Effect of H3PO4 on the PbSO4/PbO2 electrode in H2SO4 solutions

The influence of H₃Po₄ small additions on the polarization behaviour of the PbSO₄/PbO₂ electrode, in H₂SO₄ solutions of different concentrations, was studied using the microelectrode technique and the coulometric method. The reference electrode was the Hg/Hg₂SO₄/H₂SO₄ electrode. The potentiogram analysis and the coulometric measurements show that the main effect of the H₃PO₄ is the drastic passivation of the electrode processes, within the potential range under study, ie the +800 mV to +1650 mV potential range. The Q_a and Q_c electricity amounts are of about 5–10 times smaller in the H₂SO₄ solution with H₃PO₄ addition, compared with the H₂SO₄ solution free of H₃PO₄ addition. A high increase of the oxygen evolution overvoltage could also be seen. Taking into account the ratio between the H₃PO₄ addition amount, in % (w/o) and the PbO₂ active mass per unit surface area, from the lead acid batteries, we can conclude that the small H₃PO₄ additions do not decrease the capacity of the active mass of the battery, but they hinder the insulator film formation between the active mass and the grid.     

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.     

Reduction of nitric oxide at a platinum cathode in an acidic solution

The reduction of nitric oxide at a platinum electrode in 4 M H₂SO₄ was investigated by the measurement of potential/current relations and by the determination of the current efficiencies for the hydroxylamine, nitrous oxide, ammonia, hydrazine and hydrogen formation at fixed potentials in the potential range from 0 to ?400 mV vs sce.The potential/current curve for the reduction of nitric oxide has two waves, of which the limiting currents depend strongly on the potential and the time of the pretreatment of the electrode and on the direction of the potential change during the measurements. In the potential range of the first wave (viz ? > ?30 mV) nitric oxide is reduced only to nitrous oxide. In the whole potential range of the second wave (viz from ?50 to ?250 mV) nitric oxide is reduced to hydroxylamine, ammonia and nitrous oxide. Formation of hydrazine has not been detected.From the literature and from the relations of the current efficiencies a possible mechanism is proposed for the reduction of nitric oxide.     

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.     

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).     

Electrochemical evaluation of the surface of chalcopyrite during dissolution in sulfuric acid solution

The dissolution of a massive chalcopyrite electrode (98.1% chalcopyrite, 1.9% siderite) was studied in 0.5 M sulfuric acid solution. Different anodic potentials were applied and the behavior of the electrode was observed by means of EIS, potentiodynamic, and Mott-Schottky techniques. Electrochemical impedance spectroscopy studies at open circuit potential (around −235 mV vs. MSE) proved the existence of a thin surface layer on the electrode. This layer was stable up to 100 mV vs. MSE and was assumed to be Cu_1−xFe_1−yS₂ (y?x) based on reports from previous studies. By increasing the potential to the range of 100-300 mV vs. MSE, the previously formed layer partially dissolved and a second layer (Cu_1−x−zS₂) formed on the surface. Both of the layers showed the characteristics of passive layers at low potentiodynamic scan rate (0.05 mV s⁻¹) while at high scan rates they acted like pseudo-passive layers. However, in the potential range of 300-420 mV vs. MSE, both of these surface layers dissolved and active dissolution of the electrode started. Further increase in potential caused the formation of a CuS layer which hindered the dissolution rate of the electrode. The formation of CuS is concomitant with Fe₂(SO₄)₃ formation and the latter may act as a nucleation precursor for jarosite at higher potentials (around 750 mV vs. MSE). Jarosite precipitation on the electrode surface hindered the dissolution of chalcopyrite at higher potentials. Different equivalent electrochemical circuits were modeled for each potential range and the model regression results compared with the experimental results of EIS to determine the proposed sequence of chalcopyrite dissolution.     

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.     

LaNiTiO3-SE-based stabilized zirconium oxide mixed potentiometric SO2 gas sensor

In this paper, a mixed potential gas sensor based on YSZ solid electrolyte and LaNiTiO₃ sensing electrode was produced. The perovskite-type oxide LaNiTiO₃ was synthesized by the sol-gel method as a sensitive electrode, which was intended to detect the low-level concentration of SO₂ in the environment. After the aging process and continuous testing, it was found that the LaNiTiO₃-SE sensor had the best response value of ?27.5 mV to 5 ppm SO₂ at 510 °C, at this temperature, as low as 50 ppb SO2 still had a response of -1mV. Meanwhile, when the sensor was tested at 510 °C, it was found that the response value of the sensor showed a piecewise linear relationship with the logarithm of SO₂ concentration, with a sensitivity of -4 mV/decade for 0.05–1 ppm SO₂ and -40mV/decade for 1–100 ppm SO₂. In addition, the sensor also showed good selectivity, and the response to interference gases could be ignored such as NO₂, CO, CH₄, NH₃, H₂, ethanol, and formaldehyde. At the same time, the sensor also shows good repeatability and stability, being still relatively stable after two weeks of continuous operation at high-temperature.     

