Slow Electrode Kinetics Research Articles

Triiodide-in-Iodine Networks Stabilized by Quaternary Ammonium Cations as Accelerants for Electrode Kinetics of Iodide Oxidation in Aqueous Media.

The Zn-polyiodide redox flow battery is considered to be a promising aqueous energy storage system. However, in its charging process, the electrode kinetics of I- oxidation often suffer from an intrinsically generated iodine film (I2-F) on the cathode of the battery. Therefore, it is critical to both understand and enhance the observed slow electrode kinetics of I- oxidation by an electrochemically generated I2-F. In this article, we introduced an electrogenerated N-methyl-N-ethyl pyrrolidinium iodide (MEPI)-iodine (I2) solution, designated as MEPIS, and demonstrated that the electrode kinetics of I- oxidation were dramatically enhanced compared to an I2-F under conventional electrolyte conditions, such as NaI. We showed that this result mainly contributed to the fast electro-oxidation of triiodide (I3-), which exists in the shape of a I3--in-I2 network, [I3-·(I2)n]. Raman spectroscopic and electrochemical analyses showed that the composition of electrogenerated MEPIS changed from I3- to [I3-·(I2)n] via I5- as the anodic overpotential increased. We also confirmed that I- was electrochemically oxidized on a MEPIS-modified Pt electrode with fast electrode kinetics, which is clearly contrary to the nature of an I2-F derived from a NaI solution as a kinetic barrier of I- oxidation. Through stochastic MEPIS-particle impact electrochemistry and electrochemical impedance spectroscopy, we revealed that the enhanced electrode kinetics of I- oxidation in MEPIS can be attributed to the facilitated charge transfer of I3- oxidation in [I3-·(I2)n]. In addition, we found that the degree of freedom of I3- in a quaternary ammonium-based I2-F can also be critical to determine the kinetics of the electro-oxidation of I-, which is that MEPIS showed more enhanced charge-transfer kinetics of I- oxidation compared to tetrabutylammonium I3- due to the higher degree of freedom of I3-.

Redox behavior and surface morphology of polystyrene thermoplastic electrodes

Carbon composite electrodes consisting of graphite and a non-conductive binder are widely used due to their low cost and ease of fabrication but often suffer from slow electrode kinetics and high capacitance relative to traditional electrode materials such as gold and glassy carbon. Modifiers including metallic nanoparticles and carbon nanotubes are used to enhance electrode performance, but this complicates electrode preparation, increases cost, and limits reusability. Our group recently reported a new type of carbon composite electrodes called thermoplastic electrodes (TPEs) that use common thermoplastics mixed with graphite in a solvent processing system that give enhanced electrochemical performance. The polymers reported to date include polymethylmethacrylate, cyclic olefine copolymer, polycaprolactone (PCL), and blends of these materials. Polystyrene (PS) is an inexpensive and commonly used thermoplastic substrate, however, there are only several reports incorporating PS as a binder in carbon composite electrodes. Here, we report the first PS TPEs and show their unique electrochemical behavior relative to electrodes made with other binders. These unmodified composite materials surpass the performance of traditional electrode materials and show similar behavior to nanomaterial-modified electrodes. The capacitance and conductivity were measured for PS and PS-polycaprolactone (PCL) blends using PCL TPEs as a reference. Even small additions of PS to PCL electrodes substantially lowered capacitance and increased conductivity. The electrochemistry of PS and PS-PCL TPEs was investigated using cyclic voltammetry with various common and biologically relevant redox probes. The effect of differing molecular weight and the use of waste expanded polystyrene (EPS) were explored as well, and all PS binder TPEs were found to exhibit similar redox behavior. Optical profilometry (OP) and scanning electron microscopy (SEM) were used to examine morphological characteristics. OP showed lower surface roughness for PS electrodes than for PCL ones, explaining the lower capacitance. SEM data suggests the source of this enhanced redox behavior stems from the edge-plane rich PS TPE surfaces.

Progress of Advanced Nanomaterials in the Non-Enzymatic Electrochemical Sensing of Glucose and H2O2.

