Superprotonic Conductors Research Articles

Super–protonic conductors for solid acid fuel cells (SAFCs): a review

Fuel cell holds the promise of being environmentally friendly and becomes one of the alternatives for renewable energy. Solid acids have super–protonic behavior which allows them to become conductors. It can function at high temperatures. Hydration, on the other hand, may be used to improve the performance of the solid acid. Moreover, the conductivity, stability, and crystal structure of the solid acid compounds of the fuel cell are all influenced by the size of the electrolyte membrane. Very few works have been done on solid acid fuel cells which are still under investigation to make it one viable as well as a reliable alternative to clean, renewable energy. In general, this work will provide an overview of the variables or characteristics that affect the technical effectiveness and performance of the unique super–protonic conductors for solid acid fuel cells.

Design of Flexible Metal–Organic Framework-Based Superprotonic Conductors and Their Fabrication with a Polymer into Proton Exchange Membranes

In recent times, the deployment of metal–organic frameworks (MOFs) to develop efficient proton conductors has gained immense popularity in the arena of sustainable energy research due to the ease of structural and functional tunability in MOFs. In this work, we have focused on developing “flexible MOF”-based proton conductors with Fe-MIL-53-NH2 and Fe-MIL-88B-NH2 MOFs using postsynthetic modification (PSM) as the tool. Taking advantage of the porous nature of these frameworks, we have carried out PSM on the primary amine groups present on the MOFs and converted them to −NH(CH2CH2CH2SO3H) groups. The PSM increased the number of labile protons in the channels of the modified MOFs as well as the extent of H-bonded networks inside the framework. The modified Fe-MIL-53-NH2 and Fe-MIL-88B-NH2 MOFs, named hereafter as 53-S and 88B-S, respectively, showed proton conductivity of 1.298 × 10–2 and 1.687 × 10–2 S cm–1 at ∼80 °C and 98% relative humidity (RH), respectively. This reflects ∼10-fold and ∼5-fold increases in their proton conductivity than their respective parent MOFs. Since MOFs as such are difficult to make directly into flexible membranes, and these are essential for practical applications as proton conductors, we have incorporated 53-S and 88B-S as fillers into a robust imidazole-based polymer matrix, namely, OPBI [poly(4,4′-diphenylether-5,5′-bibenzimidazole)]. The resulting polymer–MOF mixed matrix membranes (MMMs) after doping with phosphoric acid (PA) performed as flexible proton exchange membranes (PEMs) above 100 °C under anhydrous conditions and were found to be much more efficient and stable than the pristine OPBI membrane (devoid of any filler loading). By optimizing the amount of filler loading in the membrane, we obtained the highest proton conductivity of 0.304 S cm–1 at 160 °C under anhydrous conditions.

Isotropic Anhydrous Superprotonic Conductivity Cooperated with Installed Imidazolium Molecular Motions in a 3D Hydrogen‐Bonded Phosphate Network

AbstractUtilizing molecular motion is essential for the use of anhydrous superprotonic molecular proton conductors (σ beyond 10−4 S cm−1) as electrolytes in hydrogen fuel cells. However, molecular motion contributing to the improvement of intrinsic proton conduction has been limited and little clarified in relation to the proton conduction mechanism, limiting the development of material design guidelines. Here, a salt with a three‐dimensional (3D) hydrogen‐bonded (H‐bonded) phosphate network with imidazolium cations installed inside was studied, whose components are known to exhibit molecular motions that contribute to proton conduction. Despite its anisotropic H‐bonded network, the salt exhibits isotropic anhydrous superprotonic conductivity exceeding 10−3 S cm−1 at ≈351 K, which is the first example for organic molecular crystal. Variable‐temperature X‐ray structural analysis and solid‐state 2H NMR measurements revealed significant 3D molecular motion of imidazolium cations, which accelerate proton conduction via the 3D H‐bonded phosphate network.

Isotropic Anhydrous Superprotonic Conductivity Cooperated with Installed Imidazolium Molecular Motions in a 3D Hydrogen-Bonded Phosphate Network.

