The preparation and behaviour of Ti/Au/PbO2 anodes

Ti/Au/PbO₂ electrodes have been prepared and their stability in H₂SO₄ (2–12 mol dm^–3) has been studied. It has been found that incorporation of a gold layer between the Ti substrate and the PbO₂ decreases the resistance of the electrode. The corrosion of an electrode polarized anodically increases with H₂SO₄ concentration especially above 8 mol dm^–3 H₂SO₄.     

A H2SO4 Loaded Polybenzimidazole (PBI) Membrane for High Temperature PEMFC

Sulfuric acid loaded polybenzimidazole (PBI) membranes were prepared with loading levels up to 10.58 (acid molecule per repeat unit of PBI) and characterized with Fourier transform infrared spectroscopy. Ionic conductivity of the PBI–H₂SO₄ membrane was found greater than that of the PBI–H₃PO₄ membrane. Through plane conductivity of a PBI–H₂SO₄ membrane with loading level 9.65 was >0.2 S cm^–1 at 150 °C. However, the conductivity of PBI–H₂SO₄ membrane increased greatly with increasing relative humidity. Membrane electrode assemblies using PBI–H₂SO₄ membrane exhibited better power density performances with pre‐humidified H₂ and air than that with none‐humidified gases. Polymer electrolyte membrane fuel cells with PBI–H₂SO₄ membrane in a single cell fixture demonstrated a peak power density >0.35 W cm^–2 with H₂ and air.     

Electrocatalytic oxidation of 2-chlorophenol on a composite PbO2/polypyrrole electrode in aqueous solution

The electrocatalytic oxidation of 2-chlorophenol on a composite PbO₂/polypyrrole (PPy) electrode was carried out in 0.1 m H₂SO₄ solution. The composite PbO₂/PPy electrode was developed by the codeposition of polypyrrole and PbO₂ microparticles on the PbO₂/SnO₂/Ti substrate. The PbO₂ microparticles and polypyrrole in the composite electrode were observed to be hydrophilic active-sites and hydrophobic inactive-sites, respectively. The results indicated that the conversion of 2-chlorophenol and the efficiency of electrooxidation were improved on the hydrophobic-modified PbO₂/PPy electrode. The performance for electrooxidation of 2-chlorophenol on the composite PbO₂/PPy electrode was better than that on Pt or PbO₂/SnO₂/Ti electrodes. The thicker the composite (PbO₂/PPy) layer, the more active-sites in the composite electrode and the more 2-chlorophenol could be oxidized.     

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.     

Mechanism of the action of Ag and As on the anodic corrosion of lead and oxygen evolution at the Pb/PbO(2−x)/H2O/O2/H2SO4 electrode system

When a Pb electrode, immersed in H₂SO₄ solution, is polarized anodically in the PbO₂ potential range the Pb/PbO(_2−x)/H₂O/O₂/H₂SO₄ electrode system is established. Oxygen is evolved at the oxide—solution interface. The oxygen atoms formed as intermediates diffuse into the anodic layer and oxidize the metal. Through a solid-state reaction, the metal is oxidized first to tet-PbO and then to PbO₂. By studying the changes in the rate—potential relations of the above reactions, as well as the phase and chemical composition of the anodic layer, it was possible to elucidate the effect of Ag and As on these processes. The additives were introduced into the electrode system either by alloying with lead or by dissolving them in the H₂SO₄ solution. When added to the solution, both Ag and As lower the overvoltage of the oxygen evolution reaction. They have practically no effect on the corrosion reaction under galvanostatic polarization conditions. If alloyed in the metal, Ag reduces the oxidation rate of Pb significantly, while As enhances it. Both additives lower the stoichiometric number of the anodic oxide layer, ie they retard the oxidation of PbO to PbO₂. The results of these investigations were used to develop further the model of the mechanism of the reactions proceeding during the anodic oxidation of lead in H₂SO₄ solutions.     

