Development of a finite-temperature density functional approach to electrochemical reactions

We present a computational method to calculate the electronic states of a molecule in an electrochemical environment. The method is based on our recently developed finite-temperature density functional theory approach to calculate the electronic structures at a constant chemical potential. A solvent effect is treated at the level of the extended self-consistent reaction field model, which allows considering a nonequilibrium solvation effect. An exchange-correlation functional with a long-range correction is employed in this calculation, because the functional is adjusted so that the derivative discontinuity of energy with respect to a number of electrons could be satisfied. It has been found that the derivative discontinuity condition plays a crucial role in an electrochemical system. The computational results are presented for a reaction of NO(+) + e(-) <==> NO in chemical equilibrium. Owing to the improvement in the solvation effect and the exchange-correlation functional, the calculated activation free energy is in good agreement with experimental results.     

Loading polyelectrolytes onto porous microspheres: impact of molecular and electrochemical parameters

The impact of macromolecule constitution and electrostatic dimensions on the adsorption of cationic model polyelectrolytes (PELs) onto oppositely charged porous microspheres (MSs) suspended in aqueous media is demonstrated. The contour length (L) of the PEL, the chemical structure of the substituents at the ionic group, the ionic strength of the solution (I), and the average pore radius of the microspheres (R) are considered as variable. Adsorption isotherms quantitatively reveal how PEL parameters, MS geometry, and medium characteristics affect the adsorbed amount and surface coverage. Electrostatic exclusion from pores was proved as long as the Debye length (lD) exceeded R, even if L was considerably smaller than the pore diameter. Two charge parameters (CRcalc and CRexp) and the ratio thereof (CR) were derived and served to evaluate the loading process. All three parameters are applicable to two limiting cases, first, adsorption only on the outer surface of the MS and, second, additional adsorption inside the pores. The findings are seen as valuable contributions to basic research in the field of PELs. Precise data, which were not available before, are provided for comparison with theoretical models and simulations. Overall, conclusions from this model system may be useful for technical applications such as surface modification, chromatographic processes, or materials development.     

Impact of porous electrode properties on the electrochemical transfer coefficient

The rate of an activation-controlled electrochemical reaction is determined by two key parameters, the exchange current density, io, and the transfer coefficient, alpha, which is inversely related to the Tafel slope. Assuming that the symmetry factor, beta, is 0.5, the minimum alpha value should be 0.5 for all standard reaction mechanisms, with alpha values larger than this indicating a better electrocatalytic mechanism. The primary goal of this paper is to better understand why alpha values of < 0.5 are often observed experimentally, with specific examples given for the oxygen reduction reaction. These low alpha values cannot be explained by adsorption behavior, but they can result when reactions occur within a porous electrode structure. Consistent with past literature related to Tafel slope predictions, we show that long and narrow pores, a low ionic or electronic conductivity of the electrode layer, and a high io value can cause alpha to be < 0.5, most typically 0.25. However, alpha values between 0.25 and 0.5 are also encountered in practice. We show here that such alpha values can be obtained for reactions occurring at porous films that have nonuniform properties. We also show that the overpotential range over which alpha changes from 0.5 to 0.25 can be quite broad, especially at high temperatures, and thus can be misinterpreted as a true Tafel region with a transfer coefficient between 0.25 and 0.5.     

Effect of current density on morphological,structural and optical properties of porous silicon

The morphology of porous silicon (PS) layers produced by electrochemical etching of n-type (100) silicon (Si) at different low current densities was studied using SEM, image J analysis and WSxM software. From FTIR spectroscopy analysis, the Si dangling bonds of the as-prepared PS layer have large amount of Hydrogen to form weak Si–H bonds. From Raman analysis, a full width half maximum (FWHM) of the Raman peak was gradually increased with increased current density, shifted towards lower energies due to reduce of crystallite size, the crystallite size in the PS varied from 63 nm to 20 nm depending on the current density. The optical response of the PS layer has been performed by the absorbance and Photoluminescence was studied experimentally in the visible range. The optical absorption and photo luminescence in PS is due to excitonic recombination between the defect states as well as on the surface of nanocrystals, and this was attributed to the presence of silicon hydride species which are confirmed by FTIR spectra. The red shift was observed in absorbance and Photoluminescence due to decrease in the size of Si crystallites and growth of Si=O bonds. The contact angle varied from 76° to 120.1°. From the wettability studies, the surface nature of the PS was converted from hydrophilic to hydrophobic when the current density increased.     

