Modeling of the impedance response of porous metal hydride electrodes

Porous metal hydride electrodes of the alloy MmNi_3.5-3.7Co_0.7-0.8 Mn_0.3-0.4Al_0.3-0.4 have been characterized by means of impedance spectroscopy. A mathematical model for the impedance response, including effects of diffusion of hydrogen, surface kinetics, conductivity in the metal phase and the solution phase, as well as a continuous, lognormal, particle size distribution, was implemented and fitted to the experimental results by application of a least square fitting routine. The model is based on physical parameters, thus avoiding problems related to the conventional interpretation of impedance spectra in terms of equivalent circuits. Very good agreement between experimental results and model results was obtained for a wide range of frequencies, indicating that physical parameters to a great extent can be determined under realistic operating conditions. The latter was confirmed by independent measurements of the variation in open circuit voltage with respect to the state of charge of the electrode. The model provides an improved methodology for the determination of diffusion coefficients based on electrochemical impedance data. Furthermore, the model can be applied for parametric studies of metal hydride electrodes.     

Corrosion behaviour of AB5-type hydride electrodes in alkaline electrolyte solution

The corrosion rates of LaNi₅ and LaNi_3.55Co_0.75Mn_0.4Al_0.3 hydride electrodes, soaked in alkaline electrolyte and under charge–discharge cycling, were investigated by linear polarization, Tafel polarization and a new method based on the difference between the charge and discharge capacities. The corrosion–inhibition by Ni in LaNi₅ and the influence of zincate ions on the corrosion rate of LaNi₅ and LaNi_3.55Co_0.75Mn_0.4Al_0.3 were also studied. The results showed that the hydride electrode exhibits different corrosion behaviour when immersed in alkaline electrolyte and during charge–discharge cycles. The corrosion rate of LaNi₅ was lower than that of LaNi_3.55Co_0.75Mn_0.4Al_0.3 when simply soaked in 6 M KOH solution, but higher during the charge–discharge cycles. The corrosion–inhibition afforded by Zn(OH)₄ ^2– ions on LaNi_3.55Co_0.75Mn_0.4Al_0.3 is small under quiescent conditions, but increases upon charge–discharge operation; such corrosion–inhibition improves the cycle-life of the hydride electrode.     

Improvement in the cycle life of LaB5 metal hydride electrodes by addition of ZnO to alkaline electrolyte

The improvement in the cycle life of a metal-hydride electrode, LaNi_3.35Co_0.75Mn_0.4Al_0.3, brought about by the addition of ZnO to the alkaline electrolyte has been investigated using measurements based upon in situ electrical resistance, corrosion, scanning electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction and inductively-coupled plasma-atomic emission spectroscopy. It was found that Zn is underpotentially deposited on and subsequently alloyed with the subject electrode upon repeated charge-and-discharge cycles. The presence of Zn extends the cycle life of the LaNi_3.35Co_0.75Mn_0.4Al_0.3 electrode by inhibiting the disintegration and lowering the corrosion rate of the alloy particles.     

高比容量镍氢电池的制备及电化学性能

Cylindrical nickel metal hydride （Ni-MH） battery with high specific volume capacity was prepared by using the oxyhydroxide Ni（OH）2 and AB5 type hydrogen storage alloy and adjusting the designing parameters of positive and negative electrodes. The oxyhydroxide Ni（OH）2 was synthesized by oxidizing spherical β-Ni（OH）2 with chemical method. The X-ray diffraction （XRD） patterns and the Fourier transform infrared （PT-IR） spectra indicated that 7-NiOOH was formed on the oxyhydroxide Ni（OH）2 powders, and some H2O molecules were inserted into their crystal lattice spacing. The battery capacity could not be improved when the oxyhydroxide Ni（OH）2 sample was directly used as the positive active materials. However, based on the conductance and residual capacity of the oxyhydroxide Ni（OH）2 powders, AA size Ni-MH battery with 2560 mA.h capacity and 407 W·h·L^-1 specific volume energy at 0.2C was obtained by using the commercial spherical β-Ni（OH）2 and AB5-type hydrogen-storage alloy powders as the active materials when 10% mass amount of the oxyhydroxide Ni（OH）2 with 2.50 valence was added to the positive active materials and subsequently the battery designing parameters were adjusted as well. The as-prepared battery showed 70% initial capacity after 80 cycles at 0.5C. The possibility for adjusting the capacity ratio of positive and negative electrodes from 1 ： 1.35 to 1 ： 1.22 was demonstrated preliminarily. It is considered the as-prepared battery can meet the requirement of some special portable electrical instruments.     

