Solid-state ionics in energy converters

Solid-state ionics, based on fast ionic transport in solids and characterized by anomalously large diffusion coefficients of ions at room temperature and unipolar conductivity (0.05–0.1 S cm^?1), assumed in the last few years great importance in the context of creation and subsequent explanation of mechanisms and kinetics of electrode processes during the work of chemical power sources (CPS). It is demonstrated that products of cathodic reactions possess ionic conduction in a solid phase.     

Anomalously large kinetic isotope effect

Activated diffusion of water between macromolecules in swollen cellulose is accompanied by anomalously high kinetic isotope effects of oxygen. The separation factor of heavy-oxygen water (H₂ ¹⁸O /H₂ ¹⁶O) is thousands of permilles instead of tens of permilles according to modern Absolute Rate Theory. This anomalous separation under usual conditions is disguised by the opposing process of very fast equalization to equilibrium through water-filled cellulose pores. This process is quicker by approximately 3 orders of magnitude than diffusion through the cellulose body. As a consequence, this opposition-directed equalization virtually eliminates the results of isotope separation. To reveal this anomaly it is necessary to suppress equalization, which was the primary problem for both discovery of this anomaly and its investigation. The method of investigating the anomalous separation in cellulose was developed with suppression of this negative influence. Discussion of the theoretical nature of the anomalous kinetic isotope effect is presented. This theoretical study would probably permit the discovery and use for isotope separation of the anomalously high isotope effect for other chemical elements, in particular, for those heavier than oxygen.      

Pulsed field gradient NMR study of anomalous diffusion in a lecithin-based microemulsion

Self-diffusion measurements in microemulsion systems composed of a naturally occurring soybean lecithin mixture, an aqueous phase, either water or a 1% aqueous PDADMAC solution, and isooctane were accomplished by pulsed field gradient (PFG) 1H NMR spectroscopy at oil dilution lines of low and intermediate water/lecithin ratios. The concentration-dependent diffusion data reveal water-in-oil (W/O) reverse micellar aggregates with dimensions on the nanometer scale being slightly smaller at low water content. With increasing micellar volume fractions, both hydrodynamic as well as direct interactions between particles significantly slow aggregate diffusion. The surfactant mean square displacements (msd's) in dilute and concentrated polymer-free systems studied as a function of diffusion time (20-1000 ms) are characterized by a crossover from Gaussian diffusion, due to slow aggregate motion, to anomalously enhanced diffusion, due to fast surface-bulk surfactant exchange at intermediate times revealing weak, barrier-controlled adsorption behavior. Upon addition of the polycation PDADMAC, the diffusion characteristics change to exclusively superdiffusive behavior with surfactant msd scaling with time as t(3/2) over the entire time range studied. This is caused by surfactant molecules performing Levy walks along the surface of reverse micelles mediated by the dilute bulk. The bulk-mediated surface diffusion is a consequence of the diffusion-controlled micelle-bulk exchange dynamics induced by interactions of PDADMAC with surfactant headgroups.     

Contribution of rotational diffusion to pulsed field gradient diffusion measurements

NMR diffusion experiments employing pulsed field gradients are well established as sensitive probes of the displacement of individual nuclear spins in a sample. Conventionally such measurements are used as a measure of translational diffusion, but here we demonstrate that under certain conditions rotational motion will contribute very significantly to the experimental data. This situation occurs when at least one spatial dimension of the species under study exceeds the root mean square displacement associated with translational diffusion, and leads to anomalously large apparent diffusion coefficients when conventional analytical procedures are employed. We show that in such a situation the effective diffusion coefficient is a function of the duration of the diffusion delay used, and that this dependence provides a means of characterizing the dimensions of the species under investigation.     

Domain model for anomalously fast diffusion

Electrostatic calculations are performed in idealized β-Al₂O₃, containing domains with displaced sodium ions. The average additional energy per sodium ion is calculated for several sizes and shapes of the domains. The movement of the domain walls is considered to be essential in the process of anomalously fast conduction. The electrostatic energy contribution to the activation energy for wall movement is calculated. Some rules are formulated which might permit the selection of possible super ionic conductors by electrostatic calculations in ionic compounds.     

Kinetics of surface segregation of bismuth atoms at the interface of a mechanically renewable Ag-Bi alloy electrode with a surface-inactive electrolyte solution

The time effects observed on mechanically renewed electrodes of eutectic-type Ag-Bi alloys in NaF solutions in the potential range of ideal polarizability are studied by the impedance method and cyclic voltammetry. The changes in the electric double layer (EDL) capacitance observed with the increase in the time of the electrode-solution contact from the moment of electrode renewal indicate that the process of surface enrichment with Bi atoms occurs. The analysis of obtained data leads to a conclusion that the surface segregation of Bi proceeds by the mechanism of surface diffusion, which provides the anomalously high diffusion rates as compared with the processes of volume diffusion in the solid phase. The results of studies of the surface segregation of bismuth on the Ag-Bi alloy/electrolyte interface are compared with the Auger-spectroscopic data for the interface of the same alloy with vacuum. Assumptions are put forward and substantiated that allow the differences in the kinetics of surface segregation processes observed on different interfaces to be consistently interpreted.     