Electrochemical behaviour of titanium in H2SO4-MnSO4 electrolytes

The corrosion of titanium in H₂SO₄ electrolytes (0.001-1.0 M) at temperatures from ambient to 98 °C has been investigated using steady-state polarization measurements. Four distinct regions of behaviour were identified, namely active corrosion, the active-passive transition, passive region and the dielectric breakdown region. The active corrosion and active-passive transition were characterized by anodic peak current (i_m) and voltage (E_m), which in turn were found to vary with the experimental conditions, i.e., d(log?(im))/dpH=−0.8±0.1 and dE_m/dpH which was −71 mV at 98 °C, −58 mV at 80 °C and −28 mV at 60 °C. The activation energy for titanium corrosion, determined from temperature studies, was found to be 67.7 kJ mol⁻¹ in 0.1 M H₂SO₄ and 56.7 kJ mol⁻¹ in 1.0 M H₂SO₄. The dielectric breakdown voltage (E_d) of the passive TiO₂ film was found to vary depending on how much TiO₂ was present. The inclusion of Mn²⁺ into the H₂SO₄ electrolyte, as is done during the commercial electrodeposition of manganese dioxide, resulted in a decrease in titanium corrosion current, possibly due to Mn²⁺ adsorption limiting electrolyte access to the substrate.     

Adhesion improvement of electroless copper to PC substrate by a low environmental pollution MnO2–H3PO4–H2SO4–H2O system

A low environmental pollution etching system, MnO₂–H₂SO₄–H₃PO₄–H₂O colloid, was used to investigate surface etching performance of polycarbonate (PC) as a replacement for the chromic acid etching solution. The effects of H₂SO₄ concentrations, H₃PO₄ concentrations and etching times upon the surface topography, surface chemistry and surface roughness were studied. With the appropriate etching treatment, the surface average roughness (R_a) of PC substrates increased from 3 to 177 nm, and the adhesion strength between the electroless copper and PC substrate also reached 1.10 KN m⁻¹. After the etching treatment, the PC surface became hydrophilic and the surface contact angle decreased from 95.2° to 24.8°. The intensity of C–O groups increased and the new functional groups (–COOH) formed on the PC surface with the etching treatment, which improved the adhesion strength between PC substrate and elctroless copper film.     

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.     

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.     

PTFE-F-PbO2 电极在H2SO4溶液中的析氧行为

F-PbO2 electrode and polytetrafluoroethylene （PTFE） doped F-PbO2 electrode （PTFE-F-PbO2） were prepared on a plexiglas sheet substrate by a series of procedure including chemical and electrochemical depositions. The electrochemical activities of these two electrodes for oxygen evolution （OE） reaction were examined by electrochemical tests. In comparison with F-PbO2, PTFE-F-PbO2 electrode exhibited larger active surface area and higher oxygen vacancy deficiency, which resulted in its higher electrocatalytic activity for OE. In addition, both exchange current density and activation energy of the electrodes for OE were calculated in terms of active surface area. The values of exchange current density and activation energy in 0.5 mol·L^-1 H2SO4 aqueous solution were 1.125×10^ -3 mA·cm^-2 and 18.62 kJ·mol^-1 for PTFE-F-PbO2, and 8.384×10^-4 mA·cm^- 2 and 28.98 kJ·mol^-1 for F-PbO2, respectively. Because these values are calculated on the basis of the active surface areas of the electrodes, the enhanced activity of PTFE-F-PbO2 can be attributed to an increase in oxygen vacancy deficiency of PbO2 due to doping by PTFE. The influence of PTFE adulteration on the activity of PbO2 film electrode for OE was investigated in detail in this study.     