Nanoparticles produced by borohydride reduction as precursors for metal hydride electrodes

The performance of electrodes, prepared from amorphous CoxByHz nanoparticles without additives, in 20% KOH solution was tested by means of cyclic voltammetry and chronopotentiometry. Peaks, assigned to hydrogen adsorption and desorption, are observed in the cyclic voltammograms. After charging, hydrogen atoms occupy different types of sites in the substrate, from which electrochemical desorption occurs. An increase in hydrogen content as a result of repeated cycling was established. Discharge capacity of the electrodes, estimated from the chronopotentiometric discharge curves obtained, is about 250mAhg–1. The observed changes in hydrogen and boron content, due to electrochemical treatment, indicate that the electrode material is an active participant in the whole electrochemical process. Hypotheses for the reaction mechanism are proposed.     

Fabrication and evaluation of 1 Ah silver/metal hydride cells

Silver/metal hydride (Ag/MH) cells of about 1 Ah capacity have been fabricated and their discharge characteristics at different rates of discharge, faradaic efficiency, cycle life and a.c. impedance have been evaluated. These cells comprise metal–hydride electrodes prepared by employing 60 m powder of an AB₂-Laves phase alloy of nominal composition Zr_0.5Ti_0.5V_0.6Cr_0.2Ni_1.2 with PTFE binder on a nickel-mesh substrate as the negative plates and commercial-grade silver electrodes as the positive plates. The cells are positive limited and exhibit two distinct voltage plateaus characteristic of two-step reduction of AgO to Ag during their low rates of discharge between C/20 and C/10. This feature is, however, absent when the cells are discharged at C/5 rate. On charging the cells to 100% of their capacity, the faradaic efficiency is found to be 100%. The impedance of the Ag/MH cell is essentially due to the impedance of the silver electrodes, since MH electrodes offer negligible impedance. The cells may be subjected to a large number of charge–discharge cycles with little deterioration.     

Low temperature electrochemical properties of LaNi4.6−xMn0.4Mx (M = Fe or Co) and effect of oxide layer on EIS responses in metal hydride electrodes

Iron is a key element in the development of Co-free AB₅-type hydrogen storage alloys. The aim of this work is to systematically investigate the effects of Fe and Co on the electrochemical properties of LaNi_4.6−xMn_0.4M_x (M = Fe or Co, x = 0, 0.25, 0.5 and 0.75) hydrogen storage alloys under relatively low temperatures (273, 253 and 233 K). The results showed that substitution of Fe for Ni reduced the low temperature electrochemical performance much more seriously than that of Co. Exchange current density (I₀), charge-transfer resistance (R_ct) and hydrogen diffusion coefficient (D) were determined based on the study of linear polarization, electrochemical impedance spectrum (EIS) and galvanostatic discharge, respectively. Both the hydrogen diffusion in the bulk of alloy particles and the electrochemical reaction at the alloy electrolyte interface were found to be greatly limited as the decrease of temperature. During the EIS analysis, interestingly, we found that the semicircle in the high frequency region increased dramatically with the decrease of temperature. The electrochemical process corresponding to this semicircle was proposed to be related to the oxide layer on the surface of alloy particles. Novel explanations of EIS response in metal hydride electrodes were proposed accordingly.     

An electrochemical investigation of sintered thick metal hydride electrodes for oxygen-metal hydride semi-fuel cell applications

Based on a conventional pasting method, a sintering process was applied to fabricate thick metal hydride electrodes for oxygen–metal hydride semi-fuel cell. It was found that, after sintering, the activation and the high-rate dischargeability of the electrodes are greatly improved. The sintering parameters were optimized by measuring the electrochemical properties of the metal hydride electrode sintered in the temperature range 650–900 °C for 10–60 min under mixed argon and hydrogen gases. It was found that to make a thick metal hydride electrode, the use of perforated copper foil as a current collector is better than using perforated nickel strip and copper mesh. A new type of structure is designed for the thick metal hydride electrode, that is, by folding the perforated foil that had been prepasted with active materials into a sandwich structure. This fold-type sandwich electrode was further clamped and held together by copper mesh.     

Comparative study on electrochemical techniques for determination of hydrogen diffusion coefficients in metal hydride electrodes

The constant potential discharge technique, constant current discharge technique and electrochemical impedance spectroscopy were employed to determine the hydrogen diffusion coefficient in a metal hydride electrode of MlNi_3.65Co_0.75Mn_0.4Al_0.2 alloy with various depth of discharge (DOD) at room temperature and with a specific DOD at various temperatures. The results were compared and the advantages and disadvantages of these techniques were also discussed.     

