Although the pulse-current electrodeposition method is a commonly used technique, it has not been widely employed in electrode preparation. This method was applied to sintered nickel electrodes in a nickel salt solution containing additives. The active material that was obtained, nickel hydroxide, was studied using different characterization techniques. Electrodes impregnated with pulse current had higher capacity than those impregnated with continuous current. The active material is homogeneous and compact with optimum loading and good performance during discharge. These characteristics would provide a large amount of energy in a short time due to an increase in the electrode kinetic reaction.
Nickel hydroxide has many applications in the positive electrodes of alkaline cells such as nickel cadmium (Ni-Cd), nickel hydrogen (Ni-H2), nickel metal hydride (Ni-MH), and nickel iron (Ni-Fe) cells [
The chemical pasting of a mixture of active material on a support conductor is used to prepare the positive electrodes of alkaline batteries [
When the electrode is charged, nickel hydroxide is oxidized to nickel oxyhydroxide. During discharge the reaction is reversed, and Ni(OH)2 has good cyclability and high energy density, both of which are important characteristics for alkaline batteries. To increase the energy density in each of these batteries, it is necessary to improve the performance of the positive electrode [
The Ni(OH)2/NiOOH charge-discharge process is believed to be a solid-state, proton intercalation/deintercalation reaction [
The most widely used technique in the construction of electrodes for space battery applications is cathodic electrodeposition, where the precipitation occurs in one step [
These anions can be produced on the negative electrode by the electrolytic decomposition of water:
During the immersion processes, Ni(NO3)2 solutions with lower concentrations of metal nitrate are added in order to incorporate additives such as Cd, Co, Zn, or Al [
Pure Ni(OH)2 is essentially an electrical insulator, so the effects of coprecipitated cobalt hydroxide increase the conductivity [
The aim of the present work was to determine whether the application of an intermittent train of pulses improves the electrochemical performance of nickel hydroxide electrodes. Only a few papers in the literature have explored these properties [
The impregnation consists of an electrochemical process in aqueous solution. During this process many parameters must be controlled to keep the impregnation results consistent.
The temperature was controlled automatically at 70°C with the heating circulator. The solution contains 1.8 M nickel nitrate and 0.2 M cobalt nitrate.
The sintered nickel plate was pretreated in boiling distilled water for 15 min and then heated at 350°C for 30 min in order to remove impurities.
The sintered nickel plate (6.25 cm2 and empty volume of 0.36 cm2) [
Impregnation cell assembly.
Continuous current was applied using 64 mAcm−2 amplitude for 3 h. The parameters of intermittent pulse trains were 64 mAcm−2 amplitude; time ON: OFF is indicated in Table
Main characteristics of Ni(OH)2 electrodes.
Electrode/pulse (sec : sec) | Theoretical capacity |
Discharge capacity |
Specific capacity |
Utilization |
Loading |
---|---|---|---|---|---|
CP1 (on: 180/off: 90) | 188.7 | 135.1 | 206.9 | 71.6 | 1.74 |
CP2 (on: 90/off: 45) | 167.1 | 174.1 | 291.3 | 104.2 | 1.66 |
CP3 (on: 12/off: 6) | 202.9 | 203.2 | 289.2 | 100.2 | 1.94 |
CP4 (on: 6/off: 3) | 161.1 | 183.9 | 321.2 | 114.2 | 1.59 |
CC (continuous current) | 197.4 | 157.1 | 229.9 | 79.5 | 1.9 |
(*) The theoretical capacity is calculated by considering the transfer of 1 electron (289 mAhg−1) of active material.
(a) The discharge capacity values listed in Table
All electrochemical experiments were performed in a conventional glass cell in 6M KOH solution that was prepared from AnalaR grade potassium hydroxide and twice-distilled water. The counterelectrode was a nickel mesh. A Hg/HgOss reference electrode in 6M KOH, which was the same as the measuring solution, was used as the reference electrode. Charge-discharge cycles at a 0.5 C rate (C = theoretical capacity, 289 mAhg−1) were performed (ARBIN BT 2000). All the potentials in this paper are referred to as the Hg/HgOss electrode. The electrodes were discharged until 0.2 V; this potential was considered 100% of state of discharge (SOD).
The morphology and chemical composition of the prepared nickel hydroxide electrode were characterized with a JEOL JSML5510LV scanning electron microscope (SEM) and by energy dispersive spectroscopy (EDS).
The main characteristics of the different electrodes are listed in Table
After impregnation, the electrodes were rinsed in deionized water and formed using the Eagle-Picher formation procedure [
Figure
Constant current discharge curves for electrodes impregnated with constant current and pulse current.
