367 results found
Ma Y, Sikdar D, He Q, et al., 2020, Self-assembling two-dimensional nanophotonic arrays for reflectivity-based sensing, Chemical Science, ISSN: 2041-6520
<p>We propose a nanoplasmonic platform that can be used for sensing trace levels of heavy metals in solutions <italic>via</italic> simple optical reflectivity measurements at the liquid–liquid interface.</p>
Kornyshev A, Pendry J, Sikdar D, 2020, Nanoparticle meta-grid for enhanced light extraction from light emitting devices, Light: Science and Applications, Vol: 9, ISSN: 2047-7538
Based on a developed theory, we show that introducing a meta-grid of sub-wavelength-sized plasmonic nanoparticles (NPs) into existing semiconductor light-emitting-devices (LEDs) can lead to enhanced transmission of light across the LED-chip/encapsulant interface. This results from destructive interference between light reflected from the chip/encapsulant interface and light reflected by the NP meta-grid, which conspicuously increase the efficiency of light extraction from LEDs. The “meta-grid”, should be inserted on top of a conventional LED chip within its usual encapsulating packaging. As described by the theory, the nanoparticle composition, size, interparticle spacing, and distance from the LED-chip surface can be tailored to facilitate maximal transmission of light emitted from the chip into its encapsulating layer by reducing the Fresnel loss. The analysis shows that transmission across a typical LED-chip/encapsulant interface at the peak emission wavelength can be boosted up to ~99%, which is otherwise mere ~84% at normal incidence. The scheme could provide improved transmission within the photon escape cone over the entire emission spectrum of an LED. This would benefit energy saving, in addition to increasing the lifetime of LEDs by reducing heating. Potentially, the scheme will be easy to implement and adopt into existing semiconductor-device technologies, and it can be used separately or in conjunction with other methods for mitigating the critical angle loss in LEDs.
McEldrew M, Goodwin ZAH, Bi S, et al., 2020, Theory of ion aggregation and gelation in super-concentrated electrolytes, Journal of Chemical Physics, Vol: 152, Pages: 1-19, ISSN: 0021-9606
In concentrated electrolytes with asymmetric or irregular ions, such as ionic liquids and solvent-in-salt electrolytes, ion association is more complicated than simple ion-pairing. Large branched aggregates can form at significant concentrations at even moderate salt concentrations. When the extent of ion association reaches a certain threshold, a percolating ionic gel network can form spontaneously. Gelation is a phenomenon that is well known in polymer physics, but it is practically unstudied in concentrated electrolytes. However, despite this fact, the ion-pairing description is often applied to these systems for the sake of simplicity. In this work, drawing strongly from established theories in polymer physics, we develop a simple thermodynamic model of reversible ionic aggregation and gelation in concentrated electrolytes accounting for the competition between ion solvation and ion association. Our model describes, with the use of several phenomenological parameters, the populations of ionic clusters of different sizes as a function of salt concentration; it captures the onset of ionic gelation and also the post-gel partitioning of ions into the gel. We discuss the applicability of our model, as well as the implications of its predictions on thermodynamic, transport, and rheological properties.
Bi S, Banda H, Chen M, et al., 2020, Molecular understanding of charge storage and charging dynamics in supercapacitors with MOF electrodes and ionic liquid electrolytes, Nature Materials, Vol: 19, Pages: 552-558, ISSN: 1476-1122
We performed constant-potential molecular dynamics simulations to analyse the double-layer structure and capacitive performance of supercapacitors composed of conductive metal-organic framework (MOF) electrodes and ionic liquids. The molecular modelling clarifies how ions transport and reside inside polarized porous MOFs, and then predicts the corresponding potential-dependent capacitance in characteristic shapes. The transmission line model was adopted to characterize the charging dynamics, which further allowed evaluation of the capacitive performance of this class of supercapacitors at the macroscale from the simulation-obtained data at the nanoscale. These 'computational microscopy' results were supported by macroscopic electrochemical measurements. Such a combined nanoscale-to-macroscale investigation demonstrates the potential of MOF supercapacitors for achieving unprecedentedly high volumetric energy and power densities. It gives molecular insights into preferred structures of MOFs for accomplishing consistent performance with optimal energy-power balance, providing a blueprint for future characterization and design of these new supercapacitor systems.