Utilizing molecular motion is essential for the use of anhydrous superprotonic molecular proton conductors (σ beyond 10-4 S cm-1 ) as electrolytes in hydrogen fuel cells. However, molecular motion contributing to the improvement of intrinsic proton conduction has been limited and little clarified in relation to the proton conduction mechanism, limiting the development of material design guidelines. Here, a salt with a three-dimensional (3D) hydrogen-bonded (H-bonded) phosphate network with imidazolium cations installed inside was studied, whose components are known to exhibit molecular motions that contribute to proton conduction. Despite its anisotropic H-bonded network, the salt exhibits isotropic anhydrous superprotonic conductivity exceeding 10-3 S cm-1 at ≈351 K, which is the first example for organic molecular crystal. Variable-temperature X-ray structural analysis and solid-state 2 H NMR measurements revealed significant 3D molecular motion of imidazolium cations, which accelerate proton conduction via the 3D H-bonded phosphate network.

Multivariate Sulfonic-Based Titanium Metal-Organic Frameworks as Super-protonic Conductors.

The proton-conducting performances of a microporous Ti-based metal-organic framework (MOF), MIP-207, were successfully tuned using a multicomponent ligand replacement strategy to gradually introduce a controlled amount of sulfonic acid groups as a source of Brönsted acidic sites while keeping the robustness and ecofriendly synthesis conditions of the starting material. Typically, multivariate sulfonic-based solids MIP-207-(SO3H-IPA)x-(BTC)1-x were prepared by combining various ratios of trimesate 1,3,5-benzenetricarboxylate (BTC) moieties and 5-SO3H-isophthalate (SO3H-IPA). The best sulfonic-MOF candidate that combines structural integrity with high proton conductivity values (e.g., σ = 2.6 × 10-2 S cm-1 at 363 K/95% relative humidity) was further investigated using ab initio molecular dynamics simulations. These calculations supported that the -SO3H groups act as proton donors and revealed that the proton transfer mechanism results from the solvation structure of protons through the fast Zundel/hydronium interconversion along the continuous H-bonded network connecting the adsorbed water molecules.

Deterioration of hydrogen-bonded superprotonic conductors belonging to CsHSO4–CsH2PO4–H2O salt system: a single-crystal neutron diffraction investigation

Single-crystal neutron diffraction investigation on Cs4(HSO4)3(H2PO4) and Cs6H(HSO4)3(H2PO4)4 superprotonic crystals, belonging to CsHSO4–CsH2PO4–H2O salt system, is under taken to elucidate the precise hydrogen atom positions in these crystals. The investigation revealed that these crystals are very sensitive to the ambient conditions and can undergo deterioration due to fluctuation in air moisture content. Cs6H(HSO4)3(H2PO4)4 crystals are more stable as compared to Cs4(HSO4)3(H2PO4). Crystal structure of Cs6H(HSO4)3(H2PO4)4 is obtained both before and after deterioration, it is found that the asymmetric O–HO hydrogen bond between the PO4 and SO4 ions of this crystal becomes stronger after deterioration. This led to the shrinkage of the unit cell, and most likely prevented further deterioration. Diabatic state model for hydrogen bonds is used to obtain the energy contour for the O–H···O hydrogen bond of Cs6H(HSO4)3(H2PO4)4 crystal. The influence of this change in the hydrogen bonding on the proton conduction ability of the crystal is discussed.

Materials with the CrVO4 structure type as candidate superprotonic conductors.

Proton conducting oxides have the potential to improve the efficiency of solid oxide fuel cells and electrolyzers, yet many oxide structures remain relatively unexplored for the ability to conduct protons. To accelerate the search for novel proton-conducting oxides, we have performed a computational screen of the proton migration energy in 41 different commonly-occurring oxide structure types. The results of this screen, which are supported by a comprehensive set of density functional theory calculations, indicate that known materials with the CrVO4 structure type have an average migration energy for proton diffusion of less than 0.2 eV, with several known materials having calculated migration energies below 0.1 eV. These results indicate that materials with the CrVO4 structure type, which to our knowledge have not been previously explored as candidate proton conductors, may exhibit very high proton conductivity that surpasses that of leading proton-conducting oxides. We present the results of our screen as well as diffusion dimensionality analysis and thermodynamic stability analysis for materials with the CrVO4 structure.

On the mean kinetic energy of the proton in strong hydrogen bonded systems.