Potentiometric SO2 gas sensor based on a thick film of Ca2+ ion conducting solid electrolyte

A planar miniaturized SO₂ sensor based upon a thick film of Ca²⁺ ion conductor-CaO·0.6MgO·6Al₂O₃ (CMA) with a Na₂SO₄ auxiliary electrode and a Pt/O₂ reference electrode was fabricated and tested. The thick film was fabricated by screen-printing CMA ink on an alumina substrate and then fired at 1823 K. The solid electrolyte was interfaced with a sodium sulphate auxiliary phase containing Pt paste and the sensor showed a good SO₂ response at 873–1073 K. The electromotive force (emf) values obtained were linearly dependent upon the logarithm values of SO₂ concentration in a range of 10–500 ppm. Both the electrodes were exposed to the same test gas thus eliminating the need to separate the electrode chambers.     

Hydrodynamic voltammetric studies of the oxygen reduction at gold nanoparticles-electrodeposited gold electrodes

The electrocatalytic reduction of oxygen at Au nanoparticles-electrodeposited Au electrodes has been studied using rotating disk electrode (RDE) voltammetry in 0.5 M H₂SO₄. Upon analyzing and comparison of the limiting currents data obtained at various rotation speeds of this RDE with those obtained at the bulk Au electrode, an effective value of the number of electrons, n, involved in the electrochemical reduction of O₂ was estimated to be ca. 4 for the former electrode and ca. 3 for the bulk Au electrode at the same potential of −350 mV versus Ag/AgCl/KCl(sat.). This indicates the higher possibility of further reduction and decomposition of H₂O₂ at Au nanoparticles-electrodeposited Au electrode in this acidic medium. The reductive desorption of the self-assembled monolayer of cysteine, which was formed on the Au nanoparticles-electrodeposited Au electrode, was used to monitor the change of the specific activity of the bulk Au electrode upon the electrodeposition of the Au nanoparticles.     

Cathodic reduction of oxygen on copper and brass

In this investigation, the electrochemical reduction of oxygen on copper and brass has been studied using the ring-disc electrode technique in solutions containing chloride, sulphate and nitrate anions and ammonium cation. The E—I curves obtained in the forward and reverse direction of polarization for copper and brass in NaCl, Na₂SO₄ and NH₄Cl solutions are similar in nature and show two waves. However in (NH₄)₂SO₄ and NH₄Cl solutions are similar in nature and show two waves. The rate of oxygen reduction is the highest in ammonium sulphate for both copper and brass whereas it is the lowest in NH₄Cl in the case of copper. In Na₂SO₄ and NaCl solutions, the rate of oxygen reduction is higher on copper than on brass. Apparent Tafel slopes for oxygen reduction obtained for copper and brass vary from 65 mV to 240 mV depending upon the medium.The two steps observed with copper disc electrode have been identified as due to the reduction of oxygen to hydrogen peroxide and reduction of H₂O₂ to OH^? ion or water depending upon the pH of the solution. In acid chloride and sulphate media no H₂O₂ was detected, which suggests direct reduction to H₂O. The diagnostic plots of I_d/I_rυs ω^{?$\text{1}\text{2}$} employed by Bockris et al indicate that in Na₂SO₄ the reduction of oxygen to H₂O₂ takes place in a parallel reaction whereas in (NH₄)₂SO₄ both direct reduction of O₂ to water (or OH^? ion) and the reduction through intermediate H₂O₂ occur.     

Single ion activity coefficients based on the electrical double-layer model. Na2SO4 aqueous solutions

Single ion activity coefficients for Na₂SO₄ aqueous solutions have been determined by emf measurements of the cells: sce/Na₂SO₄ (aq., m)/SO^2?₄-reversible electrode and sce/Na₂SO₄ (aq., m)/Na⁺- selective electrode; coupled with electric potential difference measurements for the cell: Hg (σ = const)/Na₂SO₄ (aq., m)/sce. The extra-thermodynamic assumption behind this approach is that the electric potential drop across the compact layer at the Hg/solution interface in the absence of ionic specific adsorption is strictly independent of the electrolyte concentration. Results show that γ₊ is higher than γ_?, somewhat less than expected on the basis of the simple difference in ionic charge. γ₊ and γ_? calculated with the aid of Stokes, Robinson and Bates' hydration theory have been found to deviate the other way round. In the light of these results some considerations can be made on the accuracy of Gouy—Chapman diffuse layer calculations.     

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.     

