Electroreduction of molecular oxygen on preferentially oriented platinum electrodes in acid solution

The oxygen electroreduction reaction was studied on two different preferentially oriented ((111)-type and (100)-type and on a conventional polycrystalline (PC) platinum rotating disc electrodes in acid solutions at 30 °C. At low overpotentials, Tafel lines of –0.060 V decade^–1 were obtained on the three electrodes in oxygen-saturated 1.0m H₂SO₄ and 1.0m H₂SO₄ + y m K₂SO₄ (0 y 1). At high over-potentials the usual Tafel slope of -0.120V decade^–1 was observed on both (111)-type and PC platinum electrodes in 1.0m H₂SO₄, whereas a slope of –0.165V decade^–1 was found on (100)-type platinum. In oxygen-saturated 1.0m H₂SO₄ the surface coverage by O-containing adsorbates on (100)-type platinum was greater than on both (111)-type and PC platinum. Rotating ring-disc electrode data showed that a higher amount of H₂O₂ was produced on (100)-type platinum than on the other platinum surfaces. The overpotential against current density plots are influenced by the anion concentration depending on the type of preferentially oriented platinum.     

Development of a novel redox flow battery for electricity storage system

A novel cylindrical battery which uses carbon fibres with high specific surface area as electrodes and a porous silica glass with high chemical stability as membrane has been fabricated. The results obtained from electrolysis of 0.5 M K₃Fe(CN)₆–0.5 M KCl and of 85 mM V(IV)–1 M H₂SO₄ indicate that the cell possesses excellent electrolytic efficiency. As a redox flow battery (RFB) its performance was investigated by employing all-vanadium sulfate electrolytes. The results of the cyclic voltammetry measurements indicate that at a glassy carbon electrode the electrochemical window for 2 M H₂SO₄ solution could reach 2.0 2.4 V. Constant current charging–discharging tests indicate that the batteries could deliver a specific energy of 24 Wh L^–1 at a current density of 55 mA cm^–2. The open-circuit cell voltage, after full charging, remained constant at about 1.51 V for over 72 h, while the coulombic efficiency was over 91%, showing that there was negligible self-discharge due to active ions diffusion through the membrane during this period.     

Amorphous nickel-valve metal-platinum group metal alloy electrodes for hydrogen-oxygen sulphuric acid fuel cells

Powder catalysts were prepared by immersion of amorphous Ni-40Zr and Ni-40Ti alloys containing a few at % of platinum group elements in HF solution. This treatment led to preferential dissolution of the valve metal and nickel with a consequent formation of microcrystalline alloy powders consisting of concentrated platinum group elements and some nickel and valve metal. Porous gas-diffusion electrodes prepared by using these alloy catalyst powders were employed for electrochemical reduction of oxygen and oxidation of hydrogen in 1 M H₂SO₄ at 25°C. The activity of the electrodes prepared from the amorphous alloys containing Pt–Ru, Pt–Rh, Pt and Pd for oxygen reduction was considerably higher than that of the platinum black electrode. Oxidation of hydrogen occurred readily close to the equilibrium potential. Amorphous alloy electrodes containing Pt–Ru, Pt–Rh and Pt were more active than the platinum black electrode for the hydrogen oxidation.     

Electrochemical behavior of pyrite in sulfuric acid solutions containing silver ions

A systematic electrochemical study of pyrite in H₂SO₄ solutions containing dissolved silver was undertaken to gain more information about the transfer of silver ions to pyrite and their role in enhancing the direct oxidation of pyrite. The results of cyclic voltammetry experiments provide additional evidence of the formation of metallic silver on the FeS₂ surface under open-circuit conditions. A pyrite electrode held at the open-circuit potential for 2 h in the presence of 10^–3 m Ag⁺ exhibits a large and sharp anodic peak at about 0.7V. The current associated with this peak is the result of the dissolution of metallic silver deposited during the initial conditioning period. There is no evidence of silver deposition without preconditioning until the potential drops below about 0.6V for Ag⁺ concentrations ranging from 10^–4 to 10^–2 m. However, subsequent silver deposition appears to be very sensitive to the dissolved silver concentration in this range. There is also evidence that the state of the pyrite surface has a pronounced influence on its interaction with silver ions. Agitation has also been found to have a significant effect on the electrochemistry of the Ag–FeS₂ system.     