Zaboronsky AO, Kornyshev AA, 2020, Ising models of charge storage in multifile metallic nanopores., Journal of Physics: Condensed Matter, Vol: 32, Pages: 1-12, ISSN: 0953-8984
Ising type models of charging of conductive nanopores with ions have already been proposed and investigated for single file cylindrical or single layer slit nanopores. In such pores, the state of ions, the coulombic interactions of which are exponentially screened by their images in pore walls, was named superionic. In the present work we extend the analysis of the superionic state to nanopores that can accommodate multiple rows of ions. By grouping multiple charges in the same row into 'supercharges', we map the arrangement of ions in polarised electrodes on a multi-row Ising model in an external field. We investigate one-, two- and three-row cases, which we solve exactly, using a purpose-built semi-numerical transfer matrix method. For pores of different radii, which can accommodate the corresponding number of ion rows, we calculate the dependence of the electrical capacitance and stored energy density on electrode potential. As in charging the single file pores, we find that in narrower pores higher energy densities can be achieved at low applied potentials, while wider pores perform better as the voltage is increased.
Ma Y, Sikdar D, Fedosyuk A, et al., 2020, Electrotunable nanoplasmonics for amplified surface enhanced Raman spectroscopy, ACS Nano, Vol: 14, Pages: 328-336, ISSN: 1936-0851
Tuning the properties of optical metamaterials in real time is one of the grand challenges of photonics. Being able to do so will enable a new class of photonic materials for use in applications such as surface enhanced Raman spectroscopy and reflectors/absorbers. One strategy to achieving this goal is based on the electrovariable self-assembly and disassembly of two-dimensional nanoparticle arrays at a metal liquid interface. As expected the structure results in plasmonic coupling between NPs in the array but perhaps as importantly between the array and the metal surface. In such a system the density of the nanoparticle array can be controlled by the variation of electrode potential. Due to the additive effect, we show that less than 1 V variation of electrode potential can give rise to a dramatic simultaneous change in optical reflectivity from ~93 % to ~1 % and the amplification of the SERS signal by up to 5 orders of magnitude. The process allows for reversible tunability. These concepts are demonstrated in this manuscript, using a platform based on the voltage-controlled assembly of 40 nm Au-nanoparticle arrays at a TiN/Ag electrode in contact with an aqueous electrolyte. We show that all the physics underpinning the behaviour of this platform works precisely as suggested by the proposed theory, setting the electrochemical nanoplasmonics as a promising new direction in photonics research.
Di Lecce S, Kornyshev AA, Urbakh M, et al., 2020, Electrotunable lubrication with ionic liquids: the effects of cation chain length and substrate polarity., ACS Applied Materials and Interfaces, Vol: 12, Pages: 4105-4113, ISSN: 1944-8244
Electrotunable lubrication with ionic liquids (ILs) provides dynamic control of friction with the prospect to achieve superlubrication. We investigate the dependence of the frictional and structural forces with 1-n,2-methyl-imidazolium tetrafluoroborate [C n MIM]+[BF4]- (n = 2, 4, 6) ILs as a lubricant on the molecular structure of the liquid, normal load, and polarity of the electrodes. Using non-equilibrium molecular dynamics simulations and coarse-grained force-fields, we show that the friction force depends significantly on the chain length of the cation. ILs containing cations with shorter aliphatic chains show lower friction forces, ∼40% for n = 2 as compared to the n = 6 case, and more resistance to squeeze-out by external loads. The normal load defines the dynamic regime of friction, and it determines maxima in the friction force at specific surface charges. At relatively low normal loads, ∼10 MPa, the velocity profile in the confined region resembles a Couette type flow, whereas at high loads, >200 MPa, the motion of the ions is highly correlated and the velocity profile resembles a "plug" flow. Different dynamic regimes result in distinctive slippage planes, located either at the IL-electrode interface or in the interior of the film, which ultimately lead, at high loads, to the observation of maxima in the friction force at specific surface charge densities. Instead, at low loads the maxima are not observed, and the friction is found to monotonously increase with the surface charge. Friction with [C n MIM]+[BF4]- as a lubricant is reduced when the liquid is confined between positively charged electrodes. This is due to better lubricating properties and enhanced resistance to squeeze out when the anion [BF4]- is in direct contact with the electrode.
