Phase transition in porous electrodes

It is shown by Monte Carlo simulation that electrochemical thermodynamics of electrolytes in a porous electrode is qualitatively different from that in the bulk with a planar electrode. In particular, first order phase transitions occur in porous electrodes when the pore size is comparable to the ion size of the electrolytes: as the voltage is increased from zero, the surface charge density and the ion density in the porous electrodes discontinuously change at a specific voltage. The critical points for those phase transitions are identified.     

Electrochemistry and capacitive charging of porous electrodes in asymmetric multicomponent electrolytes

We present porous electrode theory for the general situation of electrolytes containing mixtures of mobile ions of arbitrary valencies and diffusion coefficients (mobilities). We focus on electrodes composed of primary particles that are porous themselves. The predominantly bimodal distribution of pores in the electrode consists of the interparticle or macroporosity outside the particles through which the ions are transported (transport pathways), and the intraparticle or micropores inside the particles, where electrostatic double layers (EDLs) are formed. Both types of pores are filled with electrolyte (solvent plus ions). For the micropores we make use of a novel modified-Donnan (mD) approach valid for strongly overlapped double layers. The mD-model extends the standard Donnan approach in two ways: (1) by including a Stern layer in between the electrical charge and the ions in the micropores, and (2) by including a chemical attraction energy for the ions to go from the macropores into the micropores. This is the first paper where the mD-model is used to model ion transport and electrochemical reactions in a porous electrode. Furthermore we investigate the influence of the charge transfer kinetics on the chemical charge in the electrode, i.e., a contribution to the electrode charge of an origin different from that stemming from the Faradaic reaction itself, e.g. originating from carboxylic acid surface groups as found in activated carbon electrodes. We show that the chemical charge depends on the current via a shift in local pH, i.e. ??current-induced charge regulation.?? We present results of an example calculation where a divalent cation is reduced to a monovalent ion which electro-diffuses out of the electrode.     

Monte Carlo simulation of electrolytes in the constant voltage ensemble

The authors studied the structural, electrostatic, and electromechanical properties of the terlamellar structure composed of the anode, the cathode, and the electrolyte layer separating them. They used the Monte Carlo simulation technique in the constant voltage ensemble, where the electrical potential difference between the anode and the cathode is introduced as an external field. For ions, they used the primitive models of different sizes and valences in order to investigate how they affect the physical properties when an electrical field is applied between the electrodes. For electrodes, they used impermeable and permeable models, which mimic planar and porous electrodes, respectively. The asymmetry between the anions and the cations in size or valence was found to be responsible for the asymmetry in the concentration profile, the potential drop, and the stress distribution, in comparing the anode and the cathode sides. The charging/discharging process in the planar and porous electrodes is discussed at molecular level.     

锂离子电池多孔电极理论的回顾与新思考

本文总结了Newman多孔电极理论的基本内容,提出若干改进思路. 提出基于离子-空穴耦合传输机制描述浓电解质中的离子输运过程,在此基础上引入离子-电子耦合转移反应的思想处理电极材料中的离子传输问题,并通过计算嵌锂材料的离子扩散系数验证其合理性. 总结了描述多孔电极多尺度结构的相关理论和技术,表明均质化方法和基于结构重建的介观模拟方法均能给出比较合理的有效输运参数,从而提高多孔电极理论模拟结果的准确性.     

An advanced model framework for solid electrolyte intercalation batteries

Recent developments of solid electrolytes, especially lithium ion conductors, led to all solid state batteries for various applications. In addition, mathematical models sprout for different electrode materials and battery types, but are missing for solid electrolyte cells. We present a mathematical model for ion flux in solid electrolytes, based on non-equilibrium thermodynamics and functional derivatives. Intercalated ion diffusion within the electrodes is further considered, allowing the computation of the ion concentration at the electrode/electrolyte interface. A generalized Frumkin-Butler-Volmer equation describes the kinetics of (de-)intercalation reactions and is here extended to non-blocking electrodes. Using this approach, numerical simulations were carried out to investigate the space charge region at the interface. Finally, discharge simulations were performed to study different limitations of an all solid state battery cell.     

Carbon materials for supercapacitor application

The most commonly used electrode materials for electrochemical capacitors are activated carbons, because they are commercially available and cheap, and they can be produced with large specific surface area. However, only the electrochemically available surface area is useful for charging the electrical double layer (EDL). The EDL formation is especially efficient in carbon pores of size below 1 nm because of the lack of space charge and a good attraction of ions along the pore walls. The pore size should ideally match the size of the ions. However, for good dynamic charge propagation, some small mesopores are useful. An asymmetric configuration, where the positive and negative electrodes are constructed from different materials, e.g., activated carbon, transition metal oxide or conducting polymer, is of great interest because of an important extension of the operating voltage. In such a case, the energy as well as power is greatly increased. It appears that nanotubes are a perfect conducting additive and/or support for materials with pseudocapacitance properties, e.g. MnO(2), conducting polymers. Substitutional heteroatoms in the carbon network (nitrogen, oxygen) are a promising way to enhance the capacitance. Carbons obtained by one-step pyrolysis of organic precursors rich in heteroatoms (nitrogen and/or oxygen) are very interesting, because they are denser than activated carbons. The application of a novel type of electrolyte with a broad voltage window (ionic liquids) is considered, but the stability of this new generation of electrolyte during long term cycling of capacitors is not yet confirmed.     