Mechanism of sulfur poisoning on supported noble metal catalyst — The adsorption and transformation of sulfur on palladium catalysts with different supports

A series of supported palladium catalysts (Pd/Al₂O₃, Pd/MgO and Pd/TiO₂) were prepared by the impregnating method and treated with H₂S, H₂ +O₂ or O₂, among which H₂S is used as a poison and H₂ +O₂ or O₂ are as purging atmospheres. The S^2– species in the supports was introduced by means of mechanically mixing Na₂S with the supports or catalysts. X-ray photoelectron spectroscopy (XPS) was employed to determine the changes in the chemical states of oxygen, palladium and sulfur in the catalysts before and after the treatment, while infrared (IR) spectroscopy was used to measure the SO^2– ₄ group produced in the catalysts and supports. The results show that on MgO and TiO₂ carriers whose acidities are weak, there exist two kinds of oxygen species, one is the lattice oxygen, the other one is the active species of oxygen. The latter can oxidize the S^2– into SO^2– ₄ even at room temperature in air. Because of the weak acidities and smaller specific surface area of MgO and TiO₂, the S^2– is liable to adsorb on the catalysts and to transform into SO^2– ₄. But for the case of Al₂O₃ support its acidity is rather strong, and its surface oxygen species under the experimental conditions is not so active as that in MgO and TiO₂ carries. The poison H₂S on the Al₂O₃ support only experiences a process of physical adsorption-desorption. In Pd/Al₂O₃ catalyst, the negatively charged sulfur ions are not so easily adsorbed and transformed as those in Pd/MgO and Pd/TiO₂. It is also implied that the properties of the carriers are related to the ability of self-regeneration of the corresponding catalysts. Pd/Al₂O₃ catalyst is more able to self-regenerate than Pd/MgO and Pd/TiO₂ catalyst.     

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.     

Self-Assembly of Metalloporphyrin-L-Cysteine Modified Gold Electrode

A novel composite film containing metalloporphyrins was fabricated by in situ electrochemical scanning on an L-cysteine self-assembled monolayer modified gold electrode. SEM and ATR-FTIR were used to characterize the structure of the film. The electrochemical properties were investigated through techniques such as a.c. impedance, cyclic voltammetry and chronocoulometry. The porphyrin-L-cysteine film showed no peak in the first cycle, while each of the composite films derived from three different metalloporphyrin-L-cysteines presented a pair of reversible redox peaks in 1.0 mol L^–1 H₂SO₄. These peaks correspond to the rapid redox process of the metal. The supporting electrolyte and its pH value influenced the stability and sensitivity of the composite film. Cupric-porphyrin-L-cysteine film showed good catalytic activity for the reduction of H₂O₂. The catalytic current was linear to H₂O₂ concentration in the range 1.0 × 10^–6 to 3.0 × 10^–5 mol L^–1, with a correlation coefficient of 0.9995. The detection limit was 1.0 × 10^–7 mol L^–1 at a signal to noise ratio of 3. The relative standard deviation was calculated as 2.4% for solutions containing 1.0 × 10^–5 mol L^–1 H₂O₂(n= 11).     

Electrosynthesis of hydrogen peroxide in acidic solutions by mediated oxygen reduction in a three-phase (aqueous/organic/gaseous) system Part I: Emulsion structure, electrode kinetics and batch electrolysis

The mediated electrosynthesis of H₂O₂ in acidic solutions (pH 0.9–3.0) was investigated in a three-phase, aqueous/organic/gaseous system using 2-ethyl-9,10-anthraquinone (EtAQ) as mediator (redox catalyst). The main hydrogen peroxide producing route is the in situ mediating cycle: EtAQ electroreduction–homogeneous oxidation of anthrahydroquinone (EtAQH₂). The organic phase was composed of tributylphosphate solvent (TBP) with 0.2 M tetrabutylammonium perchlorate (TBAP) supporting electrolyte, 0.06 M tricaprylmethylammonium chloride (A336) surface active agent, and 0.1–0.2 M EtAQ mediator. Part I of this two part work deals with the physico-chemical characteristics of the emulsion electrolyte (e.g., ionic conductivity, emulsion type, H₂O₂ partition between the aqueous and organic phases), and kinetic aspects (both electrode and homogenous) of the mediation cycle. Furthermore, batch electrosynthesis experiments are presented employing reticulated vitreous carbon cathodes (specific surface area 1800 m² m^–3) operated at superficial current densities of 500–800 A m^–2. During 10 h batch electrolysis involving the emulsion mediated system with O₂ purge at 0.1 MPa pressure, H₂O₂ concentrations in the range 0.53–0.61 M were obtained in 0.1 M H₂SO₄ (pH 0.9) and 2 M Na₂SO₄(acidified to pH3). The corresponding apparent current efficiencies were from 46 to 68%. Part II of the present work describes investigations using flow-by fixed-bed electrochemical cells with co-current upward three-phase flow.     

