A New Solid-State Proton Conductor: The Salt Hydrate Based on Imidazolium and 12-Tungstophosphate

We report the structure and charge transport properties of a novel solid-state proton conductor obtained by acid-base chemistry via proton transfer from 12-tungstophosphoric acid to imidazole. The resulting material (henceforth named Imid3WP) is a solid salt hydrate that, at room temperature, includes four water molecules per structural unit. To our knowledge, this is the first attempt to tune the properties of a heteropolyacid-based solid-state proton conductor by means of a mixture of water and imidazole, interpolating between water-based and ionic liquid-based proton conductors of high thermal and electrochemical stability. The proton conductivity of Imid3WP\ub74H2O measured at truly anhydrous conditions reads 0.8 7 10-6 S cm-1 at 322 K, which is higher than the conductivity reported for any other related salt hydrate, despite the lower hydration. In the pseudoanhydrous state, that is, for Imid3WP\ub72H2O, the proton conductivity is still remarkable and, judging from the low activation energy (Ea = 0.26 eV), attributed to structural diffusion of protons. From complementary X-ray diffraction data, vibrational spectroscopy, and solid-state NMR experiments, the local structure of this salt hydrate was resolved, with imidazolium cations preferably orienting flat on the surface of the tungstophosphate anions, thus achieving a densely packed solid material, and water molecules of hydration that establish extremely strong hydrogen bonds. Computational results confirm these structural details and also evidence that the path of lowest energy for the proton transfer involves primarily imidazole and water molecules, while the proximate Keggin anion contributes with reducing the energy barrier for this particular pathway

A long-chain protic ionic liquid inside silica nanopores: Enhanced proton mobility due to efficient self-assembly and decoupled proton transport

We report enhanced protonic and ionic dynamics in an imidazole/protic ionic liquid mixture confined within the nanopores of silica particles. The ionic liquid is 1-octylimidazolium bis(trifluoromethanesulfonyl)imide ([HC8Im][TFSI]), while the silica particles are microsized and characterized by internal well connected nanopores. We demonstrate that the addition of imidazole is crucial to promote a proton motion decoupled from molecular diffusion, which occurs due to the establishment of new N-H⋯N hydrogen bonds and fast proton exchange events in the ionic domains, as evidenced by both infrared and1H NMR spectroscopy. An additional reason for the decoupled motion of protons is the nanosegregated structure adopted by the liquid imidazole/[HC8Im][TFSI] mixture, with segregated polar and non-polar nano-domains, as clearly shown by WAXS data. This arrangement, promoted by the length of the octyl group and thus by significant chain-chain interactions, reduces the mobility of molecules (Dmol) more than that of protons (DH), which is manifested by DH/Dmolratios greater than three. Once included into the nanopores of hydrophobic silica microparticles, the nanostructure of the liquid mixture is preserved with slightly larger ionic domains, but effects on the non-polar ones are unclear. This results in a further enhancement of proton motion with localised paths of conduction. These findings demonstrate significant progress in the design of proton conducting materials via tailor-made molecular structures as well as by smart exploitation of confinement effects. Compared to other imidazole-based proton conducting materials that are crystalline up to 90 \ub0C or above, the gel materials that we propose are useful for applications at room temperature, and can thus find applications in e.g. intermediate temperature proton exchange fuel cells

Mono- and bis-imidazolidinium ethynyl cations and the reduction of the latter to give an extended bis-1,4-([3]cumulene)-p-carbo-quinoid system

Sherpa Romeo yellow journal. This is the peer reviewed version. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived VersionsAn extended π-system containing two [3]cumulene fragments separated by a p-carbo-quinoid and stabilized by two capping N-heterocyclic carbenes (NHCs) has been prepared. Mono- and bis-imidazolidinium ethynyl cations have also been synthesized from the reaction of an NHC with phenylethynyl bromide or 1,4bis(bromoethynyl)benzene. Cyclic voltammetry coupled with synthetic and structural studies showed that the dication is readily reduced to a neutral, singlet bis-1,4-([3]cumulene)-p-carbo-quinoid due to the πaccepting properties of the capping NHCsYe

¹²⁵Te NMR Probes of Tellurium Oxide Crystals: Shielding-Structure Correlations

The local environments around tellurium atoms in a series of tellurium oxide crystals were probed by ¹²⁵Te solid-state NMR spectroscopy. Crystals with distinct TeO_<i>n</i> units (<i>n</i> from 3 to 6), including Na₂TeO₃, α-TeO₂ and γ-TeO₂, Te₂O­(PO₄)₂, K₃LaTe₂O₉, BaZnTe₂O₇, and CsYTe₃O₈ were studied. The latter four were synthesized through a solid-state process. X-ray diffraction was used to confirm the successful syntheses. The ¹²⁵Te chemical shift was found to exhibit a strong linear correlation with the Te coordination number. The ¹²⁵Te chemical-shift components (δ₁₁, δ₂₂, and δ₃₃) of the TeO₄ units were further correlated to the O–Te–O-bond angles. With the aid of ¹²⁵Te NMR, it is likely that these relations can be used to estimate the coordination states of Te atoms in unknown Te crystals and glasses

