Studying the process of ionization of hydrogen in conditions of its forced delivery to a porous nickel oxide electrode

The hydrogen ionization process is studied experimentally on an industrial sintered nickel oxide electrode in models of sealed nickel-metal hydride batteries. It is shown that the hydrogen ionization rates that are reached during overcharge by high current densities in conditions of forced gas delivery into the electrode pores (up to 40 mA cm^?2) exceed the self-discharge rate of a nickel-hydrogen battery by two orders of magnitude. Up to 70% of hydrogen delivered into the compact assembly block undergoes ionization during forced charge of models of sealed nickel-metal hydride batteries with a closed hydrogen cycle. Two independent methods (potentiostatic and manometric) are used to determine the relationship between rates of hydrogen ionization with the degree of the electrode filling with gas and perform estimation of the process intensity at a unit reaction surface. It is established that, in conditions of forced gas delivery, practically all the hydrogen oxidation current is generated at the surface of the nickel oxide electrode beneath thin films of an electrolyte solution at the rate of 4–5 mA cm^?2. It is shown that the hydrogen oxidation rate on a nickel oxide electrode filled in part by gas is independent of the electrode potential, probably because of a tangible contribution made by diffusion limitations to the overall hampering of the process.     

Dynamics of metal deposition onto a porous electrode of poor initial conductivity, with the electrode operating in a direct-flow regime at high solution flow rates

The process of metal electrodeposition onto a porous matrix with poor initial conductivity is studied with the aid of a dynamic model for a porous electrode (PE), which was designed earlier and which was complemented with a block for calculating local conductivity of the solid phase. It is established that, despite a very low initial metal deposition rate, the final weight of the deposit inside the PE in the electrolysis conditions under consideration is greater than that inside a PE with a high conductivity of the solid phase. It is demonstrated that the additional metal amount is localized largely in the rear part of the PE and undergoes deposition chiefly in the initial electrolysis stage, specifically, until the instant of full metallization of the porous matrix and the PE conversion into an equipotential electrode. Specific features characterizing variations in the metal’s deposition rate in the course of its deposition onto a low-conductivity porous matrix and possible reasons for such variations are considered.     

Dynamics of the filling of a porous flow-through electrode,which is operating in a circulation mode,with metal

A model, developed previously for describing the filling of a porous flow-through electrode (PFE) with a metallic deposit, is used to demonstrate that, in contradistinction to a straight-through mode, a decrease in the concentration of metal in a circulating solution (approximately by an order of magnitude in the course of the time period required for filling a critical cross-section of PFE with metal) leads to a change both in the direction of the spatial redistribution of metal inside the porous matrix and in the dynamics of the variation of its basic parameters. In a straight-through mode, the metal distribution inside a PFE is defined by the action of two opposite factors (development of a surface inside the working layer L _ef at the expense of the growth of the diameter of fibers and, vice versa, its shrinking at the expense of a decrease in L _ef with time and the expulsion of the process of deposition of metal in the direction of the front end of PFE) and is characterized by nonmonotonous dependences of the current efficiency and the outlet concentration of metal on time. The predominant tendency in the case of a circulation mode is different: L _ef rises with time, which leads to a displacement of the process of the metal deposition in the direction of the rear end of PFE (at rear solution input), increase of maximal amount of deposit by approximately 1.4 times at L > L _ef, as well as to dependences of the current efficiency and the ratio between the concentrations of metal at the outlet and inlet of PFE that are monotonously decreasing with time. At the expense of a continuous variation of the effective working surface area of PFE and the mass transfer coefficient with time, the circulation mode is characterized by a nonlinear dependence of the logarithm of the metal concentration in the circulating solution on the electrolysis duration. A comparison of indicators that are characterizing the dynamics of the filling of a PFE with metal in the course of galvanostatic and potentiostatic modes of electrolysis is performed. It is established that the application of a potentiostatic mode of electrolysis under the conditions that provide for the predominance of the target reaction is accompanied by a slight decrease in the maximum quantity of metal, but a very significant decrease (by 5–6 times) in the specific spendings of electric energy.__________Translated from Elektrokhimiya, Vol. 41, No. 4, 2005, pp. 452–459.Original Russian Text Copyright © 2005 by Maslii, Poddubnyi, Medvedev.     

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.     

Effect of solution flow rate and direction on copper electrodeposition process from sulfuric acid electrolyte on porous electrode with high electric conductivity

The effect of solution flow direction and rate on the dynamics of copper electrodeposition onto a premetallized coal-graphite VINN-250 material from a dilute copper sulfate sulfuric acid solution is experimentally studied in the direct-flow mode. A light effect of solution supply on the copper deposit final mass for high and low solution flow rates is found, while the effect of this parameter on the metal distribution within the porous electrode is significant. The most uniform copper deposit distribution throughout the porous electrode is observed in the case of intermediate solution flow rates at its rear supply. The obtained experimental data agree qualitatively with the earlier published mathematical simulation results. Small systematic deviations from numeric calculations can be due to the process disregarded in the mathematical model: the effect of gas phase formed within the porous cathode due to the simultaneous hydrogen evolution.     