Oxygen reduction on poly(4-vinylpyridine)-modified ordinary pyrolytic graphite electrodes with adsorbed cobalt tetra-sulphonated phthalocyanine in acid solutions

Poly(4-vinylpyridine) (PVP) has been used to modify ordinary pyrolytic graphite (OPG) electrodes with adsorbed cobalt tetra-sulphonated phthalocyanine (CoTsPc) in acid solutions. These modified electrodes were prepared in different manners and characterized by cyclic voltammetry. Their electrocatalytic activity for oxygen reduction and stability in 0.05M H₂SO₄ solutions was examined at room temperature. The OPG/CoTsPc/PVP modified electrodes were found to be more active for oxygen reduction in 0.05M H₂SO₄ solutions as compared to the electrode with adsorbed CoTsPc on OPG without PVP. The increase in activity is due to the formation of an adduct between PVP and CoTsPc. U.v.-visible and FTIR studies provide evidence for such adduct formation. Over a 10 h period the activity of the OPG/CoTsPc/PVP system was essentially constant while that of OPG/CoTsPc without polymer decreased by a substantial amount (about 37%). The PVP layer inhibits the diffusion of the CoTsPc and/or Co out of the complex into the solution phase. The stability of the OPG/CoTsPc/PVP system may also be due to low solubility of the adduct between PVP and CoTsPc in a 0.05M H₂SO₄ solution. Thicker films of PVP decreased the diffusion limiting oxygen reduction current. The effect of pH of the electrolyte solution on the activity of such PVP-modified electrodes for oxygen reduction has also been investigated.     

Chemical evolution of CO2 by electrocatalytic oxidation of vanadium-impregnated coke anode material

Baked carbon containing impregnated vanadium may be electrochemically oxidized to CO₂ in 1 M H₂SO₄ at 80–90% current efficiency during prolonged electrolysis (>20 h) at 70°C under an applied potential of 1.0 V versus saturated calomel electrode (SCE). The vanadium is electrocatalytically maintained in the highest oxidation state with an activation energy of 44–80 kJ mol^–1 at temperatures up to 80°C.     

The pH-dependence of thermally-coated Ti/TiO2 electrodes in the absence and presence of nitrobenzene

Cyclic voltammetric measurements on thermally coated Ti/TiO₂ electrodes in 0.05–3.0 m H₂SO₄, Na₂SO₄ and NaOH media were carried out to evaluate the cause of pH sensitivity of these electrodes. The electrode activity decreased in the order H₂SO₄ > NaOH > Na₂SO₄. Ionic strength had a significant influence on the electrode activity. In the redox catalysis of nitrobenzene reduction the pH dependence increased significantly. The electrode activity again decreased in the same order. In acid media the TiO₂ surface is protonated. In alkaline media OH- ions are specifically adsorbed on the TiO₂ surface. The concentration of these charged species, as well as the ease of counter ion transport through the charged layers during surface redox reactions in the oxide layer, is identified as the cause of the pH sensitivity of the electrodes. In acid media proton transport in the film occurs through a surface Grothus-like mechanism. In alkaline medium proton abstraction by OH^– ions from the trapped H₂O molecules may facilitate proton transport.     

Electrooxidation of stainless steel AISI 304 in carbonate aqueous solution at pH 8

The electrochemical behaviour of stainless steel AISI 304 (SS304) has been investigated in deaerated 0.1–1 m NaHCO₃ solutions at pH 8 using a rotating disc electrode. The polarization curves are characterized by a broad range of passivity at low potentials (–0.8 to 0.3 V), a depassivation region at 0.4 V vs SCE and, at high potentials (0.5 to 0.85 V), a passive region before oxygen evolution. In the low potential range, the SS304 electrode behaves like a Cr-rich metallic phase, and the dissolution of Fe²⁺ ions into the solution is hindered by the formation of a Cr₂O₃ layer. As the potential reaches 0.4V, the oxidation-dissolution of Cr(iii) oxide/hydroxide to CrO₄ ² ions occurs, with the participation of bicarbonate/carbonate as a catalyst in the dissolution reaction. Since the chromium oxide/hydroxide dissolution and subsequent surface enrichment of iron oxides occur, the applied potential, exposure time and oxidation charge have a considerable effect on the passive film properties. At high potentials, the presence of a passive film of iron oxides/hydroxides or oxyhydroxides plays a key role in the SS304 passivity with the presence of Fe(vi) species incorporated or adsorbed into the passive films. Colouration of the SS304 surface is observed in the second passive region. A film of a uniform gold colour formed on SS304, mild steel 1024 and iron in carbonate and borate solutions at pH 8. The colour of the electrode surfaces remain unchanged in air and in solutions at positive potential but it disappears at open-circuit potential or is easily reduced in the first negative-going potential scan.     

