Superprotonic Phase Transition 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.

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).

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.

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.

Anhydrous proton conduction of 0.6CsHSO4-0.4H4SiW12O40 (CHS-WSiA) composite materials fabricated by dry and wet mechanical ball milling

Abstract Anhydrous proton conductive 0.6CsHSO4-0.4H4SiW12O40(CHS-WSiA) composites (in molar ratio) were fabricated by dry and wet mechanical milling. From X-ray diffraction (XRD), the diffraction peak with the highest intensity of WSiA shifted to higher angle from 25.5° to 26.2° in the composites synthesized via dry and wet mechanical ball milling, which indicated that H+ ions in WSiA were partly substituted with Cs+ ions in CHS via ion-exchange reaction to form CHS-WSiA composites. The structural characterizations and conductivity of CHS-WSiA composites fabricated by CHS and WSiA were studied by XRD, thermogravimetry analysis, field emission scanning electron microscopy, and electrochemical impedance analysis (EIS), respectively. The proton conductivities of the composites were largely increased via high chemical interaction between CHS and WSiA, particularly in the low temperature region under a superprotonic phase-transition of CHS. The highest proton conductivity of 9.41 ×10-4 S cm-1 was achieved at 170°C under dry nitrogen atmosphere for 0.6CHS-0.4WSiA composites obtained by wet milling process.

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.

On the isostructural and superprotonic Cs5H3(SO4)4·xH2O transformations: physical or chemical nature?

For over 20 years, researchers have agreed that when pentacesium trihydrogen tetrasulfate hydrate (Cs5H3(SO4)4·xH2O) is heated through 141 °C, the observed conductivity increase corresponds to a physical transformation: a first-order superprotonic phase transition. A careful high-temperature phase behavior examination of this acid salt was performed by means of simultaneous thermogravimetric and differential scanning calorimetry, conventional and modulated differential scanning calorimetry, and impedance spectroscopy. The results present evidence that this transformation is of chemical, instead of physical nature. The conductivity increase is an exclusive consequence of a partial thermal decomposition, where liquid water (dissolving part of the surface salt) and hygroscopic cesium pyrosulfate (Cs2S2O7), as decomposition products, behave like a polymer electrolyte membrane where the proton transport mechanism includes the vehicle type, using hydronium (H3O+) as a charge carrier. Additionally, it was found that the intermediate temperature transformation (so-called isostructural phase transition) at around 87 °C is also of chemical nature.

Revealing Local Dynamics of the Protonic Conductor CsH(PO3H) by Solid-State NMR Spectroscopy and First-Principles Calculations

A joint study incorporating multinuclear solid-state NMR spectroscopy and first-principles calculations has been performed to investigate the local structure and dynamics of the protonic conductor CsH(PO3H) in the paraelectric phase. The existence of the superprotonic phase (>137 °C) is clearly confirmed by NMR, in good agreement with the literature. The variable-temperature 1H, 2H, and 31P NMR data further reveal a distribution of motional correlation times, with isotropic rotation of the phosphite ion being observed below the superprotonic phase transition for a small but gradually increasing subset of anions. This isotropic rotation is associated with fast local protonic motion, with the distribution of correlation times being tentatively assigned to internal defects or surface adsorbed H2O. The phosphite ion dynamics of the majority slower subset of phosphite ions is quantified through analysis of variable-temperature 17O spectra recorded from 34 to 150 °C, by considering a model for the pseudo C3 rot...

Acid-base chemistry and proton conductivity of CsHSO4, CsH2PO4 and their mixtures with N-heterocycles

Caesium hydrogen sulfate (CsHSO4) and caesium dihydrogen phosphate (CsH2PO4) are solid acids that undergo superprotonic phase-transitions at about 140 and 230°C, respectively. As a result, the proton conductivity is increased by several orders of magnitude. However, the practical operational temperature range is narrow due to decomposition of the high-conductivity phases. For CsHSO4, it is known that this window can be extended to lower temperatures by addition of carefully selected N-heterocycles. The present work investigates if the same approach can be used to extend the practical operating temperature range of CsH2PO4 as well. Binary mixtures of CsH2PO4 with 1,2,4-triazole, benzimidazole or imidazole were prepared by means of mechanochemical synthesis. Mixtures based on CsHSO4 were prepared as a basis for a comparative discussion. It was found that CsHSO4 formed organic-inorganic salts, while CsH2PO4 formed heterogeneous mixtures with the N-heterocycles due to its weaker acidity. At a N-heterocycle content of 30mol%, enhanced proton conductivity was observed for both solid acids at temperatures below their superprotonic phase transitions.

