Nature Of Ion Transport Research Articles

Elucidating Mixed-Ion Conduction in Spinel Cathode Materials for Mg Batteries

The magnesium battery (utilizing the reversible reactivity of Mg2+) offers the potential to surpass the Li-ion battery in terms of energy-density and cost, thereby enabling next-generation devices such as lower-cost and higher-range electric vehicles.1 A crucial component defining the energy density of the Mg battery is the cathode, which must display reversible Mg intercalation and redox chemistry at sufficiently high capacities and potentials. Of all known cathode materials, the Mg spinels MgM2O4 (where M = Cr, Mn, V) have conclusively demonstrated high capacity and high potentials vs Mg2+/Mg.2–5 The spinel structure possesses 3D networks of diffusion channels, with a combination of theoretical and experimental studies suggesting relatively low barriers for Mg2+ diffusion.5,6 However, reversible Mg2+ intercalation kinetics have proven to be extremely inhibitive in oxide cathode materials in general, with large overpotentials (and voltage hysteresis) observed on charge and discharge. While the origin of this effect is still a matter of debate, it has been suggested that sluggish bulk transport of Mg2+ can partially account for the hysteresis observed, although surprisingly little is known about the nature of ion transport (both electrons and Mg2+) in these systems.In this presentation, we discuss a combination of experimental and theoretical tools used to extract key ion and electron transport properties across a range of Mg-containing spinels, Mg(Cr1−x M)2O4, where M = Mn, V, Ti. We reveal the structure-composition-property relationships that emerge across this range of compounds and discuss the relative and cooperative role that ion and electron motion play in limiting conduction kinetics in these materials. This study represents a detailed treatment of bulk ion and electron transport in these systems, and provides analytical and theoretical tools to further study electrode materials for such next-generation energy storage devices.

(Keynote) Electrolyzer Development and Process Optimization: Paving the Road for Industrial Carbon-Dioxide Electroreduction

Electrochemical reduction of CO2 is a promising method for converting a greenhouse gas into value-added products, utilizing renewable energy. Novel catalysts, electrode assemblies, and cell configurations are all necessary to achieve economically appealing performance. In this talk, I am going to present a zero gap electrolyzer cell, which converts gas phase CO2 to products without the need for any liquid catholyte. I will show how to optimize the cell components, to simultaneously provide industrially relevant performance, such as high partial current density (up to 800 mA cm-2), low cell voltage (−2.8-3.2 V), high conversion efficiency (up to 45 %), and high selectivity for CO production (over 95%).I will also demonstrate our CO2 electrolyzer cell, where multiple layers are connected into an electrolyzer stack, thus scaling up the electrolysis process. The operation of the cell was validated using both silver nanoparticle and copper nanocube catalysts. Evenly distributing the CO2 gas among the layers (parallel connection), the operation of the cell stack was identical to the sum of multiple single-layer cells. When passing the CO2 gas through the layers one after the other (serial gas connection), the CO2 conversion efficiency was increased remarkably.Finally, I will show how the cell can operate with pure water feed on the anode side, therefore avoiding the use of alkaline anolyte. A carefully designed activation protocol will be presented, which allows the cell to operate at high current density, using pure water feed. The effect of temperature and flow rate on the operation will also be presented. Finally, the nature of ion-transport through the anion exchange membrane as a function of the operational conditions will be discussed.

(Invited) Electrolyzer Development and Process Optimization: Paving the Road for Industrial Carbon-Dioxide Electroreduction

