Region Of Electric Double Layer Research Articles

Non-isothermal electroosmotic flow of a viscoelastic fluid through a porous medium in a microchannel

The influence of the temperature-dependent viscosity on the electroosmotic flow through a porous medium in a microchannel of parallel flat plates. At the end, an imposed electric field induces Joule heating and temperature gradients, modifying the viscosity of the fluid. The Phan-Thien–Tanner and Darcy–Forchheimer models describe the viscoelastic fluid through the porous media. Consequently, the electric double layer (EDL) overlaps at the center plane of the microchannel, causing an electrically charged porous matrix and modifying the zeta potential along the microchannel walls. Therefore, we define a zeta potential ratio of the porous matrix and that at the microchannel walls. Unlike similar investigations, the boundary condition for the zeta potential varies locally due to the temperature field, modifying the interaction among hydrodynamics, electrical, viscoelastic, thermal and porosity effects. A modified Péclet number is defined as a function of the porosity and thermal conductivity of the fluid and an effective charge density is determined based on the Poisson–Boltzmann equation, accounting for the pore size and the zeta potential ratio. The set of equations are solved numerically considering the lubrication approximation theory. We found that the influence of the temperature-dependent viscosity increases the shear stresses at the EDL region, particularly where the temperature rises to maximum values. Also, the overlapping of the EDL occurs mainly for low values of permeability. Finally, the pressure gradient involved in the Darcy law can be altered through the zeta potential ratio.

Eliminating lithium dendrites via dependable ion regulation of charged nanochannels

Reviving lithium metal anode is of great significance to developing high-energy-density batteries after addressing the lithium dendrite. Constructing functional separator with charged nanochannel configuration has been proven to be an effective way to inhibit nucleation of lithium dendrites due to the achievement of a fast and selective lithium ion transport. While the ion regulation capability of existing functional separators cannot be maximized because their nanochannels are much larger than the electric double layer (EDL) regions that dominate ion transport. Considering the tiny EDL thickness (∼1 nm) formed in the electrolyte, the sulfonate-rich covalent organic framework (COF) is developed as functional separator to achieve a dependable ion regulation based on the nano-confined channel (∼1.55 nm) and surface negative charges. Consequently, a high Li+ transference number tLi+ (0.85) and sufficient ionic conductivity (0.53 mS cm−1) are obtained simultaneously, which effectively alleviates dendrite nucleation and growth of lithium metal anode for over 1000 h. Moreover, the principle for fast and selective lithium-ion transport is revealed from a new perspective of EDL region embedded in the nanochannel. Overall, this work demonstrates a strategy to eliminate lithium dendrites via ion regulation of charged nanochannels, which is anticipated to promote the practical application of lithium metal batteries.

Enabling an Inorganic-Rich Interface via Cationic Surfactant for High-Performance Lithium Metal Batteries

HighlightsCetyltrimethylammonium cations can shield the repelling force on anions to attract more anions into electric double layer region during lithium plating process, facilitating the formation of inorganic-rich solid-state electrolyte interphase (SEI).An inorganic-rich (N/F-containing species) structure of SEI can be evidenced by the in-depth analysis.The cycling lifetime of Li||Li symmetric cell in the designed electrolyte can be extended from 500 to 1300 h. Moreover, full cells with a high cathode mass loading of >10 mg cm-2 can be stably cycled over 180 cycles.

Ultrafine PtCo alloy by pyrolysis etching-confined pyrolysis for enhanced hydrogen evolution

Zeolitic imidazolate framework-67 (ZIF-67) has been widely used as a precursor to developing efficient PtCo alloy catalysts for hydrogen evolution reaction (HER). However, traditional in-situ pyrolysis strategies involve complicated interface structure modulating processes between ZIF-67 and Pt precursors, challenging large-scale synthesis. Herein, a “pyrolysis etching-confined pyrolysis” approach is developed to design confined PtCo alloy in porous frameworks of onion carbon derived from ZIF-67. The confined PtCo alloy with Pt content of only 5.39 wt% exhibits a distinct HER activity in both acid (η10: 5 mV and Tafel: 9 mV dec−1) and basic (η10: 33 mV and Tafel: 51 mV dec−1) media and a drastic enhancement in stability. Density functional theory calculations reveal that the strong electronic interaction between Pt and Co allows favorable electron redistribution, which affords a favorable hydrogen spillover on PtCo alloy compared with that of pristine Pt(111). Operational electrochemical impedance spectroscopy demonstrates that the Faraday reaction process is facilitated under acidic conditions, while the transfer of intermediates through the electric double-layer region under alkaline conditions is accelerated. This work not only offers a universal route for high-performance Pt-based alloy catalysts with metal–organic framework (MOF) precursors but also provides experimental evidence for the role of the electric double layer in electrocatalysis reactions.