Oxygen reduction on stainless steel

Oxygen reduction was studied on AISI 304 stainless steel in 0.51 m NaCl solution at pH values ranging from 4 to 10. A rotating disc electrode was employed. It was found that oxygen reduction is under mixed activation-diffusion control. The reaction order with respect to oxygen was found to be one. The values of the Tafel slope depend on the potential scan direction and pH of the solution, and range from – 115 to – 180 mV dec^–1. The apparent number of electrons exchanged was calculated to be four, indicating the absence of H₂O₂ formation.Nomenclature B =0.62 nFcD ^2/3 –^1/6 - c bulk concentration of dissolved oxygen (mol dm^–3) - D molecular diffusion coefficient of oxygen (cm² s^–1) - E electrode potential (V) - EH standard electrode potential (V) - E _H ⁰ Faraday constant (96 500 As mol^–1) - I current (A) - j current density (A cm^–2) - j _k kinetic current density (A cm^–2) - j _L limiting current density (A cm^–2) - m reaction order with respect to dissolved oxygen molecule - M molar mass (g mol^–1) - n number of transferred electrons per molecule oxygen - density (g cm^–3) - kinematic viscosity (cm² s^–1) - angular velocity (s^–1)     

The inhibition of corrosion and hydrogen embrittlement of AISI 410 stainless steel

A study was carried out on the inhibition of corrosion and hydrogen embrittlement of AISI 410 stainless steel by two organic inhibitors, namely benzotriazole and benzonitrile. Tensile testing, scanning electron microscopy, weight loss measurements and potentiodynamic polarization were the techniques used for this study. Tensile tests showed that 410 steel is highly susceptible to hydrogen stress cracking. Scanning electron microscopic observations of fracture surfaces showed a brittle quasi-cleavage type of failure when the steel was hydrogen charged from 0.5m H₂SO₄. Both inhibitors reduced hydrogen induced ductility loss though the fracture mode was unaltered. They showed increasing inhibition efficiencies for corrosion as well as cathodic hydrogen evolution as their concentration in H₂SO₄ increased from 3.9×10^–5 m to 8.4×10^–3 m. Benzonitrile was found to be a more efficient inhibitor than benzotriazole for AISI 410 stainless steel exposed to 0.5m H₂SO₄.     

Characterization of hydrogen evolution on cobalt electrodeposits in water electrolysis

The hydrogen evolution reaction (HER) on cobalt electrodes has been investigated in both alkaline (30 wt% KOH) and acid (1m H₂SO₄) media at 25°C. The electrocatalytic cobalt materials were produced under different electrodeposition conditions, namely deposition in the absence or presence of bubbling oxygen or nitrogen gas with two gas flow rates (80 and 230 ml min^–1) and at different current densities (50–800 A m^–2) and deposition in a cobalt powder-containing bath. It has been shown that the electrocatalytic behaviour of the cobalt deposits can be significantly affected by deposition current density via a change of surface area of the cobalt deposits. A considerable HER overpotential decrease (up to 150mV) has been achieved on the highly porous and active cobalt electrodes deposited in the presence of bubbling oxygen and chloride ions in deposition solution. However, the HER overpotential was increased on the cobalt electrodes deposited with bubbling nitrogen in the bath.     

The influence of catalyst-supporting methods on electrochemical activity and the resultant stability of air electrodes activated with iron phthalocyanine

To improve the performance of air electrodes, the dependence of iron phthalocyanine (FePc) catalytic effects on preparation methods was examined. The methods used were mixture (Electrode 1), impregnation (Electrode 2) and direct synthesis (Electrode 3). Electrodes 2 and 3 showed higher potentials during cathodic polarization up to 10 mA cm^–2 than Electrode 1. The rate of chemical destruction of H₂O₂ decreased in the order Electrode 3 > Electrode 2 > Electrode 1. Electrode 3 showed the smallest potential drop for a discharge at 10 mA cm^–2, 0.09 V after 50 h. However, the potential of Electrode 2 decreased with discharge, becoming 0.09 V lower than that of Electrode 3 after a 50 h discharge at 10mA cm^–2. Once the potential drop occurred, the potential was not recovered by resting or by drying the electrode. The potential drop may be caused by deactivation of FePc. One possible reason for such deactivation is the presence of H₂SO₄, which remained on the electrode after impregnation of the FePc-H₂SO₄ solution.     