Quaternary ammonium bromide surfactant adsorption on low-index surfaces of gold. 1. Au(111)

The coadsorption of the anionic and cationic components of a model quaternary ammonium bromide surfactant on Au(111) has been measured using the thermodynamics of an ideally polarized electrode. The results indicate that both bromide and trimethyloctylammonium (OTA(+)) ions are coadsorbed over a broad range of the electrical state of the gold surface. At negative polarizations, the Gibbs surface excess of the cationic surfactant is largely unperturbed by the presence of bromide ions in solution. However, when the Au(111) surface is weakly charged the existence of a low-coverage, gaslike phase of adsorbed halide induces an appreciable (~25%) enhancement of the interfacial concentration of the cationic surfactant ion. At more positive polarizations, the coadsorbed OTA(+)/Br(-) layer undergoes at least one phase transition which appears to be concomitant with the lifting of the Au(111) reconstruction and the formation of a densely packed bromide adlayer. In the absence of coadsorbed halide, the OTA(+) ions are completely desorbed from the Au(111) surface at the most positive electrode polarizations studied. However, with NaBr present in the electrolyte, a high surface excess of bromide species leads to the stabilization of adsorbed OTA(+) at such positive potentials (or equivalent charge densities).     

The Thermodynamics of Insertion Electrochemical Electrodes—A Team Play of Electrons and Ions across Two Separate Interfaces

Insertion electrochemical electrodes exhibit simultaneous electron and ion transfer, with the two transfers proceeding across different interfaces. Herein the thermodynamics of the overall electrochemical electrode reaction is discussed with respect to the thermodynamics of these two charge‐transfer equilibria. This Minireview includes insertion electrochemical systems where the redox centers are in a solid phase and the ions are transferred between that phase and a solution, and also systems where the redox centers are in a liquid phase that is immiscible with another liquid phase and ions are transferred between the two liquid phases. The Minireview is intended to spark similar studies on battery materials to improve their performance.     

电解液离子与炭电极双电层电容的关系

以酚醛树脂基纳米孔玻态炭(NPGC)为电极, 通过微分电容伏安曲线的测试, 研究了水相体系电解液离子与多孔炭电极双电层电容的关系. 结果表明, 稀溶液中, 多孔炭电极的微分电容曲线在零电荷点(PZC)处呈现凹点, 电容降低, 双电层电容受扩散层的影响显著；若孔径小, 离子内扩散阻力大, 电容下降更为迅速, 扩散层对双电层电容的影响增大. 而增大炭材料的孔径或电解液浓度, 可明显减弱甚至消除扩散层对电容的影响. 炭电极的单位面积微分电容高, 仅表明孔表面利用率高, 如欲获得高的电容量, 还要有大的比表面积. 离子水化对炭电极的电容产生不利影响, 选用大离子和增大炭材料的孔径, 可有效降低离子水化对炭电极电容性能的影响.     

Supercapacitive Swing Adsorption of Carbon Dioxide

An electrical effect, the supercapacitive swing adsorption (SSA) effect is reported, which allows for reversible adsorption and desorption of carbon dioxide by capacitive charge and discharge of electrically conducting porous carbon materials. The SSA effect can be observed when an electrically conducting, nanoporous carbon material is brought into contact with carbon dioxide gas and an aqueous electrolyte. Charging the supercapacitor electrodes initiates the spontaneous organization of electrolyte ions into an electric double layer at the surface of each porous electrode. The presence of this double layer leads to reversible, selective uptake and release of the CO₂ as the supercapacitor is charged and discharged.     

Electrochemistry at nanoporous interfaces: new opportunity for electrocatalysis

Physical and electrochemical features of nanoporous electrodes arising from their morphology are presented in this perspective. Although nanoporous electrodes have been used to enhance electrocatalysis for several decades, the origin of their capability was understood on the basis of enlarged surface area or crystalline facet. However, considerable attention should be paid to the fact that nano-confined space of nanoporous electrodes can significantly affect electrochemical efficiency. Molecular dynamics in nano-confined spaces is capable of offering much more chances of interaction between a redox molecule and an electrode surface. The mass transport in the nanoporous electrode depends on various pore characteristics such as size, shape, charge, connectivity, and symmetry as well as molecular properties such as size, charge, and kinetics. Moreover, when the pore size is comparable to the thickness of an electric double layer (EDL), the EDLs overlap in the porous structure so that electrochemically effective surface area is not the same as that of the real electrode surface. These unique properties come from simply nanoporous structure and suggest new opportunity to innovative electrocatalysts in the future.     