PHYSICAL ANALOGS AND PERFORMANCE OF A BOX MODEL FOR COMPOSITION AND GROWTH OF H2SO4/H2O AND HNO3/H2SO4/H2O AEROSOL IN THE STRATOSPHERE

A physical box model simulating the aerosol particle evolution along air mass trajectories is developed to provide a tool for interpreting the local observations of stratospheric aerosols (i.e., polar stratospheric clouds). The model calculates the composition and the size distributions of H₂SO₄/H₂O and HNO₃/H₂SO₄/H₂O liquid droplets. The parameterization of the physical processes affecting the dynamics of HNO₃ and H₂SO₄ solid hydrates and ice particle size distributions is also included, but not used. This work is restricted to some speculations about the liquid to solid transition, according to existing theories. The evolution of liquid particles is simulated taking into account nucleation, diffusive condensation/evaporation and coagulation. This paper reports the physical and numerical details of the model, which are discussed within the framework of the current understanding of the stratospheric aerosol physics. Performance and limitations of the model are discussed on the basis of the evolution of particle size, and composition along synthetic air mass thermal histories. Size distributions and size-dependent acid weight fractions of the liquid stratospheric aerosols consisting of HNO₃/H₂SO₄/H₂O are calculated in the cases of air mass thermal histories with different cooling rates and with rapid temperature fluctuations.     

Carbon electrodes with phthalocyanine catalyst in acid medium

The catalytic properties of polymeric phthalocyanines with Fe and Co as central atoms for the electroreduction of oxygen in 0.5–2.3m H₂SO₄ were studied. No noticeable dependence of the electrode potential on the concentration of H₂SO₄ was found. The electroactivity of the catalyst with a central Fe atom undergoes considerable deterioration under the given conditions, whereas the stability of the catalyst with a central Co atom is very good and the potential of an electrode containing 30% catalyst in the active mass is 100 mV more positive than that of an electrode with 13% platinum, both at 40 mA cm^–2. The electrode performance depends markedly on the sort of carbon substrate, showing a parallelism with respect to oxygen electrodes in alkaline medium. The gold mesh current collector can be replaced by the addition of carbon black to the active layer.     

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).     

Hydrogen evolution on RuxTi1−xO2 in 0.5 M H2SO4

Hydrogen evolution from 0.5 M H₂SO₄ on Ti electrodes coated with a Ru_xTi_1−xO₂ (x=0.04-0.5) layer has been studied by potentiostatic polarisation, cyclic voltammetry and ac-impedance spectroscopy. The results indicate that after a certain activation period the performance of such an electrode coating is comparable to platinum. The addition of small amounts of sodium molybdate increased the capacitance of the electrode and a reduction of the performance was observed. Increasing the temperature of the pure electrolyte from 20 to 75 °C caused an increase in the rate of the hydrogen evolution and in addition an increase of the electrode capacitance. The electrodes have been found to be rather tolerant to chloride and Fe²⁺ ions, and could hence be promising candidates as catalyst materials for solid polymer water electrolysis systems. From steady state measurements the Tafel slopes were found to change from −105 mV/decade for pure titanium to about −40 mV/decade for the (RuTi)O₂ coated electrodes. The exchange current densities were calculated from the steady state curves, as well as from impedance data, to be about 10⁻⁴ A cm⁻² after activation.     

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.     

Flattening of oxidized diamond (111) surfaces with H2SO4/H2O2 solutions

Changes in the topography of a diamond (111) surface with atomically flat and wide terraces, caused by immersion in HNO₃/H₂SO₄ and H₂SO₄/H₂O₂ solutions were investigated by atomic force microscopy. We observed surface roughening from the HNO₃/H₂SO₄ treatment, and flattening of the HNO₃/H₂SO₄ treated surface from the H₂SO₄/H₂O₂ treatment. This suggests that the H₂SO₄/H₂O₂ treatment is an effective wet-process for preparing atomically flat oxidized diamond (111) surfaces.     