High-performance carbon electrodes for acid methanol—air fuel cells

High-performance, Teflon-bonded carbon electrodes, catalysed with highly dispersed platinum metal, have been developed for oxygen reduction in H₂SO₄. Surface-treated Vulcan XC-72 carbon has been used as the substrate material. The electrodes can be loaded with current densities of 1.1 A cm^–2 intermittently and 900 mA cm^–2 for extended periods without serious degradation. The performance of these electrodes in the presence of methanol has also been examined.     

Transition metal catalysts for porous carbon air-electrodes in neutral chloride electrolytes

Porous carbon air-electrodes activated by CoPC, FePHP, PdPC, PtPC and metal-free PC were studied in 2 mol dm^–3 NaCl electrolyte. The activity of heat-treated carbon activated by CoPC was compared to carbon activated byin situ formed Co oxide. All the results show that the main catalytic activity of the catalyst used comes from the central metal atom used, while the chelate or oxide structures serve predominantly to keep the metal in the stable form at the carbon surface. Metal-free phthalocyanine does not show any catalytic activity for oxygen reduction.     

Oxygen reduction on Ru-oxide pyrochlores bonded to a proton-exchange membrane

Oxygen reduction at a gas-fed, porous, ruthenium-pyrochlore electrode attached to a Dow Developmental Fuel Cell Membrane was measured in solutions of various pH. Electrode assemblies containing high surface area Pb₂Ru_2–x Pb _x O_7–y or Bi₂Ru_2–x Bi _x O_7–y with different amounts of Teflon content with/without the incorporation of Dow gel in the active part of the electrode with/without a CO₂-treated Vulcan XC-72 carbon substrate were tested. The oxide pyrochlores were found to be chemically stable and to show their lowest overpotential if separated from a 2.5 M H₂SO₄ proton reservoir by the membrane. Interesting oxygen reduction activity at room temperature was obtained with the Pb₂Ru_1.74Pb_0.26O_7–y electrode bonded with 22% by weight Teflon and incorporating 5% by weight Dow gel. The performance of the oxides against B-site Pb concentration and a measurement of the surface charge on the particles indicate that, in this configuration, the active sites for the oxygen reduction reaction are OH^– species at the O-site positions of the A₂B₂O₆O_1–y pyrochlores, especially the bridging oxygen with one Ru and one Pb near neighbour, i.e. Pb–O_b–Ru. Evidence that oxide particles precipitated on CO₂-treated carbon transfer electrons to the substrate is also presented.     

Cathode catalysts for fuel cell development: A theoretical study based on band structure calculations for tungsten nitride and cobalt tungsten nitrides

Band structure calculations were performed for tungsten nitride, cobalt tungsten nitrides, and platinum slabs. The major requirements for the development of a superior cathode catalyst are: (1) that the Fermi level of the cathode catalyst is close to the energy level of the lowest unoccupied molecular orbital of O₂, the lowest unoccupied atomic orbital of an oxygen atom, and the lowest unoccupied atomic orbital of a hydrogen atom so that they can readily interact with one another; and (2) that the cathode catalysts have smaller ΔE value which represent the difference between the Fermi level and the peak position of the density of states of the O_p orbital of O₂ adsorbed on the catalyst. The active site structures of cobalt tungsten nitrides for activation of the oxygen reduction reaction were found to have the surface structure of Co–O–Co, which lowered the unoccupied orbital of the oxygen atom to approximately that of the Fermi level. However, this structure concomitantly lowered the Fermi level, which resulted in an increase in ΔE. Consequently, the optimal cathode catalyst regarding the surface conformation contains a Co–O–Co structure that is dispersed on the surface of the cobalt tungsten nitride. The cobalt tungsten oxynitride exhibited a catalytic activity for the oxygen reduction reaction. A linear dependence is observed between the ΔE and the oxygen reduction reaction offset potentials of the tungsten nitride, cobalt tungsten nitride, cobalt tungsten oxynitride, and platinum.