Levy A, Bazant M, Kornyshev A, 2020, Ionic activity in concentrated electrolytes: Solvent structure effect revisited, Chemical Physics Letters, Vol: 738, Pages: 1-5, ISSN: 0009-2614
We revisit the role of the local solvent structure on the activity coefficients of electrolytes within a nonlocal dielectric function approach. We treat the concentrated electrolyte as a dielectric medium and suggest an interpolation formula for its nonlocal dielectric response. The water dielectric response is approximated based on MD simulations and experimental data, that gives strong over-screening and oscillations in the potential, which are absent in the standard “primitive model” predictions. We obtain mathematically tractable closed-form expressions for the activity coefficients, in reasonable agreement with experimental data.
Sikdar D, Kornyshev A, 2019, An electro-tunable Fabry–Perot interferometer based on dual mirror-on-mirror nanoplasmonic metamaterials, Nanophotonics, Vol: 8, Pages: 2279-2290, ISSN: 2192-8606
Mirror-on-mirror nanoplasmonic metamaterials, formed based on voltage-controlled reversible selfassembly of sub-wavelength-sized, metallic nanoparticles (NPs) on thin metallic-film electrodes, are promisingcandidates for novel electro-tunable optical devices. Here, we present a new design of electro-tunable Fabry–Perotinterferometers (FPI), in which two parallel mirrors — each composed of a monolayer of NPs self-assembled on athin metallic electrode — form an optical cavity, which is filled with aqueous solution. The reflectivity of the cavitymirrors can be electrically adjusted, simultaneously or separately, via a small variation of electrode potentials thatwould alter the inter-NP separation in the monolayers. To investigate optical transmittance from the proposed FPIdevice we develop a nine-layer-stack theoretical model, based on our effective medium theory and multi-layer Fresnelreflection scheme, which produces excellent match when verified against full-wave simulations. We show that strongplasmonic coupling among silver NPs forming monolayer on a thin silver-film substrate makes reflectivity of eachcavity-mirror highly sensitive to the inter-NP separation. Such a design allows continuous tuning of the multiple,narrow and intense transmission peaks emerging from an FPI cavity via electrotuning the inter-NP separation in-situ— reaping the benefits from both inexpensive bottom-up fabrication and energy efficiency of tuning.
Pivnic K, Bresme F, Kornyshev AA, et al., 2019, Structural forces in mixtures of ionic liquids with organic solvents, Langmuir: the ACS journal of surfaces and colloids, Vol: 35, Pages: 15410-15420, ISSN: 0743-7463
Using molecular dynamics simulations, we study the impact of electrode charging and addition of solvent (acetonitrile, ACN) on structural forces of the BMIM PF6 ionic liquid (IL) confined by surfaces at nanometer separations. We establish relationships between the structural forces and the microscopic structure of the confined liquid. Depending on the structural arrangements of cations and anions across the nanofilm, the load-induced squeeze-out of liquid layers occurs via one-layer or bilayer steps. The cations confined between charged plates orient with their aliphatic chain perpendicular to the surface planes and link two adjacent IL layers. These structures facilitate the squeeze-out of single layers. For both pure IL and IL-ACN mixtures, we observe a strong dependence of nanofilm structure on the surface charge density, which affects the simulated pressure–displacement curves. Addition of solvent to the IL modifies the layering in the confined film. At high electrode charges and high dilution of IL (below 10% molar fraction), the layered structure of the nanofilm is less well defined. We predict a change in the squeeze-out mechanism under pressure, from a discontinuous squeeze-out (for high IL concentrations) to an almost continuous one (for low IL concentrations). Importantly, our simulations show that charged electrodes are coated with ions even at low IL concentrations. These ion-rich layers adjacent to the charged plate surfaces are not squeezed out even under very high normal pressures of ∼5 GPa. Hence, we demonstrate the high performance of IL–solvent mixtures to protect surfaces from wear and to provide lubrication at high loads.