Non-enzymatic sensing has been in the research limelight, and most sensors based on nanomaterials are designed to detect single analytes. The simultaneous detection of analytes that together exist in biological organisms necessitates the development of effective and efficient non-enzymatic electrodes in sensing. In this regard, the development of sensing elements for detecting glucose and hydrogen peroxide (H2O2) is significant. Non-enzymatic sensing is more economical and has a longer lifetime than enzymatic electrochemical sensing, but it has several drawbacks, such as high working potential, slow electrode kinetics, poisoning from intermediate species and weak sensing parameters. We comprehensively review the recent developments in non-enzymatic glucose and H2O2 (NEGH) sensing by focusing mainly on the sensing performance, electro catalytic mechanism, morphology and design of electrode materials. Various types of nanomaterials with metal/metal oxides and hybrid metallic nanocomposites are discussed. A comparison of glucose and H2O2 sensing parameters using the same electrode materials is outlined to predict the efficient sensing performance of advanced nanomaterials. Recent innovative approaches to improve the NEGH sensitivity, selectivity and stability in real-time applications are critically discussed, which have not been sufficiently addressed in the previous reviews. Finally, the challenges, future trends, and prospects associated with advanced nanomaterials for NEGH sensing are considered. We believe this article will help to understand the selection of advanced materials for dual/multi non-enzymatic sensing issues and will also be beneficial for researchers to make breakthrough progress in the area of non-enzymatic sensing of dual/multi biomolecules.

Identification of Mechanistic Subtleties that Apply to Voltammetric Studies at Boron-Doped Diamond Electrodes

The α-[S2W18O62]4–/5– and α-[S2W18O62]5–/6– polyoxometalate (POM) reduction processes at boron-doped diamond (BDD) disk electrodes in acetonitrile (0.50 M [n-Bu4N][PF6]) have been investigated using large-amplitude Fourier transformed alternating current (FTAC) voltammetry. The origins of subtle differences in experimental data and simulated data modeled assuming the Butler–Volmer relationship with ion-paring implicitly included, planar diffusion and a uniform surface are considered. Parameters estimated are the apparent heterogeneous electron-transfer kinetics (kapp0′), apparent formal reversible potential (Eapp0′), and apparent charge transfer coefficient (αapp′). The electrode kinetic parameters, in contrast to theoretical predictions of the model employed, are dependent on frequency, POM concentration, and the data analysis method. Reasons for nonconformance to the model include the adsorption of POM on graphite-like sp2-bonded carbon impurities, limitations in the availability of charge carriers in BDD, and the method of incorporating ion pairing when slow electrode kinetics apply. Masking of the sp2 carbon-rich (edge) region of the BDD disk provided FTAC voltammetric data that complied much more closely with the simulated data. Nevertheless, data analysis still produces a concentration dependence in the estimated kapp0′ values, which is considered in terms of the assumption of an infinite number of charge carriers. Conclusions derived from this study are likely to be generally applicable to electrode kinetic investigations at BDD electrodes.

One step infiltration induced multi-cation oxide nanocatalyst for load proof SOFC application

Decreasing the operating temperature leads to an issue of high polarization that results in slow electrode kinetics. Infiltration of multi-cation oxide nanoparticle catalyst layer over the cathode backbone can provide low activation energy in catalyzing several electrochemical processes. The adoption of cetrimonium bromide(CTAB)-amino acid (glycine) route can become an efficient route to infiltrate particle with proper stoichiometry due to three-dimensional network forming nature owing to the zwitterionic form of the amino acid. Herein, we report a novel route for the development of discrete Sm0.5Sr0.5CoO3-δ(SSC) nanoparticles on the cathode backbone. In particular, the full cell shows enhancement in electrochemical property with a power density of 1.57 W cm−2 at 700 °C and 100 h durability test under 1 A cm−2. These observations indicate that the selection of the CTAB-amino acid route for the infiltration process of SSC nanoparticle on the surface La0.6Sr0.4Co0.2Fe0.8O3-δ(LSCF6428) backbone is a significant step towards electrochemically viable SOFC performance improvement.

