Studies concerning charged nickel hydroxide electrodes. II. Thermodynamic considerations of the reversible potentials

In this paper the thermodynamics of mixing are applied to account for the independence of the discharge potential of the nickel hydroxide electrode as a function of nickel oxidation state. The constant potential region is considered to arise from the formation of a pair of co-existing solid solutions having a composition predetermined by the magnitude of the interactions between the oxidized and reduced species. From considerations of the excess-energy terms, it can be shown for a symmetrical potential/ composition profile, that the constant potential region is identical with the standard potentialE ₀. The influence of asymmetry on the changes inE ₀ are discussed. Consideration has also been made of the influence of dissociation of oxidized and/or reduced species on the potential determining equations. The removal of n-type defects from the nickel(II)-rich phase on discharge is considered to be responsible for the observed secondary discharge plateau at potentials 300 mV more cathodic than normal. This non-equilibrium behaviour can be explained in terms of a mixed pn-semiconducting material.List of symbols E electrode potential at constant pH(V) - E ₀ standard electrode potential (V) - R the gas constant (J K^–1 mole^–1) - F the Faraday constant (C g-equiv^–1) - T the absolute temperature (K) - ^aH⁺ proton activity in the electrolyte - a _z activity of oxidized species z - a _y activity of reduced species y - _H+ chemical potential of the proton - _e chemical potential of the electron - _z ^o standard chemical potential of species z - _z chemical potential of species z - _y ^o standard chemical potential of species y - _y chemical potential of species y - _H,e chemical potential of the proton/electron pair - x _y orx mole fraction of reduced species y - x _z mole fraction of oxidized species z - G _M total free energy of mixing (J mole^–1) - G _R free energy of reaction (J mole^–1) - G _I free energy of mixing under ideality (J mole^–1) - G _E excess free energy (J mole^–1) - A,A _i andB _i interaction energy parameters (J mole^–1) - x _u mole fraction of y in co-existing phase u - x _v mole fraction of y in co-existing phase v - _y activity coefficient of undissociated reduced species y - _z activity coefficient of undissociated oxidized species z - _y ^± mean ionic activity coefficient of y - _z ^± mean ionic activity coefficient of z - _y activity coefficient of y in phase u - _z ^u activity coefficient of z in phase u - _y ^v activity coefficient of y in phase v - _z ^v activity coefficient of z in phase v - I current (A) - S cross-sectional area (cm²) - L conductor length (cm)     

Studies concerning charged nickel hydroxide electrodes. VI. Voltammetric behaviour for pre-cycled electrodes

In this study cyclic voltammetry/coulometry has been combined with oxygen volume measurements to identify the species giving the multiple anodic and cathodic peaks for a pre-cycled -Ni(OH)₂ starting material. A complex sequence of 1 and 2 electron transfer reactions involving several U/V and U/V coexisting phase pairs has been identified. This system is complicated by overlap of the various processes in some cases and also the chemical transformation of unstable -phases. As many as six anodic and four cathodic processes can be encountered depending on the charging history.     

Studies concerning charged nickel hydroxide electrodes IV. Reversible potentials in LiOH,NaOH, RbOH and CsOH

The variation of reversible potential E_r with log a_moh and has been studied for several nickel hydroxide/oxyhydroxide couples in various alkali hydroxides. Both activated and deactivated -phase couples show only a small dependence ofE _r with log_moh (or where known) in LiOH, NaOH, RbOH and CsOH electrolytes. The change in MOH content on oxidation/reduction is found to be about 0.1 mol MOH per two-electron transfer and is the same as found previously in KOH. These results confirm that the bulk oxidized -phase lattice is devoid of alkali cation although a small quantity may be adsorbed by the surface. On the other hand both activated and deactivated /-phase couples show a marked dependence of 0.45 mol MOH per two-electron transfer in LiOH, NaOH and RbOH (at concentrations > 0.5 m), also in good agreement with earlier data for KOH. On the basis of these results a general stoichiometry can be inferred for the -phase, namely M_0.32NiO₂ · 0.7H₂O where M=Li⁺, Na⁺, K⁺ or Rb⁺. Measurements imply that the Cs⁺ ion or the Rb⁺ ion at low concentration (<0.5 m) do not enter the interlayer structure of the -phase. This behaviour is thought to be related to the low Rb-O and Cs-O bond strengths afforded by the -phase structure.     

