Activation Energy For Ion Diffusion Research Articles

Entropy Drives Accelerated Ion Diffusion upon Carbon Dioxide Expansion of Electrolytes.

Carbon dioxide-expanded liquids, organic solvents with high concentrations of soluble carbon dioxide (CO2) at mild pressures, have gained attention as green catalytic media due to their improved properties over traditional solvents. More recently, carbon dioxide-expanded electrolytes (CXEs) have demonstrated improved reaction rates in the electrochemical reduction of CO2, by increasing the rate of delivery of CO2 to the electrode while maintaining facile charge transport. However, recent studies indicate that the limiting behavior of CXEs at higher CO2 pressures is a decline in solution conductivity due to reduced polarity, leading to poorer charge screening and greater ion pairing. In this article, we employ molecular dynamics simulations to investigate the energetic driving forces behind the diffusive properties of an acetonitrile and tetrapropylammonium hexafluorophosphate (TPrAPF6) CXE with increasing CO2 concentration. Our results indicate that entropy drives solvent and electrolyte diffusion with increasing CO2 pressure. The activation energy of ion diffusion increases with higher concentrations of CO2, indicating that increasing the temperature may improve solution conductivity in these systems. This trend in the activation energies is traced to stronger cation-anion Coulombic interactions due to weaker solvent screening at high CO2 concentrations, suggesting that the choice of ion may provide a route to diminish this effect.

Activation Energy of Ion Diffusion in an Electrode Material: Theoretical Calculation and Experimental Estimation with LiCoVO4 as an Example

The development of electrode materials for metal-ion batteries is a complex and resource-demanding process. The optimization of this development process requires a combination of theoretical and experimental methods. The former is used to predict the properties of materials and the latter to confirm them. Thus, it is very important to understand how the results of the modeling and experiment are related. In this study, we compare the results of determining the activation energies of lithium ion diffusion in cobalt(II)-lithium vanadate(V), which we obtained by calculations from first principles within the framework of density functional theory (DFT), with the experimental results, which we achieved by applying electrochemical methods such as cyclic voltammetry and galvanostatic and potentiostatic pulses. Based on the experimental and theoretical data obtained for LiCoVO4, we hypothesize that the limitation of the practically realizable capacity of the material at about 1/3 of the theoretical one is due to its structural limitations that lead to the impossibility of involving all lithium ions in the current-forming process. This reason is fixed by the simulation results, but is not detected by the experimental results.

Voltage-driven motion of nitrogen ions: a new paradigm for magneto-ionics

Magneto-ionics, understood as voltage-driven ion transport in magnetic materials, has largely relied on controlled migration of oxygen ions. Here, we demonstrate room-temperature voltage-driven nitrogen transport (i.e., nitrogen magneto-ionics) by electrolyte-gating of a CoN film. Nitrogen magneto-ionics in CoN is compared to oxygen magneto-ionics in Co3O4. Both materials are nanocrystalline (face-centered cubic structure) and show reversible voltage-driven ON-OFF ferromagnetism. In contrast to oxygen, nitrogen transport occurs uniformly creating a plane-wave-like migration front, without assistance of diffusion channels. Remarkably, nitrogen magneto-ionics requires lower threshold voltages and exhibits enhanced rates and cyclability. This is due to the lower activation energy for ion diffusion and the lower electronegativity of nitrogen compared to oxygen. These results may open new avenues in applications such as brain-inspired computing or iontronics in general.

Magic-angle-spinning-induced local ordering in polymer electrolytes and its effects on solid-state diffusion and relaxation NMR measurements.

