Electrolysis of molten salt solutions containing PbSO4

Low faradaic yields of Pb were found when electrolysing melts containing PbSO₄ in solvents of either LiCl-KCl eutectic or Li₂SO₄-K₂SO₄-Na₄SO₄ eutectic and employing low current densities. For all current densities employed, the decrease in current efficiency became greater as the lead sulphate concentration was increased but the influence of other sulphates was less pronounced. From studies of the reaction of lead with the melt and a chronopotentiometric study of cathodic processes, we have found the following two reactions occur:

1. Cathodic deposition of lead by the reversible reaction Pb²⁺ +2e?Pb
2. Secondary non-electrochemical reaction of the electrodeposit 4Pb+5PbSO₄→PbS+4PbO.PbSO₄

The rate of the latter reaction is sufficient to account for the lowering in current efficiencies. No evidence could be found to support direct reduction of the sulphate anion.     

Potential of the Pb,PbSO4/H2SO4 electrode at temperatures up to 240°C

Standard lead—lead sulphate electrode potential was determined over the temperature range 20–240°C from emf measurements of the Pb, PbSO₄H₂SO₄ (0.05M)K₂SO₄KClHCl(0.1M)/AgCl, Ag and Pb, PbSO₄H₂SO₄(m)K₂SO₄H₂SO₄(0.05M)PbSO₄, Pb cells where m = 0.005, 0.01, 0.1 and 0.5 M. To this effect lead—lead sulphate electrode potential was calculated using the temperature relationship of the standard silver—silver chloride electrode potential and activity coefficients of hydrochloric acid determined by Greeley et al. at temperatures up to 260°C. Diffusion potentials occurring at the phase boundaries in the cells under investigation were calculated using the Henderson's equation. Values of the standard lead—lead sulphate electrode potential were determined by extrapolation of the E°′ function to the zero ionic strength which was calculated using the second sulphuric acid dissociation constant determined by Lietzke et al. at temperatures up to 300°C. The standard electrode potential was described in the temperature range 20–240°C by the following relationship: E°_{Pb, PbSO4/SO^2?4}(V) = 0.040-0.00126T. A change in entropy ΔS° of the electrode reaction Pb + SO^2?₄ = PbSO₄ + 2e^? is constant in this temperature range and is ?243 JK^?1 mol^?1 (?1018 cal K^?1 mol^?1).     

The structure of the Pb/PbSO4 electrode in the reduced state and the changes produced by lignin derivatives and BaSO4

A scanning electron microscope investigation of the structure of the reduced Pb/PbSO₄ electrode revealed that lignin prevented the extensive growth of dendritic lead single crystals that occurred in its absence. It was further shown that the smooth surfaced, large diameter dendritic branches, present in additive-free electrodes were replaced by smaller, more uniform, rough surfaced and porous particles in the presence of lignin. No significant changes occurred in the lead structure when BaSO₄ alone was added. Lignin and BaSO₄, used together, produced a lead structure with greater surface area and porosity than did lignin alone.     

Hydrated structures in the anodic layer formed on lead electrodes in H2SO4 solution

SEM and TEM observations of the corrosion layer obtained during the potentiostatic oxidation of lead electrodes in H₂SO₄ solution have shown that, at potentials above 1.00 V vs Hg/Hg₂SO₄, a lead dioxide layer is formed with crystal and gel-like (hydrated) structures. The crystal zones of the corrosion layer contain - and -PbO₂ crystals. Applying controlled thermal degradation it has been established that hydrated zones (denoted as PbO(OH)₂) comprise about 10% of the corrosion layer. For comparison, the lead dioxide active mass of the lead-acid battery is hydrated over 30%. On prolonged polarization of the lead dioxide electrode at 1.50 V, the basic electrochemical reaction that takes place is oxygen evolution. It has been suggested that this reaction occurs mainly at the interface crystal/gel-like zones. On opening the circuit, the electrode potential reaches the equilibrium potential for the PbO₂/PbSO₄ system within a rather long period. This potential decay is related to the diffusion of oxygen through the bulk of the corrosion layer (probably through its hydrated zones) to the solution and to the metal. A suggestion is made that hydrated zones are also involved in the oxygen reaction.     

Electro-reduction of hydrogen peroxide on iron tetramethoxy phenyl porphyrin and lead sulfate electrodes with application in direct borohydride fuel cells

Electroreduction of hydrogen peroxide in acidic medium is reported onto carbon-supported iron tetramethoxy phenyl porphyrin (FeTMPP/C) as well as carbon-supported lead sulphate (PbSO₄/C) electrodes. Both the catalytic electrodes can sustain electroreduction of hydrogen peroxide in direct borohydride fuel cells using hydrogen peroxide as oxidant but PbSO₄/C electrode shows catalytic activity.     