The electrochemical study of a chromium plating bath. II. Chromium metal and surface film formation

The investigation of the electrode reactions occurring at metal and vitreous carbon cathodes in the standard chromium electroplating solution, 200 g dm^?3 CrO₃ and 2 g dm^?3 H₂SO₄, has been continued and this paper considers the mechanism of deposition of metallic chromium and the role of surface films in this process. It is confirmed that chromium deposition occurs at potentials negative to ?1.6 V versus Hg/Hg₂SO₄ and under galvanostatic or potentiostatic conditions, the current efficiency is 30–45%. Moreover at a vitreous carbon electrode, potential step experiments lead to risingI-t transients characteristic of instantaneous nucleation and three dimensional phase growth. At potentials just prior to metal deposition the potential sweep and step experiments show clear evidence for the formation of a strongly passivating film (this may be in addition to an existing less passivating layer). The important of the film varies with H₂SO₄ concentration and at intermediate concentrations, potential step experiments lead to unusual oscillatingI-t transients.     

Electrochemical characterization of carbon nanotubes as electrode in electrochemical double-layer capacitors

Carbon nanotubes uniformly 50 nm in diameter were directly grown on graphite foil. Cyclic voltammetry (CV) shows that the carbon nanotube/graphite foil electrode has a high specific capacitance (115.7 F/g at a scan rate of 100 mV/s) and exhibits typical double-layer behavior. A rectangular-shaped CV curve persists even at a scan rate of 100 mV/s in 1.0 M H₂SO₄ aqueous solution, which suggests that the carbon nanotube electrode could be an excellent candidate as the electrode in electrochemical double-layer capacitors. In addition, the influence of the potential scan rate, aging, and the electrolyte solution on the specific capacitance of nanotube electrodes was also studied.     

The oxygen,oxygen/sulphur dioxide and oxygen/carbon dioxide electrodes in molten lithium sulphate—potassium sulphate eutectic

Open-ciruit potential, potentiostatic and voltammetric measurements of the O₂, O₂/SO₂ and O₂/CO₂ electrodes have been made in molten Li₂SO₄K₂SO₄ eutectic at 625°C. In the absence of SO₂ the overall reaction of the O₂ electrode is the reduction of O₂ to O^2? through the formation of O^2?₂ ions. In the presence of SO₂ the overall reaction is the reduction of SO₂ and O₂ to SO^2?₄, probably through the formation of SO^2?₃. The overall reaction of the O₂/CO₂ electrode is similar to that of the same electrode in molten carbonates, ie the reduction of CO₂ and O₂ to CO^2?₃. The transfer coefficients and exchange currents have been estimated for these reactions on platinum and gold.     

Use of the Nasicon/Na2SO4 couple in a solid state sensor for SO x (x=2,3)

The e.m.f. of a concentration cell for SO _x (x=2,3)-O₂ incorporating Nasicon as the main solid electrolyte has been measured in the temperature range 720 to 1080 K. The cell arrangement can be represented as, $$Pt, O'_2 + SO'_2 + SO'_3 \left| {Na_2 SO_4 \left\| {\left. {Nasicon} \right\|} \right.} \right.\left. {Na_2 SO_4 } \right|SO''_3 + SO''_2 + O''_2 , Pt$$ The Na₂SO₄ acts both as an auxiliary electrode, converting chemical potentials of SO _x and O₂ to equivalent sodium potentials, and as an electrolyte. The presence of Na₂SO₄ provides partial protection of Nasicon from chemical reaction with gas mixtures containing SO _x . The open circuit e.m.f. of the cell is in close agreement with values given by the Nernst equation. For certain fixed inlet gas compositions of SO₂+O₂, the e.m.f. varies non-linearly with temperature. The intrinsic response time of the cell to step changes in gas composition is estimated to vary from ～2.0 ksec at 723K to ～ 0.2 ksec at 1077K. The cell functions well for large differences in partial pressures of SO₃(p″_{SO 3}/p′_{SO 3}≈10⁴) at the electrodes.     

Multilayer oxide formation on platinum

The growth of multilayer oxide (oxide II) on smooth Pt was investigated as a function of cd and temperature in 0·5 and 1 M H₂SO₄. The pretreatment of the electrode is an essential feature in the forming of oxide 11: eg depending on the history of the electrode, oxide 11 was formed in 0·5 H₂SO₄ at cd of 100 mA/cm² at temperatures above 14 or 22°C. During the oxidation the potential—time curve shows a maximum, characteristic for the forming of oxide II.     

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.     

A study of the Ag(s)/Ag2 SO4+Na2 SO4(l)1102 reference electrode

The high temperature electrode Ag(s)/Ag₄SO₄+Na₂SO₂(l) contained in a mullite membrane is a suitable reference electrode for measuring sodium activities in Na₂SO₄ melts. Several Na concentration cells have been studied at 900° C to establish the thermodynamic basis for the Ag(s)/10 mol% Ag₂SO₄ + Na₂SO₄ electrode. Part of this work involved the determination of the ratio of activity coefficients of Ag₂SO₄ and Na₂SO₄ in melts that were dilute in Ag₂SO₄. When the non-ideal solution behaviour was taken into account, very good agreement was obtained between predicted and measured cell voltages for acidic Na₂SO₄ melts.