Magic-angle-spinning (MAS) enhances sensitivity and resolution in solid-state nuclear magnetic resonance (NMR) measurements. MAS is obtained by aerodynamic levitation and drive of a rotor, which results in large centrifugal forces that may affect the physical state of soft materials, such as polymers, and subsequent solid-state NMR measurements. Here, we investigate the effects of MAS on the solid-state NMR measurements of a polymer electrolyte for lithium-ion battery applications, poly(ethylene oxide) (PEO) doped with the lithium salt LiTFSI. We show that MAS induces local chain ordering, which manifests itself as characteristic lineshapes with doublet-like splittings in subsequent solid-state 1 H, 7 Li, and 19 F static NMR spectra characterizing the PEO chains and solvated ions. MAS results in distributions of stresses and hence local chain orientations within the rotor, yielding distributions in the local magnetic susceptibility tensor that give rise to the observed NMR anisotropy and lineshapes. The effects of MAS were investigated on solid-state 7 Li and 19 F pulsed-field-gradient (PFG) diffusion and 7 Li longitudinal relaxation NMR measurements. Activation energies for ion diffusion were affected modestly by MAS. 7 Li longitudinal relaxation rates, which are sensitive to lithium-ion dynamics in the nanosecond regime, were essentially unchanged by MAS. We recommend that NMR researchers studying soft polymeric materials use only the spin rates necessary to achieve the desired enhancements in sensitivity and resolution, as well as acquire static NMR spectra after MAS experiments to reveal any signs of stress-induced local ordering.

A novel proton conducting ionogel electrolyte based on poly(ionic liquids) and protic ionic liquid

As opposed to the traditional hydrogels and organogels, ionogels are a new class of gel family where the liquid phase, percolating through the solid phase, consists of an ionic liquid (IL). The properties inherent in the ionic liquids, including high ionic conductivity, low-volatility and wide electrochemical stability window, are imparted to the ionogels. In this work, a novel proton conducting ionogel polymer electrolyte (iGPE) based on a poly (ionic liquid) Poly(diallyldimethylammonium) bis(trifluoromethanesulfonyl)imide, ([PDADMA][TFSI]) and protic ionic liquid, N,N-diethylmethylammonium triflate ([DEMA][Tf]) is prepared. The influence of the protic ionic liquid content on the physical, electrochemical, and ion transport properties of the iGPE has been investigated by differential scanning calorimetry (DSC), electrochemical impedance spectroscopy (EIS), and multi-nuclear solid-state and pulse-field gradient nuclear magnetic resonance. Upon addition of 60 wt% protic IL, the conductivity of the iGPE can be enhanced to 5.8 mS/cm at 100 °C, which is signifiantly higher than that of Nafion at 120 °C (0.2 mS/cm). Solid-state and PFG NMR measurements show that although the incorporation of [PDADMA][TFSI] leads to decreased ion dynamics, it promotes proton dissociation, resulting in a lower activation energy for ion diffusion compared to the neat protic IL. Evidence of proton activity in the iGPEs was obtained by cyclic votammetry, making this material a promising candidate as a proton conducting membranes in fuel cell applications.

Diffusion Mechanisms for Ions over Hydroxylated Surfaces: Cu on γ-Al2O3

The energies involved in the diffusion of Cu2+ and Cu+ over hydroxylated I-alumina were modeled with density functional theory using explorative molecular dynamics. This is the first time that the mechanism for diffusion of ions over hydroxylated surfaces is studied. It is found that the crucial requirement for feasible activation energies for ion diffusion is the prevention of charge separation. This can be realized either by counterion codiffusion or proton contradiffusion. Furthermore, the effects of the cation valency, hydroxylation level, and nature of the counterions were studied and general trends for diffusing cations on hydroxylated surfaces were postulated. At full hydroxylation, all charge compensation is performed by proton contradiffusion, while at intermediate hydroxylation levels, a combination of proton contradiffusion and counterion codiffusion occurs. Finally, energy barriers for codiffusion are related to the bonding strength of the counterions to the surface, which depends on the counterion and the hydroxylation level.

Photoinduced Stark Effects and Mechanism of Ion Displacement in Perovskite Solar Cell Materials.