Dynamics of the filling of a porous cathode by the deposited metal: Modeling the process and analyzing the case of high cathode conductance and low solution depletion degree

A dynamic model for a porous electrode is designed on the basis of a one-dimensional representation of the electrode in the form of parallel filaments. The method takes into account the alterations in the local values of the filament diameter (and, correspondingly, in the effective conductances of phases), porosity, the velocity of a linear flow, and the mass transfer coefficient for the deposited metal ions, which occur in the course of the metal electrodeposition. For the simplest version of dynamics, at a high initial conductance of the electrode and a small solution depletion degree, the method predicts the following specific features: (i) the development of the working surface area and an increase in the current efficiency for the metal associated with it, (ii) a decrease in the metal penetration depth into the electrode with time and the metal localization near the most loaded end, and (iii) an irregular change in the current efficiency and concentration of the metal at the exit out of the electrode.Translated from Elektrokhimiya, Vol. 41, No. 3, 2005, pp. 333–342.Original Russian Text Copyright © 2005 by Maslii, Poddubnyi, Medvedev.     

Performance and scaling of a 1.2 V/1.5 Ah nickel/metal hydride cell to a 6 V/1.5 Ah battery

A 1.2 V/1.5 Ah positive-limited nickel/metal hydride cell has been studied to determine its charge-discharge characteristics at different rates in conjunction with its AC impedance data. The faradaic efficiency of the cell is found to be maximum at ∼70% charge input. The cell has been scaled to a 6 V/1.5 Ah battery. The cycle-life data on the battery suggest that it can sustain a prolonged charge-discharge schedule with little deterioration in its performance. Received: 17 November 1998 / Accepted: 5 February 1999     

Dynamics of metal electrodeposition inside porous electrodes: Effect of the oxidant reduction reaction

Effect of the more electropositive reaction of the oxidant reduction on the metal deposition process inside a porous electrode (PE) for a direct-flow potentiostatic regime of electrolysis is studied with the aid of a mathematical model that was developed previously. It is shown that the marked worsening of dynamic indicators characterizing the process (decrease in the rate of deposition and final weight of metal, its localization in a narrow layer near the front end of PE) is caused not only by the worsening of potential distribution inside PE at the expense of the oxidant reduction reaction but also by anodic dissolution of the metal deposit in cathodically unprotected areas of PE. The effect various factors exert on the dynamics of the emergence and development of an anodic zone inside a cathodically polarized PE and possible ways to suppress it are considered.     

Effect of oxygen vacancies in anodic titanium oxide films on the kinetics of the oxygen electrode reaction

The effect of oxygen vacancies in the anodic oxide film on passive titanium on the kinetics of the oxygen electrode reaction has been studied by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Oxide films of different donor density were prepared galvanostatically at various current densities until a potential of 20.0 V_SHE was achieved. The semiconductive properties of the oxide films were characterized using EIS and Mott-Schottky analysis, and the thickness was measured using ellipsometry. The film thickness was found to be almost constant at ∼44.7 ± 2.0 nm, but Mott-Schottky analysis of the measured high frequency interracial capacitance showed that the donor (oxygen vacancy) density in the n-type passive film decreased sharply with increasing oxide film formation rate (current density). Passive titanium surfaces covering a wide range of donor density were used as substrates for ascertaining relationships between the rates of oxygen reduction/evolution and the donor density. These studies show that the rates of both reactions are higher for passive films having higher donor densities. Possible explanations include enhancement of the conductivity of the film due to the vacancies facilitating charge transfer and the surface oxygen vacancies acting as catalytic sites for the reactions. The possible involvement of surface oxygen vacancies in the oxygen electrode reaction was explored by determining the kinetic order of the OER with respect to the donor concentration. The kinetic orders were found to be greater than zero, indicating that oxygen vacancies are involved as electrocatalytic reaction centers in both the oxygen evolution and reduction reactions. This paper was submitted in honor of the many contributions to electrochemistry that have been made by Professor Boris Grafov. The article is published in the original.     