Besides having greater capacity than CC (Table
To study the performance of the electrodes, the response of the discharge capacity of the electrode versus different discharge currents was also taken into account. To estimate the electrode capacity, the discharge current values ranged from C/20 to 3 C in order to observe the behavior of the electrode under strict discharge conditions and lower current values.
We used different discharge currents, maintaining the charge current and the charging time, C/2, for 2.5 h.
Figure
Discharge capacity versus discharge current rate.
Table
In some cases, a mass of active material was deposited but was not used in the discharge reactions.
For other electrodes, the number of exchanged electrons per metal atom (NEE) was greater than 1 (>289 mAhg−1), a number that may be surprising. Since the reaction Ni(OH)2/NiOOH involves the exchange of a unique electron, this observation comes from the fact that upon extended charging, 125% of the initial capacity assuming a number of NEE of 1,
Audemer et al. [
The electrode impregnated with a high-frequency pulse current shows the best performance of all electrodes tested. This situation can be directly related to the morphology of the deposit of active material as evidenced in previous work [
A low current (C/20) of discharge produces self-discharge because there are parallel reactions [
The optimum electrode was chosen from different electrodes impregnated with different current frequencies. The cyclability and discharge capacity were taken into account as parameters by selection. These two parameters are very important in the performance of batteries. The electrode CP3 is one of the best electrodes and was selected to be compared with electrode CC.
So far, no relationship has been found between frequency and electrochemical performance.
Figure
Variation of the discharge capacity versus number of cycles.
Each cycle begins with 150 min charge at a C/2 rate followed by a C/2 rate discharge to 0.2 V versus Hg/HgOss.
A stable discharge capacity is observed with cycle life until cycle 500, after which the capacity drops sharply. One of the reasons is electrode swelling, which changes the electrode thickness after several numbers of cycles. Therefore, the cycle life of the electrode can be correlated with the electrode expansion: the greater the electrode swelling is, the faster the capacity decay of the electrode results.
Zhang and Hou-Tian have performed the impregnation by applying intermittent currents, ON:OFF pulses, with the aim of reducing the activation and polarization by concentrating polarization on the cathode. In this way, smaller active material particles were formed, thus increasing the active surface area of the electrode and achieving greater use of the deposited material [
Photomicrographs (Figures
Sintered nickel plate (a) 100x (b) 3500x.
The morphology of the deposited material is shown in Figures
SEM micrographs of the electrode surface (CC), with nickel hydroxide as the active material; (a) 120x and (b) 3500x.
SEM micrographs of the electrode surface (CP3), with nickel hydroxide as the active material; (a) 120x and (b) 3500x.
The deposits made by applying a continuous current Figure
Figure
Chemical properties of different types of electrode samples.
Analysis (% w/w) | ||
---|---|---|
Ni | Co | |
Electrode CC | 41.63 | 04.61 |
Electrode CP3 | 30.06 | 03.20 |
The loading (gcm−3) from sample CC (continuous current) was 10% larger than that of sample CP3 (pulse current). The surface morphology of the deposited material in the first sample is homogeneous and compact. Although we achieve a greater loading, this value does not guarantee a better nickel electrode capacity.
In the case of pulse-current deposition, the active material is deposited along the shape of the porous substrate and leaves visible gaps or pores. This leads to an increased deposition of the surface active material, achieving a higher utilization rate. It can also be observed that the deposited material has a spongy appearance, generating an electrode with a larger active surface area and smaller particle size.
Table
All electrodes that were electrochemically impregnated with pulse current showed optimum capacity, similar to loading values found in the literature, with the active material producing a larger active surface area. When the impregnation current is continuous, the active material forms a homogeneous and compact surface with optimum loading but poor performance during discharge. Furthermore, the electrodes that were impregnated with different pulse currents produce loading similar to that of the electrodes impregnated with continuous current, although their active material is deposited following the shape of the porous substrate. This type of deposition produces a larger active surface area that has the highest utilization percentage. The particle size influences the electrochemical performance. When the particle size decreases, the amount of adsorbed water molecules on the surface of the particles increases. This surface water is thought to improve nickel hydroxide particle wettability, thus resulting in an enhanced proton transport within the active mass during the charge-discharge process and a better utilization of the electrode material.
This work was supported by CONICET, Consejo Nacional de Investigaciones Científicas y Técnicas of Argentina; Agencia Nacional de Promoción Científica y Tecnológica; CIC, Comisión de Investigaciones Científica de la Provincia de Buenos Aires; and Universidad Nacional de La Plata.