence of the 1H NMR spectra was observed with two different hydrogen-bonded networks in the CDP structure. The systematic evolution of the lineshape with temperature dependence indicates hydrogen hopping in the chemical structure of hydrogen-bonded networks in a solid acid lattice. The proton conduction under two types of hydrogen-bonds—interchain and intrachain—in CsH2PO4 in the chemical environments of PO4 tetrahedra is discussed.

First universality of conductivity spectra in superprotonic (NH4)3H(SO4)2 single crystal

We present the frequency study of electric conductivity in the superprotonic single crystal in a wide temperature range covering the superprotonic phase I and the low conducting, ferroelastic phase II. The data below microwave frequencies are analyzed using the Summerfield scaling method to check for presence of the first universality. The scaled conductivity obtained from the raw experimental data plotted versus scaled frequency do not show the universality because it contains, in a low temperature range, steps related to relaxational dielectric contribution. The contribution, evidenced by a temperature study of frequency dependence of ε″, presumably comes from polar ammonia and therefore does not reflect properties of the hopping motion of mobile ions. To get rid of it, the conductivity is calculated using the impedance data obtained from the fitting procedure of the Nyquist plots. The resulting scaled ac conductivity plot forms a single curve in a wide temperature range. Thus, (NH4)3H(SO4)2 meets criteria for the first universality, despite the fact that it undergoes the structural, superionic phase transition in the temperature range studied.

Superprotonic CsH2PO4 in dry air

The first observation of a stable superprotonic CsH2PO4 (CDP) phase in the absence of high humidity and high pressure is reported. Temperature- and time-resolved impedance spectroscopy data show that the superprotonic conductivity of a CDP pellet measured in dry air (22%rh) in a hermetically sealed chamber holds at σ∼1.5 × 10−2 S cm−1 over a timespan t = 50 h at a temperature T = 260 °C. Nyquist plots confirm the superprotonic nature of the conduction and x-ray diffraction data reveal that no dehydration of the CDP superprotonic phase occurs under the above-mentioned conditions.

The Changes of Thermal, Dielectric, and Optical Properties at Insertion of Small Concentrations of Ammonium to K3H(SO4)2 Crystals

The structure of (K1–x(NH4)x)3H(SO4)2 crystals with a low ammonium concentration and the behavior of their thermal, optical, and dielectric properties in a temperature range of 275–500 K have been investigated to clarify the influence of doping on the phase transition kinetics. An examination of unit-cell parameters of (K1 – x(NH4)x)3H(SO4)2 single crystals has confirmed the existence of a superprotonic phase transition at a temperature of ≈450K. The conducting properties of single-crystal and polycrystalline samples have been studied.