Effect of solution volume on the rate of metal recovery and its distribution in porous cathode

By means of earlier developed dynamic model of porous electrode, numerical analysis is given of the effect of the division into smaller parts of optimal volume V _opt of metal-containing solution circulating through the porous electrode, which ensures both achieving of the metal preset recovery and the prescribed final porosity in the critical cross-section of the porous cathode, most strongly filled with the deposit. It is shown that the result depends significantly on the presence (or absence) of oxidant ions in the solution. In the absence of oxidant, the division of V _opt is desirable; it entails some improving of the distribution uniformity and increase in the deposit eventual mass. On the contrary, in the presence of oxidant such a division brings about marked negative consequences: (1) the metal deposit stronger localization at the porous electrode front edge and (2) decrease in the deposit’s final mass. It was shown that these phenomena result from the formation of anodic zone in the porous electrode rear side in the initial stage of the processing of the second and subsequent portions of solution. This is promoted by low polarization in this part of the porous electrode and the presence of a soluble metal deposited here during the final stage of electrolysis of the solution preceding portion.     

Effect of the electrode material on the oxygen reduction at the nonstoichiometric lanthanum fluoride/electrode interface

The voltammetry method with a linear potential scan is used for investigating the effect the electrode material (Ni, Co, electrodes on the basis of cobalt oxides modified with carbon) exerts on the reduction of gaseous oxygen at interfaces solid fluoride-conducting electrode LaF₃:Eu²⁺/electrode, O₂, and conjugated processes. Properties of the modified electrodes are characterized by the impedance spectroscopy, scanning electron microscopy, and x-ray photoelectron spectroscopy methods. The oxygen reaction is irreversible at the LaF₃:Eu²⁺|Ni (or Co) interfaces. At the interface of LaF₃:Eu²⁺ with modified electrodes Co (C n at %), where n = 5 and 9, mobile forms of oxygen are reversible and the reduction of gaseous and chemisorbed oxygen is controlled by diffusion with different effective kinetic parameters.     

Effect of rate and direction of solution flow on metal deposition in porous electrodes: The final weight and distribution of the deposit

Using an earlier-developed dynamic model for a porous flow-through electrode (PFE) with a high initial conductivity, the effect of the solution’s flow rate (0.05–10 cm/s) and direction on the final metal weight m _f and uniformity of the metal distribution in the porous matrix is studied. It is found that m _f increases with increasing flow rate. However, the dependence is nonmonotonic: it peaks at intermediate flow rates. The peak is most pronounced in the case of rear supply. At high and very low flow rates, m _f is independent of the flow direction. In the first case, the metal distribution profiles almost coincide, while in the second case they are mirror-opposite. The deposit weight correlates well with the index of uniformity of its distribution: all other factors being equal, the more uniform the deposit distribution in PFE, the larger the m _f. These effects are explained by taking into account the joint effect of profiles of cathodic polarization and concentration of metal ions in PFE.     

Dynamics of variations in the metal deposit distribution in porous flow-through electrodes: Effect of solution flow velocity and direction

Dynamics of variations in the metal deposit distribution in a porous flow-through electrode (PFE) and a number of integral indicators of the process are studied as a function of the velocity of solution flow and the direction of its supply with the aid of a dynamic model for PFE. It is established that qualitative character of variations in the above parameters with time depends on which of two factors (distribution of polarization or concentration of metal ions) predominantly defines the metal electrodeposition process inside PFE. It is shown that employing reverse of solution flow through PFE with the aim of increasing the weight of metal deposited in it and the uniformness of its distribution gives positive effect only in the case where the metal distribution for opposite supply directions is different.     

In situ monitoring of the rising of aqueous solution meniscus for a partially immersed silver electrode during electrochemical reduction of oxygen

Rising phenomena of aqueous solution meniscus were found for the silver electrode of a 5 MHz piezoelectric quartz crystal (PQC) partially immersed in Na₂SO₄, NaClO₄, HClO₄ and NaF aqueous solutions at oxygen reduction potentials, respectively. A detailed study revealed that a decrease in contact‐angle hysteresis (or a contact‐angle decrease) and a continued collection of the water product at the solid‐gas‐solution interface during oxygen reduction, rather than the electrocapillary effect and an agitation effect induced by the oscillation of PQC, are responsible for the meniscus‐rising phenomena. In addition, in situ determination of the immersed height of a partially immersed Ag electrode was studied on the basis of simultaneous measurements of the electroacoustic admittance and electrochemical impedance.     

Platinum alloying effects on the behavior of a metal hydride electrode

This is a study of the alloy structure, cycling life, and reaction kinetics of LaNi_4.7–x Sn_0.3Pt _x (x=0 and 0.1) metal hydride electrodes, using X-ray diffraction, X-ray absorption spectroscopy, electrochemical charge/discharge cycling, and electrochemical impedance spectroscopy. It is seen that the presence of platinum in the alloy causes an increase of the cycle life and a decrease in the hydrogen equilibrium pressure, activation time, charge storage capacity, and the rate of capacity decay during multicycling. XANES results are consistent with a decrease in the Ni oxidation in the Pt-containing alloy after the electrode cycling, indicating a protection introduced by Pt against Ni oxidation. It was also found that the catalytic activity of charge/discharge is improved with Pt alloying, a factor exclusively related to an increase of the active area due to higher alloy pulverization.     