Dynamics of codeposition of metals inside porous electrode operating in direct-flow mode

Using an extended dynamic model of liquid flow-through porous electrode (PE), the effect of kinetics of deposition of individual components and conditions of potentiostatic electrolysis on the dynamics and final parameters (the cathodic deposit weight, the ratio between the amounts of components, and the spatial distribution of components) of codeposition of two metals M₁ and M₂ is studied. An equipotential PE operating in the direct-flow mode in the absence of anodic dissolution of electronegative component M₂ is considered. The effects of concentration, exchange currents, a difference between the equilibrium potentials M₁ and M₂, a prescribed voltage on PE, and solution flow velocity and direction are analyzed. It is shown that, for this version of codeposition of metals, the rates of M₁ and M₂ deposition averaged over the PE width are constant in time. However, this does not mean that their local deposition rates are constant. The general tendency is that the metal deposition rate on the rear part of PE decreases with the time, whereas the deposition rate on the frontal zone of PE, which is closer to the anode, increases. As a result, the final profiles for M₁ and M₂, which are calculated for equal deposition times taking into account and ignoring the redistribution of current during the deposition, differ essentially.     

Structural and electrochemical properties of a porous nanostructured SnO2 film electrode for lithium-ion batteries

A B₂O₃-doped SnO₂ thin film was prepared by a novel experimental procedure combining the electrodeposition and the hydrothermal treatment, and its structure and electrochemical properties were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) analysis, energy dispersive X-ray (EDX) spectroscopy and galvanostatic charge–discharge tests. It was found that the as-prepared modified SnO₂ film shows a porous network structure with large specific surface area and high crystallinity. The results of electrochemical tests showed that the modified SnO₂ electrode presents the largest reversible capacity of 676 mAh g^?1 at the fourth cycle, close to the theoretical capacity of SnO₂ (790 mAh g^?1); and it still delivers a reversible Li storage capacity of 524 mAh g^?1 after 50 cycles. The reasons that the modified SnO₂ film electrode shows excellent electrochemical properties were also discussed.     

In situ electrochemical spectroscopy for boron-doped diamond electrode reactions: recent progress and perspectives

Over the past years, great attention has been given to the developments of boron-doped diamond (BDD) materials in various fields because of the advantages of electrochemical features, such as large potential range and low background current. This minireview aims to present the recent progress of in situ electrochemical spectroscopy for BDD electrode reactions. After a concise state of the widely used in situ electrochemical spectroscopy techniques, including in situ electrochemical Raman, infrared, and electron paramagnetic resonance spectroscopy, the current progress of BDD electrode reactions using in situ electrochemical spectroscopy has been summarized. Finally, challenges and perspectives for the tendency of the BDD study via in situ electrochemistry are provided, of which several potential electrochemical combined technologies relating to the mechanism exploration of BDD are proposed.     

95Mo nuclear magnetic resonance parameters of molybdenum hexacarbonyl from density functional theory: appraisal of computational and geometrical parameters

Solid-state (95)Mo nuclear magnetic resonance (NMR) properties of molybdenum hexacarbonyl have been computed using density functional theory (DFT) based methods. Both quadrupolar coupling and chemical shift parameters were evaluated and compared with parameters of high precision determined using single-crystal (95)Mo NMR experiments. Within a molecular approach, the effects of major computational parameters, i.e. basis set, exchange-correlation functional, treatment of relativity, have been evaluated. Except for the isotropic parameter of both chemical shift and chemical shielding, computed NMR parameters are more sensitive to geometrical variations than computational details. Relativistic effects do not play a crucial part in the calculations of such parameters for the 4d transition metal, in particular isotropic chemical shift. Periodic DFT calculations were tackled to measure the influence of neighbouring molecules on the crystal structure. These effects have to be taken into account to compute accurate solid-state (95)Mo NMR parameters even for such an inorganic molecular compound.     