Dudka M, Kondrat S, Bénichou O, et al., 2019, Superionic liquids in conducting nanoslits: A variety of phase transitions and ensuing charging behavior, The Journal of Chemical Physics, Vol: 151, ISSN: 0021-9606
We develop a theory of charge storage in ultranarrow slitlike pores of nanostructured electrodes. Our analysis is based on the Blume-Capelmodel in an external field, which we solve analytically on a Bethe lattice. The obtained solutions allow us to explore the complete phase diagramof confined ionic liquids in terms of the key parameters characterizing the system, such as pore ionophilicity, interionic interaction energy,and voltage. The phase diagram includes the lines of first- and second-order, direct and re-entrant phase transitions, which are manifestedby singularities in the corresponding capacitance-voltage plots. Testing our predictions experimentally requires monodisperse, conductingultranarrow slit pores, to permit only one layer of ions, and thick pore walls, to prevent interionic interactions across the pore walls. However,some qualitative features, which distinguish the behavior of ionophilic and ionophobic pores and their underlying physics, may emerge infuture experimental studies of more complex electrode structures.
Kornyshev A, Sikdar D, Weir H, 2019, Optical response of electro-tuneable 3D superstructures of plasmonic nanoparticles self-assembling on transparent columnar electrodes, Optics Express, Vol: 19, Pages: 26483-26498, ISSN: 1094-4087
Electrically tuneable, guided self-assembly of plasmonic nanoparticles (NPs) at polarized, patterned solid–liquid interfaces could enable numerous platforms for designing nanoplasmonic optical devices with new tuneable functionalities. Here, we propose a unique design of voltage-controlled guided 3D self-assembly of plasmonic NPs on transparent electrodes, patterned as columnar structures — arrays of vertical nanorods. NP assembly on the electrified surfaces of those columnar structures allows formation of a 3D superstructure of NPs, comprising stacking up of NPs in the voids between the columns, forming multiple NP-layers. A comprehensive theoretical model, based on quasi-static effective medium theory and multilayer Fresnel reflection scheme, is developed and verified against full-wave simulations for obtaining optical responses — reflectance, transmittance, and absorbance — from such systems of 3D self-assembled NPs. With a specific example of small gold nanospheres, self-assembling on polarized zinc oxide columns, we show that the reflectance spectrum can be controlled by the number of stacked NP-layers. Numerical simulations show that peak reflectance can be enhanced up to ~1.7 times, along with spectral broadening by a factor of ~2 — allowing wide range tuning of optical reflectivity. Smaller NPs with superior mobility would be preferable over large NPs for realizing such devices for novel photonic and sensing applications.
Vasilyev OA, Kornyshev AA, Kondrat S, 2019, Connections matter: On the importance of pore percolation for nanoporous supercapacitors, ACS Applied Energy Materials, Vol: 2, Pages: 5386-5390, ISSN: 2574-0962
Nanoporous supercapacitors play a key role in energy storage and thereby attract growing interest from the research community. Development of porous electrodes for supercapacitors is of the paramount importance, but their characterization still remains a challenge. Herein, we analyze two examples of the popular carbide-derived and activated carbon electrodes from the point of view of interpore connectivity and ion permeation. Due to limited percolation, the effective porosity, as seen by an ion, decreases with an increase in the ion size, which can reduce the stored energy density substantially. Our results highlight the importance of high quality well-percolated porous electrodes for supercapacitors and suggest that the interpore connectivity is an important characteristic to consider when optimizing the existing and developing new electrode materials.
Kondrat S, Vasilyev OA, Kornyshev AA, 2019, Feeling your neighbors across the walls: How interpore ionic interactions affect capacitive energy storage., Journal of Physical Chemistry Letters, Vol: 10, Pages: 4523-4527, ISSN: 1948-7185
Progress in low-dimensional carbon materials has intensified research on supercapacitors with nanostructured/nanoporous electrodes. The theoretical and simulation work so far has focused on charging single nanopores or nanoporous networks and the effects due to ionic interactions inside the pores, while the effect of interpore ion-ion correlations has received less attention. Herein, we study how the interactions between the ions in the neighboring pores across the pore walls affect capacitive energy storage. We develop a simple lattice model for the ions in a stack of parallel-aligned nanotubes, solve it by using the perturbation and "semi-mean-field" theories, and test the results by Monte Carlo simulations. We demonstrate that the interpore ionic interactions can have a profound effect on charge storage; in particular, such interactions can enhance or diminish the stored energy density, depending on the sign of like-charge interactions. We also find that charging can proceed either continuously or via a phase transition. Our results call for more detailed investigations of the properties of carbon pore walls and suggest that tuning their electrostatic response may be promising for the rational design of an optimal supercapacitor.
