Quantized double-layer charging of highly monodisperse metal nanoparticles

We describe unprecedented resolution of electrochemically observed quantized double layer (QDL) charging, attained with use of reduced solution temperatures and with an annealing procedure that produces hexanethiolate monolayer protected gold clusters (C6 MPCs) with a high level of monodispersity in charging capacitance, C(CLU). The spacing DeltaV = e/C(CLU) on the electrochemical potential axis between one electron changes in the electronic charge of nanoscopic metal particles is determined by their effective capacitance C(CLU). The high monodispersity of the C6 MPCs with Au(140) cores facilitates (a) detailed rotated disk and cyclic voltammetric measurements, (b) simulation of QDL waveshapes based on assumed reversible, multivalent redox-like behavior, (c) determination of nanoparticle diffusion rates, and (d) observation of as many as 13 changes in the MPC charge state, from MPC(6-) to MPC(7+). The single electron QDL charging peaks are quite evenly spaced (DeltaV constant) at potentials near the MPC potential of zero charge, but are irregularly spaced at more positive and negative potentials. The irregular spacing is difficult to rationalize with classical double layer capacitance ideas and is proposed to arise from a correspondingly structured (e.g., not smooth) density of electronic states of the nanoparticle core, resulting from its small HOMO/LUMO gap and incipiently molecule-like behavior.     

Quantized spectroelectrochemical behaviour of monolayer-protected gold cluster films assessed by reflectance spectroelectrochemical quartz crystal microbalance

The spectral changes occurring in multilayer films of hexanethiolate monolayer-protected Au₁₄₇ clusters (C6–Au₁₄₇ MPCs) as a consequence of quantized MPC core charging have been investigated in aqueous solutions using a multiresponse technique, UV–vis reflectance spectroelectrochemical quartz crystal microbalance (SEQCM). The joint technique, a combination of UV–vis near-normal incidence reflectance spectroelectrochemistry and electrochemical quartz crystal microbalance, has enabled us to follow both reflectance and gravimetric changes taking place in the MPC film concurrently with each single electron transfer event. Reversible film reflectance drops were observed upon anodic MPC charging, which were linearly dependent on the MPC charge state. The values of the formal potential and number of electrons transferred in each charging step, determined from the potential dependence of the reflectance changes, proved that the spectral features were induced by the discrete charging of the MPCs. Simultaneously, the gravimetric signal monitored with EQCM yielded values of the number of MPC-bound electrolyte ions as a function of the MPC redox state, both during voltammetric and potential step charging of the MPC films. Additionally, the dynamics of electron transfers in these multilayer MPC films has been investigated by electrochemical impedance spectroscopy (EIS). Thus, the film capacitance, the resistance to charge transfer, and the electron-transfer rate constant for MPC oxidation have been estimated.     

Electrochemical characteristics of poly(ophenylendiamine) film electrodes in phosphatic solution

The electrochemical characteristics of poly(o-phenylendiamine) (POPD) film modified electrodes have been investigated using different electrochemical techniques.The main interest is focused on the effect of potential and film thickness on the electrode process.Good agreement has been found for the apparent diffusion coefficient estimated by chronocoulometry and impedance spec-troscopy.The charge transfer process within POPD films is diffusion processes at negative and positive overpotentials and electron hopping mechanism at formal potential.The POPD film conductivity of the oxidized state is better than that of the reduced state.For all electrode processes,the H+ may penetrate the film/electrolyte interface and take part in charge transfer or protonation-deprotonation of phenazine rings.     