Organometallic halide perovskites (OMHPs) have recently emerged as a promising class of materials in photovoltaic technology. Here, we present an in-depth investigation of the physics in these systems by measuring the photoinduced absorption (PIA) in OMHPs as a function of materials composition, excitation wavelength, and modulation frequency. We report a photoinduced Stark effect that depends on the excitation wavelength and on the dipole strength of the monovalent cations in the A position of the ABX3 perovskite. The results presented are corroborated by density functional theory calculations and provide fundamental information about the photoinduced local electric field change under blue and red excitation as well as insights into the mechanism of light-induced ion displacement in OMHPs. For optimized perovskite solar cell devices beyond 19% efficiency, we show that excess thermalization energy of blue photons plays a role in overcoming the activation energy for ion diffusion.

Using ellipsometry with lock-in detection to measure activation energy of ion diffusion in ionic and mixed conductors

We describe a technique for measuring the activation energy of ion diffusion in ionic and mixed ionic/electronic conductors. The technique is based on monitoring small changes in refractive index near the interface of a semitransparent gold electrode with the sample surface. Constant bias voltage is applied to the sample to weakly perturb the distribution of charge carriers near this front electrode. The relaxation process induced by bias removal is probed by applying alternating voltage and monitoring by ellipsometry with lock-in detection the changes in the refractive index dominated by changes in material polarizability. Since the ionic contribution to the total material polarizability is much larger than that of electrons or protons, the diffusion of ions can be distinguished. Measurements were made as a function of temperature on single crystals of 8mol% Y-stabilized zirconia (YSZ8), on ceramic pellets of 20mol% Gd doped CeO2 (GDC20) and on single crystals of 0.03mol% Fe-doped SrTiO3 (Fe-STO). In YSZ8, a single moving species (oxygen vacancies) with activation energy of 0.8eV was detected. The “wet” and “dry” states of GDC20 can be clearly distinguished: in the “wet” state there are mobile species other than oxygen vacancies, most likely protons. In Fe-doped SrTiO3, the proposed technique can reliably measure the activation energy of oxygen ion diffusion on a background of the much larger electronic conductivity.

Sintering of 4YSZ (ZrO 2 + 4 mol% Y 2O 3) nanoceramics for solid oxide fuel cells (SOFCs), their structure and ionic conductivity

In this work 4YSZ nanoparticles were produced with a variant of the sol–gel method, the particles were sintered using two different heating schemes, and the properties of the resulting pellets investigated. XRD spectra show the crystal structure of the particles to be mainly cubic, and TEM pictures reveal their size to be around 20 nm. The relative densities of the sintered pellets are found with the Archimedes method to lie between 87 and 97% of the theoretical material density, and the grain size, using thermal etching and SEM imaging, found to lie between 0.2 and 5 μ m, depending on the sintering temperature and the presence of impurities. The pellets were subjected to electrical impedance spectroscopy making it possible to distinguish the internal crystal and grain boundary impedances due to their different time-constants. With the “brick layer model” the approximate grain boundary thickness was found to lie between 4 and 8 nm, depending on the sintering conditions and on the presence of impurities. The specific resistances of the grain interiors and the grain boundaries, and the activation energies for ion diffusion in both, are presented. The ionic conductivities of the pellets are compared with literature values, and are found to compare favorably with these, at some temperatures even with data for 8YSZ, which is the standard material used for electrolytes due to its higher ionic conductivity, but which is also mechanically weaker than 4YSZ.

Effect of size confinement on CdSe nanocrystals in a GeO 2 glass matrix characterized by photoacoustic spectroscopy

Change in the electronic states due to quantum confinement effect was studied for CdSe nanocrystals in a GeO 2 glass matrix by photoacoustic (PA) spectroscopy. The PA technique is a photothermal method which is useful for optical absorption characterization of opaque samples. In the PA method, one can detect an acoustic energy directly proportional to the thermal energy induced by nonradiative processes. The lowest excited energy of CdSe nanocrystals in a GeO 2 glass by quantum confinement effect shifts to lower energy region with the increase of annealing time and temperature, indicating the increase of nanocrystal size. The nanocrystal growing is a diffusion process and the thermal activation energy of ion diffusion during nanocrystal growth is determined to be 1.7 eV.

