Processes in solid state at anodic oxidation of a lead electrode in H2SO4 solution and their dependence on the oxide structure and properties

During anodic oxidation of the lead electrode in H₂SO₄ solution in PbO potential region an anodic deposit is formed containing a dense layer of tet-PbO and a porous layer of PbSO₄. The rate of oxidation of Pb to tet-PbO is determined from the transport of O^2? ions through the tet-PbO layer by a vacancy mechanism.On illumination of the electrode with white light photoelectrochemical processes proceed in the anodic layer leading to the transformation of tet-PbO into PbO_n where (1 < n < 2). PbO_n is a solid electrolyte with semiconductor properties. The processes of photoelectrochemical oxidation of tet-PbO to αPbO₂ are discussed on the basis of the band energy scheme of semiconductors. These photoelectrochemical investigations show that at oxidation of tet-PbO to αPbO₂ in solid state the highest energy barrier is the band gap.In the PbO₂ potential region the PbO_n transforms into a semiconductor with electrone conductivity. The anodic corrosion of the Pb electrode in this region is discussed on the basis of semiconductor properties of the oxide. The corrosion proceeds in two stages. During the first Pb is oxidized to tet-PbO and at the second stage tet-PbO is oxidized in solid state to αPbO₂. For the second process the highest energy barrier is the band gap. Its overcoming is realized by surface states forming at the oxide/solution interface during the reactions of oxygen evolution. These are O^? radicals and O atoms. They penetrate into the oxide layer by an oxygen vacancy mechanism and oxidize the Pb to tet-PbO as well as the tet-PbO to αPbO₂. The processes in solid state in the PbO_n layer are favoured by the layer crystal structure and by the similar shape of the unit cells of tet-PbO and αPbO₂.     

A novel flow battery: A lead acid battery based on an electrolyte with soluble lead(II): Part VII. Further studies of the lead dioxide positive electrode

Extensive cycling of the soluble lead flow battery has revealed unexpected problems with the reduction of lead dioxide at the positive electrode during discharge. This has led to a more detailed study of the PbO₂/Pb²⁺ couple in methanesulfonic acid. The variation of the phase composition (XRD) and deposit structure (SEM) have been defined as a function of current density, Pb²⁺ and H⁺ concentrations, deposition charge and temperature as well as the consequences of charge cycling. Pure α-PbO₂, pure β-PbO₂ and their mixtures can be deposited from methanesulfonic acid media. The α-phase deposits as a more compact, smoother layer, which is well suited to charge cycling. While the anodic deposition of thick layers of PbO₂ is straightforward, their reduction is not; the complexities are explained by an increase in pH within the pores of the deposit. The results suggest that operating the battery at lead(II) concentrations <0.3 M and elevated temperatures should be avoided.     

Electrodeposition of lead dioxide on carbon substrates from a high internal phase emulsion (HIPE)

PbO₂ coatings on carbon electrode substrates were produced by anodic electrodeposition from a stationary high internal phase emulsion (HIPE) with the bath components added in the water phase. The deposits have a distinct structure consisting of 10–50 m high pyramidal aggregates, pitted with smaller pores. This is attributed to the growth of the deposits through the aqueous network of the HIPE emulsion that is, through the tortuous paths formed by its interconnected water cavities. The coatings thus produced are characterised by enhanced electrochemical activity towards the PbO₂/PbSO₄ transformation.     

Polyaniline as cathodic material for electrochemical energy sources: The role of morphology

