Compact Double Layer Research Articles

A reactive force field approach to modeling corrosion of NiCr alloys in molten FLiNaK salts

The interface between NiCr alloys and FLiNaK molten salt exhibits complex corrosion behavior, mainly driven by intricate chemical interactions involving Cr and F−ions. Understanding these dynamic reactions is crucial for developing effective corrosion mitigation strategies to ensure the long-term durability of Ni-based alloy components in molten salt technologies. However, obtaining molecular-level understanding through experiments is challenging. To address this, we utilize reactive molecular dynamics simulations enabled by a reactive force field, ReaxFF, to investigate detailed reaction dynamics at the atomic level. We first present the development of the ReaxFF parameter set for Ni/Cr/F/Li/Na/K based on extensive first-principles calculations. With this force field, we achieve a strong agreement for the structure of FLiNaK molten salt by comparing the pair distribution functions with experimental and simulation results. Furthermore, our simulations successfully reproduce the experimental phenomenon of Cr dissolution in fluoride salt, with the corrosion rate depending on the alloy and salt compositions. Particularly, our simulations reveal that increasing the concentration of Li can enhance the formation of a compact double layer, mitigating Cr dissolution. This work enables a fundamental understanding of the interfacial behavior between fluoride salt and NiCr alloys.

Design of a compact ultra-high frequency antenna for partial discharge detection in oil immersed power transformers

Oil-immersed Power transformers are considered to be one of the most essential parts of the electrical system and partial discharges (PD) are one of the dangerous problems that face transformer insulation. PD is localized dielectric breakdown of an insulation degradation or defect in an oil power transformer. Ultra-high frequency (UHF) detection technique can be utilized in PD detection inside oil-filled power transformers, by capturing the electromagnetic waves generated by PD using an antenna. This research presents a design for a compact double layer printed UHF antenna to detect and discriminate various types of PD activity in a power transformer. PD activity in the transformer has been proposed in this paper by using four different artificial insulation defects, metal protrusion defect in air medium, metal protrusion defect in an oil medium, surface discharge defect, and oil-paper void defect. The presented design guarantees a small-size wideband antenna for detection. The designed antenna has small/simple size and ultra-wide frequency passband from 1.4 to 1.85 GHz, 2.2 to 2.65 GHz, and 3.2 to 3.4 GHz. The normalized power frequency spectra of the captured UHF-PD signals of the four defective models showed that every PD type has its signature, and this could help in the discrimination among different types on site.

Modeling of electrochemical oxide film growth-a PDM refinement

The Point Defect Model (PDM) is known for over 40 years and has brought deeper insight to the understanding of passivity. During the last decades it has seen several changes and refinements, and it has been widely used to analyze growth kinetics of different alloys. Nevertheless, the model has been based on still unconfirmed assumptions, as constant and potential independent electric field strength. To overcome this limitation, we introduce a Refined PDM (R-PDM) in which we replace those assumptions by using additional equations for charge distribution including new physically valid boundary conditions based on considering finite dimensions for the defects by introduction of two defect layer at the film boundaries and by calculating the potential drop at the surface of the film towards the solution over the compact double layer. The calculations by the R-PDM show that the original PDM assumptions are only valid for very specific parameter combinations of oxide film growth and vacancies transport and cannot generally be taken for granted. We believe our findings of electric field and potential drop dependency on the external potential to pave the way for a more realistic description of passive layer formation.

Compact SQUID Realized in a Double-Layer Graphene Heterostructure.

2D systems that host 1D helical states are advantageous from the perspective of scalable topological quantum computation when coupled to a superconductor. Graphene is particularly promising for its high electronic quality, its versatility in van der Waals heterostructures, and its electron- and hole-like degenerate 0th Landau level. Here we study a compact double-layer graphene SQUID (superconducting quantum interference device), where the superconducting loop is reduced to the superconducting contacts connecting two parallel graphene Josephson junctions. Despite the small size of the SQUID, it is fully tunable by the independent gate control of the chemical potentials in both layers. Furthermore, both Josephson junctions show a skewed current-phase relationship, indicating the presence of superconducting modes with high transparency. In the quantum Hall regime, we measure a well-defined conductance plateau of 2e2/h indicative of counter-propagating edge channels in the two layers.