A note on the impedance of reduced PbO2 layers on some lead alloys in sulphuric acid

The impedances of reduced layers of PbO₂ in sulphuric acid are measured at potentials in the range between initial discharge and complete reduction to lead. The electrode behaviour is fairly polarizable when the electrode is remote from the PbSO₄/Pb potential. Near the reversible potential the electrode behaves as a porous electrode under charge-transfer and diffusion control.     

An Electrochemical and XPS Study of Ag–Pb Binary Alloys incorporated in Nafion® Coatings

Glassy carbon electrodes coated with thin films of Nafion^® metalized with silver and lead species were investigated by cyclic voltammetry, chronoamperometry and X-ray photoelectron spectroscopy (XPS). Metalization of Nafion^® film was accomplished by dipping the coated electrodes in 3 mM AgNO₃ + 3mM Pb(NO₃)₂ solution for 10 min. The resulting chemically modified electrodes were electrochemically characterized toward the oxidation of amino compounds in carbonate solutions buffered at pH 10. Under chronoamperometric experiments carried out at a constant applied potential of 0.95 V vs SCE, the linear range (r ²>0.995) was determined to be at least three decades and the limit of detection range from 26M (ethylamine) and 65M (tert-butylamine), for the investigated amino compounds. The perm-selective properties of the Nafion^® film with respect to anion species were investigated toward the electrooxidation of ethylamine in presence of large concentration of chloride ions. The XPS analysis revealed heterogeneous distribution of the catalytic species dispersed in the metalized Nafion^® film. Thus, a comparison of the spectra of Ag3d and Pb4f acquired at various take-off angles, indicates an increase in the atomic ratio Ag:Pb and a notable enrichment of lead oxide species in the outer surface of the film when compared with the bulk membrane coated electrode.     

Effect of gelling on the open circuit potential against time transients of Pb/PbSO4 electrodes at various states of charge

The technique of open-circuit potential relaxation from a prior prepolarized state has been employed to study the kinetics of the Pb/PbSO₄ electrode in 4.81 m sulphuric acid. The significant features governing the relaxation have been quantitatively analysed to obtain a correlation between state-of-charge of the electrode and its potential-recovery time constant. The effect of immobilizing the electrolyte has also been included due to its relevance in making sealed maintenance-free batteries. The study provides a method for estimating the variation of exchange current density of the Pb/ PbSO₄ and hydrogen evolution reactions at various states-of-charge with and without added gelling agents.This paper is dedicated to the memory of the late Professor S. Sathyanarayana.     

The electrochemical oxidation of Pb to form PbSO4

The anodic oxidation of solid Pb in H₂SO₄ to form PbSO₄ has been investigated by rotating disc, potentiostatic pulse and ac impedance measurements. The system is compared with measurements on Pb(Hg) in H₂SO₄ and with Pb(Hg), Hg, solid Pb in HClO₄ solution. It is suggested that a precipitation mechanism to form PbSO₄ occurs under some conditions prior to the solid state reaction.     

The influence of different factors on the kinetics of potentiostatic forming of pasted positive lead electrodes

The influence of such factors are the surface active material, H₃BO₃ and mixing of H₂SO₄ on the potentiostatic forming of positive lead electrodes with the production paste was investigated. The potentiostatic forming of pasted positive plates with 4PbO.PbSO₄ has been examined.     

Morphological examination of the effect of antimony on the electrochemistry of lead

An SEM study of the oxidation of PbSO₄ on Pb and Pb-Sb alloys is presented. The formation of a duplex layer of PbO₂ on the antimonial alloy is established and the primary layer of PbO₂ on Pb-Sb is shown to be analogous to the single layer on pure lead.     

Ac behaviour of the lead dioxide electrode: antimony effect

The impedance of PbO₂ electrodes in 8 N H₂SO₄ has been measured in a wide range of frequencies of (0.01–10,000 Hz) and applied potentials (500–1500 mV/Hg/Hg₂SO₄). Both α- and β-allotropic forms of pure and antimony doped lead dioxide have been taken into account. The effect of antimony added to the electrolyte has been considered.In the range of PbO₂ reducton potential, at least four contributions to the electrode impedance, characterized by different relaxation times, have been found. Two of them appear to be related to the electrochemical process taking place in two consecutive steps, a third one can be assigned to solid state diffusion of oxygen and/or lead towards the reaction zones and the last one refers to the resistance of a thin layer of PbSO₄.     