Polyaniline layers of different morphologies ranging from open and “sponge-like” structures to compact and “pebble-like” surfaces were synthesized from perchlorate solutions and employed as cathode in the galvanic cell with Zn anode and NH₄Cl/ZnCl₂ electrolyte. Cathodic properties of synthesized layers were investigated by the constant current charging/discharging method in 500 cycles. Specific charge capacities and specific energies obtained form the current-time curves strongly depend on the morphology of investigated layers and discharge conditions. The results unambiguously show that charging/discharging reaction of polyaniline layers is limited to relatively thin layer at polymer/solution boundary. Specific charge capacities are inversely related to both the polymer thickness and the discharge current density. In the limit of zero current densities the specific charge capacity as high as 245 A h kg⁻¹ could be achieved for porous structures of polyaniline layers. Specific capacitance higher than 400 F g⁻¹ obtained at 2 mA cm⁻² current density makes polyaniline a promising material for the application in electrochemical supercapacitors. The electrochemical behaviour of the layers was investigated by cyclic voltammetry and electrochemical impedance spectroscopy before and after 500 cycles of charging/discharging experiments. Both, cyclic voltammetry and electrochemical impedance spectroscopy showed that some polyaniline layers develop an increased charged transfer resistance at the carbon support/polymer interface during charging/discharging process. The increased charge transfer resistance does not affect the overall specific charge of the layers. The low-frequency capacities in impedance spectra are attributed to charging/discharging of polymer/electrolyte interface and seem to be related to the specific charge capacities obtained by extrapolation to zero current density discharge reaction.     

Electrochemical noise analysis of O2 evolution on PbO2 and PbO2-matrix composites containing Co or Ru oxides

Electrochemical noise measurements have been performed on various electrode materials during oxygen evolution reaction. Fluctuations of both electrode potential and electrolyte resistance, recorded under galvanostatic polarization, have been analysed in order to separate ohmic and non-ohmic contributions. The anode materials compared in this study were PbO₂ and composite oxides consisting of a PbO₂ matrix and variable amounts of catalytic dispersed phases (Co₃O₄, RuO₂ and hydrous Co oxides); the composite oxide layers were generally obtained by anodic codeposition of fine particles (typically 0.1-1 μm in size) with a PbO₂ matrix electrochemically grown by Pb²⁺ oxidation. The power spectral density (PSD) of the electrolyte resistance fluctuations is very similar for PbO₂ and composite electrodes, suggesting a similar gas evolution profile. On the contrary, the potential PSD is markedly different for PbO₂ and composites containing catalytic particles. In the latter case, the potential fluctuations are entirely due to ohmic effects, while non-ohmic components are clearly dominant in the case of O₂ evolution on pure PbO₂, probably because of the much higher activation overpotential. Furthermore, on PbO₂ electrodes a plateau was obtained at intermediate frequencies (0.1-10 Hz), which was tentatively explained by bubble coalescence phenomena.     

Preparation and characterization of PbO2 electrodes doped with different rare earth oxides

PbO₂ electrodes doped with rare earth oxides (Re-PbO₂), including Er₂O₃, Gd₂O₃, La₂O₃ and CeO₂, were prepared by anodic codeposition in order to investigate the effect of rare earth oxide dopants on the properties of PbO₂ electrodes. The physicochemical properties of the Re-PbO₂ electrodes were analyzed by spectral methods and electrochemical measurements. The surface morphology of the Re-PbO₂ electrodes held the characteristics of the dopants and the crystal grain of PbO₂. The crystal structure of the PbO₂ electrodes was also influenced by doping with different rare earth oxides. The presence of Er₂O₃ and La₂O₃ in the PbO₂ films could enhance the direct anodic oxidation, which was helpful to mineralize 4-chlorophenol. The 4-chlorophenol decay on the Re-PbO₂ electrodes was analyzed and good fitting was found using the relation for the pseudo-first order reaction. Of the electrodes examined, the Er-PbO₂ electrode exhibited the best performance for the degradation of 4-chlorophenol. The removal rates of COD and 4-chlorophenol during the 9 h electrolysis at a current density of 20 mA cm⁻² were 80.7 and 100%, respectively, with the current efficiency being 16.0-10.1%.     

Carbon/PbO2 asymmetric electrochemical capacitor based on methanesulfonic acid electrolyte

A new hybrid electrochemical capacitor based on an activated carbon negative electrode, lead dioxide thin film and nanowire array positive electrode with an electrolyte made of a lead salt dissolved in methanesulfonic acid was investigated. It is shown that the maximum energy density and specific capacity of the C/PbO₂ nanowire system increase during the first 50 cycles before reaching their maximum values, which are 29 Wh kg⁻¹ and 34 F g⁻¹, respectively, at a current density of 10 mA cm⁻² and a depth of discharge (positive active electrode material) of 3.8%, that corresponds to a 22C rate. This is 7–8 times higher than the corresponding maximum values reached with a C/PbO₂ thin film cell operated in the same conditions. After an initial activation period, the performances of the C/PbO₂ nanowire system stay constant and do not show any sign of degradation during more than 5000 cycles. For comparison, the C/PbO₂ thin film system exhibits a 50% decrease of its performances in similar conditions.     