The reaction of acrolein with acetylperoxyl radicals in the gas‐phase

A rate constant for the epoxidation of acrolein by acetylperoxyl radicals has been determined to be k₄ = (1.3 ± 0.9) × 10⁴ dm³mol⁻¹s⁻¹ at 383 K, which is anomalously fast in comparison with the epoxidation of alkenes. Abstraction of the acyl hydrogen atom from acrolein by acetylperoxyl radicals at 393 K was found to be at least 60 times slower than from acetaldehyde and at least three orders of magnitude slower than abstraction of the acyl hydrogen atom of the epoxide of acrolein. The fast rate for epoxidation of acrolein and the slow rate for hydrogen abstraction provide an explanation for the anomalously slow rate for the autoxidation of acrolein and suggests that acrolein formed during the autoxidation of alkene will react further to give its epoxide, and not exclusively by abstraction of the acyl hydrogen atom as was previously accepted. © 1999 John Wiley & Sons, Inc., Int J Chem Kinet 31: 277–282, 1999     

The interpretation of pressure-dependent very-low-pressure pyrolysis experiments

Unimolecular rate data from systems such as very-low-pressure pyrolysis (steady-state flow) and static experiments where gas-gas collisions compete with gas-wall collisions must be interpreted in terms of reactant inhomogeneity arising from finite diffusion rates, rather than using the usual assumption of a well-stirred reactor. An integrodifferential equation describing this process is derived, and a numerical variational solution applicable to weak gas-gas collisions is presented. This gives a powerful method for obtaining collisional energy transfer probabilities from such experiments. Previously reported data (on cyclobutane and cycloheptatriene reactions) are reinterpreted to give conventional values for average energy transfer, replacing the anomalously low collisional efficiencies proposed previously.     

Dynamic structures of aqueous oxalate and the effects of counterions seen by 2D IR

Two dimensional vibrational echo spectra of oxalate in the carboxylate asymmetric stretch region in D(2)O show two transitions having anomalously slow spectral diffusion and a third transition having relaxation properties typical of the free carboxylate ion. Quantitative analysis of the frequency shifts of the carboxylate asymmetric stretch modes caused by a singly charged cation in the oxalate hydration shell supports that ion pairs can be responsible for these new transitions. Experimental evidence and DFT calculations are consistent with oxalate forming a mixture of "side-on" and "end on" contact ion pairs wherein the carboxylate groups are protected from mobile heavy water molecules.     

Nuclear magnetic resonance diffusion with surface relaxation in porous media

Nuclear magnetic resonance (NMR) diffusion simulations with surface relaxation were performed numerically in unconsolidated and consolidated porous media by a random walk technique. Two uniform and nonuniform models of surface relaxation were proposed and compared. The apparent diffusion coefficient and extinction function were determined and studied in the fast, slow and intermediate diffusion regimes of relaxation. According to theoretical predictions, it was observed that the extinction function does not depend on surface relaxivity parameter rho 2 in the slow diffusion regime. The apparent diffusion coefficients are independent of rho 2 in the fast diffusion regime and tend to be superposed onto a single curve in the slow one. The evolution of the apparent diffusion coefficients is gathered by a reduced representation in the fast diffusion regime.     

Interfacial instabilities affect microfluidic extraction of small molecules from non-Newtonian fluids

As part of a project to develop an integrated microfluidic biosensor for the detection of small molecules in saliva, practical issues of extraction of analytes from non-Newtonian samples using an H-filter were explored. The H-filter can be used to rapidly and efficiently extract small molecules from a complex sample into a simpler buffer. The location of the interface between the sample and buffer streams is a critical parameter in the function of the H-filter, so fluorescence microscopy was employed to monitor the interface position; this revealed apparently anomalous fluorophore diffusion from the samples into the buffer solutions. Using confocal microscopy to understand the three-dimensional distribution of the fluorophore, it was found that the interface between the non-Newtonian sample and Newtonian buffer was both curved and unstable. The core of the non-Newtonian sample extended into the Newtonian buffer and its position was unstable, producing a fluorescence intensity profile that gave rise to the apparently anomalously fast fluorophore transport. These instabilities resulted from the pairing of rheologically dissimilar fluid streams and were flowrate dependent. We conclude that use of non-Newtonian fluids, such as saliva, in the H-filter necessitates pretreatment to reduce viscoelasticity. The interfacial variation in position, stability and shape caused by the non-Newtonian samples has substantial implications for the use of biological samples for quantitative analysis and analyte extraction in concurrent flow extraction devices.     