The mean atomic kinetic energies of the proton, Ke(H), and of the deuteron, Ke(D), were calculated in moderate and strongly hydrogen bonded (HB) systems, such as the ferro-electric crystals of the KDP type (XH2PO4, X = K, Cs, Rb, Tl), the DKDP (XD2PO4, X = K, Cs, Rb) type, and the X3H(SO4)2 superprotonic conductors (X = K, Rb). All calculations utilized the simulated partial phonon density of states, deduced from density functional theory based first-principle calculations and from empirical lattice dynamics simulations in which the Coulomb, short range, covalent, and van der Waals interactions were accounted for. The presently calculated Ke(H) values for the two systems were found to be in excellent agreement with published values obtained by deep inelastic neutron scattering measurements carried out using the VESUVIO instrument of the Rutherford Laboratory, UK. The Ke(H) values of the M3H(SO4)2 compounds, in which the hydrogen bonds are centro-symmetric, are much lower than those of the KDP type crystals, in direct consistency with the oxygen-oxygen distance ROO, being a measure of the HB strength.

Interrelationship between Number of Mobile Protons, Diffusion Coefficient, and AC Conductivity in Superprotonic Conductors, CsHSO4 and Rb3H(SeO4)2

Using quasielastic neutron scattering (QENS), we investigated the proton dynamics for two superprotonic conductors, CsHSO4 and Rb3H(SeO4)2. To evaluate the self-diffusion coefficients and the number of mobile protons on both superprotonic and normal phases, we focused on proton dynamics not only in the phase above Tc, but also in the phase below Tc. In Rb3H(SeO4)2, the self-diffusion of protons was observed even below the Tc phase. In contrast to popular belief, no large changes in the self-diffusion coefficients were observed across Tc. Nevertheless, the increase in the number of mobile protons across Tc was about 14.5 times, which was estimated from the integrated intensity of QENS spectra, and this change could not account for the increased magnitude of proton conductivity, which is about 500 times. As a large translational self-diffusion coefficient has not been reported in previous works by QENS experiments, there are still unknown factors that contribute to the Nernst–Einstein relation that need to ...

Phase behavior and superprotonic conductivity in the Cs1−xRbxH2PO4and Cs1−xKxH2PO4systems

The solid acid compound CsH2PO4 (CDP) adopts a cubic structure of high conductivity above 228 °C, rendering it attractive as a fuel cell electrolyte for intermediate temperature operation. This superprotonic phase is stable from the phase transition temperature, Ts, to the dehydration temperature, Td, where the latter depends on water vapor pressure (e.g. Td = 290 °C at pH2O = 0.8 atm). In this work we examine the possibility of modifying these temperatures and thereby, amongst other characteristics, fuel cell operating conditions by introduction of Rb and K as substituents for Cs in CDP. The phase behavior of the Cs1−xRbxH2PO4 and Cs1−xKxH2PO4 pseudo-binary systems are determined by in situ X-ray diffraction (XRD) and thermal analysis. It is found that RbH2PO4 (RDP) and CDP are entirely miscible both below and above the transition to the cubic phase. With increasing Rb concentration, Ts increases and Td decreases. In contrast, K has limited solubility in CDP, with a 27 at.% solubility limit in the cubic phase, and both Ts and Td decrease with K content. The eutectoid temperature in the Cs1−xKxH2PO4 system is 208 ± 2 °C and the K solubility decreases sharply below this temperature. In both systems, conductivity decreases monotonically with increasing substituent concentration. Furthermore, even after normalization for cation size, the impact of K is greater than that of Rb, suggesting local disruptions to the proton migration pathway, beyond global changes in unit cell volume. Although this investigation shows unmodified CDP to remain the optimal fuel cell electrolyte material, the study provides a possible framework for elucidating proton transport mechanisms in superprotonic conductors.

Lithium hydroxide dihydrate: A new type of icy material at elevated pressure

We show that, in addition to the known monohydrate, LiOH forms a dihydrate at elevated pressure. The dihydrate involves a large number of H-bonds establishing chains along the <001> direction. In addition, the energy surface exhibits a saddle point for proton locations along certain O interatomic distances, a feature characteristic for superprotonic conductors. However, MD simulations indicate that LiOH · 2H(2)O is not a superprotonic conductor and suggest the relevant interpolyhedral O-O distances being too large to allow for proton transfer between neighboring Li-coordinated polyhedra at least on the time scale of the MD-simulations.

Probing the Nanodomain Origin and Phase Transition Mechanisms in (Un)Poled PMN-PT Single Crystals and Textured Ceramics.