Highly flexible supercapacitors with manganese oxide nanosheet/carbon cloth electrode

We report a simple and cost-effective synthesis of hierarchically porous structure composed of Birnessite-type manganese dioxide (MnO₂) nanosheets on flexible carbon cloth (CC) via anodic electrodeposition technique. Petal-shaped MnO₂, having sheet thickness of a few nm and typical width of 100 nm, with a strong adhesion on CC is observed. This hierarchically porous MnO₂–CC hybrid structure dose exhibit not only excellent capacitance properties, such as up to 425 F g⁻¹ in specific capacitance, but also high crack resistance owing to its efficient release of bending stress, as observed by cyclic voltammetry and galvanostatic charge/discharge measurements under different curvature of bending configurations. Furthermore, flexible supercapacitors based on this kind of MnO₂ nanosheet/CC electrode showed significantly improved stability in capacitive performance over 3000 cycles under the bending test, which is highly promising for future applications in flexible energy storage device.     

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.     

The fabrication of lead dioxide layers on a titanium substrate

It is shown that stable and active lead dioxide on titanium anodes (including mesh electrodes) may be fabricated by appropriate pretreatment of the substrate and a strategy involving the anodic deposition of two PbO₂ layers—a thick underlayer with the lead dioxide doped with fluoride and iron and a top layer with the lead dioxide doped with bismuth. Periods on open circuit, especially in solutions containing an oxidisable organic or inorganic species must, however, be avoided since all forms of PbO₂ are reducible unless protected by an anodic current; the rates of the reduction does depend on the medium, the dopants in the PbO₂ and the morphology of the layer. It is also shown that, as at gold substrates, the morphology of the PbO₂ layer on titanium (consequently, their properties including adhesion and electrocatalytic activity) depend strongly on the deposition conditions including the bath composition (including Pb(II) concentration, acid concentration, concentration and choice of dopant ion), temperature, current density and deposition charge.     

Dependence of the phase composition of the anodic layer on oxygen evolution and anodic corrosion of lead electrode in lead dioxide potential region

At potentials more positive than 1300 than 1300 mV with respect to a Hg/Hg₂SO₄ electrode the partial currents of lead corrosion and oxygen evolution in 7_n H₂SO₄ follow the Tafel's dependence. X-ray investigations and wet analyses of the anodic layer show that it consists of tet-PbO and α PbO₂. Besides, at potentials more negative than 1530 mV, β PbO₂ forms at the oxide/solution interface.It is established that lead anodic corrosion proceeds in two stages. During the first Pb is oxidized to tet-PbO. During the second stage tet-PbP is oxidized to PbO₂. If this process is performed in solid state, α PbO₂ forms. If PbO is dissolved and then oxidized, β PbO₂ crystals are formed. The oxidation of tet-PbO to α PbO₂ proceeds at different rates in the bulk of the oxide and at its surface with the solution. At potentials more positive than 1530 mV the oxidation of tet-PbO to α PbO₂ at the surface of the oxide with the solution leads to the dropping off of part of the oxide layer.The oxidation of Pb and PbO is carried out under the action of O atoms and O^? radicals which evolve at the oxide/solution interface and penetrating the oxide reach the metal/oxide surface.     