Water molecules mute the dependence of the double-layer potential profile on ionic strength.

We present the results of molecular dynamics simulations of a nanoscale electrochemical cell. The simulations include an aqueous electrolyte solution with varying ionic strength (i.e., concentrations ranging from 0-4 M) between a pair of metallic electrodes held at constant potential difference. We analyze these simulations by computing the electrostatic potential profile of the electric double-layer region and find it to be nearly independent of ionic concentration, in stark contrast to the predictions of standard continuum-based theories. We attribute this lack of concentration dependence to the molecular influences of water molecules at the electrode-solution interface. These influences include the molecular manifestation of water's dielectric response, which tends to drown out the comparatively weak screening requirement of the ions. To support our analysis, we decompose water's interfacial response into three primary contributions: molecular layering, intrinsic (zero-field) orientational polarization, and the dipolar dielectric response.

Thermal conductivity effect on thermophoresis of charged spheroidal colloids in aqueous media.

Thermophoresis of spheroidal colloids in aqueous media under the thermal conductivity effect is analyzed. The thermophoretic velocity and the thermodiffusion coefficient of spheroidal colloids have been formulated for extremely thin electric double layer (EDL) cases. Furthermore, a numerical thermophoretic model is built for arbitrary EDL thickness cases. The parametric studies show that the thermal conductivity mismatch of particle and liquid gives rise to a nonlinear temperature region around the spheroid, with the thickness close to the minor semiaxis. When the EDL region is thin relative to such nonlinear temperature region, the thermal conductivity effect on the thermophoresis of spheroidal colloids is significant, which strongly depends on the ratio of the minor semiaxis to the EDL thickness, the thermal conductivity ratio of particle to liquid, and the particle aspect ratio. Finally, to estimate the thermodiffusion coefficient of spheroidal colloids with arbitrary thermal conductivity, electrolyte concentration, and particle shape, the average dimensionless axial temperature gradient on the spheroidal equator plane in the EDL region is proposed.

Molecular Dynamics Simulation of the Interfacial Structure and Differential Capacitance of [BMI+][PF6−] Ionic Liquids on MoS2 Electrode

MoS2 nanomaterials and ionic liquids (ILs) have attracted tremendous interest as the prime electrodes and electrolytes of supercapacitors. However, the corresponding charge storage mechanisms have not yet been clearly understood. Herein, we study the molecular-level energy storage mechanisms of the MoS2 electrode in imidazolium ionic liquid ([BMI+][PF6−]) using molecular dynamics (MD) simulation. The electric double-layer (EDL) structures of MoS2 electrodes in [BMI+][PF6−] electrolytes are comprehensively studied in terms of number density, MD snapshots, separation coefficient, and ion-electrode interaction energy. Based on this, the electric potential and electric field distributions are calculated by integrating Poisson equations. Importantly, a bell-shaped differential capacitance profile is proposed, different from the U-shaped curve from the conventional Gouy–Chapman theory. Especially, it can be well reproduced by the differential charge density curve in the Helmholtz layer. This indicates that the capacitive behaviors of the MoS2 electrode are primarily determined by the counterion population/structure in the EDL region 5.0 Å from the electrode surface. In the end, ion diffusion coefficients within different interfacial EDL regions are evaluated, revealing that dynamics are significantly suppressed by ~50% in the Helmholtz region. However, the dynamics can be recovered to a bulk state with the ion position 10 Å away from the electrode surface. The as-obtained insights reveal the charge storage mechanisms of MoS2 in ILs, which can provide useful guidance on improving the energy density of MoS2 supercapacitors.