Outstanding electrical properties of solids are often due to the composition heterogeneity and/or the competition between two or more sublattices. This is true for superionic and superprotonic conductors and supraconductors, as well as for many ferroelectric materials. As in PLZT ferroelectric materials, the exceptional ferro- and piezoelectric properties of the PMN-PT ((1−x)PbMg1/3Nb2/3O3−xPbTiO3) solid solutions arise from the coexistence of different symmetries with long and short scales in the morphotropic phase boundary (MPB) region. This complex physical behavior requires the use of experimental techniques able to probe the local structure at the nanoregion scale. Since both Raman signature and thermal expansion behavior depend on the chemical bond anharmonicity, these techniques are very efficient to detect and then to analyze the subtitle structural modifications with an efficiency comparable to neutron scattering. Using the example of poled (field cooling or room temperature) and unpoled PMN-PT single crystal and textured ceramic, we show how the competition between the different sublattices with competing degrees of freedom, namely the Pb-Pb dominated by the Coulombian interactions and those built of covalent bonded entities (NbO6 and TiO6), determine the short range arrangement and the outstanding ferro- and piezoelectric properties.

Engineering the Next Generation of Solid State Proton Conductors: Synthesis and Properties of Ba3−xKxHx(PO4)2

A new series of compounds with general chemical formula Ba_(3−x)K_(x)H_(x)(PO_4)_2 has been successfully prepared. This particular stoichiometry was targeted as a candidate solid-state proton conductor because of its anticipated structural similarity to known M_(3)H(XO_4)_2 superprotonic conductors (M = Cs, Rb, NH4, K; X = Se, S) and to the known trigonal compound Ba_(3)(PO_4)_2. The materials were synthesized from aqueous solution using barium acetate, dipotassium hydrogen phosphate, and potassium hydroxide as starting materials. Through variations in the initial solution stoichiometry or the synthesis temperature, the final stoichiometry could be controlled from x ~ 0.5 to ~1. X-ray powder diffraction, energy dispersive spectroscopy chemical analysis, ^(1)H magic angle spinning (MAS) nuclear magnetic spectroscopy, and thermogravimetric analysis were all employed to establish potassium and proton incorporation. The diffraction data confirmed crystallization of a trigonal phase, and chemical analysis showed the (Ba+K):P ratio to be 3:2, consistent with the target stoichiometry. The conductivity of the Ba_(3−x)K_(x)H_(x)(PO_4)_2 materials, as measured by A.C. impedance spectroscopy, is about 3 orders of magnitude greater than the end-member Ba_(3)(PO_4)_2 material with only a slight dependence on x, however, it is substantially lower than that of typical superprotonic conductors and of the M_(3)H(XO_4)_2 materials in particular. The close proximity of Ba to the hydrogen bond site is proposed to explain this behavior. At 250 °C, the conductivity is 2.4 × 10^(−5) S/cm for the composition x = 0.80, which, when combined with the water insolubility and the relatively high thermal stability, may render Ba_(3−x)K_(x)H_(x)(PO_4)_2 an attractive alternative in selected electrochemical applications to known superprotonic conductors.

Pressure effect on phase transitions in MeHAO4 superprotonic conductors (A = S, Se and Me = NH4, Rb, Cs)

The review is devoted to the behavior of some compounds of the hydrogensulfates and hydrogenselenates families under pressure. Many of these crystals undergo unusual phase transitions at ambient pressure and moderate temperatures. These transitions result in the formation of networks of dynamically disordered hydrogen bonds providing fast proton transport through the crystal structure. New additional high proton conducting (superprotonic) states can also be produced by applying pressure. Moreover, even if no superprotonic phases are formed in the compound at ambient pressure, such phases can become thermodynamically stable at high pressures, as it was demonstrated for NH4HSO4 and RbHSO4 crystals.

Solid-hydroxide protonic conductors: Superionic conductivity, phase transitions, isotopic effect, and self-organized microheterogeneity

New members of the family of inorganic solid-state protonic conductors are synthesized on the basis of KOH, NaOH, and H2O. The equimolar eutectics KOH/NaOH and KOH/KOH · H2O and the monohydrate KOH · H2O are superprotonic conductors in the temperature ranges 360–458, 360–370, and 320–420 K, respectively, with the conductivities of above 1 mS/cm and activation energies below 0.4 eV. A characteristic feature of the eutectics is the presence of anomalies in the temperature dependences of the conductivity and heat capacity in the range 360 ± 1 K. The isotopic effect of the protonic conductivity for KOH · H2O and for the high-temperature form of KOH/NaOH is found to be 1.40 ± 0.15. The role played by the self-organized microheterogeneity of the KOH/NaOH and KOH/KOH · H2O solid eutectics in the substantial increase in the conductivity and in the change of its pattern as compared to the individual KOH and NaOH is discussed.