Scanning electrochemical microscopy determination of organic soluble MPC electron-transfer rates

In this paper, we describe a novel method for measuring the forward heterogeneous electron-transfer rate constant (kf) through the thiol monolayer of gold monolayer protected clusters (MPCs) in solution using scanning electrochemical microscopy (SECM). Applying the equations for mixed mass-transfer and electron-transfer processes, we develop a new formula using only the diffusion coefficient and the tip radius and use it as part of a new method for evaluating SECM approach curves. This method is applied to determine the electron-transfer rates from a series of SECM approach curves for monodisperse hexanethiol MPCs and for polydisperse hexanethiol, octanethiol, decanethiol, dodecanethiol, and 2-phenyethylthiol gold MPCs. Our results show that as the alkanethiol length increases the rate of electron transfer decreases in a manner consistent with the previously proposed tunneling mechanism for the electron transfer in MPCs. However, the effective tunneling coefficient, Beta, is found to be only 0.41 A-1 for alkanethiol passivated MPCs compared to typical values of 1.1 A-1 for alkanethiols as self-assembled monolayers on two-dimensional gold substrates. Similar SECM approach curve results for Pt and Au MPCs indicate that the electron-transfer rate is dependent mostly on the composition of the thiol layer and not on differences in the core metal.     

Arylthiolate-protected silver quantum dots

This paper describes a new, organic-soluble 4-tert-butylbenzyl mercaptan (BBT) monolayer-protected silver cluster (AgBBT MPC) as the first example of a dissolved silver nanoparticle that exhibits quantized one-electron double layer charging (QDL) voltammetry. Polydisperse AgBBT MPCs made by two different synthetic protocols, but with similar average core diameters (2.1 nm), exhibit sharply differing electrochemistry and optical absorbance spectra. A two-phase procedure (organic/aqueous, termed Prep A-AgBBT) produced MPCs exhibiting a 475 nm surface plasmon absorbance and QDL voltammetry. Neither property was seen for MPCs made by a single-phase procedure, termed Prep B-AgBBT. The difference is thought to reflect poor passivation to oxide formation in the latter Prep B procedure, which is supported by X-ray photoelectron spectroscopy results. Thermogravimetry, mass spectra, and electrochemistry results suggest an average stoichiometric formula of Ag140BBT53, but transmission electron microscopy shows that the products are also polydisperse and include polycrystalline aggregates. Dry, cast films of both Ag MPC preparations on interdigitated array electrodes exhibit low electron hopping conductivity, compared to Au MPCs.     

Characteristics of high efficiency dye-sensitized solar cells

Impedance spectroscopy was applied to investigate the characteristics of dye-sensitized nanostructured TiO2 solar cells (DSC) with high efficiencies of light to electricity conversion of 11.1% and 10.2%. The different parameters, that is, chemical capacitance, steady-state transport resistance, transient diffusion coefficient, and charge-transfer (recombination) resistance, have been interpreted in a unified and consistent framework, in which an exponential distribution of the localized states in the TiO2 band gap plays a central role. The temperature variation of the chemical diffusion coefficient dependence on the Fermi-level position has been observed consistently with the standard multiple trapping model of electron transport in disordered semiconductors. A Tafel dependence of the recombination resistance dependence on bias potential has been rationalized in terms of the charge transfer from a distribution of surface states using the Marcus model of electron transfer. The current-potential curve of the solar cells has been independently constructed from the impedance parameters, allowing a separate analysis of the contribution of different resistive processes to the overall conversion efficiency.     

Electrochemical impedance spectroscopic analysis of dye-sensitized solar cells

Electrochemical impedance spectroscopy (EIS) has been performed to investigate electronic and ionic processes in dye-sensitized solar cells (DSC). A theoretical model has been elaborated, to interpret the frequency response of the device. The high-frequency feature is attributed to the charge transfer at the counter electrode while the response in the intermediate-frequency region is associated with the electron transport in the mesoscopic TiO2 film and the back reaction at the TiO2/electrolyte interface. The low-frequency region reflects the diffusion in the electrolyte. Using an appropriate equivalent circuit, the electron transport rate and electron lifetime in the mesoscopic film have been derived, which agree with the values derived from transient photocurrent and photovoltage measurements. The EIS measurements show that DSC performance variations under prolonged thermal aging result mainly from the decrease in the lifetime of the conduction band electron in the TiO2 film.     