Adsorption and electrochemical activity: an in situ electrochemical scanning tunneling microscopy study of electrode reactions and potential-induced adsorption of porphyrins

The effect of adsorption on molecular properties and reactivity is a central topic in interfacial physical chemistry. At electrochemical interfaces, adsorbed molecules may lose their electrochemical activity. The absence of in situ probes has hindered our understanding of this phenomenon and electrode reactions in general. In this work, classical electrochemistry and electrochemical scanning tunneling microscopy (EC-STM) were combined to provide molecular level insight into electrochemical reactions and the molecular adsorption state at the electrolyte-electrode interface. The metal-free porphyrin 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine (TPyP) adsorbed on Au(111) in 0.1 M H(2)SO(4) solution was chosen as a model system. TPyP is found to irreversibly adsorb on Au(111) over a wide range of potentials, from -0.25 to 0.6 V(SCE). The adsorption state of TPyP has a dramatic effect on its electrochemistry. Preadsorbed, oxidized TPyP displays no well-defined cathodic peaks in cyclic voltammograms in sharp contrast to solution-phase TPyP. Our present work provides direct, molecular level evidence of the electrochemically "invisible" species. Electrochemical activity of absorbed species is recovered by allowing the oxidized molecule sufficient time (tens of minutes) to reduce. The redox state of adsorbed TPyP also affects the nature of the adsorption. Oxidized species can apparently only form monolayers. However, multilayers, stable enough to be imaged by STM, can form when the adsorbed TPyP is in the reduced state. This suggests that by controlling the electrochemistry one can either promote or suppress the formation of multilayers.     

Peculiarities of metal electrodeposition inside a flow-through porous electrode with low initial conductivity in oxidant containing solution

The earlier developed dynamic model of a flow-through electrode is used for studying how the variations in initial conductivity of a porous matrix κ_s,ini and a metal deposit affect the rate of metal deposition from an oxidant-containing solution for the direct-flow operation mode of the porous electrode. It is found that in contrast to an oxidant-free solution in which the decrease of κ_s,ini improves the uniformity of deposit distribution inside the porous cathode and increases the deposit final mass m _f, the opposite situation is observed in the presence of an oxidant, namely, a decrease in κ_s,ini, under otherwise similar conditions reduces the deposit mass and leads to its specific spatial distribution. The final metal deposit is divided into two separate fragments (rear and front) with a region of low conductivity of the initial porous matrix in between. Dynamics of the current and metal redistribution within the porous electrode, the reasons for the formation and stabilization of the rear fragment of coating, the correlation between the metal deposition rate and changes in the anodic zone position and intensity are discussed. It is shown that with the appearance of a specific profile of deposit distribution, the dependence of m _f on the metal conductivity develops a limit that differs considerably from the deposit final mass for an equipotential porous electrode.     

Thermodynamic and kinetic characteristics of intermediates of electrode reactions: Determination by direct and combined electrochemical methods

The potentialities of electrochemical and combined methods for the determination of thermodynamic characteristics and kinetic parameters of intermediates are analyzed. The electrochemical methods include polarography, versions of voltammetry, and chronopotentiometry. The combined methods combine nonelectrochemical (or indirect) generation of intermediates with electrochemical methods of their subsequent investigation: photomodulation voltammetry, method of polarography of pulsed-radiolysis products, method of indirect electrolysis, electrochemiluminescence, and a group of laser photoemission methods. Theoretical foundations and basic advantages and disadvantages of the laser photoemission methods (the measurement of photocurrents in potentiostatic conditions—method of time resolved voltammograms obtained under chopped illumination and the measurement of the kinetics of variations in the potential of an electrode after illuminating it by a pulsed laser—in coulostatic conditions) are described in detail. The potentialities of the laser photoemission methods for the determination of thermodynamic (standard potentials E ⁰ of redox pairs R/R^-, standard adsorption free energies) and kinetic (values of rate constants W ⁰ at an equilibrium potential, bulk lifetime of radicals Rand products of their reduction R^-) characteristics of intermediates are demonstrated by studying a trifluoromethyl radical.Translated from Elektrokhimiya, Vol. 41, No. 2, 2005, pp. 142–156.Original Russian Text Copyright © 2005 by Krivenko, Kotkin, Kurmaz.This revised version was published online in April 2005 with corrections to the article note and article title and cover date.     