Ma Y, Sikdar D, Fedosyuk A, et al., 2019, An auxetic thermo-responsive nanoplasmonic optical switch., ACS Applied Materials and Interfaces, ISSN: 1944-8244
Development and use of metamaterials have been gaining prominence in large part due to the possibility of creating platforms with 'disruptive' and unique optical properties. However, to date the majority of such systems produced using micro or nanotechnology, are static and can only perform certain target functions. Next-generation multifunctional smart optical metamaterials are expected to have tuneable elements with the possibility of controlling the optical properties in real time via variation in parameters such as pressure, mechanical stress, voltage, or through non-linear optical effects. Here, we address this challenge by developing a thermally controlled optical switch, based on the self-assembly of poly(N-isopropylacrylamide)-functionalised gold nanoparticles on a planar macroscale gold substrate. We show that such meta-surfaces can be tuned to exhibit substantial changes in the optical properties both in terms of wavelength and intensity, through the temperature-controlled variation of the interparticle distance within the nanoparticle monolayer as well as its separation from the substrate. This change is based on temperature induced auxetic expansion and contraction of the functional ligands. Such a system has potential for numerous applications, ranging from thermal sensors to regulated light harnessing.
Sikdar D, Ma Y, Kucernak AR, et al., 2019, Nanoplasmonic metamaterial devices as electrically switchable perfect mirrors and perfect absorbers, Conference on Lasers and Electro-Optics (CLEO), Publisher: IEEE, Pages: 1-2, ISSN: 2160-9020
We introduce nanoplasmonic metamaterial devices — electrically-switchable between perfect- mirror/absorber states — based on voltage-controlled assembly/disassembly of gold nanoparticles on silver films. These are investigated using effective-medium-theory, verified with simulations and experiments.
Montelongo Y, Sikdar D, Ma Y, et al., 2019, Author Correction: Electrotunable nanoplasmonic liquid mirror., Nat Mater, ISSN: 1476-1122
In the version of this Article originally published, the last sentence of the acknowledgements incorrectly read 'L.V. acknowledges the support of a Marie Skodowska-Curie fellowship (N-SHEAD)'; it should have read 'L.V. and D.S. acknowledge the support of Marie Skłodowska-Curie fellowships, N-SHEAD and S-OMMs, respectively'.
Feng G, Kornyshev A, Goodwin Z, 2019, Free and bound states of ions in ionic liquids, conductivity, and underscreening paradox, Physical Review X, Vol: 9, ISSN: 2160-3308
Free and Bound States of Ions in Ionic Liquids, Conductivity, and Underscreening ParadoxGuang Feng,1,* Ming Chen,1 Sheng Bi,1 Zachary A.H. Goodwin,2,3 Eugene B. Postnikov,4 Nikolai Brilliantov,5,6* Michael Urbakh,7,* and Alexei A. Kornyshev3,8*1State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Room 321, Power Building, 1037 Luoyu Road, Wuhan, Hubei 430074 China2Department of Physics, Imperial College London, South Kensington Campus, SW7 2AZ London, United Kingdom3Department of Chemistry, Imperial College London, Molecular Sciences Research Hub, White City Campus, W12 0BZ London, United Kingdom4Theoretical Physics Department, Kursk State University, Radishcheva Str., 33, Kursk 305000, Russia5Department of Mathematics, University of Leicester, University Road, Leicester LE1 7RH, United Kingdom6Skolkovo Institute of Science and Technology, Moscow 121205, Russia7School of Chemistry, The Sackler Faculty of Science, Tel Aviv University, Ramat Aviv, 69978 Tel Aviv, Israel8Thomas Young Centre for Theory and Simulation of Materials, Imperial College London, South Kensington Campus, SW7 2AZ London, United Kingdom*Correspondence: firstname.lastname@example.org (GF), email@example.com (MU), firstname.lastname@example.org (NB) and email@example.com (AAK) ABSTRACTUsing molecular dynamics simulations and theoretical analysis of velocity autocorrelation functions, we study ion transport mechanisms in typical room temperature ionic liquids. We show that ions may reside in two states – free and bound with an inter-state exchange; we investigate quantitatively the exchange process and reveal new qualitative features of this process. To this end we propose a dynamic criterion for free and bound ions, based on the ion trajectory density and demonstrate that this criterion is consistent with a static one, based on interionic distances. Analyzing the trajectories of individual cations and anions, we estimate t
Sikdar D, Ma Y, Kucernak AR, et al., 2019, Nanoplasmonic Metamaterial Devices as Electrically Switchable Perfect Mirrors and Perfect Absorbers
© 2019 The Author(s) 2019 OSA. We introduce nanoplasmonic metamaterial devices - electrically-switchable between perfect-mirror/absorber states - based on voltage-controlled assembly/disassembly of gold nanoparticles on silver films. These are investigated using effective-medium-theory, verified with simulations and experiments.