Interactive MD-DFT Model to Predict the Multi-Component Electrolyte Reduction within the Electrical Double Layer

A typical liquid electrolyte for lithium (Li)-ion and Li-metal batteries is a liquid mixture with salt, solvent, and additives. The reduction products of the electrolytes form the solid-electrolyte interphase (SEI) layer, whose composition, heterogeneity and thickness largely determine the Li plating and stripping processes and thus the cycling performance. It is highly desirable to obtain atomistic insights into the reduction reactions of the electrolytes to help predict and control the formation of the SEI layer. The reduction reactions of the liquid electrolyte occur near the charged electrode surface within the electric double layer (EDL). The electrolyte structures within EDL must change under an electric field, which impacts the electrolyte reduction and SEI formation. This has been rarely considered in previous theoretical investigations of the reduction potentials and reduction products.In this work, we have developed an interactive model combining molecular dynamics (MD) simulations and density functional theory (DFT) calculations to predict the multi-component electrolyte reduction within the EDL.[1] First, MD simulations, which can deal with thousands of atoms and even more, are used to capture the dynamics and statistics of the EDL structures. Then DFT calculations are used to compute the reduction products and corresponding reduction potentials of the representative clusters in the EDL. Finally, a formulation was proposed to calculate averaged reduction potential based on DFT-calculated reduction potentials of all the possible Li+-coordinated structures and their distribution probabilities obtained through MD analyses.We have applied this new interactive MD-DFT model to two types of essential multi-component electrolytes, the carbonate-based electrolyte (LiPF6@EC:EMC) and the ether-based electrolyte (LITFSI@DOL:DME). We find in both electrolytes the Li+ ions within the EDL tend to have less coordination with other electrolyte species (salt anion, solvent and additive) compared to those in the bulk electrolyte. For LiPF6@EC:EMC, we reveal that FEC can enter into the EDL region and is favorable to be reduced, which helps form the F-containing SEI component (e.g., LiF). In addition, we have studied the effects of the FEC additive on LiTFSI@DOL:DME at various temperatures. We find that, at -40 °C, the number of F atoms that come near the electrode is largely increased after adding FEC, while no big change is observed at 20 °C. This explains that adding FEC is more effective in forming stable F-containing SEI and in promoting cycling performance at low temperatures.[2] Our simulations have also explained why ether based electrolyte works better than carbonate base electrolyte for Li metal anode. Reference: [1] Qisheng Wu,Yue Qi. Manuscript under preparation, 2022. [2] Akila C. Thenuwara, et al. ACS Energy Lett. 2020, 5, 2411−2420

Voltammetric investigation of anodic and cathodic processes at Au(hkl)|ionic liquid interfaces

• The electrochemical behaviors of 8 ionic liquids (ILs) were studied on Au( hkl ). • Pyrrolidinium-based ILs had the widest electrical double-layer region. • The potential windows depended on Au( hkl ) and the ions comprising the ILs. • The desorption/re-adsorption of iodine for 2 iodide-based ILs depended on Au( hkl ). • In terms of the peak potential for iodine desorption: Au(100) < Au(111) < Au(110) . In this work, the effect of the crystallographic orientation of gold on the interfacial processes of six non-halide-based ionic liquids (ILs) and two iodide-based ILs was systematically investigated by linear sweep voltammetry (LSV), cyclic voltammetry (CV), and double potential step chronocoulometry (DPSCC). The cation and anion of the IL affected both the width of the electrical double-layer (EDL) region ( E dl ) and the electrochemical potential window ( E pw ) of the ILs. The shape of the CV profile in the EDL region was unique for each Au( hkl )|IL interface. The reductive desorption and re-adsorption of iodine adatoms on gold in 1-alkyl-3-methylimidazolium iodide ([C n mim][I], n = 4 or 6) were also surface-sensitive. The reduction peak current density was proportional to the scan rate, suggesting that the reaction involves adsorbed iodine. The amount of iodine adatoms desorbed during the initial cathodic scan was identical to the monolayer coverage of iodine on gold single crystal electrodes. The peak potential for reductive desorption was estimated to follow the ensuing order from negative to positive: [C 6 mim][I] < [C 4 mim][I] and Au(100) < Au(111) < Au(110) . Following reductive desorption of iodine, the cations were partly attached to the gold surface. The adsorption strength of [C n mim] + followed an identical order: Au(100) < Au(111) < Au(110) .