The influence of electrochemical treatment on electrode reactions for vanadium redox-flow batteries

Through targeted and reproducible electrochemical treatment of glassy carbon electrodes,investigations have been carried out on the electrochemical behaviour of the oxidation of V~(2+),VO~(2+)and the reductions of VO_2~+,VO~(2+)and V~(3+)in order to pretreat electrodes specifically for use in vanadium redox flow batteries and,if possible,to treat them in situ.For this purpose,a glassy carbon electrode was treated potentiostatically for a period of 30 s at different potentials in the range of 500 mV–2000 mV vs.Hg/Hg_2SO_4in2 M H_2SO_4and then linear sweep voltammograms were performed in the different vanadium-containing solutions.With this method,it could be shown that all reactions are extremely surface sensitive and the reaction speeds changed by several decades.The reaction rates increased significantly in all reactions compared to polished electrodes and had an optimum treatment potential of approx.1600 mV vs.Hg/Hg_2SO_4,although the oxidation reaction of V~(2+)and the reduction reactions of V~(3+)and VO~(2+)had opposite tendencies to oxidation of VO~(2+)and the reduction of VO_2~+in the area of low treatment potentials.In the former,the kinetics increased and in the latter,they decreased.In addition,causes were investigated using confocal microscopy and XPS.No correlation was found to the roughness or size of the stretched surfaces,although these changed significantly as a result of the treatment.XPS measurements gave indications of a dependence on hydroxyl groups for the oxidation of VO~(2+)and the reduction of VO_2~+,while for the reactions of oxygen-free cations and the reduction of VO~(2+)weak indications of a dependence on carboxyl groups were obtained.     

Dependence of metal contact exchange dynamics on electrochemical parameters of electrode processes

Effect of kinetic parameters of electrode processes on the growth dynamics and structural characteristics of the friable metal deposit, which forms via its contact deposition from an aqueous solution, is studied. A model concept is based on the electrochemical processes proceeding at the metal/solution interface. Dependences of the deposition dynamics and deposit characteristics on the exchange currents of depositing and dissolving metals and on the hydrogen exchange currents at the surfaces of both metals are studied.     

A polarographic cell system with a novel temperature-controlled reference electrode for the determination of kinetic parameters of electrode reactions

An improved experimental arrangement for the determination of kinetic data relating to polarographic reductions is described. The significant feature is the constant-temperature reference electrode; its construction and use in this context are described. A comparison is made of the more realistic results obtained by these means, with those obtained with a more conventional cell where the temperature of the reference electrode is varied with that of the working solution.     

Effects of temperature and current density on the porous structure of InP

Journal of Solid State Electrochemistry - The electrochemical oscillations that occur during the etching of InP and the three-dimensional (3D) porous structure of the resulting material were...     

Decreasing current ramp technique for the study of fast electrode reactions

A new technique for measuring rate constants of moderately fast electrode reactions is proposed. The input is a decreasing current ramp of the form i(t) = Δi(1 ? t/τ). The overpotential transient-response expected for the activation/diffusion model is confirmed through experiments on the system K₄Fe(CN)₆+K₃Fe(CN)₆/Pt. The rate constant and the transfer coefficient are obtaine presence of several interesting features (a maximum, a zero crossing etc.) in the η?t profile makes this technique almost unique.     

气体电极反应动力学的薄膜旋转圆盘电极方法研究

薄膜旋转圆盘电极方法是一种常用的评价气体物质在纳米电催化剂上的反应活性的方法,但是在数据分析过程中经常忽视了气体反应物在催化剂层中到活性位点的传质可能对估算的反应动力学参数的影响.本文以氧电极反应为例,使用薄膜旋转圆盘电极研究了不同担载量Pt/C电极的氧还原活性.实验结果表明,根据Koutecky-Levich方程求算相同电位下的"表观动力学电流密度"(对Pt活性面积归一化的mA/cm2Pt)或比质量电流(mA/μg Pt)随Pt担载量的减小而增大,说明在估算动力学电流时不能忽略O2在催化剂层中的扩散传质,而气体在催化剂层中的传质与催化剂层的结构、厚度、纳米催化剂的分散度等密切相关.建议在使用薄膜旋转圆盘电极方法来研究纳米催化剂气体电极反应活性时,首先系统考察担载量、分散度与催化剂层厚的影响,然后根据不同担载量催化剂归一化后的动力学电流密度(或比质量电流)-电势曲线是否重合来验证得到的是否是真实的动力学电流,从而得到更为准确的评价结果.     

Effect of trialkylammonium salts and current density on the electrosynthesis of hydrogen peroxide from oxygen in a gas-diffusion electrode in acid solutions

The influence exerted by the nature of cation of a supporting electrolyte and by the current density on the electroreduction of oxygen to hydrogen peroxide in acid K₂SO₄ solutions (pH 0.9–1.4) in gas-diffusion hydrophobized carbon black electrodes with varied electrolyte porosity was studied.