Modelling the electric double layer at electrode/electrolyte interfaces

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Toward better electrode/electrolyte interfaces in the ionic-liquid-based rechargeable aluminum batteries

The past decade has witnessed the germination of rechargeable aluminum batteries(RABs)with the colossal potential to enact as a device for the large scale energy storage and conversion.The Majority of investigations are dedicated to the exploration of suitable cathode materials,while less is known about the electrode/electrolyte interfaces that determine the electrochemistry of batteries.In this perspective,we will highlight the significance of electrode/electrolyte interface for RABs,in overall kinetics and capacity retention.Emphasis will be laid on the complicated yet basic understandings of the phenomena at the interfaces,including the dendrite growth,surface Al2O3 and solid–electrolyte-interphase(SEI).And we will summarize the reported practice in effort to build better electrode/electrolyte interfaces in RAB.In the end,outlook regarding to the challenges,opportunities and directions is presented.     

Molecular dynamics and phase transitions in phospholipid monolayers at liquid-liquid interfaces

Stablen-hexadecane/water andn-tetradecane/water macroemulsions containing monolayers of natural (egg yolk lecithin, EY) and synthetic (dimyristoylphosphatidylcholine, DMPC) phospholipids at liquid-liquid interfaces were prepared. The existence of the monolayers was proved by studying the reduction kinetics of a surface-active spin probe with ascorbate anions. Spin labeled derivatives of stearic acid in which the nitroxide group is locared at different distances from the polar head (5-, 12-, and 16-doxylstearic acids) were used to study the temperature dependences of the molecular ordering, rotational mobility, and local polarity in the monolayers in emulsions and also in bilayers in liposomes obtained from the same lipids. In the EY monolayers, the degree of spin probe solubilization is higher, while the order parameters (S) and rotational correlation times (τ) are lower than those in EY bilayers. The differences between these parameters for mono- and bilayers increase with an increase in the distance of the reporter group from the aqueous phase. In the DMPC monolayers, a first-order phase transition was detected by measuring the temperature dependences ofS and τ. The temperature region of the phase transition in monolayers is shifted to lower temperatures with respect to that for bilayers and depends on the nature of the oil phase. It was concluded that the phospholipid monolayers in emulsions incorporate hydrocarbon molecules, whose concentration in the DMPC monolayers increases on going from the low-temperature (gel) to the high-temperature (liquid crystal) phase. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 418–425, March, 1998.     

Phase transitions in two-dimensional colloidal particles at oil/water interfaces

Enhanced digital video microscopy is applied to study the equilibrium structure of a two-dimensional charged sulfate-polystyrene particle (2 mum in diameter) monolayer at decane/water interfaces. When the surface density is decreased, a sequential phase transition, pure solid phase-->pure hexatic phase-->liquid-hexatic-coexisting phase-->pure liquid phase, is observed. In addition, the transition between liquid and hexatic phases is first order, while the solid-hexatic phase transition is second order. The temperature effect on this two-dimensional melting transition is discussed by performing the experiments at three different temperatures. The Voronoi [J. Reine Angew. Math. 134, 198 (1908)] construction is applied to analyze the defect structure in the two-dimensional particle monolayer. The pair interaction potential of the two-dimensional colloidal particles is found to be a very long range repulsion and to decay with distance to the power of -3.     

Influence of ammonium nitrate in phase transitions of Langmuir and Langmuir-Blodgett films at air/solution and solid/solution interfaces