Correlations between radical distributions and structural defects of squalane and poly(methyl methacrylate) glasses in the oxidation kinetics of radicals

The kinetics have been studied for two processes whose limiting stage is the diffusion of molecular oxygen: (i) oxidation of radicals after irradiation; and (ii) quenching of phenanthrene phosphorescence. The processes were studied in glassy squalane and poly(methyl methacrylate) matrices. Comparison between the kinetic properties of these processes allowed us to conclude about predominant stabilization of radicals near structural defects of glass. In squalane this manifests itself in anomalously high value of reaction radius for oxidation of radicals and in poly(methyl methacrylate) in quantitative difference in the parameters of distribution in rate constants for processes (i) and (ii).     

SPEEDY: spin-echo enhanced diffusion filtered spectroscopy. A new tool for high resolution MAS NMR

The pulsed field gradient diffusion edited experiment, bipolar LED, has been combined with the Carr-Purcell-Meiboom-Gill (CPMG) spin-echo sequence for the analysis of solid phase resin samples. Spin-echo enhanced diffusion filtered spectroscopy (SPEEDY), when optimized, filters both the compounds that demonstrate fast diffusion rates as well as the compounds that demonstrate fast T(2) relaxation rates. Using this technique, compounds that are not covalently attached to the resin are not observed and contributions from the resin matrix are greatly attenuated. The interpretation of the resulting spectrum is more readily accessible. This technique lessens the importance of completely removing reaction residues or the wash solvents simply for analytical evaluation. The utility of the combined filtering scheme was demonstrated by the implementation into a NOESY sequence.     

Thermodynamics and kinetics of electrochemical Li+ intercalation in pyrophyllite

The possibility of directly using the natural mineral pyrophyllite for the efficient generation of Li⁺ intercalation current is demonstrated experimentally. The dependences of changes in the Gibbs energy and the entropy of the intercalation reaction on the degree of the guest lithium load are analyzed. A distinctive feature of the intercalation kinetics in Li _x Al₂(OH)₂[Si₂O₅]₂ is the anomalously high diffusion coefficients of lithium cations at x > 0.3.     

Interpretation of apparent activation energies for electron transport in dye-sensitized nanocrystalline solar cells

Electron transport in dye-sensitized nanocrystalline solar cells appears to be a slow diffusion-controlled process. Values of the apparent electron diffusion coefficient are many orders of magnitude smaller than those reported for bulk anatase. The slow transport of electrons has been attributed to multiple trapping (MT) at energy levels distributed exponentially in the band gap of the nanocrystalline oxide. In the MT model, release of immobile electrons from occupied traps to the conduction band is a thermally activated process, and it might therefore be expected that the apparent electron diffusion coefficient should depend strongly on temperature. In fact, rather small activation energies (0.1-0.25 eV) have been derived from time and frequency resolved measurements of the short circuit photocurrent. It is shown that the MT model can give rise to such anomalously low apparent activation energies as a consequence of the boundary conditions imposed by the short circuit condition and the quasi-static relationship between changes in the densities of free and trapped electrons. This conclusion has been confirmed by exact numerical solutions of the time-dependent generation/collection problem for periodic excitation that provide a good fit to experimental data.     

Pulsed field gradient NMR study of phenol binding and exchange in dispersions of hollow polyelectrolyte capsules

The distribution and exchange dynamics of phenol molecules in colloidal dispersions of submicron hollow polymeric capsules is investigated by pulsed field gradient NMR (PFG-NMR). The capsules are prepared by layer-by-layer assembly of polyelectrolyte multilayers on silica particles, followed by dissolution of the silica core. In capsule dispersion, (1)H PFG echo decays of phenol are single exponentials, implying fast exchange of phenol between a free site and a capsule-bound site. However, apparent diffusion coefficients extracted from the echo decays depend on the diffusion time, which is typically not the case for the fast exchange limit. We attribute this to a particular regime, where apparent diffusion coefficients are observed, which arise from the signal of free phenol only but are influenced by exchange with molecules bound to the capsule, which exhibit a very fast spin relaxation. Indeed, relaxation rates of phenol are strongly enhanced in the presence of capsules, indicating binding to the capsule wall rather than encapsulation in the interior. We present a quantitative analysis in terms of a combined diffusion-relaxation model, where exchange times can be determined from diffusion and spin relaxation experiments even in this particular regime, where the bound site acts as a relaxation sink. The result of the analysis yields exchange times between free phenol and phenol bound to the capsule wall, which are on the order of 30 ms and thus slower than the diffusion controlled limit. From bound and free fractions an adsorption isotherm of phenol to the capsule wall is extracted. The binding mechanism and the exchange mechanism are discussed. The introduction of the global analysis of diffusion as well as relaxation echo decays presented here is of large relevance for adsorption dynamics in colloidal systems or other systems, where the standard diffusion echo decay analysis is complicated by rapidly relaxing boundary conditions.     