Electrochemical activation and electrocatalytic enhancement of a hydride-forming metal alloy modified with palladium, platinum and nickel

The influence of the addition of polycrystalline Pt, Pd and Ni powders into hydride-forming metal alloy electrodes on the activation process, rate-capability and cycling stability has been evaluated. The metal powder additives were found to improve the activation rate and to decreased the overpotentials of the charge and discharge processes. The electrochemical properties of these modified electrodes were analysed in terms of a simplified form of a previously developed model. Electrochemical impedance spectroscopy measurements were performed to allow the validation of the model and the identification of kinetic and physicochemical parameters.     

Characterization of surface fouling of Ti/IrO2 electrodes in 4-chlorophenol aqueous solutions by electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy (EIS) was used as the main technique coupled with cycling voltammetry (CV) to characterize the surface fouling of a conventional Ti/IrO₂ in 4-CP aqueous solutions caused by the electropolymerization of chlorinated phenol. Capacitive information of polymeric films formed was successfully derived from both the on-line and off-line impedance measurements and was used to characterize the surface fouling of IrO₂ electrodes. Results showed that the fouling extent at IrO₂ electrode decreased when its heating temperature was increased. With increasing the anodic potential, the surface fouling was enhanced firstly and then weakened, reaching the highest extent at 0.9 V. More positive potentials were believed to further oxidize the formed films and thereby to reactivate the deactivated electrode surface. With the increase of positive potential, the regeneration was enhanced, but no entire recovering could be achieved after the reactivation even at very high potentials.     

Theory of the electrochemical impedance of macrohomogeneous porous electrodes

An equation for the impedance of a macrohomogeneous porous electrode is derived for the general case (finite electrode thickness and resistivities of the solid and liquid phases), and it is shown how to obtain from it the general solution for the transient state. The electrode model is represented by a transmission line equivalent circuit, and the theory is further generalized by including an arbitrary time delay (a hindrance) of the charge transfer process at the pore surface. A reformulation leads to the definition of an appropriate dielectric function. Two hindrance processes are considered, namely control of the charge transfer by finite diffusion and control by both diffusion and formation of a pseudocapacity (ie charge storage) on a molecular scale. The resulting impedance characteristics for selected parameter values are discussed and preliminary results of fitting experimental data with the new theory are presented.     

Preparation and electrochemical property of three-phase gas-diffusion oxygen electrodes for metal air battery

Three-phase gas-diffusion oxygen electrodes for metal air battery were prepared and characterized. Nano-structured γ-MnO₂ catalysts were synthesized by solid state redox reaction of two compounds, Mn(CH₃COO)₂·4H₂O and C₂H₂O₄·2H₂O. Their crystal phase, morphologies and particle size were characterized by XRD, TEM, respectively. The electrochemical property of three-phase gas-diffusion oxygen electrodes composed of nano-structured γ-MnO₂ catalysts for oxygen reduction was examined by using the linear polarization method in a neutral solution. Besides, the surface morphologies of the catalytic layer of three-phase gas-diffusion oxygen electrodes were also investigated by SEM. Experimental results revealed that these kinds of three-phase gas-diffusion oxygen electrodes have excellent electrochemical performance. The optimal proportion of nano-structured γ-MnO₂ catalysts in the catalytic layer was 60 wt.%. Three-phase gas-diffusion oxygen electrodes composed with nano-structured γ-MnO₂ catalysts appear to be a highly possible candidate for applications in neutral solution metal air battery.     

Electrochemical impedance characterization of FeSn2 electrodes for Li-ion batteries

This work reports the electrochemical characterization of a micro-scale FeSn₂ electrode in a lithium battery. The electrode is proposed as anode material for advanced lithium ion batteries due to its characteristics of high capacity (500 mAh g⁻¹) and low working voltage (0.6 V vs. Li). The electrochemical alloying process is studied by cyclic voltammetry and galvanostatic cycling while the interfacial properties are investigated by electrochemical impedance spectroscopy. The impedance measurements in combination with the galvanostatic cycling tests reveal relatively low overall impedance values and good electrochemical performance for the electrode, both in terms of delivered capacity and cycling stability, even at the higher C-rate regimes.     

Interfacial synthesis of porous MnO2 and its application in electrochemical capacitor

In this paper, the porous manganese dioxide (MnO₂₎ was prepared by an interfacial reaction of potassium permanganate in water/ferrocene in chloroform. The surface area and pore distribution of MnO₂ can be controlled by simply adjusting the reaction time and the content of surfactant in the aqueous phase. The electrochemical performance of the prepared MnO₂ was evaluated as an electrode material for supercapacitors by the means of cyclic voltammetry and galvanostatic charge-discharge tests. Electrochemical tests results indicated that the pore size plays an important role at high charge-discharge rate, the sample with a large pore size shows a better rate capability, while the sample with a small pore size but large surface area delivers a large capacitance at low current rate.     