Preparation and electrochemical properties of pitch-based carbon foam as current collectors for lead acid batteries

Pitch-based carbon foam with large pore size in the cell walls was prepared and evaluated as possible current collectors of lead acid batteries. Cyclic voltammetry and galvanostatic charge-discharge experiments were performed on the material to evaluate its electrochemical properties. The carbon foam is electrochemically stable in the voltage range where the negative electrodes of lead acid batteries operate, while oxygen evolution at the foam occurs below the voltage related to the oxidation of PbSO₄ to PbO₂. Initial charge-discharge characterization of the carbon foam coated with lead oxide pastes shows that the carbon foam is suitable for use as negative current collectors of lead acid batteries, but cannot be used as positive current collectors due to the oxygen evolution. To use the carbon foam as positive current collectors, further study is necessary to increase the overpotential for the oxygen evolution.     

Fundamentals of lead-acid cells. IX. The effect of alloying with antimony on the electrochemical properties of solid lead

The electrochemistry of the Pb-Sb (5 wt %)/H₂SO₄ system has been followed using linear sweep voltammetry and the data compared with that from the Pb/H₂SO₄ system. Pre-cast lead rods aged for more than a month were used throughout in an effort to follow established industrial practice. The two areas covered are: (a) The rate of attainment of constant response of the cycled electrodes, and (b) the behaviour of stabilized electrodes. The results show that antimonial lead is corroded to a much greater extent than pure lead. Active material retention is also discussed in view of the differences between antimonial and pure lead.     

Reactions at lead electrodes in 5 mol dm−3H2SO4 near the equilibrium potential

The nature of the anodic reactions of lead near the PbSO₄/Pb equilibrium potential depends strongly upon the state of the metal. By an electrochemical etching process a lead surface is produced which participates in a solution controlled dissolution process; a simple chemical etching process results in a solid state process. The natures of these processes are investigated and discussed. The addition of expander materials to the electrolyte solution apparently inhibits both the solution and the solid state processes.On study leave from Chemistry Department, University of Technology, Dresden.     

Solid state electro-oxidation processes on lead and lead alloys in the PbO2/PbSO4 regions

The results are presented of potential step experiments in 5M sulphuric acid on lead and two lead alloys of some commercial importance. Differences in the nucleation and growth processes of PbO₂ on PbSO₄ are observed as a result of the presence of the alloying ingredient. The effect of Sb is very marked. Some reasons for the observations are discussed.Nomenclature A time dependent rate of nucleation - F Faraday constant - i _m maximum current - k ₁ parallel rate constant - k ₂ orthogonal rate constant - M molecular weight - n ₀ initial number of nuclei - t _m time corresponding to maximum current - z number of electrons transferred - density On study leave from Department of Chemistry, Helsinki University of Technology, Finland.     

Influence of temperature on electroreduction of anodically formed passive films on lead electrodes in H2SO4 solutions Part I: Electroreduction of PbS04 layers

Combined voltammetry and potentiostatic current transient techniques have been employed to study the electroreduction processes of anodically formed lead sulphate layers on polycrystalline lead electrodes in concentrated sulphuric acid solutions in the 0–50° C temperature range. Data analysis using parametric identificiation procedures and nonlinear fit routines has demonstrated that the kinetics of the electroreduction processes of the Pb(II)-containing surface species can be interpreted in terms of complex nucleation and growth mechanisms. The electroreduction of the PbSO₄ layer involves mainly an instantaneous nucleation and 3D growth mechanism under diffusion control. The activation energy of the different reaction stages can be calculated from Arrhenius plots derived from the corresponding kinetic parameters. This contributes to a better understanding of the whole cathodic reaction scheme as well as to the identification and characterization of the various stages taking place in the electroreduction processes.     

Stoichiometric activity coefficients of aqueous H2SO4 solutions at 298·15°K

Earlier emf data on cells using the PbSO₄/Pb(Hg) electrode have been used, in conjunction with a recent determination of the standard potential of this electrode, to calculate stoichiometric activity coefficients of H₂SO₄ in water at 298·15°K.     

Morphology of PbSO4 formed on solid Pb in concentrated sulphuric acid: a note pertaining to a recent paper

A scanning electron microscopy (SEM) study of the formation of PbSO₄ on Pb in H₂SO₄ at concentrations greater than 5 mol dm^?3 is presented. The production of a tight and mechanically sound PbSO₄ film in concentrated H₂SO₄ postulated in an earlier communication is confirmed.     

Hydrothermal synthesis and crystal structure of a luminescent 3D lead(II) coordination polymer [Pb(4-cca)]n (4-cca = 4-carboxycinnamate)

A novel lead(II) coordination polymer [Pb(4-cca)]_n (1) was prepared by the hydrothermal reaction of Pb(NO₃)₂ with 4-carboxycinnamic acid (4-H₂cca) and structurally characterized. Single-crystal X-ray diffraction revealed that 1 has a 3D supramolecular architecture constructed by 2D [Pb(COO)₂]_n layers pillared by –Ph–CHCH– spacers. The thermal and luminescent properties of 1 in the solid state were also investigated.