普鲁士蓝膜修饰电极的电化学阻抗谱

测量了应用电化学方法制备的不同厚度的普鲁士蓝膜修饰电极的循环伏安行为与电化学阻抗谱.由所得到的循环伏安图讨论了普鲁士蓝修饰膜的氧化还原过程,并对相关的Nyquist图进行了解析,提出了相应的等效电路.在此基础上计算出较薄膜中普鲁士蓝/普鲁士白电化学反应的表观速率常数和表观扩散系数,讨论了膜厚度对电荷扩散的影响.当膜相对较薄时,电极过程主要由动力学过程控制;当膜达到一定厚度时,电荷在膜中的扩散速率受到限制,电极过程由动力学过程和电荷扩散过程共同控制,证实了文献报导的普鲁士蓝膜修饰电极为多层空间分布电荷传递模型.     

Electrochemical Impedance Analysis of Thermogalvanic Cells

Thermogalvanic cells(also known as thermo-electrochemical cells) that convert waste heat energy to electricity are a new type of energy conversion device. However, the electron transfer kinetics and mass transfer of redox couples have not been thoroughly studied. Here, the ion reaction and charge transport in thermogalvanic cells are investigated by electrochemical impedance analysis. We first propose the detailed impedance model followed experimental verification on three types of electrode materials. Parameters including kinetic rate constants and ion diffusion coefficients for the electrodes are obtained by fitting the impedance data. Our study shows explicitly that impedance analysis can provide useful information on selecting suitable electrode materials for thermogalvanic cells.     

Electrochemical properties of LiFePO4-multiwalled carbon nanotubes composite cathode materials for lithium polymer battery

LiFePO₄-multiwalled carbon nanotubes (MWCNTs) composites were prepared by a hydrothermal method followed by ball-milling and heat treating. Cyclic voltammetry, ac impedance and galvanostatic charge/discharge testing results indicate that LiFePO₄-MWCNTs composite exhibits higher discharge capacity and rate capability than pure LiFePO₄ at high-rate at room temperature. It is demonstrated that the added MWCNTs not only increase the electronic conductivity and lithium-ion diffusion coefficient but also decrease crystallite size and charge transfer resistance of LiFePO₄-MWCNTs composite.     

On the Criteria of Instability for Electrochemical Systems

Both cyclic-voltammetry-based and impedance-based experimental criteria that have been developed recently for the oscillatory electrochemical systems are critically appraised with two typical categories of oscillators.Consistent conclusions can be drawn by the two criteria for the category of oscillators that involve the coupling of charge transfer mainly with surface steps(e.g.ad-and desorption)such as in the electrooxidation of C1 organic molecules.Whereas,impedance-based criterion is not applicable to the category of oscillators that involve the coupling of charge transfer mainly with mass transfer(e.g.diffusion and convection) such as in the Fe(CN)6^3- reduction accompanying periodic hydrogen evolution.The reason is that the negative impedance cannot include the feedback information of convection mass transfer induced by the hydrogen evolution.However,both positive and negative nonlinear feedbacks,i.e., the diffusion-limited depletion and convection-enhanced replenishment of the Fe(CN)6^3- surface concentration,that coexist between the bistability,i.e.,Fe(CN)6^3- reduction with and without hydrogen evolution at lower and higher potential sides respectively,are all reflected in the crossed cyclic voltammogram(CCV).It can be concluded that the voltammetry-based criterion(in time domain)is more intuitive,less time-consuming and has a wider range of applications than the impedancebased one (in frequency domain).     

染料敏化太阳电池阻抗特性研究

电化学阻抗谱(EIS)是染料敏化太阳电池(DSC)领域中最重要的研究手段之一。本文详细介绍了EIS在DSC研究中的理论模型、实验方法、内部电荷传输和转移过程、阻抗信息提取和动力学过程解析的最新研究进展;综述了其在光阳极、电解液体系、对电极、稳定性、新结构设计等DSC各个研究领域中的应用,特别总结了DSC内部各个组成部分的阻抗特性。最后,对这些方面存在的问题进行了评论,并对未来新材料和电池机理的深入研究进行了展望。     

掺杂电活性离子聚吡咯膜内的电荷传输

用循环伏安法和计时库仑法研究了掺杂亚铁氰化钾离子的聚哟咯膜内的电荷传输问题。实验结果表明，聚吡咯膜内的电荷传递可以处理成电子在膜内的扩散模型，电荷传递速度可以 用电子表观扩散系数（Ｄａｐｐ）来表征。Ｄａｐｐ的大小由聚合物膜的结构、电活怀离子之间的过及对离了在膜中的运动决定。     