Studies concerning charged nickel hydroxide electrodes. VIII. The relative potentials of the β-/γ -nickel oxy hydroxide reduction processes

In this communication it is demonstrated that a partly chargedβ-Ni(OH)₂/β-NiOOH couple can oxidizeα-Ni(OH)₂ to theγ-phase. This observation is in accordance with the order of the reversible potentials and illustrates the greater stability of theγ-phase in 7 mol dm^?3 KOH. Contrary to several claims in the literatureβ-NiOOH undergoes reduction over a range of potentials from 400 to 300 mV wrt Hg/HgO/KOH whilst theγ-phase containing Ni⁴⁺ species undergoes reduction usually at less positive potentials (below 300 mV). Internal oxidation/reduction reactions between various α/γ and β/β-phase couples can be expected to take place in partly charged electrodes during open circuit stand periods in addition to oxygen evolution. Ageing of oxidized phases may also lead to changes in free energies. Both types of process may lead to shifting of peaks after open circuit stand periods.     

Studies concerning charged nickel hydroxide electrodes. VII. Influence of alkali concentration on anodic peak positions

After repetitive potential cycling employing a high positive potential limit (>700 mV wrt Hg/HgO/ KOH) three anodic and one cathodic peak can be observed using aβ-Ni(OH)₂ starting material. Anodic peaks found at 425, 470 and 555 mV in 5 mol dm^?3 KOH shift to less positive potentials as the alkali concentration is increased appearing at 365, 410 and 455 mV respectively in 12.5 mol dm^?3 KOH. Four anodic processes involving various pairs of coexisting phases within both theβ andα-/γ-phase system can be identified as summarized below in order of increasing positive potential: Peak A $$\begin{gathered} Peak A{\text{ }}U_\alpha ^A \to {\text{ }}V_\gamma ^A \hfill \\ Peak B{\text{ }}U_\beta ^B \to {\text{ }}V_\beta ^B \hfill \\ {\text{ }}\mathop C\limits^ + {\text{ }}U_\alpha ^C \to {\text{ }}V_\gamma ^C \hfill \\ Peak E{\text{ }}V_\beta ^B \to {\text{ }}V_\gamma ^E \hfill \\ \end{gathered} $$ Observed shifts in anodic and cathodic peak potentials are consistent with the known influence of alkali and water activity on the reversible potentials for the above processes.     

Studies on the lithium and potassium uptake of nickel hydroxide electrodes

The extent of lithium and potassium uptake by both- and-Ni(OH)₂ electrodes has been studied by atomic absorption/emission spectroscopy. It was found that lithium ions are preferentially taken up into the bulk of the electrodes. The increase in the end-of-charge potential on constant current charging in lithiated electrolyte compared with pure potassium hydroxide is attributed to the bulk incorporation of lithium into the Ni(OH)₂ electrodes. X-ray studies have indicated no change in the phases present in the lithiated electrolytes either in the charged or discharged states. Cyclic voltametric measurements to 100 cycles showed no significant benefit due to lithium ions in terms of charge retention.     

Discharge behaviour of plane nickel hydroxide electrodes

Plane nickel oxide electrodes were prepared by electrochemical deposition of nickel hydroxide on flexible nickel strips. Compact and adherent deposits were obtained, with thickness up to 4 × 10^?3 cm. They were cycled in 6 M KOH solution, and owing to the flexibility of the supporting nickel strips, did not show detaching. The results obtained from galvanostatic discharge curves fit the Peukert equation iⁿt = constant, giving an n value of 1.23 for the 1 × 10^?3 cm thick film and a value of 1.10 for the 4 × 10^?3 cm think film. The diffusion coefficients of moving species are in the order of magnitude of 10^?5 ?10^?6 cm²s^?1; the rate of discharge does not seem to be limited by proton diffusion. The proton diffusion in solid phase is not the only factor to be taken into account in the discharge process; diffusion and migration OH^? and H₂O species play an important role in plane thick nickel oxide film discharge.     