Radial basis function interpolation supplemented lattice Boltzmann method for electroosmotic flows in microchannel.

Large gradients of physical variables near the channel walls are characteristic of EOF. The previous numerical simulations of EOFs with the lattice Boltzmann method (LBM) utilize uniform lattice and are not efficient, especially when the electric double layer (EDL) thickness is significantly smaller than the channel height. The efficient LBM simulation of EOF in microchannel calls for a nonuniform mesh which is dense in the EDL region and sparse in the bulk region. In this article, we formulate a radial basis function (RBF)-based interpolation supplemented LBM (ISLBM) to solve the governing equations of EOF, that is, the Poisson, Nernst-Planck, and Navier-Stokes equations, in a nonuniform mesh system. Unlike the conventional ISLBM, the RBF-ISLBM determines the prestreaming distribution functions by using the local RBF-based interpolation over circular supporting regions and is particularly suitable for nonuniform meshes. The RBF-ISLBM is validated by the EOFs in infinitely long and finitely long microchannels. The results show that the RBF-ISLBM possesses excellent robustness and accuracy. Finally, we use the RBF-ISLBM to simulate the EOFs with the hitherto highest electrokinetic parameter, κa, defined by the ratio of channel height a to EDL thickness κ-1 , in LBM simulations of EOF.

Control of Mass Flow-Rate of Viscoelastic Fluids Through Time-Periodic Electro-Osmotic Flows in a Microchannel

Abstract In the present research, we address the implications of the pulsating electric field on controlling mass flow-rate characteristics for the time-periodic electro-osmotic flow of a viscoelastic fluid through a microchannel. Going beyond the Debye-Hückel linearization for the potential distribution inside the Electric Double Layer, the Phan-Thien-Tanner constitutive model is employed to describe the viscoelastic behaviour of the fluid. The analytical/semi-analytical expressions for the velocity distribution corresponding to a steady basic part, and a transient perturbed part are obtained by considering periodic pulsations in the applied electrical field. Our results based on sinusoidal pulsations reveal that enhanced shear thinning characteristics of the viscoelastic fluids show higher amplitude of pulsations with the oscillations in the velocity gradients primarily contrived within the Electric Double Layer region. The amplitude of mass flow rates increases with increasing the viscoelastic parameter , whereas, the phase lag displays a reverse trend. The analysis for an inverse problem is extended where the required magnitude of electric field pulsations for a target mass flow rate in the form of sinusoidal pulsations. It is found that with increasing shear-thinning characteristics of the viscoelastic fluid, there is a progressive reduction in the required electric field strength to maintain an aimed mass flow rate. Besides, required electric fields for controlled mass flow with triangular and trapezoidal pulsations are also determined.

Temperature distribution and entropy generation during Darcy–Forchheimer–Brinkman electrokinetic flow in a microfluidic tube subject to a prescribed heat flux

Electrokinetically modulated flow through a hydrophobic microtube embedded in a Darcy–Forchheimer porous medium is investigated in this paper. The steric effect has been taken into account in the electrical double layer (EDL) region. Heat transfer is analysed in the case of Darcy–Forchheimer–Brinkman flow subject to Joule heating. The flow is supposed to take place under the combined influence of electroosmosis and imposed pressure gradient. The governing nonlinear partial differential equations for electric potential, fluid flow and heat transfer are solved numerically by developing an iterative finite difference method that has second order accuracy. The thermal efficiency is discussed under the purview of the second law of thermodynamics. Influences/impact of different physical parameters on velocity, temperature and entropy are investigated and demonstrated graphically. The paper shows that with an increase in the steric effect, the electrokinetic velocity diminishes and that thermal irreversibility is very high in the electrical double layer region, but it reduces drastically in the neighbourhood of the central region of the microtube. Results of the study are likely to be of profuse interest in the design and development of microfluidic devices that deal with critical types of fluid transport mechanism in non-Darcian porous media.