Electrochemical behaviors of novel electroactive Au nanoparticles protected by self-assembled monolayers

There has been substantial recent interest in studying monolayer-protected gold clusters (MPCs) owing to their diverse applications. The present work is an electrochemical study of novel gold nanoparticles covered with a monolayer of mercapto-dodecanol ended chloro-dicyano-quinone (HS-C12O-CDQ), which was adsorbed on the electrode (CDQ-MPCs film). Our findings reveal a redox behavior for CDQ-MPCs film similar to the solution electrochemistry of dichloro-dicyano-quinone. Furthermore, a diffusion-like mechanism was found for electron transfer, which may have occurred due to proton diffusion towards or outwards the electrode through the film casted. Chronoamperometry confirmed diffusion behavior of the ET process. Finally, EIS was used to find the rate constant of ET process for the redox reaction that occurred and the contribution of MPCs in total interfacial capacitance.     

Application of 2-(3,4-dihydroxyphenyl)-1,3-dithialone self-assembled monolayer on gold electrode as a nanosensor for electrocatalytic determination of dopamine and uric acid

The electrooxidation of dopamine (DA), uric acid (UA) and their mixture on a gold electrode modified by a self-assembled monolayer of 2-(3,4-dihydroxyphenyl)-1,3-dithialone has been studied by cyclic voltammetry (CV), chronoamperometry and differential pulse voltammetry (DPV). CV was used to investigate the redox properties of the modified electrode at various scan rates and the apparent charge transfer rate constant (k(s)), and transfer coefficient (α) were calculated. The mediated oxidation of DA at the modified electrode under the optimum condition (pH = 7.0) in CV occurs at a potential about 220 mV less positive than that of the unmodified gold electrode. The values of electron transfer coefficients (α), catalytic rate constant (k) and diffusion coefficient (D) were calculated for DA, using electrochemical methods. DPV exhibited a linear dynamic range over the concentration range of 0.2-250.0 μM and a detection limit (3σ) of 0.07 μM for DA. The modified electrode was used for simultaneous determination of DA and UA by DPV. The results showed that the electrode is highly efficient for the catalytic electrooxidation of DA and UA, leading to a remarkable peak resolution (~350 mV) for two compounds. The electrode was used for the determination of DA in an injection sample.     

放电温度对LiNi3/8Co2/8Mn3/8O2电化学性能的影响

采用X射线衍射（XRD）、X射线光电子能谱（XPS）、恒流充放电、循环伏安及交流阻抗法，研究了放电温度对LiNi3/8Co2/8Mn3/8O2的倍率特性、锂离子扩散及电荷传递的影响.结果表明， 提高放电温度可显著改善LiNi3/8Co2/8Mn3/8O2的放电容量与倍率放电性能.尽管温度升高使电荷传递活性与锂离子扩散速度都增加，但电荷传递活化能比锂离子扩散活化能大一倍多，表明电荷传递步骤是其电化学反应控制步骤.温度对其电荷传递的影响大于对锂离子扩散的影响.温度升高，电荷传递速率加快，电化学嵌入-迁出反应加速，是其放电容量与倍率放电特性显著改善的主要原因.     

Large amplitude Fourier transformed ac voltammetry at a rotating disc electrode: a versatile technique for covering Levich and flow rate insensitive regimes in a single experiment