Theory of porous electrodes—XVI. The nickel hydroxide electrode

A model of the positive plate of a nickel—cadmium accumulator is proposed, based on the known electrochemical behaviour of hydrated nickel oxides and on transport equations similar to those used previously for the cadmium electrode. The mathematical treatment of this model allows to predict the theoretical discharge characteristics, which are compared with those measured on a real electrode. A comparison of the calculated and measured discharge time suggests that the composition of the oxidised form is intermediate between NiO_1.5 and NiO_1.8.     

Textural and structural studies on nickel hydroxide electrodes. II. Turbostratic nickel (II) hydroxide submitted to electrochemical redox cycling

The textural and structural modifications involved in electrochemical redox cycling of turbostratic nickel (II) hydroxide has been investigated using X-ray diffraction and electron microscopy methods. It was found that during the first cycles, different phenomena compete: redox reactions which occur in the solid state, and ageing reactions via the solution. For the first galvanostatic charge performed at the C/5 rate in 4.5 N KOH, the direct oxidation of (II) to (III) and the ageing of (II) to (II) via the solution followed by the oxidation to (III) are in competition. The study of the discharge mechanism shows that the direct reduction (III)(II) is parallel to the reduction (III)(II) and the ageing of the turbostratic hydroxide via the solution. After the first cycle it was established that the alpha-generated (II) active phase consisted of a mixture of two kinds of particles, the oxidation of which follows two paths: (II)/(III) for the thicker particles and for the thinner (II)/(III), but these latter (II) particles aged via the solution by Oswald ripening and the (II)/(III) couples swung to (II)/(III).     

Performance improvement of pasted nickel electrodes with an addition of ball-milled nickel hydroxide powder

Spherical β-Ni(OH)₂ was modified by a low-cost method of normal ball milling (NBM), and the physical properties of both ball-milled and un-milled Ni(OH)₂ were characterized by transmission electron microscopy, specific surface area, particle size distribution and X-ray diffraction. It was found that NBM could obviously increase the surface area, decrease the particle and crystallite size, and reduce the crystallinity of β-Ni(OH)₂, which were advantageous to the improvement of the electrochemical activity of Ni(OH)₂. NBM also lowered the packing density and flowability of Ni(OH)₂, as revealed by the measurements of tapping density and angle of repose. Electrochemical performances of pasted nickel electrodes with an addition of ball-milled Ni(OH)₂ to spherical Ni(OH)₂ as the active material were investigated, and were compared with those of the pure spherical Ni(OH)₂ electrodes. Charge/discharge tests showed that ball-milled Ni(OH)₂ addition could enhance the charging efficiency, specific discharge capacity, discharge voltage and high-rate capability of the electrodes. This performance improvement could be attributed to a more compact electrode microstructure, better reaction reversibility and lower electrochemical impedance, as indicated by scanning electron microscopy, cyclic voltammetry and electrochemical impedance spectroscopy. Thus, it was an effective method to modify the microstructure and improve the electrochemical properties of pasted nickel electrodes by adding an appropriate amount of ball-milled Ni(OH)₂ to spherical Ni(OH)₂ as the active material.     

Electrocatalytic oxidation of urea by nanostructured nickel/cobalt hydroxide electrodes

The present paper describes the catalytic oxidation of urea performed by nickel hydroxide and nickel/cobalt hydroxide modified electrodes by using both electrodeposited films and nanoparticles. The incorporation of Co foreign atoms leads to a slight increase in sensitivity besides the shift in redox process, avoiding the oxygen reaction. Nanostructured Ni₈₀Co₂₀(OH)₂ was synthesized by sonochemical route producing 5 nm diameter particles characterized by high-resolution transmission electron microscopy (HRTEM) being immobilized onto electrode by using the electrostatic Layer-by-layer technique, yielding attractive modified electrodes for sensor development.     