Interface equilibrium modeling of all-solid-state lithium-ion thin film batteries

Dynamic modeling of all-solid-state batteries (SSBs) has gained strong research interest in recent years. However, the intrinsic equilibrium state, important for the exchange current calculation and the interface resistance analysis is hardly addressed. For the first time, we introduce an advanced MPNP-FBV model which takes the electrical double layer (EDL) structure and the unoccupied regular lattice sites (vacancies) into account to illustrate the fundamental phenomena at the equilibrium state. This general model is verified from the chemical perspective and the interface experimental results in our previous investigations, which indicates that the model is able to calculate the exchange current and the charge transfer resistance from fundamental material properties. The influence on concentration and electrostatic potential by the different EDL structures, the state of charge (SOC) and the diffusivity are also investigated. Results show that the total electrostatic potential drop at the equilibrium state is related to the free enthalpy difference between the two materials. Furthermore, the charge transfer resistance with the diffuse double layer structure is higher than that with the compact double layer.

Oxidation and ablation protection of double layer HfB2-SiC-Si/SiC-Si coating for graphite materials

In order to protect graphite from oxidation and ablation, novel double layer HfB2-SiC-Si/SiC-Si coatings were designed and fabricated by a joint process of slurry method and gaseous silicon infiltration on graphite substrates. X-ray diffraction analysis and scanning electron microscopy were used to study the phase composition and microstructure of the as-fabricated coatings. The tests of oxidation and oxyacetylene flame were conducted to assess the oxidation and ablation resistance ability of the HfB2-SiC-Si/SiC-Si coatings. Results indicated that the coated samples can endure 752 h and 1200 h of oxidation at 900 °C and 1500 °C with weight gains of 0.099% and 0.26%, respectively, whereas the coated sample had a weight gain of 0.34% after 230 h of oxidation at 1600 °C. The compact double layer structure of coating resulted in the attractive anti-oxidation property at low temperature (900 °C), while the good oxidation and exfoliation resistance at high temperatures (1500–1600 °C) were ascribed to the formed compound silicate oxide layer comprising SiO2 and HfSiO4 on the coating surface. Meanwhile, the fabricated coating exhibited good ablation resistance property, after ablation for 90 s with a heat flux of 2.38 MW/m2, the mass and linear ablation rates of the coated sample were 0.36 mg/s and −0.31 μm/s, respectively. The formed compound oxides containing HfO2 and SiO2 on the surface acted as a thermal barrier that prevented the further consumption of coated sample.

The structure of the electrochemical double layer: Ag(111) in alkaline electrolyte

The structure of the electrochemical double layer at the interface between a Ag(111) electrode and 0.1 M KOH electrolyte has been probed using in-situ surface X-ray scattering (SXS). Detailed modeling of the SXS data at negative potential (E = − 1.0 V versus SCE) is consistent with the presence of an hydrated K + cation layer at a distance of 4.1 ± 0.3 Å from the Ag surface and at positive potential (E = − 0.2 V), indicates that the presence of OH ad stabilizes the hydrated K + cations through a non-covalent interaction forming a compact double layer structure in which the Ag-K + distance is reduced to 3.6 ± 0.2 Å.

Three-dimensional structure and IgE-binding properties of mature fully active Der p 1, a clinically relevant major allergen

Der p 1 is a 25-kd allergen with cysteine protease activity. Sensitization to Der p 1 affects a large proportion of individuals with allergy, resulting in rhinitis, asthma, and/or atopic dermatitis. We determined the Der p 1 crystallographic structure to understand the relationships among structure, function, and allergenicity. Recombinant pro-Der p 1 was produced in Pichia pastoris and allowed to mature spontaneously before purification by a 2-step procedure. Protease activity was checked by using a fluorogenic peptide substrate. Allergenicity was analysed by IgE binding assays and basophil activation test. The determination of the 3-dimensional structure was obtained by X-ray crystallography at 1.9 A resolution. The recombinant protein is fully active and expresses an allergenicity equivalent to its natural counterpart. Der p 1 exhibits a cysteine protease fold typical of the papain family, has a magnesium binding site, and forms dimers with a large interface. The crystal lattice shows that the dimers are tightly packed in a compact double layer of proteins. Such an assembly likely exists in dry fecal pellets, the natural form of allergen exposure, and appears ideal to interact with cell surface and trigger allergic inflammation. We present here the 3-dimensional structural features of mature fully active Der p 1, one of the main allergens involved in human allergic diseases. This opens the possibility to evaluate the importance of enzymatic activity in pathology and possible new therapeutic interventions.