Conductometric discrimination of electro-inactive metal ions using nanoporous electrodes

We investigated conductometric analysis of electro-inactive metal ions at high concentration based on nanoporous electrodes by electrochemical impedance spectroscopy (EIS). The three dimensional interconnected nanoporous Pt (L₂-ePt) was found to enable significantly sensitive and selective conductometric detection of alkali and alkaline earth metal ions of high concentration at low frequency without any additional surface modification, which can be hardly done by flat Pt. The extremely large surface area of L₂-ePt remarkably suppressed the electrode impedance and the pore effect was additional positive contribution to selective ion sensing by conductometry at low frequency. Importantly, L₂-ePt allowed recognition of fractional ratio reversal of Na⁺ to K⁺ ions in a mixed solution at physiological concentration maintaining the constant total ionic strength. The results suggest the possibility of real time extracellular monitoring of instantaneous ion exchange near ion channels of a cell membrane such as action potential propagation along axons in a neuronal system.     

Enhanced electrochemical properties of nonstoichiometric amorphous Mg2Ni1.3 electrodes

Nonstoichiometric amorphous Mg₂Ni_1.3 alloys were synthesized by mechanically milling crystalline Mg₂Ni alloy with Ni powders. In comparison with the stoichiometric material, the nonstoichiometric Mg₂Ni_1.3 phase showed a higher discharge capacity because of the amorphization of the alloy. Surface modification with graphite was also carried out for further improvement of its electrode performance. The coated powders showed a better cyclic stability because the graphite protected the Mg from oxidation. The rate capability (RC) and discharge capacity of electrodes was also markedly improved with graphite coating due to the excellent electrical conductivity and electrocatalytic activity of graphite.     

Characterization and electrochemical studies of Nafion/nano-TiO2 film modified electrodes

A nano-TiO₂ film from stable aqueous dispersion has been modified on a glassy carbon electrode (GCE), and was characterized by scanning electron microscopy (SEM) and surface-enhanced Raman spectroscopy (SERS). This nanostructured film exhibits an ability to improve the electron-transfer rate between electrode and dopamine (DA), and electrocatalyze the redox of DA. The electrocatalytical behavior of DA was examined by cyclic voltammetry (CV). Combined with Nafion, the bilayer-modified electrode (N/T/GCE) gives a sensitive voltammetric response of DA regardless of excess ascorbic acid (AA). Electrochemical impedance spectroscopy (EIS) at a fixed potential was performed at variously treated GCEs. The mechanism of the electrode reaction of DA at N/T/GCE and the equivalent circuits of different GCEs have been proposed.     

Electro-oxidation of hydrazine on electrodes modified with vitamin B12

In this paper we report a kinetic study of the electro-oxidation of hydrazine catalyzed by vitamin B₁₂ pre-adsorbed on an ordinary pyrolytic graphite electrode. Kinetic parameters were determined by linear sweep voltammetry and rotating-disk electrode polarization curves. The order of the reaction is 1 in OH⁻ ions and Tafel plots give slopes of 80 mV/decade. A possible redox-catalysis mechanistic scheme is proposed.     

Electrochemical determination of the diffusion coefficient of cations into Chevrel phase-based electrochemical transfer junction by potential step chronoamperometry and impedance spectroscopy

The molybdenum chalcogenides Mo₆X₈ (X = S, Se) offer the possibility of intercalation/de-intercalation processes by chemical or electrochemical way. Besides the different applications of so-called Chevrel phases, we have proposed an electrochemical transfer junction for selective recovery of metallic cations in the perspective of recycling of industrial liquid mineral wastes. Thus, the knowledge of the diffusion properties of cations in the Chevrel phases is essential. Here we report on the electrochemical determination of diffusion coefficients of Co²⁺, Ni²⁺, Fe²⁺, Cd²⁺, Zn²⁺, Mn²⁺ and Cu²⁺ for Mo₆S₈ and Mo₆Se₈ matrices. Experiments were realized on samples with compactness of 50% and 96–98%. They point out that the lower compactness is unfavorable to the mobility of the cobalt ions. From potential step chronoamperometry and electrochemical impedance spectroscopy, the diffusion coefficients were found around 10⁻⁹ cm² s⁻¹, even 10⁻⁶ cm² s⁻¹ for copper. These results confirm the high mobility of transition metal ions in studied phases and complete the data for Co, Fe or Mn–Mo₆S₈ system and Mn–Mo₆Se₈ system. For the sulfide phase, the following sequence for is observed Ni < Co < Fe < Cd < Zn < Mn ? Cu and can be explained in regards with structural considerations and repulsion effects for copper.