Structure and electrochemical properties of nickel hydroxide electrodes with cobalt additives

In this article,cobalt additives are introduced into nickel hydroxide electrodes by two incorporation methods—co-precipitated cobalt hydroxide during the nickel hydroxide synthesis or post-added CoO with nickel hydroxide. The results of X-ray diffraction, cyclic voltammetry, electrochemical impedance spectroscopy, and charge–discharge tests indicate that (i) the diffraction peaks show a decrease in intensity and increase in the half peak breadths for Ni(OH)₂ with co-precipitated cobalt hydroxide; (ii) the electrochemical activity of nickel hydroxide can be improved by both incorporated cobalt and the effects of post-added CoO are more notable; (iii) CoOOH derived from post-added CoO is not stable in the KOH electrolyte when the potential of the Ni(OH)₂ electrode is lowered and its reduction product may be inactive, thus results in an irreversible capacity loss of nickel-metal-hydride battery after over-discharge-state storage.     

Flow-through and flow-by porous electrodes of nickel foam. I. Material characterization

This paper deals with the characterization of three nickel foams for use as materials for flow-through or flow-by porous electrodes. Optical and scanning electron microscope observations were used to examine the pore size distribution. The overall, apparent electrical resistivity of the reticulated skeleton was measured. The BET method and the liquid permeametry method were used to determine the specific surface area, the values of which are compared with those known for other materials.Nomenclature a _e specific surface area (per unit of total volume) (m^–1) - a _s specific surface area (per unit of solid volume) (m^–1) - (a _e)_BET specific surface area determined by the BET method (m^–1) - (a _e)_Ergun specific surface area determined by pressure drop measurements (m^–1) - mean pore diameter (m) - mean pore diameter determined by optical microscopy (m) - mean pore diameter using Ergun equation (m) - e thickness of the skeleton element of the foam (m) - G grade of the foam (number of pores per inch) - P/H pressure drop per unit height of the foam (Pa m^–1) - r electrical resistivity ( m) - R _h hydraulic pore radius (m) - T tortuosity - mean liquid velocity (m s^–1) Greek symbols mean porosity - circularity factor - dynamic viscosity (kg m^–1 s^–1) - liquid density (kg m^–3) - pore diameter size dispersion     

Dynamic monitoring of structural changes in nickel hydroxide electrodes during discharge in batteries

The nickel hydroxide electrode is used as the positive plate of many rechargeable battery systems such as the nickel/cadmium, nickel/hydrogen, and nickel/metal hydrides. The electrochemical energy storage in the nickel hydroxide electrodes is related to the reversible characteristics of the redox couple nickel hydroxide/ox hydroxide. In the present work we describe the use of the electrochemical impedance spectroscopy (EIS) technique as a tool to characterize the dynamic behaviour of nickel hydroxide electrodes at different states of discharge (SOD) in KOH 7 M electrolytic solutions. The parameter identification procedure allows the estimation of the active area per unit volume, the solution conductivity as well as diffusion and kinetic constants related to the process, that represent very important parameters to evaluate the electrode performance.     

Theory of porous electrodes—XVII. Correction for anodic and cathodic reaction rates for nickel hydroxide electrode

The theory of the porous nickel hydroxide electrode presented in the preceeding communication was corrected by considering the dependence of the anodic and cathodic reaction rates on the concentration of current carriers in the solid phase. The influence of this correction on the calculated E-t discharge curves is not sufficient to account for the obseerved discrepancies between these curves and the measured ones, which are probably due to phenomena on the contact between the current collector and the active material and among the particles of the electrode mix.     