Numerical analysis of thermal conductivity effect on thermophoresis of a charged colloidal particle in aqueous media

It is well known that colloidal particles dispersed in aqueous media usually are naturally charged and thus an electrical double layer (EDL) is formed near the surface of the charged particles. Here, we report a numerical analysis of the thermal conductivity effect on thermophoresis of a charged colloidal particle in aqueous media. The proposed numerical model includes the coupling energy equation, Nernst-Planck equation, Poisson equation, and Stokes equation. The numerical simulations reveal that the temperature distributes non-linearly around the particle surface due to mismatch of the thermal conductivity of particle and liquid, and such non-linear temperature distribution has a profound effect on the thermophoresis of the charged particle. When the non-linear temperature region is much thinner than the EDL region (or thick EDL cases), the thermodiffusion coefficient of the particle is similar to that of the “normal” particle (namely a particle with an equal thermal conductivity to the liquid), and the thermal conductivity effect on the thermophoresis is negligible. Yet, when the non-linear temperature region is much thicker than the EDL region (or thin EDL cases), the thermal conductivity effect becomes significant, and the particle thermodiffusion coefficient decreases with increasing the thermal conductivity ratio of particle to liquid. Finally, a parameter, the average dimensionless axial temperature gradient along the radial axis (r) in the EDL region, is derived to approximately estimate the thermophoretic behavior of a charged particle with arbitrary thermal conductivity and EDL thickness.

Transient characteristics of electric double layer charging and the associated induced-charge electrokinetic flow

This paper presents a detailed numerical analysis of the transient characteristics of electric double layer (EDL) charging and the associated induced-charge electrokinetic (ICEK) flow around an ideally polarizable cylinder. To this end, we solved numerically the coupled Poisson-Nernst-Planck and Navier-Stokes equations with the finite element method. The numerical simulation provides an unprecedented full-field (including the EDL region) characterization of the transient evolutions of ion transport, electric potential, and fluid flow during the EDL charging. The simulation results show that the EDL charging is driven by the electric current normal to the cylinder surface. With EDL being charged, the charge density in the EDL counteracts the local external electric field on the cylinder surface to reduce the electric current, which then leads to the slowing down of the EDL charging. At the steady state, the EDL becomes fully charged and the charge density in EDL exactly counteracts the external electric field, and then the EDL charging stops. During the EDL charging, the interaction of the external electric field with the charge density in the EDL drives the liquid in the EDL to move first, and then as time evolves, the liquid in the bulk electrolyte sets in motion because of the momentum transfer between the EDL and the bulk. These findings are conducive to the understanding of the transient dynamics of ICEK phenomena around polarizable objects.

Probing double layer structure at Au/[BMIm]BF4 interface by molecular length-dependent SERS Stark effect

Room temperature ionic liquids (RTILs) played an important role in the electrochemistry because of the advantageous properties. Due to the different electrochemical behavior from the dilute solution, it was still remained a significant challenge to obtain the insight into the interfacial structure. Although the spectroelectrochemistry offered the rich spectroscopic information for obtaining the interfacial structure at molecular level, the lack of the spatial information of those techniques at nano/subnanometer scale led the difficulties in the determination of the spatial structure of double layer. Here, the surface enhanced Raman spectroscopy (SERS) Stark tuning rates of CN stretching frequency were employed to determine the electric double layer structure on Au electrode, and probes with different lengths, involving cyanide ion (CN−), thiocyanate (SCN−), 3-aminopropanenitrile (3-APN) and 6-aminohexanenitrile (6-AHN), were used as rulers to measure the spatial structure of double layer. In this approach, SERS combined with electrochemical control was performed to distinguish the orientation of surface species and detect the vibrational frequency of the nitrile stretching mode. For shorter probes (such as CN− and SCN−), the nitrile group were located in the electric double layer region, resulting in measureable Stark tuning rates (dνCN/dE), while for longer probe molecules (such as 3-APN and 6-AHN), the interfacial potential completely decayed before reaching the nitrile group location, resulting in the negligible Stark tuning rates. The vibrational Stark tuning rates suggested that the potential drop occurred over a range of 5–6Å from the metal surface into the bulk phase of ionic liquids. The results demonstrated that the overall potential mostly dropped across the first layer and only a small fraction across the second layer. The preliminary results allowed us to obtain an insight into the physical picture of the interfacial structure of RTILs/metal system.