Centri‐Voltammetric Folic Acid Detection

AbstractThe need for practical detection of folic acid (FA) has been increased day by day. For this reason in this work, a two steps electroanalytical technique, centri‐voltammetry was utilized for FA detection for the first time. In order to get rid of the slow electrode kinetics of FA oxidation, the working electrode was modified with graphene‐Pt hibrid nanomaterial. Also for increasing the sensitivity, 1‐Chloro‐2‐[2‐(2‐methoxyethoxy) ethoxy]ethane (TEG−Cl) and 1,2‐di{2‐[2‐(2‐methoxyethoxy)ethoxy] ethoxy}‐4‐nitrobenzene (4NC−NO2) was used as a carrier material. After the characterization of graphene‐Pt hybrid nanomaterial, experimental parameters like, 4NC−NO2 amount, adsorption time, centri‐voltammetric parameters like centrifuge time and speed were optimized. After that, analytical characteristics such as linear range, relative standard deviation (R.S.D), limit of detection (LOD) and limit of quantification (LOQ) values were found. In this manner, linear range was obtained for FA between 1.0 μM–1000 μM with the equations of (R2=0.9977). R.S.D value was calculated for 0.83 mM FA (n=3) as 1.86 % while LOD and LOQ values were found as 1.00 μM and 3.34 μM respectively. After the examination of interference effect of substances like ascorbic acid and uric acid, established centri‐voltammetric technique was enforced for FA detection in pharmateutical tablets. As a result, the recovery value was calculated as 96.4 %.

Metal Phosphide-Embedded Phosphorus-Based Nanocomposite Anode Materials for Rechargeable Batteries

The development of high-performance rechargeable batteries including advanced lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) has been accelerated due to the rapid growth of electric vehicles and stationary large-scale energy storage systems. As one of promising alloying-based anode candidates, elemental phosphorus has attracted much attention among researchers because of its significantly high theoretical specific capacity of 2595 mA h g-1 in both LIBs and SIBs, low-cost, and environmental benignity. However, there are shortcomings such as the large volume change during alloying/dealloying reactions and low conductivity (10-14 S cm-1) of phosphorus, which results in poor cycle performance and slow electrode kinetics. Most approaches aiming at the development of phosphorus-based anode materials have focused on overcoming these difficulties through the utilization of composite structure with nanoscale carbonaceous materials as conductive buffer matrices. While these composites have shown improved cycle and rate performance of phosphorus, they suffer from low initial charge/discharge efficiency and low energy density due to the high carbon content in the structure. In this work, phosphorus-based nanocomposites containing both metal phosphide structure-reinforcing buffer and carbon network have been synthesized by a simple mechanical ball milling method. Morphological characterization shows that metal phosphide particles are well-mixed with phosphorus and further dispersed in carbon matrix. The resultant phosphorus-based nanocomposites exhibit a significantly improved electrochemical performance compared with amorphous red phosphorus, with a high initial Coulombic efficiency of over 80% in both LIB and SIB systems. Combined structural and electrochemical investigation demostrates that the performance improvement is mainly attributed to the presence of conductive phosphide inclusions that mitigates structural degradation and offers high conductivity.