Electron, ion, and photon beam induced reduction of SnO2(110): Power dissipation thresholds

Incident electrons, ions, and photons stimulate the desorption of surface oxygen from the (110) surface of SnO2. In the case of incident electrons, this oxygen desorption provides an extremely sensitive in situ measure of electron beam effects, by means of the surface conductivity properties of the substrate. Energy relaxation processes other than stimulated desorption also follow the initial excitation, and raise the effective local temperature. Subsequent temperature-dependent changes in the surface and subsurface composition and structure provide characteristic material-dependent power dissipation thresholds, which can be detected with great sensitivity. At the (110) surface of the ionic semiconductor SnO2, the thermal threshold or activation energy for ion diffusion is apparently lowered by an electrostatic field, induced by the electron beam in the space-charge region.

Effect of Size Confinement on the Electronic States of CdS Cluster in a Germanium Oxide Matrix

Change in the electronic state due to size confinement was studied for the CdS cluster embedded in GeO2 glass by photoacoustic, optical absorption and photoluminescence spectroscopies. The cluster size increases with increasing annealing temperature and time. The activation energy of ion diffusion during cluster growth is 0.45 eV. When the cluster size becomes less than about twice the exciton size, the energy bands become discrete. The energies of 1st and 2nd excited states increase with decreasing cluster size. The size dependence of the ist excited state determined experimentally coincides with the calculation. The energy of the surface state also shifts in parallel with the shift of the 1st excited state. When the cluster size decreases, the intensity of the edge emission decreases faster than that of the surface emission.

7Li NMR spectra, nuclear relaxation, and lithium ion motion in alkali silicate, borate, and phosphate glasses

Investigations of the 7Li nuclear magnetic relaxation time T 1 and of the broad line spectra over a wide temperature range are reported for binary and ternary alkali silicate, alkali borate, and alkali phosphate glasses. Some measurements in low effective fields ( T 1D, T 1ϱ) have also been made. Second-moment data yield information concerning the distribution of the alkali ions in the glass. Motional narrowing is governed by a mixture of dipolar and quadrupolar effects. There are two relaxation mechanisms, one due to diffusion, which is operative between approximately 250 K and the transformation temperature, and the other one operative below. The high-temperature Zeeman relaxation is dominated by the time-dependent coupling of the lattice to the quadrupole moment, the dipole relaxation by the dipole-dipole interaction. The temperature dependences can be explained only by a distribution which incorporates decreasing values of correlation times. Typical quantities such as the activation energy for ion diffusion and the width parameter are reported and discussed in terms of the glass composition. Anomalous relaxation at low temperatures has been found also to be related to the distribution of correlation times, and it is suggested that the relaxation be described by two-level tunneling modes in a double minimum potential. T 1/ T 1D ratios of the order of 100 have indicated the existence of slow motions in this region.

Magnetic aging effect of the Fe3O4-γ⋅Fe2O3 solid solution

During slow oxidation of magnetite powder in the air, we can obtain Fe3O4-γ⋅Fe2O3 solid solutions. There are expressed in the formula ?? x/3Fe3-x/3O4, where ?? is the cation vacancy and (x) is the degree of oxidation. The composition which corresponds to (x)=0.65 shows an abnormal increase in coercive force. After the sample is prepared, its coercive force increases gradually ; this phenomenon is a kind of aging effect.The present author tries to describe this abnormal phenomenon of magnetic properties and discuss the origin and mechanism of the effects. The author applies the "nucleation model" to the aging effect. The oxidation does not proceed uniformly in the magnetite particle and some structural defects act as the nuclei of oxidation. These nuclei behave supermagnetically when their sizes are small, but, when they grow up to the range of single domain size, their coercive force increases. The aging effect takes place by the diffusion of ferric ions, cation vacancies and electrons. The activation energy for ion diffusion decreases because this solid solution is an electron conductive substtance. Further experiments and discussions will be published.