Distribution of a Soluble Product During Its Electrosynthesis in the Porous Hydrophobic Electrode

The distribution of the concentration of a product, which is soluble in the electrolyte, inside a porous hydrophobic electrode is calculated. During the electrosynthesis under inner-kinetic conditions, the electrode obeys regularities observed in hydrophobic electrodes of fuel cells. The product distribution depends mainly on the polarization at the electrode front, the electrolyte conductivity, and the diffusion coefficient, and slightly depends on the electrode structure. An approximate equation describing the distribution of the product concentration inside the electrode is suggested.     

添加剂氟代乙烯碳酸酯对锂离子电池低温性能影响的机制研究

本文通过磷酸铁锂/碳电池研究了电解液添加剂氟代乙烯碳酸酯（FEC）对电池低温性能的影响. 电池充放电实验证明,FEC添加剂能够在负极表面形成良好的固体电解质界面层（SEI）. 电解液中添加5% FEC后,电池-40 ^oC低温放电容量保持率可以从31.7%提高至43.7%,还提高了电池放电电压平台. 交流阻抗测试表明,FEC的加入能够有效降低电池的界面传荷阻抗（R_ct）. 参比电极测试表明,其主要是降低了碳负极的低温极化.     

The Effect of Supporting Electrolyte Concentration on Zinc Electrodeposition Kinetics from Methimazole Solutions

The kinetic parameters of Zn²⁺ ion electroreduction in sodium perchlorate used as the supporting electrolyte on the mercury electrode in the presence of methimazole were determined using electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and DC polarography. The two‐stage nature of this process was demonstrated. Both steps are catalysed by a methimazole. The size of the acceleration effect depends on the concentrations of methimazole and the supporting electrolyte. The acceleration of the electrode process involves the formation of active complexes between the depolarizer ions and methimazole on the electrode surface. These complexes facilitate the exchange of charge between the electrode and zinc ions during electroreduction process. The change of the hydrating sphere of the zinc ion is also important here. This in turn depends on its oxidation state and the concentration of the supporting electrolyte.     

A new surface structural approach to ion adsorption: tracing the location of electrolyte ions

Electrolyte ions differ in size leading to the possibility that the distance of closest approach to a charged surface differs for different ions. So far, ions bound as outersphere complexes have been treated as point charges present at one or two electrostatic plane(s). However, in a multicomponent system, each electrolyte ion may have its own distance of approach and corresponding electrostatic plane with an ion-specific capacitance. It is preferable to make the capacitance of the compact part of the double layer a general characteristic of the solid-solution interface. A new surface structural approach is presented that may account for variation in size of electrolyte ions. In this approach, the location of the charge of the outersphere surface complexes is described using the concept of charge distribution in which the ion charge is allowed to be distributed over two electrostatic planes. It was shown that the concept can successfully describe the pH dependent proton binding and the shift in the isoelectric point (IEP) in the presence of variety of monovalent electrolyte ions, including Li(+), Na(+), K(+), Cs(+), Cl(-), NO(-)(3), and ClO(-)(4) with a common set of parameters. The new concept also sheds more light on the degree of hydration of the ions when present as outersphere complexes. Interpretation of the charge distribution values obtained shows that Cl(-) ions are located relatively close to the surface. The large alkali ions K(+), Cs(+), and Rb(+) are at the largest distance. Li(+), Na(+), NO(-)(3), and ClO(-)(4) are present at intermediate positions.     

Dressed ion theory of size-asymmetric electrolytes: effective ionic charges and the decay length of screened Coulomb potential and pair correlations

The effects of ionic size asymmetry on long-range electrostatic interactions in electrolyte solutions are investigated within the primitive model. Using the formalism of dressed ion theory we analyze correlation functions from Monte Carlo simulations and the hypernetted chain approximation for size asymmetric 1:1 electrolytes. We obtain decay lengths of the screened Coulomb potential, effective charges of ions, and effective permittivity of the solution. It is found that the variation of these quantities with the degree of size asymmetry depends in a quite intricate manner on the interplay between the electrostatic coupling and excluded volume effects. In most cases the magnitude of the effective charge of the small ion species is larger than that of the large species; the difference increases with increasing size asymmetry. The effective charges of both species are larger (in absolute value) than the bare ionic charge, except for high asymmetry where the effective charge of the large ions can become smaller than the bare charge.     

一种新型氧化还原电解液电化学电容器体系

以含有Fe3+/Fe2+离子对的H2SO4溶液为电解液, 以多孔炭做电极材料, 就Fe3+/Fe2+离子对在多孔炭纳米孔隙中的电化学行为及准电容效应进行了探讨. 循环伏安测试结果表明, Fe3+/Fe2+离子对在多孔炭电极纳米孔隙中发生了可逆的电化学反应. 恒流充放电结果发现, 加入Fe3+/Fe2+使得充放曲线出现对称的充放电平台, 有效地提高了电化学电容器(EC)的电能存储容量, 其单电极比电容最高达174 mAh•g−1, 比单纯的H2SO4电解液的比电容高109 mAh•g−1, 且有着良好的循环稳定性. 根据实验现象及结果, 探讨了Fe3+/Fe2+离子对在EC电极上的充放电机理, 并提出了一种新的概念——氧化还原电解液电化学电容器.