Effect of equilibrium potential on anodic peaks of a reversible electrode process in conditions of stripping voltammetry at a linear alteration of potential on a thin-film mercury electrode

The full contour of a stripping-voltammetry peak for a reversible electrode process in conditions of boundedly semi-infinite and symmetrical diffusion on a thin-film mercury electrode at a linearly altering potential is calculated with an exact explicit equation allowing for the equilibrium-potential effect in a broad range of values of parameter H (which are defined by the film thickness, potential scan rate, and diffusion coefficient). The height, position of maximum, and full width at half-maximum of anodic peaks are evaluated as a function of parameter H and equilibrium potential. The latter is shown to exert substantial influence on the parameters and shape of anodic peaks.To the Centennial of B.N. KabanovDeceasedTranslated from Elektrokhimiya, Vol. 41, No. 1, 2005, pp. 69–75.Original Russian Text Copyright © 2005 by Nazarov, Stromberg, Larionova.     

新型粘合剂的金属氢化物电极研究——化学处理对电极性能的影响

镍氢电池;苯乙烯;金属氢化物电池;丁二苯-苯乙烯共聚物;丁酮浸渍;新型粘合剂的金属氢化物电极研究—化学处理对电极性能的影响     

氧气在阴极的电还原及其应用研究进展

本文综述了氧阴极电还原的研究进展，着重介绍了阴极表面修饰对该过程的影响以及氧阴极电还原在有机物电合成与废水处理方面的研究进展。     

The effect of water on the transport of oxygen through nylon-6 films

The effect of presence of water on the transport of oxygen through films of Nylon-6 was evaluated at 5, 23, and 40°C by permeation experiments. Through the oxygen permeability experiments it was found that the diffusion of oxygen through Nylon-6 is not a simple Fickian process and the total diffusion process can be expressed by a bimodal diffusion mechanism. Permeability, solubility, and diffusion coefficients were determined as a function of water activity for both mechanisms. The effect of sorbed water on the oxygen diffusion and solubility in the polymer is presented as a function of the state of water in the polymer and as a result of the molecular competition between water and oxygen. © 1994 John Wiley & Sons, Inc.     

Catalysis of the electrochemical oxygen reduction in room-temperature ionic liquids on a pyrolytic graphite electrode by iron-containing superoxide dismutase

Catalysis of the electrochemical oxygen reduction reaction (ORR) on a pyrolytic graphite electrode (PGE) by iron-containing superoxide dismutase (Fe-SOD) is investigated for the first time using cyclic voltammetry and electrochemical impedance spectroscopy. The study is carried out in three room-temperature ionic liquids (RTILs), namely, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF₄), 1-propyl-3-methylimidazolium tetrafluoroborate (PMIBF₄), and 1-butyl-3-methylimidazolium tetrafluoroborate (EMIBF₄). The results demonstrate that in EMIBF₄, Fe-SOD exhibits the most satisfactory catalysis for ORR, with the standard rate constant of ORR on bare PGE, k _s, increasing from 3.9 to 5.1 times 10⁻³ cm s⁻¹, while in PMIBF₄ and BMIBF₄ containing Fe-SOD k _s increases from 2.6 to 3.6 and from 1.4 to 2.2 times 10⁻³ cm s⁻¹, respectively. In addition to the increased k _s, adding Fe-SOD renders the formal potential of ORR more positive. To accelerate the electron transfer, multi-walled carbon nanotubes (MWCNTs) are employed to modify PGE, consequently, yielding the dramatically increased peak current and k _s. For MWCNTs-modified PGE in EMIBF₄ free of Fe-SOD, k _s increases from 3.9 to ∼7.1 times 10⁻³ cm s⁻¹. The ORR catalysis by Fe-SOD in the presence of Fe-SOD is also evidenced by the formal-potential shift in the positive direction. With MWCNTs accounting for the larger k _s and Fe-SOD being responsible for the formal-potential shift, the catalysis of ORR is satisfactory. Chronocoulmetry experiments proved that some Fe-SOD could be adsorbed on PGE. After analyzing the results, dismutation of superoxide anion O ₂ ⁻ by Fe-SOD is thought to be the main reason for the formal-potential shift. The different polarity of RTILs is probably partly responsible for different k _s obtained in different RTILs. Basing on an earlier proposition, the catalysis of ORR by MWCNTs in RTILs is discussed. Published in Russian in Elektrokhimiya, 2007, Vol. 43, No. 9, pp. 1137–1146. The text was submitted by the authors in English.