The effect of ammonium nitrate on the phase transitions in Langmuir films of amphiphiles-stearic acid, stearyl amine (STAM), stearyl alcohol, dihexadecylphosphate, and the quarternized ammonium salt dioctadecyldimethylammonium bromide have been studied at air/water interface and in local ordering of their Langmuir-Blodgett films (LB films). The study shows that except for the stearyl amine (STAM) all other monolayers exhibit a liquid-expanded to liquid-condensed transition with slight expansion in area in the presence of ammonium nitrate. STAM monolayers show a new phase transition, which possibly arises due to the differently ionized amino groups, and change in solvation sheath due to an ion-dipole type interaction between the amino groups and the ammonium ion in the subphase. Mixed films of the amine with the acid and alcohol did not show such intermediate phases indicating that competing H-bonds between polar groups themselves and dipolar couplings between the polar groups and ammonium nitrate play a major role in the organization of the molecules at the interface. The above effect resulting in a change in the local order is borne out by Brewster angle micrographs (BAM) of the Langmuir films of STAM at air/solution interface. Such behavior is also seen at solid/liquid interfaces where the polar component of surface energy undergoes a drastic change for the amine films transferred onto solid substrates from the air/ammonium nitrate solution interface.     

The growth of compact layers at the electrode interface: Part IV. Fractal character of uracil films at the mercury-aqueous electrolyte interface

Compact layers of uracil films grown on the mercury electrode-aqueous solution interface were studied by means of time-resolved FFT impedance spectroscopy. The films are characterized by the fractal dimension which evolves with time. The results are discussed in terms of the cluster-cluster aggregation model and the hole formation mechanism proceeding in the compact layer.     

The boundary element method: the simulation of voltammetry at immiscible liquid/liquid interfaces

The boundary element method is presented as an efficient and powerful method for the analysis of time-dependent electrochemical processes occurring at immiscible liquid/liquid interfaces. This paper outlines the theory and numerical details required for the development and application of two-dimensional transient diffusion models for the simulation of cyclic voltammetry behaviour at a range externally polarised immiscible liquid/liquid interfaces of differing topography. The benefits of the BEM approach are discussed, including the reduction in dimensionality brought about by the formulation procedure and complete elimination of the need for domain discretisation with the time-domain convolution approach. The versatility and efficiency of the numerical procedures are examined with respect to a number of liquid/liquid interface geometries and a series of working curves established to quantify the influence of interface topography on the observed voltammetric behaviour.     

Monitoring electrode/electrolyte interfaces of Li-ion batteries under working conditions: A surface-enhanced Raman spectroscopic study on LiCoO2 composite cathodes

Lithium-ion batteries are commonly used for electrical energy storage in portable devices and are promising systems for large-scale energy storage. However, their application is still limited due to electrode degradation and stability issues. To enhance the fundamental understanding of electrode degradation, we report on the Raman spectroscopic characterization of LiCoO₂ cathode materials of working Li-ion batteries. To facilitate the spectroscopic analysis of the solid electrolyte interface (SEI), we apply in situ surface-enhanced Raman spectroscopy under battery working conditions by using Au nanoparticles coated with a thin SiO₂ layer (Au@SiO₂). We observe a surface-enhanced Raman signal of Li₂CO₃ at 1090 cm⁻¹ during electrochemical cycling as an intermediate. Its formation/decomposition highlights the role of Li₂CO₃ as a component of the SEI on LiCoO₂ composite cathodes. Our results demonstrate the potential of Raman spectroscopy to monitor electrode/electrolyte interfaces of lithium-ion batteries under working conditions thus allowing relations between electrochemical performance and structural changes to be established.     

Effects of surface heterogeneity of oxides in simple ion adsorption at oxide/electrolyte interfaces

It has been shown that the adsorption of ions at oxide/electrolyte interface should be treated in terms of adsorption on a heterogeneous solid surface. This is because the adsorption runs via formation of complexes with surface oxygen atoms. Because of the small degree of oxide surface organization, different surface oxygens may have different status with respect to electrostatic interactions and the strength of the chemical bonds formed between the surface oxygens and the adsorbed protons or metal ions. We have shown that the equations describing the equilibria of formation of surface complexes can be transformed to Langmuir-like equations for multicomponent adsorption. One can apply, therefore, for these systems theoretical methods used to describe mixed-gas adsorption on heterogeneous solid surfaces.     