(Invited) Triple Phase Boundary Reaction in a Mixed-Conducting SOFC Cathode

Solid oxide fuel cells (SOFCs) attract attentions as high-efficiency energy-conversion devices. In order to establish high-performance SOFCs, the slow electrode kinetics, which is often a main cause of the energy loss, should be improved. In present SOFCs, mixed ionic-electronic conducting (MIEC) oxides, such as (La,Sr)CoO3 -δ and (La,Sr)(Co,Fe)O3 -δ , are typically used as the cathode materials. In such an MIEC cathode, it is generally believed that electrochemical oxygen reduction reaction takes place mainly at the oxide surfaces (double phase boundary, DPB) and the contribution of the reaction at the electrode/electrolyte/gas interface (triple phase boundary, TPB) is insignificant. Therefore, researches on the kinetics of SOFC cathodic reaction have mostly concerned to investigate the surface reaction on the electrode oxides. In our previous works, the reaction distributions in MIEC SOFC electrodes were experimentally evaluated by using operando micro X-ray absorption spectroscopy (operando μ-XAS) [1, 2]. For more quantitative and precise investigation of the reaction distribution, we proposed a patterned thin film electrode, which is a kind of the columnar electrode simplifying the microstructures of a porous electrode [2]. However, the patterned thin film electrode in our previous work did not have triple phase boundaries, and it was thus impossible to discuss the contribution of TPB reaction to the total reaction. In this work, a patterned thin film electrode having triple phase boundaries was fabricated. (La,Sr)CoO3- d (LSC) was chosen as the model MIEC oxide for SOFC cathodes. The reaction distribution in this new type of the patterned thin film electrode was experimentally determined by using operando μ-XAS, and the results were compared with ones obtained with the old type of the patterned thin film electrode without TPB. It was found the effective reaction area tended to shrink by the introduction of TPB to the patterned thin film electrode. This fact might suggest that the reaction at TPBs in SOFC cathodes is not negligible even when MIEC oxides are used as electrode materials.

Enzyme-free electrochemical immunosensor based on methylene blue and the electro-oxidation of hydrazine on Pt nanoparticles

Enzyme-free electrochemical sensors enable rapid, high sensitivity measurements without the limitations associated with enzyme reporters. However, the performance of non-enzymatic electrochemical sensors tends to suffer from slow electrode kinetics and poor signal stability. We report a new enzyme-free electrochemical immunosensor based on a unique competitive detection scheme using methylene blue (MB), hydrazine and platinum nanoparticles (Pt NPs). This scheme is coupled with a robust immunosandwich format employing a MB-labelled detection antibody as a non-enzymatic reporter. In the presence of the target antigen, surface-immobilized MB consumes interfacial hydrazine thereby diminishing the electro-oxidation of hydrazine on Pt NPs. Thus, the concentration of the antigen is directly proportional to the reduction in the electrochemical signal. For proof-of-concept, this sensor was used to detect Plasmodium falciparum histidine-rich protein 2 (PfHRP2), an important malaria biomarker, in unadulterated human saliva samples. Chronocoulometric measurements showed that this platform exhibits pM-range sensitivity, high specificity and good reproducibility, making it well suited for many biosensing applications including noninvasive diagnostic testing.

Environmentally-Friendly Aqueous Li (or Na)-Ion Battery with Fast Electrode Kinetics and Super-Long Life

Current rechargeable batteries generally display limited cycle life and slow electrode kinetics and contain environmentally unfriendly components. Furthermore, their operation depends on the redox reactions of metal elements. We present an original battery system that depends on the redox of I−/I3 − couple in liquid cathode and the reversible enolization in polyimide anode, accompanied by Li+ (or Na+) diffusion between cathode and anode through a Li+/Na+ exchange polymer membrane. There are no metal element–based redox reactions in this battery, and Li+ (or Na+) is only used for charge transfer. Moreover, the components (electrolyte/electrode) of this system are environment-friendly. Both electrodes are demonstrated to have very fast kinetics, which gives the battery a supercapacitor-like high power. It can even be cycled 50,000 times when operated within the electrochemical window of 0 to 1.6 V. Such a system might shed light on the design of high-safety and low-cost batteries for grid-scale energy storage. References G. L. Soloveichik, Electrochemistry: Metal-free energy storage. Nature 505, 163–165(2014). X. Dong, L. Chen, J. Liu, S. Haller, Y. Wang, Y. Xia, Environmentally-friendly aqueous Li (or Na)-ion battery with fast electrode kinetics and super-long life. Sci. Adv. 2, e1501038 (2016). Figure 1

Metallophthalocyanine/C/Nafion Composites for Electrocatalysis of Oxygen Reduction