Supporting Electrolyte and Solvent Effects on Single-Electron Double Layer Capacitance Charging of Hexanethiolate-Coated Au140 Nanoparticles

Sequential injections of single electrons (or holes) into the cores of Au(140) hexanethiolate monolayer-protected clusters (MPCs) occur at measurably different electrochemical potentials owing to the extremely small (subattofarad) values of the single MPC capacitance (C(MPC)) of the nanoparticle. The potential increment for each sequential injection is DeltaV = e/C(MPC). The dependence of DeltaV on the concentration of supporting electrolyte (from 1 to 100 mM), measured using square wave voltammetry, is shown to be caused, primarily, by changes in the diffuse double layer component (C(DIFFUSE)) of C(MPC). The dependence of C(DIFFUSE) on r(core), the radius of the nanoparticle, is considered. Additionally, significant changes in the magnitude of the compact double layer component (C(COMPACT), equivalent to the Stern layer) of C(MPC) were induced by adding hydrophobic solvent components such as hexane or dodecane or by introducing hydrophobic electrolyte ions (tetrabutyl-, tetrahexyl-, and tetraoctylammonium, perchlorate, and tetraphenylborate). These changes are interpreted as specific solvation and ion penetration of the hexanethiolate monolayer. For brevity we will refer to these phenomena as solvation/penetration phenomena.

The Reversible Redox Behaviour of Adsorbate Layers on Account of the Electrosorption Valency

AbstractThe electrosorption valency γ [9–11] was introduced regarding acid‐base and redox equilibria at low surface concentrations at excess supporting electrolyte. A formula is given for the calculation of the apparent protolytic dissociation constant of an adsorbate (Eq. (10)) and a modified Nernst equation for adsorbates being to some extent immersed in a compact double layer (Eq. (17)). Neglecting the integral term another formula was obtained for the pH dependence of the half‐wave (peak) potentials of reversible surface redox reactions (Eq. (18)). Also modified equations for the peak current (Eq. (23)) and peak width (Eq. (24)) of cyclic voltammograms involving adsorbed species were deduced. Test substances were Pyocyanine, Nile blue and Neutral red adsorbed at graphite electrodes.

The relation between electrical double layer data and electronic work functions for amalgams

The relationship between the experimental zero charge potential, Epzc, of indium and thallium amalgams and their electronic work function, Φ, is studied and discussed. For indium amalgams, the linear relationship, Epzc = Φ −const., is found to be obeyed in the surprisingly wide composition range of 0.02 < xIn < 0.6. The data for pure mercury do not fit on the line, presumably because of its low electronic density of states. Potentials at constant negative charge density, Epσc, depend on the work function in a characteristic manner, which is interpreted by taking into account the desorption of indium from the metal vb solution interface and the degree of water orientation in the compact double layer. Also, the experimental data are used to discuss the concept of “electrochemical work functions” determined as the shift between two parallel charge densities vs. potential curves. Using electrochemical work functions determined for the thallium amalgams, their behaviour is found to be quite similar to that of the indium amalgams. It appears that these amalgams constitute a most useful continuous series of metal phases to study the metal dependency of interfacial properties.

Analysis of molecular polarization and interaction in adsorbed monolayers of electrodes: Part 3. Ionic adsorption isotherm with mean field treatment of interactions involving ions and dipoles

The effective field and potential at the origin of an adsorbed lattice of ions and dipoles are evaluated. The contributions of dipoles and ions to this effective field and potential are expressed in terms of effective coordination numbers (i.e. Madelung constants), and the variation of the micropotential and microfield functions with normal distance in the compact double layer is also determined. On the basis of the mean field approximation, the coordination numbers are used to construct an isotherm describing the competitive adsorption of ions and dipoles at a metal-electrolyte interface, and an expression for the corresponding differential capacitance.

Treatment of dipole orientation in the double layer at electrodes: Analysis of conditions leading to polarization catastrophes

Molecular polarization contributions to the potential profile and differential capacity of electrode-solution interfaces are compared, on the basis of the primitive model for the double layer, according to three simple representations of the orientation of solvent dipoles at the electrode: the Watts–Tobin/BDM two-state model, the Fawcett three-state model, and a Debye–Langevin-type continuum-of-orientations model. The so-called Cooper–Harrison catastrophe, in which the induced polarization qdip of the inner layer is calculated to exceed the surface charge density qM of the electrode, is shown to arise for all models considered, and is therefore not, as supposed previously, a specific artifact of the two-state model with respect to which it was first noted. This polarization catastrophe is shown to result from an oversimplified but frequently employed representation of the polarizing field as E1=4πqM. When the field assumed to act on the molecules in the polarization expressions is represented as E1=Δφ1/d (where Δφ1 is the potential difference across the inner layer and d is its thickness), or as E1=4π(qM−qdip), i.e., taking into account the reactive polarization of the dipole layer, there result implicit equations for qdip and Δφ1 which provide self-consistent values of these quantities as a function of qM or of electrode/solution potential difference V. Anomalous behavior is then shown not to arise. These equations are shown to be formally consistent with the conditions of continuity of potential and dielectric displacement, applied to the interfaces assumed to be present in the given model of the double layer. The calculation procedures which are described may be readily extended to other models which have been proposed to describe the polarization of the compact double layer.