Bi S, Wang R, Liu S, et al., 2018, Minimizing the electrosorption of water from humid ionic liquids on electrodes, Nature Communications, Vol: 9, ISSN: 2041-1723
In supercapacitors based on ionic liquid electrolytes, small amounts of absorbed water could potentially reduce the electrochemical window of electrolytes and cause performance degradation. The same would take place if ionic liquids are used as solvents for electrocatalysis involving the dissolved molecular species. In this work, we carry out molecular dynamics simulations, with gold and carbon electrodes in typical ionic liquids, hydrophobic and hydrophilic, to study electrosorption of water. We investigate the effects of hydrophobicity/hydrophilicity of ionic liquids and electrodes on interfacial distribution of ions and electrosorbed water. Results reveal that using hydrophilic ionic liquids would help to keep water molecules away from the negatively charged electrodes, even at large electrode polarizations. This conclusion is supported by electrochemical cyclic voltammetry measurements on gold and carbon electrodes in contact with humid ionic liquids. Thereby, our findings suggest potential mechanisms for protection of electrodes from water electrosorption.
Ma Y, Zagar C, Klemme DJ, et al., 2018, A tunable nanoplasmonic mirror at an electrochemical interface, ACS Photonics, Vol: 5, Pages: 4604-4616, ISSN: 2330-4022
Designing tunable optical metamaterials is one of the great challenges in photonics. Strategies for reversible tuning of nanoengineered devices are currently being sought through electromagnetic or piezo effects. For example, bottom-up self-assembly of nanoparticles at solid | liquid or liquid | liquid interfaces can be used to tune optical responses by varying their structure either chemically or through applied voltage. Here, we report on a fully reversible tunable-color mirror based on a TiN-coated Ag substrate immersed in an aqueous solution of negatively charged Au-nanoparticles (NPs). Switching electrode potential can be used to fully control the assembly/disassembly of NPs at the electrode | electrolyte interface within a 0.6 V wide electrochemical window. The plasmon coupling between the electrode and the adsorbed NP array at high positive potentials produces a dip in the optical reflectance spectrum, creating the "absorber" state. Desorption of NPs at low potentials eliminates the dip, returning the system to the reflective "mirror" state. The intensity and wavelength of the dip can be finely tuned through electrode-potential and electrolyte concentration. The excellent match between the experimental data and the theory of optical response for such system allows us to extract valuable information on equilibrium and kinetic properties of NP-assembly/disassembly. Together with modeling of the latter, this study promotes optimization of such meta-surfaces for building electrotunable reflector devices.
Bresme F, Robotham O, Chio W-IK, et al., 2018, Debye screening, overscreening and specific adsorption in solutions of organic ions, Physical Chemistry Chemical Physics, Vol: 20, Pages: 27684-27693, ISSN: 1463-9076
Tetrabutylammonium (TBA) and tetraphenylborate (TPB) ions dissolved in dichloroethane (DCE) are widely used in electrochemistry of liquid–liquid interfaces. Unlike alkali halide solutions in water, TBA–TPB–DCE solutions feature large organic ions and a solvent with a dielectric constant almost one order of magnitude lower than that of water. This is expected to dramatically amplify the impact of ionic correlations in the properties of the solution. Here we report atomistic simulations of TBA–TPB–DCE solutions and analyze ion correlations, clustering, and charge screening effects. We target concentrations in the range of 0.01–0.25 molal (m), hence exploring concentration regimes typical for many experimental investigations. We show that the transition from monotonic to oscillatory decay of the charge density, which signals the onset of strong ion correlations, takes place in this concentration interval, leading to overscreening effects. Furthermore, we investigate the distribution and adsorption of ions at the DCE–air interface. Unlike what is observed for small inorganic ions in water at similar concentrations, we find that TPB and TBA ions accumulate near the DCE surface, resulting in significant interfacial clustering and adsorption at concentrations ∼0.25 m. TPB ions adsorb more strongly leading to free energy wells of ∼1–2 kBT. The adsorption modifies significantly the electrostatic potential of the DCE–air interface, which undergoes a shift of 0.2 V in going from pure DCE to TBA–TPB–DCE solutions at 0.25 m.