Muon dynamics in superprotonic conductors

Abstract In order to clarify the mechanism of high proton conductivity ( σ H + ) for superprotonic conductors, MHXO 4 , where M = Cs and Rb, X = S and Se, μ + SR experiments have been performed in the temperature range between 250 and 450 K using single crystal samples. Here, MHXO 4 exhibits extraordinary high σ H + at T above its structural phase transition ( T c = 414 K for CsHSO 4 ) from a low- T monoclinic phase (Phase II) to a high- T tetragonal phase (Phase I). Since the asymmetry of weak transverse field (wTF) spectrum does not reach its maximum at ambient T , muonium (Mu) state is found to exist in both CsHSO 4 and CsHSeO 4 . The Mu fraction in wTF spectrum for CsHSO 4 is still a finite value even in Phase I, while the Mu state disappears in Phase I of CsHSeO 4 . The longitudinal field μ + SR measurements for observing the Mu state and its dynamics in CsHSO 4 , show fast Mu diffusion and the conversion from Mu to diamagnetic μ + in whole T range measured. Considering the fact that the σ H + in Phase I of CsHSO 4 is about 10 times larger than that of CsHSeO 4 , the Mu formation in Phase I implies the presence of the atomic hydrogen state and play a possible crucial role for the high σ H + in Phase I of CsHSO 4 .

Study of hydrogen diffusion in superprotonic ionic conductors, MH XO 4, by μ+SR and QENS

In order to clarify the mechanism of high proton conductivity ( σ H+) for superprotonic ionic conductors, MH XO 4, where M=Cs and Rb, X=S and Se, muon-spin rotation and relaxation ( μ +SR) and quasi-elastic neutron scattering (QENS) measurements have been performed at temperatures mainly in the vicinity of T c, at which MH XO 4 undergoes a structural phase transition from a low- T orthorhombic phase (Phase II) to a high- T tetragonal phase (Phase I). The μ +SR experiment shows the presence of muonium (Mu) state even in Phase I only for CsHSO 4 (CHS), while no Mu state was found in Phase I of CsHSeO 4 (CHSe). Considering the fact that the σ H+ in Phase I of CHS is more than 10 times larger that of CHSe, this implies a relationship between the presence of the atomic hydrogen (Mu) state and high σ H+ in Phase I of CHS. According to the QENS measurements using a single crystal of CHSe, line shapes of the energy spectra along three crystallographic directions are found to be slightly anisotropic in Phase I, in contrast to the isotropic conductivity reported by AC conductivity measurements.

The relation between elasticity and the superprotonic phase transition temperature for M 3H(XO 4) 2

We have investigated the rule concerning the superprotonic phase transition temperature T c for M 3H(XO 4) 2-type superprotonic conductors. It was found that the component of spontaneous strain a 0 ( = ( b m − a m / 3 ) / 2 b m ) , where a m and b m are lattice constants far below T c , is closely related to the phase transition temperature and that the superprotonic phase transition temperature obeys the equation a 0 2 = α − β T c ( α = 9.02 × 1 0 − 5 , β = 1.74 × 1 0 − 7 K − 1 ) . This result indicates that the spontaneous strain in the a m – b m plane is strongly correlated with the transition temperature T c . Moreover, it is deduced from analysis of the free energy that this scaling rule is describing the competition between the spontaneous strain energy and the kinetic energy of the protons, that is, the energy accompanying the increase in entropy of protons in the superprotonic phase.

Conversion Of Gypsum Into A Superprotonic Conductor

Development of superprotonic conductors of inexpensive and environmentally stable inorganic materials is important for clean energy fields such as H2-O2 fuel cells and hydrogen energy technology. We will show that gypsum (CaSO4·2H2O), normally an insulator, can be easily converted into a superprotonic conductor by a new processing method using phosphoric acid.The proton conductivities of the modified gypsums reach ∼ 10−2 S cm&minus1 at room temperature. Fuel cells (1 atm dry-H2)/(air), using 1 mm thick plates of the modified gypsums, give a power density of 0.15 mW cm−2 at room temperature. The high proton conductivities of the modified gypsums are discussed in terms of their x-ray diffraction patterns and IR spectra.

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.