Electrochemical reduction of CO2 is a promising method for converting a greenhouse gas into value-added products, utilizing renewable energy. Novel catalysts, electrode assemblies, and cell configurations are all necessary to achieve economically appealing performance. In this talk, I am going to present a zero gap electrolyzer cell, which converts gas phase CO2 to products without the need for any liquid catholyte. This is the first report of a CO2 electrolyzer cell, where multiple stacks are connected, thus scaling up the electrolysis process. The operation of the cell was validated using both silver nanoparticle and copper nanocube catalysts, and the first was employed for the optimization of the electrolysis conditions. Upon this, CO formation with partial current densities above 250 mA cm−2 were achieved routinely, which was further increased to 300 mA cm−2 (with ~95 % Faradaic efficiency) by pressurizing the CO2 inlet. Evenly distributing the CO2 gas among the stacks (parallel connection), the operation of the multi-stack cell was identical to the sum of multiple single-stack cells. When passing the CO2 gas through the stacks one after the other (serial gas connection), the CO2 conversion efficiency was increased remarkably. Importantly, the presented electrolyzer simultaneously provides high partial current density, low cell voltage (−3.0 V), high conversion efficiency (up to 40 %), and high selectivity for CO production; while operating at up to 10 bar differential pressure.In the second part of the talk I will show how the cell can operate with pure water feed on the anode side, therefore avoiding the use of alkaline anolyte. A carefully designed activation protocol will be presented, which allows the cell to operate at high current density, using pure water feed. The effect of temperature and flow rate on the operation will also be presented. Finally, the nature of ion-transport through the anion exchange membrane as a function of the operational conditions will be discussed.

Negative Stefan-Maxwell Diffusion Coefficients and Complete Electrochemical Transport Characterization of Homopolymer and Block Copolymer Electrolytes

Nanostructured block copolymers are of particular interest as electrolytes in batteries with lithium metal anodes. The performance of electrolytes in batteries can be predicted only if three transport coefficients (ionic conductivity, κ, salt diffusion coefficient, D, and cation transference number, t0+) are known. We present complete electrochemical transport characterization of a microphase-separated SEO block copolymer electrolyte by reporting κ, D, and t0+ as functions of salt concentration. We compare the properties of the block copolymer electrolyte with those of PEO homopolymer electrolytes. Negative values of t0+ are observed in many cases. Recasting the transport parameters in terms of Stefan-Maxwell coefficients provides insight into the nature of ion transport in these electrolytes.

Dielectric losses and proton conductivity of polyantimonic acid membranes

Composite membranes consisting of polyvinyl alcohol and disperse particles of polyantimonic acid with different ratios of components were studied. Processes related to the relaxation and proton transport in a heterogeneous system were considered. The proton transport in the membranes was shown to occur both within the particle (along the chains of alternating wcapacitorater molecules and oxonium ions), forming macrodipoles, and in the intercrystallite space in the polymer matrix. The relaxation was described by the two-layer Maxwell–Wagner capacitor with an effective relaxation time. The relaxation activation energy obtained from the dielectric loss data coincided with the proton conductivity activation energy in the frequency-independent region, which points to the same nature of ion transport.

Mathematical models of ion transport through cell membrane channels

We discuss various models of ion transport through cell membrane channels. Recent experimental data shows that sizes of some ion channels are compared to those of ions and that only few ions may be simultaneously in any single channel. Theoretical description of ion transport in such channels should therefore take into account stochastic fluctuations and interactions between ions and between ions and channel proteins. This is not satisfied by macroscopic continuum models based on the Poisson-Nernst-Planck equations. More realistic descriptions of ion transport are offered by microscopic molecular and Brownian dynamics. We present a derivation of the Poisson-Nernst-Planck equations. We also review some recent models such as single-file diffusion and Markov chains of interacting ions (boundary driven lattice gases). Such models take into account discrete and stochastic nature of ion transport and specifically interactions between ions in ion channels.