Correlation of network structure with devitrification mechanism in lithium and sodium diborate glasses

The intermediate-range structure of the network former in lithium and sodium diborate was studied using nuclear magnetic resonance spectroscopy. Specifically, 11B{10B} rotational-echo double resonance experiments were employed to determine the distribution of dipole couplings between these isotopes and in this way determine whether the intermediate range order of the borate network was the same in the glasses as in the crystal forms of these compounds. It was found that in the lithium diborate case the networks are in fact similar between glass and crystal, while in sodium diborate they differ substantially. Because lithium diborate shows homogeneous nucleation and growth on the laboratory time scale while sodium diborate does not, it was concluded that structural similarity between glass and crystal of the glass former correlates strongly with nucleation mechanism

Structural similarity on multiple length scales and its relation to devitrification mechanism: a solid-state NMR study of alkali diborate glasses and crystals

The nature of the devitrification process is a fundamental problem in glass science and governs the ultimate stability of glass. It is hypothesized that the devitrification mechanism of a given glass composition is strongly correlated with the extent of structural similarity at the intermediate-range level between the glass and crystal phase to which it transforms. However, relatively little information is available until now to test this hypothesis because of the lack of long-range periodicity in glass and the absence of effective analysis methods. In this work Li2O-2B2O3 (L2B) and Na2O-2B2O3 (N2B) glasses, which respectively exhibit homogeneous (internal) and heterogeneous (surface) devitrification mechanisms, and plus corresponding crystals from same batches were prepared and comprehensively studied by using multiple solid state NMR techniques. Of those, 11B MAS and MQMAS experiments were applied to reveal short-range order information; 7Li and 23Na spin-echo decay and 11B{6Li} and 11B{23Na} rotational echo double resonance (REDOR) experiments were used to determine homonuclear and heteronuclear interactions that can reflect the intermediate-range order. It was found that the local bonding environments of the L2B crystal and glass are quite similar, while there are significant differences in the N2B system. Moreover, the experimental results indicated that the second moment (M2) values of both homo- and heterodipolar interactions measured for the L2B glass are very close to those of its isochemical crystal, in contrast, the M2 values for N2B glass differ significantly from those of its crystal phase. These observations indicate the existence of a strong correlation between structural similarity at both the short and intermediate length scale, and nucleation mechanis

Intermediate-range order of alkali disilicate glasses and its relation to the devitrification mechanism

There are two general mechanisms of devitrification in glass: heterogeneous nucleation of crystals from surfaces and impurities, and homogeneous nucleation from the volume. It is thought that structural similarities between glass and crystal at the intermediate-range level influence the mechanism followed, however there are few scarse experimental studies to test this hypothesis. In this paper solid-state nuclear magnetic resonance spectroscopy is used to probe intermediate-range order in sodium and lithium disilicate glasses through measurement of the second moment of the distribution of dipolar couplings. These two glasses undergo heterogeneous and homogeneous nucleation respectively. The second moments measured for the lithium glass closely follow the trend established by the layered structures of the isochemical crystalline phases while the same measurements for the sodium glass do not. This observation supports the hypothesis that glasses capable of homogeneous nucleation are structurally more similar to the resulting crystalline phases than those glasses that exhibit only heterogeneous nucleationEstados Unido

Relating ¹³⁹La Quadrupolar Coupling Constants to Polyhedral Distortion in Crystalline Structures

A broad series of crystalline lanthanum oxide-based materials has been investigated through high-field ¹³⁹La solid state nuclear magnetic resonance (ssNMR) spectroscopy and ab initio density functional theory (DFT) calculations. The ¹³⁹La NMR spectra of LaBGeO₅, LaBSiO₅, LaBO₃, LaPO₄·1.8H₂O, La₂(SO₄)₃·9H₂O, and La₂(CO₃)₃·8H₂O are reported for the first time. Both newly reported and literature values of ¹³⁹La quadrupolar coupling constants (<i>C</i>_Q) are related to various quantitative expressions of polyhedral distortion, including sphericity (Σ) and ellipsoid span (ϵ). The compounds were separated into two groups based upon their polyhedral distortion behavior: compounds with the general formula LaMO₃, where M is a trivalent cation; compounds with different general formulas. The ¹³⁹La <i>C</i>_Q of the LaMO₃ family was found to correlate best with ϵ. The ¹³⁹La <i>C</i>_Q of non-LaMO₃ compounds correlates adequately to ϵ but is better described by Σ. The ¹³⁹La isotropic chemical shift (δ_iso^CS) of the non-LaMO₃ compounds is negatively correlated with the lanthanum coordination number; there is insufficient data from the LaMO₃ compounds to draw conclusions relating to chemical shift. DFT calculations of NMR parameters prove to be a sensitive probe of the quality of input geometry, with predicted parameters agreeing with experiment except in cases where the crystal structure is suspect