Different Carbide Derived Nanoporous Carbon Supports and Electroreduction of Oxygen

It has been recognized that, depending on the heat-treatment temperatures different carbon materials exhibit different properties, such as the surface area, porosity, electrical and ionic conductivity, water uptake as well as the electrocatalytic activity. During last ten years various carbides have been used as a raw material for synthesis of the nanoporous carbons with exact pore size distribution. The main aim of this work was to investigate influence of the synthesis temperature on the various physical and electrochemical properties of nanoporous carbons, prepared from SiC, TiC, Mo2C using chlorination process at various temperatures from 600°C to 1000°C. The X-ray microtomography, X-ray diffraction, Raman spectroscopy, time-of-flight mass spectrometry, scanning electron microscopy and transmission electron methods were applied for the structural and electronic characterization of materials. Due to the different precursor carbide and chlorination temperature used, the synthesized carbons have very different morphology, i.e. mainly amorphous or partly amorphous with well-defined graphitic areas. According to the gas adsorption data measured at liquid nitrogen temperature the specific surface area of precursor carbide increase in the following order: SiC < Mo2C ≤ TiC. The total pore volume value for Mo2C is two and three times higher compared with TiC and SiC, respectively. The chemical contamination of carbon powders has been identified by the X-ray microtomography. It has been established that, the amount of contaminants in carbon powders depends strongly on the synthesis method and chlorination temperature used. The X-ray microtomography images also demonstrate that the carbon nanoparticles are partly agglomerated. The carbons with the fine particle sizes (C(TiC) and C(Mo2C)) more easily form agglomerates with various sizes, than (C(SiC)) carbon with coarse particles. The results established for carbide derived carbon (CDC) powders have been compared with commercially available ordered mesoporous carbon CMK-3 (ACS Material). Electrocatalytic activity of electrodes prepared from CDC powders has been studied by the rotating disc electrode, cyclic voltammetry and electrochemical impedance methods. Electrochemical data for nanoporous carbon electrodes show that the catalytic activity and mechanism noticeable depends on the hierarchical structure of the carbon electrode mainly, on the micro-mesoporosity and specific surface area of electrode under study. The partially graphitized carbon C(Mo2C) with a large number of edge plane sites showed higher electrocatalytic activity towards oxygen reduction compared with amorphous carbons studied. The calculated capacitance values in the electrical double layer region for the C(Mo2C) electrode are nearly eight times higher than those for C(TiC). Within oxygen (as well as surfake oxide) electroreduction and hydrogen evolution region, very high faradaic pseudocapacitance value have been calculated, demonstrating that C(Mo2C) electrodes can be used in hybride energy storage/generation devices. Acknowledgements: This work was supported by the European Spallation Source Project: Estonian Partition in ESS Instrument design, development and building and application for scientific research: SLOKT12026T; the Estonian target research: IUT20-13; the Estonian Centre of Excellence in Science: TK117T "High-technology Materials for Sustainable Development"; the Estonian Energy Technology Program: SLOKT10209T and the Materials Technology Project: SLOKT12180T. References I.Tallo, T.Thomberg, H.Kurig, K.Kontturi, A.Jänes, E.Lust, Carbon 67 (2014) 607. I.Tallo, T.Thomberg, H.Kurig, K.Kontturi, A.Jänes, E.Lust, Journal of Solid State Electrochemistry, 117 (2013) 19. A. Jänes, J. Eskusson, R. Kanarbik, E. Lust, IOP Conf.Series: Materials Science and Engineering 49 (2013) 012006. I.Tallo, T.Thomberg, A.Jänes, E.Lust, ECS Transactions, 50 (43) (2013) 3. R.Jäger, P.E.Kasatkin, E.Härk, E.Lust, Electrochemistry Communications, 35 (2013) 97. E.Lust, E.Härk, J.Nerut, K.Vaarmets, Electrochimica Acta, 101 (2013) 130. E.Härk, J.Nerut, K.Vaarmets, I.Tallo, H.Kurig, J.Eskusson, K.Kontturi, E.Lust, Journal of Electroanalytical Chemistry, 689 (2013) 176.

Potential-dependent structures investigated at the perchloric acid solution/iodine modified Au(111) interface by electrochemical frequency-modulation atomic force microscopy.