Compact and diffuse double layer interaction in unsupported system small-signal response

Previous exact results for the small-signal impedance of an unsupported electrode/material/electrode system which include effects of the finite size of charge carriers are simplified and discussed. The material contains non-recombining charges of opposite sign with the positive one immobile and uniformly distributed. General boundary conditions which encompass the range from no electrode reaction to ohmic electrode behavior are employed. In the presence of an electrode reaction, the interaction of the compact and diffuse double layers leads to considerably more complexity in the equivalent circuit than might appear in simple treatments of the supported case, in which the diffuse double layer capacitance is neglected or the compact double layer and diffuse double layer capacitances are placed in series. Two different approximate equivalent circuits made up of frequency-independent elements are found which yield remarkable agreement with the exact results over the entire frequency range of interest. The first involves the ordinary approximate circuit (OAC) previously found in the absence of compact layer effects plus a series compact layer contribution involving a parallel resonant circuit with quality factor at resonance which may approach unity. Pseudo-inductance effects are found to be extremely significant in this representation. The second approximate equivalent circuit, simpler and almost as accurate as the first, has the same form as the original OAC but with its reaction element values altered by the presence of the compact layer. For non-Butler-Volmer electrode kinetics an upper limit is found for the experimentally determinable apparent reaction rate constant, a feature of practical importance for thin films or membranes. The response of thin films and membranes, including compact layer effects, can very readily be erroneously confused with pure bulk response, yielding entirely incorrect values for the geometrical capacitance and bulk resistance of the material.

Compact double layer effects in small-signal electrical response

Compact double layer effects in small-signal electrical response

Compact double layer effects in small-signal electrical response

Compact double layer effects in small-signal electrical response

Non-mediated zero voltage conductance of hydrophobic ions through bilayer lipid membranes

A charge pulse technique applied to the study of charge transfer at metal-solution interfaces has been used to determine the capacity and the conductance of a membrane bilayer at both zero time and zero voltage. The transport of hydrophobic ion across a glycerol-monooleate bilayer (tetraphenyl borate, picrate, dipicrylamine and tetraphenyl arsonium) has been investigated by this method. A theoretical approach to the problem has been proposed based on one analogous to that used for the compact double layer at metallic electrodes.

Electrical double layer at the interface between mercury and dilute aqueous solutions of depolarizers

The impedance behaviour of dme/depolarizer systems in the potential region of limiting current was investigated with dilute aqueous solutions of some metal salts [AgNO 3, TlNO 3, Cd(NO 3) 2 etc] and TlOH. The capacitance/potential curves of metal salt solutions (pH 4–6) were found to be almost identical with those of ideally polarizable electrode systems. In contrast to the metal salt systems, the capacitance and resistance of the TlOH system (pH 11) showed a remarkable decrease and increase, respectively, at the potentials where a limiting current was observed. The impedance behaviour of the TlOH system, however, became similar to that of the metal salt systems by the successive addition of HClO 4 or NaClO 4. These experimental results suggest that a normal compact double layer is formed in depolarizer solutions if the solution contains electrochemically inactive cations such as H + or Na + in concentrations higher than about 10 −5 N. Some evidence is also presented for the possible contribution of depolarizer cations to the formation of double layer. In the TlOH system a frequency dispersion was observed, which implies that the formation of interfacial layer is affected by some processes of a kinetic nature, unless the solution contains sufficient amount of electrochemically inactive cations to form a normal compact double layer.

Compact double layer as a function of temperature and pressure

Compact double layer as a function of temperature and pressure

Short‐Pulse Techniques: II . Perturbation Times in Alkali‐Halide Systems

A new technique involving the perturbation of a system from equilibrium using very short galvanostatic pulses and miniature cells has been applied to study the electrical double layer at Pt electrodes. The perturbation time, the time required for the double layer to begin charging, was determined for hydrogen‐ and alkali metal‐halide systems and for reversible hydrogen systems in several acids and . The perturbation time was essentially independent of charging current density and of the anion in solution but was strongly dependent on the cation species and electrolyte concentration. Models to explain the absence of electrode polarization during the initialflow of charge into the double layer are discussed. A model which involves a very fast faradaic process and which is consistent with the experimental observations is proposed. It is suggested that the perturbation time, which is less than 100 nanosec for 1M solutions, represents the time required for movement of ions into or out of the compact double layer.