Electrocatalytic degradation of 4-chlorophenol on F-doped PbO2 anodes

PbO₂ and F-doped PbO₂ (F-PbO₂) anodes have been prepared by a standard thermal decomposition-electrodeposition technique. The electrochemical stability of these anodes has been investigated by the accelerated life tests in sulphuric acid solution. The results show that the service life of the F-PbO₂ anodes is almost three times longer than that of the PbO₂ anodes. Furthermore, in the degradation of 4-chlorophenol (4-CP), the F-PbO₂ anodes give a higher degradation rate than that observed for the PbO₂ anodes. The influence of F⁻ anion doping on the stability and activity of PbO₂ anodes has been discussed. With the F-PbO₂ anodes, the degradation of 4-CP is investigated according to the results of high-performance liquid chromatograph (HPLC), ionic chromatograph (IC) and cyclic voltammetry (CV). In addition, the electron spin resonance (ESR) technique using 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as the spin-trap reagent has been applied to detect free radical intermediates generated during in situ the electrocatalytic degradation of aqueous 4-CP on the F-PbO₂ anodes. ESR measurements give the direct evidence that the active species (OH) are responsible for the decomposition of 4-CP over F-PbO₂ under anodic bias potential, strongly suggesting that the electrocatalytic degradation of most organic compounds on F-PbO₂ anodes proceed via surface intermediates of water oxidation, not via direct oxidation at electrode surface. The formation mechanism of surface intermediates is also discussed.     

Microstructure,formation mechanism and antifouling property of multi-layered Cu-incorporated Al2O3 coating fabricated through plasma electrolytic oxidation

In this work, a Cu-incorporated Al₂O₃ coating has been successfully produced through the one-step plasma electrolytic oxidation (PEO) method. Unexpectedly, the resultant Cu-incorporated Al₂O₃ coating displays a multi-layered structure, which exhibits significant variations in morphology, composition and crystallinity. Compared to the compact outer layer of the PEO coating, both medium and inner layers are highly porous even the average pore size of the medium layer is obviously higher than that of the inner layer. The medium layer has a higher Cu content in comparison to both outer and inner layers. And the outer layer mainly consists of crystalline γ-Al₂O₃ phase whereas both medium and inner layers are in amorphous states. Further, it was found that Cu specie could exist as Cu₂O and CuO in the Cu-incorporated PEO coating. Finally, it was revealed that the incorporation of Cu into the porous PEO coating significantly improves the antifouling property of Al metallic substrate against SRB, which successfully prevents the formation of SRB biofilm. Hence, PEO method is a promising surface modification technique for the fabrication of antifouling coating.     

Selective electrodeposition of PbO2 on anodised-polycrystalline titanium

Electrochemical experiments on titanium electrodes were coupled with electron backscattered diffraction (EBSD) experiments. The substrates were thermally treated and electropolished in order to have flat and reproducible polycrystalline surfaces, leading to EBSD orientation mapping. Afterwards, the samples were anodised by a galvanostatic procedure. It was shown that electrodeposition of PbO₂ from a 0.5 M Pb(NO₃)₂+2.5 M HNO₃ solution occurs selectively on the near {0 0 0 1} grains, whereas lead electrodeposition occurs on all the grains, whatever their orientation. These results are discussed, taking into account the fact that on {0 0 0 1} grains, the oxide layers are thinner than on other grains. It was concluded that electrodeposition is observed locally on Ti/TiO₂ electrodes for (i) cathodic electrodeposition of metals at low overvoltage; (ii) anodic electrodeposition of PbO₂, in potentiostatic or galvanostatic conditions.     

An investigation of the surface structure of some lead dioxide and related electrodes

S.E.M. observations have been carried out on PbO₂ and related electrodes after electrochemical reaction in 5 M H₂SO₄. The morphology of the surfaces examined is strongly effected by their history, particularly their charge/discharge cycles. PbSO₄ formed by the self-corrosion process is much more porous than that formed by the electrochemical reduction of PbO₂.     

Amorphous-to-crystalline transition of silicon-incorporated anodic ZrO2 and improved dielectric properties

Sputter-deposited zirconium and Zr-16 at.% Si alloy have been anodized to various voltages at several formation voltages in 0.1 mol dm⁻³ ammonium pentaborate electrolyte at 298 K for 900 s. The resultant anodic films have been characterized using X-ray diffraction, transmission electron microscopy, Rutherford backscattering spectroscopy, glow discharge optical emission spectroscopy, and electrochemical impedance spectroscopy. The anodic oxide films formed on Zr-16 at.% Si are amorphous up to 30 V, but the outer part of the anodic oxide films crystallizes at higher formation voltages. This is in contrast to the case of sputter-deposited zirconium, on which the crystalline anodic oxide films, composed mainly of monoclinic ZrO₂, are developed even at low formation voltages. The outer crystalline layer on the Zr-16 at.% Si consists of a high-temperature stable tetragonal phase of ZrO₂. Due to immobile nature of silicon species, silicon-free outermost layer is formed by simultaneous migrations of Zr⁴⁺ ions outwards and O²⁻ ions inwards. An intermediate crystalline oxide layer, in which silicon content is lower in comparison with that in the innermost layer, is developed at the boundary of the crystalline layer and amorphous layer. Capacitances of the anodic zirconium oxide are highly enhanced by incorporation of silicon due to reduced film thickness, even though the permittivity of anodic oxide decreases with silicon incorporation.     