Thermal decomposition of nickel hydroxide

The thermal decomposition of nickel hydroxides prepared from aqueous nickel chloride solution on addition of alkali under various conditions has been examined by thermogravimetry, differential thermal analysis, X-ray diffraction study, visible and infrared spectrophotometry, magnetic susceptibility measurement and electron spin resonance spectroscopy. As a result, it is concluded that the thermal decomposition of nickel hydroxide proceeds in the following sequence:      

Studies on the effect of cobalt addition to the nickel hydroxide electrode

Ni(OH)₂ electrodes are known to take up potassium and lithium ions from electrolytes on cycling. The present study revealed lower levels of adsorption of potassium and lithium in the case of cobalt co-deposited -nickel hydroxide electrodes although similar uptake patterns were clearly visible. At room temperatures, addition of cobalt has also been found to be advantageous in so far as the electrode performance on repeated cycling in 1 M LiOH+4 M KOH is concerned. In pure KOH, however, cobalt addition has been found to have a detrimental effect on the life of the electrode although an easier charging process is observed in the initial stages. Increase in the end-of-charge potential in lithiated electrolyte was found to be a direct consequence of lithium adsorption by the cobalt co-deposited -nickel hydroxide electrodes.     

Flotation of metal hydroxide precipitates. I. Flotation of copper hydroxide

Flotation of copper hydroxide precipitate has been investigated at total initial copper concentration 10^?2M with sodium dodecylbenzenesulphonate (DBSNa) and dodecyldimethylbenzylammonium bromide (DDMBABr), both at initial concentration 10^?4M. In particular, granulometric analysis of the precipitate and measurement of its electrokinetic potential were carried out over a wide range of the acidity of aqueous precipitate suspension in order to establish essential factors governing flotation of the precipitate. Moreover, adsorption of the flotation collectors by the precipitate as well as the rate of precipitate sedimentation were measured. The cationic collector (DDMBABr) neither influenced the electrokinetic potential of copper hydroxide precipitate nor adsorbed on its surface. Consequently, no flotation of copper hydroxide was observed with (DDMBABr). On the other hand, the anionic collector (DBSNa) influenced the electrokinetic potential of copper hydroxide within the same pH range where adsorption of DBSNa on the precipitate was observed and flotation was effective. The rate of flotation varied with the pH of the aqueous suspension. This dependence was irregular and presumably governed by the aggregation of precipitate grains since the rate of flotation increased with the size of the aggregates.     

Studies concerning the growth of cadmium dendrites. I. Morphology in alkaline media

The morphology of cadmium dendrites formed during potentiostatic electrodeposition onto nickel and cadmium substrates from cadmate solutions in alkaline supporting electrolyte has been investigated. The morphology is potential dependent for deposition under convective diffusion conditions to a nickel substrate. For 1.05×10^?4 mol dm^?3 Cd(OH) ₄ ^2? /30% KOH solutions, 2D- fern dendrites are observed at an overpotential of ?150mV, needle dendrites at ?200 mV, and large ‘filled-in’ fern dendrites at ?300 mV. Similar results were found at the higher concentration, 2.4×10^?4 mol dm^?3 Cd(OH) ₄ ^2? /50% KOH, but the time taken to grow an equivalent morphology and length were reduced in proportion. Crystalline aggregate dendrites were observed on a cadmium substrate in 1.05×10^?4 mol dm^?3 Cd(OH) ₄ ^2? /30% KOH, becoming more crystalline and well defined with increase in overpotential. A significant induction time of the order 8 h was observed for all deposition onto stationary nickel and cadmium wires. Under the well-defined diffusion conditions at a rotating nickel disc electrode only one morphology, namely small ferns, was observed over a wide range of overpotentials. The current-time behaviour is presented, and the current is shown to have a (time)² dependence, indicative of progressive nucleation of dendrites. The induction time, indicated approximately by the current minima, had decreased significantly.     

Electroreduction of nitrate ions in concentrated sodium hydroxide solutions at lead,zinc, nickel and phthalocyanine-modified electrodes

The electrochemical reduction of nitrate in strongly alkaline solution has been studied using nickel, lead, zinc and iron cathodes. Intermediate formation of nitrite ion and ammonia product was observed for all electrode materials. Coating a nickel sponge electrode with phthalocyanine renders it less active toward nitrate reduction, while iron electrodes appear to be activated. Electrolysis between a lead cathode and a nickel anode is an efficient means of removing nitrate from strongly alkaline solutions.