Effects of protonation of pyridine moieties on the 2D assembly of porphyrin layers on Au(111) at electrochemical interfaces

Unique molecular assemblies of a porphyrin derivative are prepared on Au(111) by controlling the protonation/unprotonation of the pyridine groups. The porphyrin derivative, driven by the protonation of the pyridine groups, can provide characteristic assemblies with specific molecular conformations on an Au(111) surface at the electrochemical interface. In situ scanning tunneling microscopy images revealed clear differences in the adlayer structures for the unprotonated and the protonated forms of the molecules that depended upon the electrochemical potential.     

Model system for the study of 2D phase transitions and supramolecular interactions at electrified interfaces: hydrogen-assisted reductive desorption of catechol-derived adlayers from Pt(111) single-crystal electrodes

Classical electroanalytical techniques and in situ FTIR are used to study the oxidative chemisorption of catechol (o-H(2)Q) and the hydrogen-assisted reductive desorption of catechol-derived adlayers (o-Q((ads))) at nearly defect-free Pt(111) single-crystal electrodes in 0.5 M H(2)SO(4). At near equilibrium conditions (lim(upsilon-->0)) the cyclic voltammetric response does not conform to the behavior expected from classical models of molecular adsorption at electrochemical interfaces. Instead, attractive interactions play a controlling role, i.e., hydrogen-assisted displacement of o-Q((ads)) takes place as an electrochemically reversible two-dimensional (2D) phase transition controlled by collision-nucleation-growth phenomena in the presence of 2 mM o-H(2)Q((aq)). In contrast, different desorption dynamics are observed when the reductive desorption of the adlayers is carried out in clean (0 mM o-H(2)Q((aq)) supporting electrolyte. Donor-acceptor (DA) interactions between the Pt(111)/o-Q((ads)) surface adduct and o-H(2)Q((aq)) are postulated as a possible intervening mechanism leading to the observed differences in the macroscopic electrochemical responses. The results also demonstrate that in aqueous solutions it is thermodynamically feasible to shift the formal oxidation potential of catechol-metal adducts to potentials near those of molecular hydrogen via chemically reversible, nondissociative interactions, taking place as a 2D phase transition.     

Fidelity as a marker of topological phase transitions in 2D Dirac materials

Silicene, as other 2D buckled structures, is a gapped Dirac material with intrinsic spin‐orbit coupling whose band structure can be controlled by applying a perpendicular electric field. It presents a topological phase transition from a topological insulator to a band insulator at the charge neutrality point. We present in this article a characterization of this phase transition by using fidelity of Loschmidt echoes when a magnetic and two slightly different electric fields are applied by considering the time evolution of two kinds of wave packets, one with a single Gaussian profile and the other with a double Gaussian profile creating a cat state. We also show that Zitterbewegung, classical, and revival Loschmidt periods diverge close to but not exactly at the charge neutrality point and explain this behavior.     

Electrochemical behavior of PbO2/PbSO4 electrode in the presence of surfactants in electrolyte

The electrochemical behavior of PbO₂/PbSO₄ electrode is investigated in 4.5 M H₂SO₄ in presence of three surfactants, Sodium Dodecyl Sulfate (SDS), Cetyltrimethylammonium bromide (CTAB) and Sodium tripolyphosphate (STPP), using cyclic voltametry, electrochemical spectroscopy impedance and galvanostatic discharge as techniques. The micro morphology of the surface of the modified PbO₂ electrodes is examined by scanning electron microscopy. The results show that SDS and CTAB when added in the electrolyte could refine the coating particles and change the roughness of the surface of the electrode leading to a thin film of PbO₂ with amorphous character. In addition, SDS and CTAB shift the hydrogen evolution potential towards more negative values, improve the discharge capacity of the anodic layer and accelerate the charge transfer. Under cathodic polarization, CTAB presents the lowest value of the charge transfer resistance R_ct. In the contrary, STPP shifts the oxygen evolution potential towards more positive values, passivates the surface of the electrode and inhibits completely the reaction of PbO₂ formation.