Goodwin ZAH, Kornyshev AA, 2018, Theory of polymer-electrolyte-composite electroactuator sensors with flat or volume-filling electrodes, Soft Matter, Vol: 14, Pages: 7996-8005, ISSN: 1744-683X
In reverse actuation, a voltage/electrical-current signal can be generated from applying a mechanical force to an electroactuator. Such processes are of interest in touch sensing and soft robotics applications. We develop a classical density functional theory of reverse actuation for polymer-electrolyte-composite electroactuators, which treats mobile cations in the same spirit as forward actuation (curving in response to applied voltage). The proposed framework is applied to electroactuators with micro-structured porous electrodes (with cylindrical or slit pores) and flat electrodes, the dynamic response of which has to be modelled differently. Open- and short-circuit operation modes are investigated separately. A detailed analysis of the proposed theory indicates the preferred architectures for sensing, depending on the operation regimes.
McEldrew M, Goodwin ZAH, Kornyshev AA, et al., 2018, Theory of the Double Layer in Water-in-Salt Electrolytes., Journal of Physical Chemistry Letters, Vol: 9, Pages: 5840-5846, ISSN: 1948-7185
One challenge in developing the next generation of lithium-ion batteries is the replacement of organic electrolytes, which are flammable and most often contain toxic and thermally unstable lithium salts, with safer, environmentally friendly alternatives. Recently developed water-in-salt electrolytes (WiSEs), which are nonflammable, nontoxic, and also have enhanced electrochemical stability, are promising alternatives. In this work, we develop a simple modified Poisson-Fermi theory for WiSEs, which demonstrates the fine interplay between electrosorption, solvation, and ion correlations. The phenomenological parameters are extracted from molecular dynamics simulations, also performed here. The theory reproduces the WiSEs' electrical double-layer structure with remarkable accuracy.
Budkov YA, Kolesnikov AL, Goodwin ZAH, et al., 2018, Theory of electrosorption of water from ionic liquids, Electrochimica Acta, Vol: 284, Pages: 346-354, ISSN: 0013-4686
We propose and develop a classical density functional theory for the description of a minor amount of water dissolved in ionic liquid in the vicinity of an electrode. In addition to the electrostatic energy and lattice-gas mixing entropy terms, the utilised grand canonical potential contains several phenomenological terms/parameters that describe short-range interactions between ions, water molecules and the electrode. Here we investigate: (i) specific interaction of ions and molecules with the electrode, which are responsible for their specific adsorption; (ii) hydrophilicity/hydrophobicity of ions. We obtain water electrosorption isotherms as a function of the potential drop across the electrical double layer, investigate its asymmetry with respect to the sign of electrode potential, and establish the relationship between the sign of this asymmetry and hydrophobicity/hydrophilicity of cations and anions. We also calculate the effect of water electrosorption on the double layer differential capacitance which brings clear new features to its voltage dependence, some of which have been already experimentally observed.
Goodwin Z, Eikerling M, Loewen H, et al., 2018, Theory of microstructured polymer-electrolyte artificial muscles, Smart Materials and Structures, Vol: 27, ISSN: 0964-1726
Ionic electroactuator beams are promising systems for artificial muscles in microrobotics. Here a theory is developed to investigate one promising class of such systems, which employs flexible volume-filling electrodes impregnated with polymer–electrolyte. The theory provides analytical formulae for the equilibrium beam curvature as a function of voltage and structure-related operational parameters. It predicts a possible enhancement of beam curvature by orders of magnitude over that of flat electrodes. Volume-filling electrodes thus constitute one of the 'strongest' architectures for voltage-induced movement. Approximate expressions for the dynamics of tandem pore charging and beam deflection are developed to determine the maximum pore length that still warrants a sufficiently fast response time (up to 1 s). Upper bounds on applied voltage and response time constrain the maximal device thickness and curvature, and therefore, the resulting work such a device can perform.