Oxygen reduction (OR) is a critical reaction occurring at one of the electrodes of both fuel cells and metal-air batteries. However, this reaction has slow electrode kinetics and thus, the catalysis of this reaction has vital importance. The studies in literature showed that some phthalocyanine (Pc) complexes had the ability of displaying high catalytic performance for dioxygen reduction, due to their highly conjugated π-electrons systems and rich redox properties.1 There is a strong relationship between the redox properties of Pc compounds and their efficiency as OR catalyst. Thus, the determination of the electron transfer properties of novel Pc derivatives is highly important. This work involves the identification of redox properties of various novel mononuclear tetra-2,6-dimethoxy-substituted cobalt(II), iron(II), manganese(II) and nickel(II) Pcs by voltammetry and electrocolorimetry supported in situ spectroelectrochemistry in nonaqueous solution medium. It also includes determination of their suitability for usage as OR catalyst in energy conversion and/or storage devices such as fuel cells and metal-air batteries. In situspectroelectrochemical measurements provided critical support for the complete assignment of the redox processes, i.e., whether the reactions are metal- or ligand-based. The complexes displayed enriched reversible or quasi-reversible ligand and/or central metal-based one-electron redox processes. The electrocatalytic performances of the novel Pc complexes with different metal centers towards OR have been tested and compared by dynamic rotating disk and rotating ring-disk voltammetry measurements of the MPc/C(Vulcan XC-72)/Nafion modified glassy carbon electrodes in aqueous medium. In general, the complexes display different catalytic performances depending on the differences in the interactions of their metal centers with oxygen.

Orthorhombic niobium oxide nanowires for next generation hybrid supercapacitor device

Energy storage device with high energy density at high power density is of particular importance for various applications, such as hybrid vehicles. Next generation high performance hybrid supercapacitor using organic electrolyte is able to provide high operation voltage as well as enhanced energy density. However, the energy density at high power density is usually low, partly due to the slow electrode kinetics at high current densities. In this contribution, we report a facile preparation method for orthorhombic phase niobium oxide (T-Nb2O5) nanowire structure with ultra-thin carbon coating, which shows stable high rate Li+ storage ability. A prototype of 3V hybrid supercapacitor is fabricated using carbon coated T-Nb2O5 as anode and commercially available activated carbon as cathode. This hybrid device shows high energy density of 43.4Whkg−1 and a high power density of 7.5kWkg−1 with excellent stability.

Kinetic Control of Electron Transfer at Doped Zinc Oxide/Redox-active Molecule Interface for Photocurrent Rectification

Abstract Reactivity of redox-active molecules for heterogeneous electron-transfer processes at the surface of doped zinc oxide substrates depended on their energy levels. The redox-active molecules with negative redox potentials displayed a reversible electrochemical response but those with positive redox potentials were characterized by slow electrode kinetics, which was attributed to more than three times larger heterogeneous electron-transfer rate constant for the former redox-active molecules. A photovoltaic cell using methyl viologen as an acceptor molecule, TEMPOL as a donor molecule, and the doped ZnO as the transparent conducting substrate gave high photovoltage, which was ascribed to the reactivity of the redox-active molecules at the doped ZnO surface.

Analysis of the Lithium-Ion Insertion Silicon Composite Electrode/Separator/Lithium Foil Cell

The galvanostatic charge and discharge of a silicon composite electrode/separator/lithium foil cell is modeled using porous electrode theory and concentrated solution theory. The one-dimensional (flow) model is solved with COMSOL 3.5a software. Porosity changes that accompany the large molar volume changes in the lithium-silicon electrode during operation are included and analyzed. The concept of reservoir is introduced for lithium-ion cells to accommodate the displaced electrolyte (i.e. the liquid phase). Simulation results quantitatively show the importance of a high initial porosity in silicon electrodes for better utilization of active material, especially at low rates. At higher rates, the utilization becomes similar for both thicker, porous electrodes and thinner, less porous electrodes. The insensitivity to porosity and thickness at high rates is attributed to the slow electrode kinetics for the Li-Si system. Therefore, the application dictates the optimum thickness and porosity of the electrode. Moreover, the importance and need for faster electrode kinetics relative to transport limitations is quantitatively shown.

