Session K10: Dynamics of Polymers and Electrolytes in Bulk and in Confinement

Dynamics of water in metal ion intercalated graphene oxide.

When water is confined on a nanometer scale, it exhibits several unusual behaviors which include ordering, viscoelasticity, and long relaxation times. The presence of ions further changes properties of water because ions influence ordering and interactions. Different ions, according to the Hofmeister series, have different effects on ordering and dynamics of confined water. The combination of Quasi Elastic Neutron Scattering and Atomic Force Microscopy allow us to measure the dynamics and viscoelasticity of water in GO membranes at different time scales for direct comparison of dynamics and viscosity, while matching size, composition, and temperature conditions. In the talk, we would like to present how the presence of three different cations (structure former, Na+, structure "destroyer", Cs+ and a "intermediate" ion, K+) will affect the interlayer and intra-layer diffusion and viscosity of water in GO.   

Ionic Structure and Decay Length in Highly Concentrated Confined Electrolytes

Under dilute electrolyte conditions, Debye-Huckel theory predicts that the characteristic decay length scales inversely with the square root of the electrolyte concentration. Recent experiments have reported an increase in the decay length with increasing electrolyte concentration for highly concentrated electrolytes (e.g., for >1 M in aqueous NaCl). We investigate the ionic structure in electrolytes confined by homogeneously-charged planar interfaces using coarse-grained molecular dynamics simulations. Our results reveal two distinct regimes of screening behavior as the concentration is changed from 0.1 M to 2.5 M for a wide range of electrolyte systems generated by tuning the interfacial separation, surface charge density, and ionic size. For low concentrations, the integrated charge exhibits a monotonic decay with a characteristic decay length decreasing sharply with increasing concentration. For high concentrations (e.g., >1 M for aqueous NaCl), the integrated charge has a non-monotonic behavior due to the formation of structured layers of cations and anions near the interfaces, driven by enhanced steric ion-ion correlations. The associated decay length rises with increasing concentration, albeit much more slowly than the rise observed in experiments.   

Frequency Dependence of Ionic Conductivity in Large-Scale Simulations of Concentrated Electrolytes

The transport of ions in a confined environment underlies a number of important technologies that utilize separations, electrochemistry, and energy storage and delivery. The collective net motion of ions due to an electric field is quantified by the conductivity.  At low concentrations and/or low ionic strengths, the conductivity is well explained by the classical transport theories of Debye, Hükel and Onsager. However, at high concentrations and/or high ionic strengths, the screening length acquires values on the order of the ion's diameter, and the inter-ionic correlations become significant, thereby noticeably increasing the complexity of the physical problem. To address this limitation, we investigate the frequency response of concentrated electrolytes using large scale Brownian dynamics simulations coupled with Poisson's equation. We compute the complex conductivity over four orders of magnitude of frequency by applying a sinusoidal electric field whose frequency exponentially increases with time. The analysis is performed with and without inter-particle hydrodynamic interactions over a large range of ionic concentrations, allowing us to find scaling relationships for important descriptors of the frequency-dependent conductivity as a function of the Debye screening length.    

Effect of a structure breaking salt on the mobility of nanoconfined water

Structure and dynamics of water vary dramatically between bulk state, confined state, and solution impacting various processes associated with energy storage applications and the transport of ions in biological membranes. Here, using quasielastic neutron scattering technique, we have investigated the effect of confinement in 3 nm silica pores on water diffusivity in aqueous solutions of archetypical structure-making (kosmotrope) NaCl and structure-breaking (chaotrope) KCl salts, up to 1.0 M in concentration. The water diffusivity in bulk aqueous solutions in this concentration range is known to decrease very slightly in the presence of NaCl and increase very slightly in the presence of KCl. However, the water diffusivity in confined H2O-KCl increases by a factor of 2 compared to the pure water diffusivity in the same confinement. This unusually strong cumulative effect of confinement and a structure-breaking salt may have profound implications for the mobility and transport of aqueous species in nature and technological applications.   

Effects of alkali ion dopants on the diffusion mechanisms and thermal stabilities of imidazolium-based organic ionic plastic crystal

Organic ionic plastic crystals (OIPCs), in which ions show rotational disorder with its crystalline structure maintained, are one of excellent candidates for the solid-state electrolytes of rechargeable batteries. Doping alkali ions in the OIPCs matters because one may affect both the thermal stability and the ionic conductivity, both of which are desired properties for the solid-state electrolytes. In this work, we perform molecular dynamics simulations to investigate the effects of alkali ion-doping on the melting temperatures and the translational diffusion of ions in OIPCs. In our simulations, we consider lithium ion (Li⁺), sodium ion (Na⁺), and potassium ion (K⁺) as doping agents on 1-methyl-3-methylimidazolium hexafluorophosphate ([MMIM][PF₆]) OIPCs. We find from our results that as alkali ions of smaller size are doped, the larger domain of the crystal becomes disordered. Because larger distortion disables the crystal to maintain its lattice structure, Li⁺ doped crystal shows the lowest melting temperature. Also, the translational diffusion mechanism of alkali ions depends on the kind of alkali ion dopants. Li⁺ undergoes continuous diffusion through amorphous regime, whereas K⁺ undergoes hopping diffusion. Na⁺ shows an intermediate diffusion feature between Li⁺ and K⁺.   