The theory for large amplitude Fourier transformed ac voltammetry at a rotating disc electrode is described. Resolution of time domain data into dc and ac harmonic components reveals that the mass transport for the dc component is controlled by convective-diffusion, while the background free higher order harmonic components are flow rate insensitive and mainly governed by linear diffusion. Thus, remarkable versatility is available; Levich behaviour of the dc component limiting current provides diffusion coefficient values and access to higher harmonics allows fast electrode kinetics to be probed. Two series of experiments (dc and ac voltammetry) have been required to extract these parameters; here large amplitude ac voltammetry with RDE methodology is used to demonstrate that kinetics and diffusion coefficient information can be extracted from a single experiment. To demonstrate the power of this approach, theoretical and experimental comparisons of data obtained for the reversible [Ru(NH(3))(6)](3+/2+) and quasi-reversible [Fe(CN)(6)](3-/4-) electron transfer processes are presented over a wide range of electrode rotation rates and with different concentrations and electrode materials. Excellent agreement of experimental and simulated data is achieved, which allows parameters such as electron transfer rate, diffusion coefficient, uncompensated resistance and others to be determined using a strategically applied approach that takes into account the different levels of sensitivity of each parameter to the dc or the ac harmonic.     

聚阿魏酸修饰电极的电化学特性及电催化性能

研究了阿魏酸在玻碳电极表面电聚合成膜的方法和条件,测量了应用电化学方法制备不同厚度的阿魏酸修饰电极的循环伏安行为及其它电化学性质.对厚度为0.5 μm的阿魏酸膜,测得的电子转移系数为0.49,表观电极反应速率常数(ks)为6.56 s－1.扩散系数DR为7.9×108 cm2•s－1,Do为4.48×108 cm2•s－1.该修饰电极对烟酰胺腺嘌呤二核苷酸（NADH）氧化具有很好的催化作用.NADH浓度在0.01～5.0 mmol•dm－3范围内与峰电流呈现良好的线性关系.     

The Kinetics of Indium Electroreduction from Chloride Solutions

The electroreduction of indium on indium electrode (99.98%) in perchlorate-containing chloride electrolytes is studied by the methods of linear sweep and cyclic voltammetry, impedance spectroscopy, and chronoamperometry. The indium electroreduction is limited by diffusion, the reaction rate constant is 1.3 10^–4 cm/s at the indium salt concentration of 0.1 M. The values of the apparent rate constant for the charge transfer stage found by linear sweep and cyclic voltammetry and also by impedance spectroscopy are 2.37 × 10^–3, 3.62 × 10^–3, 3.06 × 10^–3 cm/s, respectively. The values of diffusion coefficient of indium(III) ions calculated according to the Cottrell equation based on chronoamperametric measurements and from the Warburg impedance found by impedance spectroscopy are in good agreement. The presence of the Gerischer impedance is stated, which suggests that a homogeneous reaction of formation of indium chloride complexes proceeds and its mechanism is chemical-electrochemical.

     

Solvent effects on charge transport through solid deposits of [Os(4,4'-diphenyl-2,2'-dipyridyl)2Cl2

Mechanically attached, solid-state films of [Os(4,4'-diphenyl-2,2'-dipyridyl)2Cl2] have been formed on gold macro- and microelectrodes and their voltammetric properties investigated. The voltammetric response of these films associated with the Os(2+/3+) redox reaction is reminiscent of that observed for an ideal reversible, solution phase redox couple only when the contacting electrolyte contains of the order of 40% v/v of acetonitrile (ACN). The origin of this effect appears to involve preferential solvation of the redox centres by acetonitrile which facilitates the incorporation of charge compensating counterions. Scanning electron microscopy reveals that voltammetric cycling in 40:60 ACN-H2O containing 1.0 M LiClO4 as the electrolyte induces the formation of microcrystals. Voltammetry conducted under semi-infinite linear diffusion conditions has been used to determine the apparent diffusion coefficient, Dapp, for homogeneous charge transport through the deposit. The dynamics of charge transport decrease with increasing film thickness but appear to increase with increasing electrolyte concentration. These observations suggest that ion transport rather than the rate of electron self-exchange limit the overall rate of charge transport through these solids. When in contact with 40:60 ACN-H2O containing 1.0 M LiClO4 as electrolyte, Dapp values for oxidation and reduction are identical at 1.7 +/- 0.4 x 10(-12) cm2 s(-1). In the same electrolyte, the standard heterogeneous electron transfer rate constant, k(o), determined by fitting the full voltammogram using the Butler-Volmer formalism, is 8.3 +/- 0.5 x 10(-7) cm s(-1). The importance of these results for the rational design of solid state redox active materials for battery, display and sensor applications is considered.