Activation Parameters Derived From a Temperature Dependent Large Amplitude ac Voltammetric Study of the Electrode Kinetics of the Cp2M0/+ Redox Couples (M = Fe, Co) at a Glassy Carbon Electrode

Heterogeneous charge transfer rate constants (k°) for the oxidation of ferrocene, Fc (k° = 0.21 cm s(-1) at 20 °C), and the reduction of the cobaltocenium cation, Cc(+) (k° = 0.12 cm s(-1) at 18 °C), in CH(2)Cl(2) (0.5 M Bu(4)NPF(6)) at a glassy carbon (GC) electrode were determined as a function of temperature using the higher harmonic components available in Fourier transformed large amplitude ac voltammetry. The measured k° values lie at the upper end of the analytically useful range available for the acquisition of meaningful kinetic data for these very fast (i.e., close to reversible) processes. Measurements of the kinetics are facilitated by the ac technique because contributions resulting from uncompensated resistance and slow electrode kinetics, both of which also vary with temperature, can be distinguished via their different contributions to individual harmonics. Values of k° measured over the temperature range of -18 to +20 °C for the Fc(0/+) and Cc(+/0) processes gave linear Arrhenius plots yielding free energies of activation (ΔG(‡)) estimates of 17.0 and 18.0 kJ mol(-1), respectively, in excellent agreement with theoretical values derived from Marcus theory. An analogous temperature dependence is indicated in a study of the Fc(0/+) process in acetonitrile (0.1 M Bu(4)NPF(6)), but the greater k° value evident makes the temperature dependence of these parameters more difficult to quantify as departures from reversibility are minimal, even in the higher harmonics.

Discrimination and Evaluation of the Effects of Uncompensated Resistance and Slow Electrode Kinetics from the Higher Harmonic Components of a Fourier Transformed Large-Amplitude Alternating Current Voltammogram

The influence of uncompensated resistance (also called the IRu effect, where I is current and Ru is uncompensated resistance) and slow electrode kinetics have been assessed for the dc and first five ac harmonics derived from Fourier transformed large-amplitude ac voltammetry. Resistance and rate constant conditions emphasized correspond to those where separation of effects attributable to either parameter is essentially impossible under conditions of dc cyclic voltammetry. Results derived from simulations and experiments demonstrate that it is relatively easy to discriminate and quantify contributions from these two effects over a wide range of values using the fourth and fifth harmonic ac components derived from single large-amplitude ac voltammetric measurement. Furthermore, these ac components also are essentially devoid of background charging current. Concepts developed initially from simulations are confirmed by experimental studies on the following: (a) the oxidation of ferrocene, in moderately resistive CH3CN and highly resistive CH2Cl2 (represents examples of IRu effect on a reversible electron-transfer process); (b) the reduction of a low 0.2 mM concentration of [Fe(CN)6]3- in the highly conductive 3 M KCl electrolyte media (case of slow kinetics with negligible IRu effect); (c) and reduction of a high 10 mM concentration of [Fe(CN)6]3- in less conductive aqueous 0.5 M KCl electrolyte media (example where the simultaneous effects of both IRu and slow kinetics need to be resolved).

Edge Effects on Reference Electrode Measurements in PEM Fuel Cells

A reference electrode specifically designed for proton exchange membrane (PEM) fuel cells (FCs) was developed. By using the Nafion membrane material as the electrolyte, this reference electrode eliminates the needs to modify the normal FC setup and can be easily integrated into FC systems. Experimental and modeling studies had been conducted to further understand the effects of misalignment in the main electrodes on the reference electrode signals. It was found that the reference sensing point is greatly affected by the relative geometry and alignment of the two electrodes on the main membrane and electrode assembly (MEA) and the nature of electrode kinetics. Even with a perfect alignment, the reference sensing point is no longer fixed at the middle of the membrane due to difference in the kinetic characteristics at the anode and the cathode. When the misalignment is greater than the membrane thickness, the potential that the reference electrode measures is overwhelmingly masked by the edge potential of the protruding electrode. Slow electrode kinetics can significantly magnify edge effects. Generally the reference sensing point varies significantly with the change of misalignment factor and cell voltage or current. The use of reference electrode as a qualitative tool to detect electrode flooding is also discussed. © 2004 The Electrochemical Society. All rights reserved.