Chemical manipulaiton of polymer structure of composite polymer electrolytes

Composite polymer electrolytes comprising garnet nanoparticles and anion salts have been synthesized and their ionic conductivity investigated. We find that ion transport maximizes for low %wt. loads of the nanoparticles and that the ionic conductivity is controlled by the garnet particle Li-molar content. Microstructural studies of the composite electrolytes, reveal that the particle chemistry modifies the polymer macrostructure resulting in the formation of high ionic conductivity channels.   

In-Depth Study of Magnetic and Thermoelectric Properties of Gadolinium Offers Insight Toward New Spin and Quantum Driven Thermoelectric Materials Design Strategies

Gadolinium, the element with the highest number of unpaired 4f electrons, provides an ideal system for exploring spin and quantum-mediated thermoelectric properties. Its thermopower shows three distinct regions at <100 K, 100-240 K, and >240 K, shaped by the phonon-drag, bipolar diffusion, magnon-drag, and quantum effects. We investigate the anomalous magneto-thermopower trends by probing the magnetic, electrical, and thermal properties versus magnetic field and temperature. Below 100K, the phonon-drag dominates thermopower, while the trend from 100K to 240K follows the diffusive multi-carrier transport properties. The Berry curvature-dominated anomalous Hall conductivity affects the carrier transport properties through modification of the group velocity. Magneto-thermopower measurements at higher temperatures reveal positive magnon-drag thermopower and a negative diffusion contribution. The multifold spin and quantum effects lead to ~50% thermopower and ~70% zT enhancement near the Curie temperature (T_C~290K) despite >70% suppression of the magnon contribution at 12T. The magnetic field-driven spin and quantum effects distilled from this system offer a fundamentally new viewpoint to extend thermoelectric research to magnetic and quantum materials.   

Spin Fluctuation: A new route for enhancing zT in thermoelectrics

Spin fluctuations (SF) have already shown improving thermopower, but enhancing thermoelectric figure-of-merit (zT) has not been reported until now. We evidence remarkable zT enhancement (~80%) from SF in the ferromagnetic CrTe near and below the Curie temperature, T_C~335 K. The ferromagnetism in CrTe originates from the collective electronic and localized magnetic moments. The temperature- and field-dependent transport properties demonstrate the profound impact of SF on the electrons and phonons. The suppression of the SF with an external magnetic field leads to a strong reduction of the thermopower and enhancement of the thermal conductivity near and below T_C. The anomalous transport properties are investigated and analyzed based on theoretical models, and a good agreement with experimental data is found. Moreover, the detailed analysis of transport properties proves that spin-wave has an insignificant contribution to the transport properties. This study is contributory to understanding the SF-mediated thermoelectric properties and designing high-performance spin-driven thermoelectric materials.   

Energy conversion with Active Dipoles

To understand the creation of electromagnetic energy (or a photonic degree of freedom) from an external energy source, an electromotive force must be generated capable of separating positive and negative charges. The separation of charges (free or bound) may be modelled as a permanent polarization, which has a non-zero electric vector curl, created by an external force per unit charge, or fictitious electric field. The resulting system forms an active physical dipole in the static case, or an active Hertzian dipole in the time dependent case. This system is the electrical dual of the magnetic solenoid described by a magnetic vector potential and excited by an electrical current. Correspondingly, the creation of an active electric dipole is a voltage source may be described by an electric flux density, which exhibits an electric vector potential and a magnetic current boundary source, within the frame work of two-potential theory without the need for the existence of the magnetic monopole. From this result we derive the Dual electric Berry phase and make the conjecture that it should be equivalent to the geometric phase that is described in modern electric polarization theory, which also describes the nature of the permanent polarization of a ferroelectric or triboelectric material. This work gives a formal meaning to the electric vector potential that defines the etectric geometric phase, and determines that a permanent polarization output voltage has both a scalar and vector potential component, which must be considered to fully describe the nature of an active electric dipole. Additionally, we show that Faraday's and Ampere's law may be derived from the time rate of change of the Aharanov-Bohm phase and the DAB phase respectively, independent of the electromagnetic gauge.   

Withdrawn

From the early stage of planet formation due to charging between dust particles to state-of-art triboelectric nanogenerator, triboelectricity has been present nearby the history of the earth and remained an unprecedented unresolved problem. Here, we introduce the rigorously formulated theoretical triboelectric series based on the thermoelectric effect. Our work suggests the first theoretical quantitative triboelectric series, determined by triboelectric factors defined as Seebeck coefficient, density, specific heat, and thermal conductivity. We have revealed that the dissipation of frictional energy forms a temperature distribution in material and thus creates an electrostatic potential difference at the interface, which initiates charge transfer. Our findings are expected to deepen not only the microscopic understanding of triboelectricity but also provide an effective strategy to increase the efficiency of triboelectric nanogenerators.