Chloride Enhances Fluoride Transport in Anion Exchange Membranes

Anion exchange membrane (AEM) fuel cells are a promising candidate to become the next-generation fuel cells. One of the most important advantages of AEM fuel cells is that they can utilize non-noble metals catalysts, because the oxygen reduction reaction is more facile in an alkaline fuel cell environment.1 Furthermore, a variety of more complex fuels can be used in these AEM fuel cells and generate high power densities at moderate operating temperatures.2 The study of ion transport in a charged polymer swollen with water is challenging, however. Transport is hindered by steric blockades, Coulombic barriers, and tortuous pathways. Molecular dynamics (MD) simulations can give insight into the nature of ion transport and suggest possible synthetic means to improve transport. Poly(vinyl benzyltrimethyl ammonium), PVBTMA, membranes have shown promising anionic conductivity. Furthermore, the benzyltrimethyl ammonium (BTMA) cations are stable in an alkaline environment, and do not suffer from Hofmann elimination because there are no b-hydrogen atoms.3,4 Because of these advantages, one of the main goals in this study was to understand ion transport in PVBTMA. To directly compare with experiments, we studied fluoride ions (19F−) as the diffusing species, because its diffusivity can be readily probed by pulsed-field gradient spin-echo NMR experiments and it can be reasonably modeled by classical force fields in MD simulations. However, one complication arose when we compared the experimental data with simulations. It turned out that the binding between the ammonium cation and the chloride ions, the natural counterions left in the syntheses, is so strong that 100% exchange with fluoride could not be accomplished experimentally. This motivated us to study how chloride can affect fluoride mobility. To our surprise, at 300 K, we found in simulations that the self-diffusion constant of fluoride increases by 60% when fluoride content decreases from 100% to 40% (and 60% chloride), and it increases by 130% when fluoride content is down to 10% (see Figure 1). We hypothesize the root cause of this enhancement effect is due to the lower surface electric field of the chloride ion than that of the fluoride ion. As a chloride approaches a cation, it loses its water solvation shells more easily, and more freed water becomes available for ion transport. The net effect is that the mobility of fluoride is facilitated. This effect is likely more general than just the fluoride/chloride case reported here. Funding for this project was provided by the Army Research Office for the Multidisciplinary University Research Initiative (MURI) under contract W911NF-10-1-0520.**Please insert the attached figure here**Figure 1: The self-diffusion constant of fluoride (DFluoride) is plotted as a function of %Fluoride. The diffusion constant increases by as much as 130 % when %Fluoride decreases from 100% to 10%.

Influence of Cationic ordering on ion transport in NASICONs: Molecular dynamics study

Abstract Molecular dynamics investigations suggest the significance of Si/P ordering on the nature of ion transport in NASICONs (acronym for Na-superionic conductor). For certain Si/P ordering predominantly two dimensional transport of Na + ions in Na 2 Zr 2 SiP 2 O 12 is predicted, which is otherwise known to be an isotropic ionic conductor. The microscopic energetic barriers and topology of the conduction channels are analyzed.

Three-dimensional nature of ion transport in thin-layer electrodeposition.

A generalized three-dimensional model for ion transport in electrodeposition is introduced. Ion transport is mainly governed by diffusion, migration, and convection. When convection prevails, in particular, in the limiting case of gravity-driven convection, the model predicts concentration shells and convection rolls and their interaction mode with a deposit tip: shell and roll bend and surround the tip forming a three-dimensional envelope tube squeezed at the deposit tip. In the limiting case of electrically driven convection, a vortex ring and an electric spherical drop crowning the deposit tip are predicted. When gravity and electric convection are both relevant, the interaction of ramified deposits, vortex tubes and rings, and electric spherical drops, leading to complex helicoidal flow, is predicted. Many of these predictions are experimentally observed, suggesting that ion transport underlying dendrite growth is remarkably well captured by our model.

Kinetics of ion depolarization of Si-MOS structures in the linear voltage sweep regime

A new approach is developed to describe isothermal ion depolarization of Si-MOS structures in the regime of linear variation of the gate electrode potential Vg. The approach is based on the experimentally proven fact of the substantially nonequilibrium nature of ion transport in such structures, with the ions initially localized on the SiO2/Si interface in shallow bulk traps. Dynamic current-voltage characteristics of the depolarization are calculated on the basis of the thermal-emission mechanism of ion transport through the barrier formed by the polarizing voltage (Vg>0). The calculated results afford a good description of the experimental data: narrow current peaks with half-width ∼kT localized near Vg=0, their temperature dependence, and the sweep rate and initial ion density at the SiO2/Si interface. On this basis, the effective ion mobility μθ=(2.5–11)×10−8 cm2/(V·s) (θ is the sticking coefficient), activation energy μ(Eμ≅0.6 eV) and depth of the bulk traps (trap energy) in SiO2 (Et≅0.2 eV) have been found in the temperature range 423–453 K. According to the available data, the values Eμ≅0.6 eV are characteristic of mobile Na+ ions.

Non-linear and fractal dynamic processes in superionics

Abstract The relaxation in superionic conductor near electrode interface has been considered with computer simulation method. Specific effects have been explained in the terms of fractal nature of ion transport in the systems of conducting channels. Our results and also the analogy with the diffusion front in percolation theory and the fractal geometry approach allow us to propose the explanation for the described non-linear effects.