High temperature phase transition and multiphase state formation in K3H(SO4)2 superprotonic conductor

EPR spectra of K3H(SO4)2 (KHS) doped with VO2+ or Mn2+ recorded at the temperature higher than about 440K are time-dependent and indicate on anomalously slow kinetics of the paraelastic/ferroelastic phase transition which takes place far below the temperature proposed by Baranov et al. (TII=463K). The formation of a multidomain state upon heating below TII is manifested as irreversible changes in EPR spectra after sample cooling to room temperature. The angular dependence of EPR spectra indicates that K3H(SO4)2 undergoes a ferroelastic phase transition between the point groups 2/m and 3¯m. The EPR spectra of KHS doped with VO2+ or Mn2+ are rapidly changed into the pseudo-powder ones after annealing above TI=480K. This points to a decomposition of the KHS crystal and a multiphase state formation at TI=480K, rather than at superprotonic phase transition, which is typical for the majority of compounds of the M3H(XO4)2 type.

Dielectric Property Conduction Mechanism and Thermal Analysis of Cu 1 . 0 9 Ni 0 . 9 1 (HSeO3)2Cl2⋅4H2O Compound

The Cu1.09Ni0.91(HSeO3)2Cl2⋅4H2O (noted CuNiSe) compound was prepared from an aqueous solution reaction and crystallizes in the orthorhombic Pnma space group. It was analyzed by single-crystal X-ray diffraction and the X-ray powder diffraction to confirm the purity of the synthesized compound. The structure of this compound consists by a three-dimensional framework structure, constructed by layers parallel to the (010) plane formed by copper/nickel octahedral and (HSeO\(_{3}^{\mathrm {-}})\) trigonal pyramids. The thermal analysis of the CuNiSe compound shows three endothermic peaks at 410, 433, and 473 K. The Raman spectra of CuNiSe have been recorded from 299 to 479 K. In addition, the dielectric evolution constant as a function of frequency and temperature revealed a ferroelectric–paraelectric phase transition about 433 K and superprotonic phase transition about 473 K. However, the conductivity evolution versus temperature confirmed the presence of these two types of phase transitions.

(Invited Plenary) Aromatic Ionomers As Alternative Fuel Cell Membranes: Issues and Possibilities

Background and objective One of the important challenges in the current fuel cell research is to develop an alternative membrane to state-of-the-art perfluorinated sulfonic acid (PFSA) ionomers. While the PFSA ionomer membranes are highly proton conductive and chemically and physically stable, high gas permeability, high cost, and environmental incompatibility of the fluorinated materials are drawbacks for the wide-spread dissemination of fuel cells. In the past couple of decades, a variety of proton conductive materials have been proposed as alternative membranes. Nonfluorinated hydrocarbon polymers, acid-doped polymers, inorganic/organic nanohybrids, solid acids with superprotonic phase transition, and acid/base ionic liquids fall into this category. Some of them are claimed to exhibit high proton conductivity, very low gas permeability, and reasonable stability. However, none of them can compete with PFSAs under a wide range of fuel cell operating conditions. The most critical issues of these alternative membranes are insufficient durability and significant dependence of the proton conductivity upon humidity. In addition, interfaces between these emerging proton conductive materials and the catalyst layers have not been well explored. In this communication, current issues (in particular, chemical and mechanical stabilities and interfacial problems) and future possibilities of aromatic ionomer membranes will be discussed. Stability issues In pursuit of better performing and more stable aromatic ionomer membranes, we have investigated the effect of a simple structure, sulfo-1,4-phenylene unit, as a hydrophilic component on the membrane properties. Unlike typical aromatic ionomer membranes, the newly designed ionomer membranes exhibited reduced water uptake and excellent mechanical stability under humidified conditions due to the absence of polar groups such as ether, ketone, or sulfone groups in the hydrophilic component. The membrane has survived several tens of thousands cycles in humidity cycling test of US DOE protocol. The high local ion concentration in the hydrophilic segments contributed to an increase in the proton conductivity especially at low humidity. Consequently, the high ion exchange capacity (IEC = 2.67 meq/g) membrane exhibited high proton conductivity under low humidity (20% RH, 7.3 mS/cm) conditions at 80 °C, which are one of the highest values reported thus far for aromatic ionomer membrane.1) Interfacial issues In the literature, a number of hydrocarbon ionomer membranes that exhibit comparable proton conductivity to PFSA membranes have been reported, however, they underperformed in operating fuel cells compared to PFSA membranes in most cases. This is partly because of the cathode performance was lower even when the same cathode catalyst layers were used. We have revealed that the double ionomer membrane composed of thin layer Nafion attached on our aromatic ionomer membrane showed higher fuel cell performance than the parent (single layer) ionomer membrane.2,3) The Nafion inter-layer improved the interfacial contact between the aromatic ionomer membrane and the catalyst layer. Based on the results, we have then designed and synthesized novel ionomer membranes composed of perfluoroalkyl and the sulfonated phenylene groups. The resulting membranes could support good electrocatalytic performance of the catalyst layer at the membrane-electrode interface due to the well-controlled finely phase-separated morphology. Acknowledgement This work was partly supported by the New Energy and Industrial Technology Development Organization (NEDO) through the SPer-FC Project. References 1) J. Miyake, T. Mochizuki, K. Miyatake, ACS Macro Lett., 4, 750-754 (2015). 2) T. Mochizuki, K. Kakinuma, M. Uchida, S. Deki, M. Watanabe, K. Miyatake, ChemSusChem, 7, 729-733 (2014). 3) T. Mochizuki, M. Uchida, H. Uchida, M. Watanabe, K. Miyatake, ACS Appl. Mater. Interfaces, 6, 13894-13899 (2014).