Pivnic K, Fajardo OY, Bresme F, et al., 2018, Mechanisms of Electrotunable Friction in Friction Force Microscopy Experiments with Ionic Liquids, JOURNAL OF PHYSICAL CHEMISTRY C, Vol: 122, Pages: 5004-5012, ISSN: 1932-7447
Using molecular dynamics simulations and a coarse-grained model of ionic liquids (ILs), we study mechanisms of electrotunable friction measured in friction force microscopy experiments, where only one layer of IL is present between the tip and the electrode (substrate). We show that the variation of the friction force with the electrode surface charge density is determined by the regime of motion of the confined IL relative to the substrate and tip. The latter depends on the strengths of the ion–substrate and ion–tip interactions and on the commensurability between the characteristic ion dimensions and lattice spacings of the substrate and tip surfaces. Related with those factors, our simulations predict two strictly different scenarios for the variation of the friction force with the electrode surface charge. Revealing mechanisms of frictional energy dissipation in nanoscale IL films offers a way for controlling friction by tuning ion–substrate interactions and electrical polarization of sliding surfaces.
Kornyshev AA, Sikdar D, Edel JB, et al., 2018, Towards Electrotuneable Nanoplasmonic Fabry–Perot Interferometer, Scientific Reports, Vol: 8, ISSN: 2045-2322
Directed voltage-controlled assembly and disassembly of plasmonic nanoparticles (NPs) at electrified solid–electrolyte interfaces (SEI) offer novel opportunities for the creation of tuneable optical devices. We apply this concept to propose a fast electrotuneable, NP-based Fabry–Perot (FP) interferometer, comprising two parallel transparent electrodes in aqueous electrolyte, which form the polarizable SEI for directed assembly–disassembly of negatively charged NPs. An FP cavity between two reflective NP-monolayers assembled at such interfaces can be formed or deconstructed under positive or negative polarization of the electrodes, respectively. The inter-NP spacing may be tuned via applied potential. Since the intensity, wavelength, and linewidth of the reflectivity peak depend on the NP packing density, the transmission spectrum of the system can thus be varied. A detailed theoretical model of the system’s optical response is presented, which shows excellent agreement with full-wave simulations. The tuning of the peak transmission wavelength and linewidth is investigated in detail. Design guidelines for such NP-based FP systems are established, where transmission characteristics can be electrotuned in-situ, without mechanically altering the cavity length.
Chen M, Goodwin ZAH, Feng G, et al., 2017, On the temperature dependence of the double layer capacitance of ionic liquids., Journal of Electroanalytical Chemistry, Vol: 819, Pages: 347-358, ISSN: 1572-6657
The temperature dependence of room temperature ionic liquids differential capacitance is studied here with both theoretical and computational methods. On the theory front, the lattice-gas mean-field theory of ionic liquids is further generalised to account for ‘ion pairing’ and ‘neutral aggregate’ formation. An anomalous temperature dependence of linear response capacitance was found, similar to that reported in earlier work. The theory also predicted that differential capacitance curves transform from a camel to bell shape with increasing temperature. Molecular dynamics simulations verified the expected transition in shape of differential capacitance curves with temperature and the dependence of linear response capacitance on temperature. Further investigation into charge density distributions revealed an ordered structure, reminiscent of oriented ion pairs and neutral aggregates, extending far enough from the electrode to control the capacitance-voltage response. It was found that these structures were dismantled with increasing temperature, as predicted by the mean-field theory.
Rochester C, Sartor A, Pruessner G, et al., 2017, "One dimensional" double layer. The effect of size asymmetry of cations and anions on charge-storage in ultranarrow nanopores-an Ising model theory, RUSSIAN JOURNAL OF ELECTROCHEMISTRY, Vol: 53, Pages: 1165-1170, ISSN: 1023-1935
We develop a statistical mechanical theory of charge storage in quasi-single-file ionophilic nanopores with pure room temperature ionic liquid cations and anions of different size. The theory is mapped to an extension of the Ising model exploited earlier for the case of cations and anions of the same size. We calculate the differential capacitance and the stored energy density per unit surface area of the pore. Both show asymmetry in the dependence on electrode potential with respect to the potential of zero charge, related to the difference in the size of the ions, which will be interesting to investigate experimentally. It also approves the increase of charge storage capacity via obstructed charging, which in these systems emerges for charging nanopores with smaller ions.
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