Wire-mesh honeycomb catalysts for selective catalytic reduction of NO with NH3

Both flat and corrugated wire mesh sheets were coated with aluminum powder by using electrophoretic deposition (EPD) method. Controlled thermal sintering of coated samples yielded uniform porous aluminum layer with a thickness of 100 μm that was attached firmly on the wire meshes. Subsequent controlled calcination formed a finite thickness of Al₂O₃ layer on the outer surface of each deposited aluminum particles, which resulted in the formation of Al₂O₃/Al double-layered composite particles that were attached firmly on the wire surface to form a certain thickness of porous layer. A rectangular-shaped wire-mesh honeycomb (WMH) module with triangular-shaped channels was manufactured by packing alternately the flat sheet and corrugated sheet of the Al₂O₃/Al-coated wire meshes. This WMH was further coated with V₂O₅-MoO₃-WO₃ catalyst by wash-coating method to be applied for the selective catalytic reduction (SCR) of NO with NH₃. With an optimized catalyst loading of 16 wt%, WMH catalyst module shows more than 90% NO conversion at 240 °C and almost complete NO conversion at temperatures higher than 300 °C at GHSV 5,000 h⁻¹. When compared with conventional ceramic honeycomb catalyst, WMH catalyst gives NO conversion higher by 20% due to reduced mass transfer resistance by the existence of three dimensional opening holes in WMH.     

Top coating of low-molecular weight polymer MALPB used for enhanced protection on anodized AZ31B Mg alloys

A protective composite coating on an AZ31B magnesium alloy was prepared by anodic oxidation to form an oxide layer, followed by single immersion in maleic anhydride-g-liquid polybutadiene (MALPB) solution to cover a polymer coating on top. As a low-molecular weight polymer with low viscosity, MALPB had a tendency to infiltrate into the pores and cracks in the anodic layer to fill the defects among oxides so that a compact layer could be formed after it was cured by its hardeners, as observed by scanning electron microscopy (SEM). This compact layer, which was composed of anodic oxides integrated with solidified MALPB, possessed thickness around 0.7 μm as detected by energy dispersive X-ray (EDX). The anodized, MALPB-coated AZ31B alloy exhibited enhancement in corrosion resistance superior to that separately coated by anodizing oxides or MALPB, as reflected by its much higher corrosion potentials (E _ccor) and lower corrosion current density (i _corr) in DC polarization tests. Based on electrochemical impedance spectroscopy (EIS) data, we conclude that the anticorrosive performance of the composite coating can be attributed to the barrier effects provided by different layers when the electrolyte passed through the MALPB layer, compact layer, and then the inner anodic layer before it reached the surface of Mg alloy, and that among them, the compact layer acted as a much more effective barrier to the electrolyte. The appearance of damaged areas on the composite coating surface after a much longer duration in a salt spray environment revealed that the life-span of the AZ31B Mg alloy could be greatly prolonged if the pores and cracks on the anodizing films were properly sealed by suitable polymers.     

EIS study of the influence of BrCl on the behavior of electrodes in Li/SOCl<Subscript>2</Subscript> cells

The influence of BrCl on the impedance response of both the lithium anode and the carbon cathode in Li/SOCl₂ cells was studied. The impedance of the lithium anode increases with storage time while the addition of BrCl to Li/SOCl₂ cells decreases the impedance. However, the porous carbon cathode shows a small film resistance before discharge. The addition of BrCl to Li/SOCl₂ cells also decreases the impedance, especially for that part of the interface reaction resistance R₂. As a rule, the film resistance of the lithium anode decreases sharply during the early period of discharge, while that of the porous carbon cathode rises rapidly. It follows that the porous carbon cathode is the rate controlling electrode during discharge.