Effect of silica on the thermal behaviour and ionic conductivity of mixed salt solid acid composites

The effect of adding silica (SiO2) to a mixed-salt solid acid composite, CsH2PO4/NaH2PO4, was studied via conductivity measurements, thermal analyses and X-ray diffraction. The conductivity value of the composites ranged from 1.1 × 10−4 S cm−1 to 7.6 × 10−3 S cm−1 for temperatures from 210 °C to 250 °C, respectively. Adding NaH2PO4 to the composite reduced the conductivity value at 230 °C due to the decomposition of NaH2PO4, which initiated at 210 °C. Dehydration of CsH2PO4 was evident at 230 °C and 270 °C, while the superprotonic phase transition was not observed. Dehydration was detected for NaH2PO4 at temperatures above 210 °C, but a thermally stable composite was produced by mixing it with CsH2PO4 and SiO2. An X-ray diffraction analysis confirmed the existence of solid acid solutions for the composites consisting of NaH2PO4:CsH2PO4 and SiO2:CsH2PO4 in molar ratios of 1:10 to 3:10. The presence of SiO2 partially obstructed the dehydration of the composite, as the diffraction intensity of the dehydration by-product Cs2H2P2O7 decreased at the end of the thermal treatment.

Structure, thermal behavior, and dielectric properties of new cesium hydrogen sulfate arsenate: Cs2(HSO4)(H2AsO4)

Synthesis and structural characterization by single-crystal X-ray diffraction method, thermal behavior, and electrical proprieties are given for a new compound with a superprotonic phase transition Cs2(HSO4)(H2AsO4). The title compound crystallizes in the monoclinic system with the P21/n space group. The structure contains zigzag chains of hydrogen-bonded anion tetrahedra that extend in the [010] direction. Each tetrahedron is additionally linked to a tetrahedron neighboring chain to give a planar structure with hydrogen-bonded sheets lying parallel to (10ī). The existence of O–H and (S/As)–O bonds in the structure at room temperature has been confirmed by IR and Raman spectroscopy in the frequency ranges 4000–400 cm−1and 1200–50 cm−1, respectively. Differential scanning calorimetry analysis of the superprotonic transition in Cs2(HSO4)(H2AsO4) showed that the transformation to high temperature phase occurs at 417 K by one-step process. Thermal decomposition of the product takes place at much higher temperatures, with an onset of approximately 534 K. The superprotonic transition was also studied by impedance and modulus spectroscopy techniques. The conductivity in the high temperature phase at 423 K is 1.58 × 10−4 Ω−1 cm−1, and the activation energy for the proton transport is 0.28 eV. The conductivity relaxation parameters associated with the high disorder protonic conduction have been examined from analysis of the M”/M”max spectrum measured in a wide temperature range. Transport properties of this material appear to be due to the proton hopping mechanism.

CsHSO4/TiP2O7 composite membrane for high temperature (>150 °C) proton exchange membrane fuel cells

Proton exchange membrane fuel cells (PEMFCs) are one of the most promising clean energy technologies converting hydrogen energy to electric power. In this study, a proton-conducting electrolyte based on a CsHSO4/TiP2O7 composite membrane and operating at > 150 °C was fabricated using the vacuum infiltration method and characterized. The electrical properties were investigated in the high temperature range of ∼ 110–190 °C, in a dry atmosphere, using an impedance analyzer. The analysis of the resultant phase relationship showed that the composite membrane exhibited the same major peaks as each individual material (TiP2O7 and CsHSO4) without any secondary phase. The relative density of the infiltrated CsHSO4/TiP2O7 composite membrane increased up to 96.76%, indicating that most of the pores present in the initial TiP2O7 supporting matrix were infiltrated with CsHSO4. It was consistent with the result of scanning electron microscopy. Moreover, the composite membranes exhibited low ionic conductivities in the low-temperature region, but at ∼ 140 °C, the ionic conductivities significantly increased because of the super-protonic phase transition of CsHSO4. The maximum conductivity (∼2.38 × 10−3 S/cm) was achieved at 190 °C under a dry Ar atmosphere.