Colloidal Supercapattery: Redox Ions in Electrode and Electrolyte

Redox chemistry is the cornerstone of various electrochemical energy conversion and storage systems, associated with ion diffusion process. To actualize both high energy and power density in energy storage devices, both multiple electron transfer reaction and fast ion diffusion occurred in one electrode material are prerequisite. The existence forms of redox ions can lead to different electrochemical thermodynamic and kinetic properties. Here, we introduce novel colloid system, which includes multiple varying ion forms, multi‐interaction and abundant redox active sites. Unlike redox cations in solution and crystal materials, colloid system has specific reactivity‐structure relationship. In the colloidal ionic electrode, the occurrence of multiple‐electron redox reactions and fast ion diffusion leaded to ultrahigh specific capacitance and fast charge rate. The colloidal ionic supercapattery coupled with redox electrolyte provides a new potential technique for the comprehensive use of redox ions including cations and anions in electrode and electrolyte and a guiding design for the development of next‐generation high performance energy storage devices.     

Modeling of surface exchange reactions and diffusion in composites including transport processes at grain and interphase boundaries

Surface exchange reactions and diffusion of oxygen in ceramic composites consisting of a dilute and random distribution of inclusions in a polycrystalline matrix (host phase) are modeled phenomenologically by employing the finite element method. The microstructure of the mixed conducting composite is described by means of a square grain model, including grain boundaries of the matrix and interphase boundaries between the inclusions and grains of the host phase. An instantaneous change of the oxygen partial pressure in the surrounding atmosphere may give rise to an oxygen exchange process, i.e., oxidation or reduction of the ceramic composite. Relaxation curves for the total amount of exchanged oxygen are calculated, emphasizing the role played by fast diffusion along the interfaces. The relaxation curves are interpreted in terms of effective medium diffusion, introducing appropriate equations for the effective diffusion coefficient and the effective surface exchange coefficient. When extremely fast diffusion along the grain and interphase boundaries is assumed, the re-equilibration process shows two different time constants. Analytical approximations for the relaxation process and relations for the separate relaxation times are provided for this limiting case as well as for blocking interphase boundaries. Furthermore, conductivity relaxation curves are calculated by coupling diffusion and dc conduction. In the case of effective medium diffusion, the conductivity relaxation curves do not deviate from those for the total amount of exchanged oxygen. On the contrary, the conductivity relaxation curves differ remarkably from the time dependence of the total amount of exchanged oxygen, when the different phases of the composite re-equilibrate with separate time constants.     

Overcoming Diffusion Limitation of Faradaic Processes: Property-Performance Relationships of 2D Conductive Metal-Organic Framework Cu3(HHTP)2 for Reversible Lithium-Ion Storage

Faradaic reactions including charge transfer are often accompanied with diffusion limitation inside the bulk. Conductive two-dimensional frameworks (2D MOFs) with a fast ion transport can combine both—charge transfer and fast diffusion inside their porous structure. To study remaining diffusion limitations caused by particle morphology, different synthesis routes of Cu-2,3,6,7,10,11-hexahydroxytriphenylene (Cu₃(HHTP)₂), a copper-based 2D MOF, are used to obtain flake- and rod-like MOF particles. Both morphologies are systematically characterized and evaluated for redox-active Li⁺ ion storage. The redox mechanism is investigated by means of X-ray absorption spectroscopy, FTIR spectroscopy and in situ XRD. Both types are compared regarding kinetic properties for Li⁺ ion storage via cyclic voltammetry and impedance spectroscopy. A significant influence of particle morphology for 2D MOFs on kinetic aspects of electrochemical Li⁺ ion storage can be observed. This study opens the path for optimization of redox active porous structures to overcome diffusion limitations of Faradaic processes.     

Finite element simulation of diffusion into polycrystalline materials

Diffusion in polycrystalline materials is investigated by means of numerical finite element simulations for constant source conditions. The grain boundaries are assumed to provide fast diffusion paths. Main emphasis is put on situations that typically occur for nanocrystals, viz. on situations in which (i) the diffusion length is significant compared with grain size, (ii) the influence of boundaries that are parallel to the surface become important in addition to the perpendicular ones. Furthermore, we treat the influence of blocking space charge layers sandwiching the core pathways and thus channeling grain boundary diffusion.