Electrochemical studies with a Cu-5wt.%Ni alloy in 0.5 M H2SO4

The electrochemical behavior of the annealed Cu-5wt.%Ni alloy in 0.5 M H₂SO₄ was studied by means of open-circuit potential (E_OCP) measurements, cyclic voltammetry, electrochemical impedance spectroscopy (EIS), and quasi-stationary linear potential sweep. The hydrodynamics of the system was also studied. This material is constituted by a single α₁ phase. The anodic behavior of a Cu-Ni alloy in H₂SO₄ consists fundamentally on the electrodissolution of Cu, its main component, and the formation of a sulfur-containing passive layer. The presence of Ni decreases the rate of Cu oxidation, mostly at high positive potentials. The impedance spectra, obtained for the unrotating electrode, can be interpreted in terms of a simple charge-transfer reaction across a surface layer. When the electrode is rotated, the occurrence of an inductive loop evidenced the existence of an adsorbed layer. All the resistance estimated from the proposed equivalent circuits diminished with the electrode rotation rate, emphasizing the influence of ion transport in the overall electrode process. The system presented two anodic Tafel slopes: 40 mV dec⁻¹ for E<255 mV and 67 mV dec⁻¹ for E>275 mV. A Tafel slope of 40 mV dec⁻¹ evidences that copper dissolution can be interpreted in terms of the mechanism proposed by Mattsson and Bockris. The second Tafel suggests that at potentials more positive than 275 mV, copper dissolves according to a mechanism that considers the disproportionation of adsorbed Cu(I) species.     

Colorimetric sensing of H2PO4− using a Pb2+ complex of a pyridylazo dye in aqueous solution

This study develops a sensitive and selective colorimetric probe for the detection of dihydrogen phosphate (H₂PO₄⁻) based on a lead ion (Pb²⁺) complex of a commercially available pyridylazo dye (PAPS) in aqueous media under physiological conditions. The complex was made by adding Pb²⁺ to PAPS in a 1:1 stoichiometric ratio (PAPS-Pb²⁺), which was accompanied by the remarkable solution change from yellow to purple. The colorimetric sensing of H₂PO₄⁻ over other anions with PAPS-Pb²⁺ occurred owing to the regeneration of original PAPS by the interaction of H₂PO₄⁻ with Pb²⁺ in a 1:2 molar ratio. Moreover, the H₂PO₄⁻ detection was less affected by the presence of other coexisting competitive anions. The colorimetric naked-eye behaviour of PAPS-Pb²⁺ rendered a useful and convenient probe for the detection of H₂PO₄⁻ via the indicator displacement assay.     

Electrochemical production of ceric sulphate in concentrated H2SO4

The electrochemical preparation of ceric sulphate has been studied in concentrated H₂SO₄(10 mol dm^?3) and we have found that the current efficiency of the reaction increases considerably if a mixed catalyst is used (Ag₂SO₄ + MnSO₄).     

A novel LiTi2(PO4)3/MnO2 hybrid supercapacitor in lithium sulfate aqueous electrolyte

In this work, carbon-coated lithium-ion intercalated compound LiTi₂(PO₄)₃ and MnO₂ have been synthesized and they deliver a capacity of 90 and 60 mAh/g in 1 M Li₂SO₄ neutral aqueous electrolyte within safe potentials without O₂ and H₂ evolution, respectively. The novel hybrid supercapacitor in which MnO₂ was used as a positive electrode and carbon-coated LiTi₂(PO₄)₃ as a negative electrode was assembled and the LiTi₂(PO₄)₃/MnO₂ hybrid supercapacitor showed a sloping voltage profile from 0.7 to 1.9 V, at an average voltage near 1.3 V, and delivers a capacity of 36 mAh/g and an energy density of 47 Wh/kg based on the total weight of the active electrode materials. It exhibits a desirable profile and maintains over 80% of its initial energy density after 1000 cycles. The hybrid supercapacitor also exhibit an excellent rate capability, even at a power density of 1000 W/kg, it has a specific energy 25 Wh/kg compared with 43 Wh/kg at the power density about 200 W/kg.