Electrochemical frequency-modulation atomic force microscopy (EC-FM-AFM) was adopted to analyze the electrified interface between an iodine modified Au(111) and a perchloric acid solution. Atomic resolution imaging of the electrode was strongly dependent on the electrode potential within the electrochemical window: each iodine atom was imaged in the cathodic range of the electrode potential, but not in the more anodic range where the tip is retracted by approximately 0.1 nm compared to the cathodic case for the same imaging parameters. The frequency shift versus tip-to-sample distance curves obtained in the electric double layer region on the iodine adlayer indicated that the water structuring became weaker at the anodic potential, where the atomic resolution images could not be obtained, and immediately recovered at the original cathodic potential. The reversible hydration structures were consistent with the reversible topographic images and the cyclic voltammetry results. These results indicate that perchlorate anions concentrated at the anodic potential affect the interface hydration without any irreversible changes to the interface under these conditions.

Effect of the Formation of Highly Ordered Platinum(II) Octaethylporphyrin Adlayer on the Surface Reconstruction of Gold and Supramolecular Assembly of Fullerenes

The structures of 2D molecular assemblies of platinum(II) octaethylporphyrin (PtOEP) on Au(111) and Au(100) surfaces were examined using in situ scanning tunneling microscopy (STM) in 0.05 M HClO4 solutions. The in situ STM observations revealed that the reconstructed rows of Au(111) and Au(100) were stabilized through the formation of highly ordered arrays by adsorption of PtOEP in a wide potential range of the electric double-layer region and that the PtOEP overlayer expanded the stable potential range for the reconstructed phase. Furthermore, the supramolecular assembly of C60 and C84 on a highly ordered PtOEP adlayer formed on Au(111) surfaces was investigated. STM images revealed site-selective supramolecular assembly of fullerenes, especially C84, which exhibited a low coverage on the PtOEP adlayer. C60 molecules exhibited a full coverage and formed highly ordered adlayers on PtOEP, whereas C84 exhibited disordered arrays. The results of this study showed that the strong π-electron donation by the PtOEP molecules stabilized the reconstructed rows of single-crystal Au planes and that the PtOEP adlayer acted as a strong electron-donating layer, aiding the formation of supramolecularly assembled C60 molecules.

Comparison of salt adsorption capacity and energy consumption between constant current and constant voltage operation in capacitive deionization

Capacitive deionization (CDI) is an electrochemically controlled desalination technology based on applying a non-faradaic process in the electrical double layer region. Because CDI performance is affected by the type of operational mode, this study examined salt adsorption capacity (deionization capacity) and energy consumption of two CDI operational modes, constant voltage (CV) and constant current (CC). The use of higher cell voltage in CV mode resulted in faster salt adsorption under a given charging time due to stronger electrostatic attraction than CC mode. However, despite of the faster salt adsorption of CV mode, CC mode showed much lower energy consumption than CV mode by 26%–30% as an identical electrical charge was consumed or an identical amount of ion removal was considered. This lower energy consumption is due to the overall lower cell voltage used in CC mode than in CV mode. This study suggests that better salt adsorption capacity and energy consumption in CDI operation may be achieved by applying appropriate CV and CC modes.

Magnetocapacitance in magnetic microtubular carbon nanocomposites under external magnetic field

A small external magnetic field of 0.072T was reported to significantly influence the specific capacitances of the supercapacitor with magnetic carbon - metal (iron, cobalt or nickel) oxides microtubular nanocomposite fabrics as flexible electrodes. The decorated metal oxide species and annealing method (microwave-assisted vs. conventional tubular annealing) were discovered to play important roles in the increase or decrease of the capacitance. All the electrodes demonstrated excellent cycling retention performances, for example, upto ~350% for nickel oxide doped carbon nanocomposite electrodes. The corresponding mechanism for the magnetic field influence on the capacitance has been interpreted in term of a synergistic effect of three internal resistances, solution resistance in electrolyte solution, charge transfer resistance at electrode/electrolyte interface and leakage resistance in electric double layer region. The applied magnetic field is able to change the solution resistance due to the magnetohydrodynamic phenomena as well as the electrode resistance (directly corresponds to charge transfer resistance) by the magnetoresistance (MR) behavior.