99/01311 Studies with porous zinc electrodes with additives for secondary alkaline batteries : Shivkumar, R. et al. J. Power Sources, 1998, 75, (1), 90–100

A method of solving the classical problem of short cycle life with nickel/zinc secondary batteries is suggested by studying the effects of the negative electrode substrate on Zn deposition. The use of different coatings on the substrate, (the so-called current-collector) is studied by testing several working electrodes (Zn, Cd and Pb) in a zinc oxide saturated potassium hydroxide electrolyte. Cyclic voltammograms show that the nucleation potential of the Zn deposition on a Pb substrate is lower than that on Zn or Cd. Cathodic polarization curves at various temperatures are also obtained and reveal that slower electrode kinetics and more Cd or Pb coating can retard shape change and dendritic growth by leading to slow electrode kinetics and uniform current distribution. These coatings are applied to a Cu mesh and the effects of coating materials on the current-collector on Zn deposition in Ni/Zn secondary batteries are also studied. Application of Pb as a coating metal on the current-collector of negative electrode retards shape change and the dendritic Zn growth and, hence, increases cycle life.

Zinc deposition during charging nickel/zinc batteries

A method of solving the classical problem of short cycle life with nickel/zinc secondary batteries is suggested by studying the effects of the negative electrode substrate on Zn deposition. The use of different coatings on the substrate, (the so-called current-collector) is studied by testing several working electrodes (Zn, Cd and Pb) in a zinc oxide saturated potassium hydroxide electrolyte. Cyclic voltammograms show that the nucleation potential of the Zn deposition on a Pb substrate is lower than that on Zn or Cd. Cathodic polarization curves at various temperatures are also obtained and reveal that slower electrode kinetics and more Cd or Pb coating can retard shape change and dendritic growth by leading to slow electrode kinetics and uniform current distribution. These coatings are applied to a Cu mesh and the effects of coating materials on the current-collector on Zn deposition in Ni/Zn secondary batteries are also studied. Application of Pb as a coating metal on the current-collector of negative electrode retards shape change and the dendritic Zn growth and, hence, increases cycle life.

Cyclic Voltammetry of Palladium(II) Complexes with Tridentate Arsine Ligands. Separation of the Two Single-Electron Transfer Steps of the Pd(II) ↔ Pd(0) Interconversion Based on Thermodynamic and Kinetic Discrimination

A detailed electrochemical study of the reduction of the five-coordinate complexes [Pd(mtas)2]2+ and [Pd(ptas)2]2+ (mtas = bis(2-(dimethylarsino)phenyl)methylarsine; ptas = bis(2-(dimethylarsino)phenyl)phenylarsine) has been undertaken in acetonitrile and dichloromethane. When cyclic voltammetry is carried out in acetonitrile at room temperature using conventional scan rates, the main voltammetric feature is a single apparently quasireversible two-electron reduction step. A small peak due to reduction of the Pd(I) dimer [Pd2(L3)2]2+ (L3 = mtas, ptas) formed in a follow-up reaction is also observed. At lower temperatures and faster scan rates, splitting of the two-electron reduction and associated oxidation peak occurs, enabling the thermodynamics and kinetics of the individual one-electron steps to be examined. The intermediate monomeric Pd(I) species are thermodynamically unstable with respect to disproportionation, but slow (relative to the experimental time scale) electrode kinetics for the Pd(I/0) couples and slow kinetics for the disproportionation reaction enable separation of the responses for the Pd(II/I) and Pd(I/0) couples. In dichloromethane, the Pd(I) complexes are thermodynamically stable over potential ranges of ca. 180 and 20 mV for [Pd(ptas)2]+ and [Pd(mtas)2]+, respectively. Examination of the electrochemical data allows rationalization of the trends in stabilities of the Pd(I) species.