H. Ménard

Impedance studies of porous lanthanum-phosphate-bonded nickel electrodes in concentrated sodium hydroxide solution

Abstract The hydrogen evolution reaction (HER) was investigated on a lanthanum-phosphate-bonded nickel (LPBN) powder electrode in 30 wt.% NaOH at 70°C using ac impedance and steady-state polarization techniques. Circuits containing one or two constant-phase elements (CPEs) in parallel with a resistance and corresponding to fractal and porous electrode models were tested in order to interpret the ac impedance data. The experimental impedance spectra were well described by the porous electrode model and the circuit containing two CPEs. The results obtained from the ac impedance and steady-state measurements allowed the mechanism and kinetics of the HER to be evaluated. Comparison of these parameters with those obtained on the polycrystalline nickel electrode in 1 M alkaline solution at 25°C indicates that an increase in activity is principally due to an increase in the real surface area.

Investigation of hydrogen evolution on Raney-Nickel composite-coated electrodes

Abstract The hydrogen discharge on the Raney nickel composite coated electrode is investigated in 30% wt NaOH aqueous solution at 70°C by ac impedance technique. The easiest pathway for the hydrogen evolution reaction (HER) is the Volmer-Heyrovsky mechanism. Both Volmer and Heyrovsky steps control the reaction. The increase in the electrocatalytic activity from 30% wt NaOH to 1.04 M NaOH at 70°C is related to a larger exchange current density of the HER.

Electrocatalytic performance of composite-coated electrodes for alkaline water electrolysis

Abstract Raney Ni, Raney Ni-Co and Raney Co composite-coated electrodes were investigated for hydrogen evolution cathodes and oxygen evolution anodes in alkaline media. The influence of the temperature, the OH − ion concentration and potential cycling on the kinetics of the hydrogen evolution reaction (HER) was investigated. The HER reaction mechanisms depends on the KOH concentration, since the latter strongly influences the Tafel slope while the apparent energy of activation is 8.4 kJ mole −1 . Potential cycling is detrimental to the HER and eventually causes cracks in the electrode material. The presence of Co in the electrode material is also detrimental to the HER. The best performances with respect to the oxygen evolution reaction (OER) were observed for Raney Ni composite electrodes whose i 0 is ≈ 100 times higher than for materials containing Co. The apparent energy of activation of the OER on Raney Ni composite electrodes is 12.2 kJ mole −1 and the Tafel slope is equal to 80 mV between 54 and 69°C in 32 ω/o KOH.

In situ micro-Raman spectroscopy to investigate pitting corrosion product of 1024 mild steel in phosphate and bicarbonate solutions containing chloride and sulfate ions

The nature of the corrosion products generated by localized attack on 1024 mild steel have been investigated in the presence of chloride and/or sulfate ions in bicarbonate and phosphate aqueous solutions. A spectroelectrochemical cell was used for in situ measurements of the Raman spectra of the corrosion products generated during pitting. These products were identified as the so-called green rust compounds. The assignment of the hydroxyl groups in green rust is confirmed by isotopic substitution. The composition of the green rust generated in bicarbonate or phosphate solution containing chloride and/or sulfate ions has been determined. A correlation between the green rust composition and the electrochemical behaviour after the initiation of pitting has been noted and discussed.

Copper Dissolution in NaHCO3 and NaHCO3 + NaCl Aqueous Solutions at pH 8

The anodic oxidation of copper in 0.001 to 0.1M NaHCO 3 and NaHCO 3 +NaCl aqueous solutions at pH 8 has been studied using a rotating disk electrode. The first oxidation product is a thin porous Cu(I) oxide film formed by a solid-state mechanism. In the solutions containing low NaHCO 3 concentrations ( 0.05M, more positive anodic potentials, or longer exposure times, further oxidation of copper take place through the thin porous Cu(I) oxide film

Kinetics of the hydrogen evolution reaction on a rhodium electrode

Abstract Kinetics of the hydrogen evolution reaction (her) were measured at a rhodium electrode in a 1 M solution of KOH at various temperatures between 5 and 75°C. The measurements were conducted at a Rh wire electrode activated before every measurement by cycling in the range of potentials between −1.0 and 0.3 V (vs. Hg/HgO/1 M KOH) until a stationary behaviour was obtained. Experiments were also carried out on the electrode kept at 25°C for three days at −1.0 V. In this case the current was by about one order of magnitude lower than that observed for the activated electrode. For both electrodes the her could be described by the Volmer-Heyrovsky mechanism. For activated electrodes the cathodic transfer coefficient α changed with temperature from 0.3 to 0.4; at deactivated electrode it was equal to 0.43. The activation resistance decreased with temperature by almost two orders of magnitude. The results were analysed according to two models of the impedance on solid electrodes. An approximation of both the Tafel plots and the logarithm of the reaction admittance vs. overpotential made the calculation of the rate constant of the slowest step possible, which was three orders of magnitudes greater than that for the nickel electrode. Total capacities of the double layer at poisoned and activated electrodes were almost the same and equal to 60–80 μF cm−2. We have concluded that traces of metallic impurities are responsible for poisoning of the electrode.

Electrochemical behaviour of tin in bicarbonate solution at pH 8

The anodic oxidation of tin in 0.1 to 1M bicarbonate solutions at pH 8 has been studied. The process may be divided into three potential regions: (1) a short active dissolution (Tafel) region; (ii) a dissolution-precipitation region; and (iii) a large region of electrode passivity. The rate-determining step of the reaction in the active-dissolution region is attributed to the diffusion of an ionic species into the solution, the diffusing species being generated at the metal surface. In the region of the first oxidation peak, the reaction rate is controlled by diffusion of CO32− species in solution. When the potential becomes more positive than −0,1 Vsce, a highly passivating (most likely SnO2) film is formed on the electrode surface.

Hydrogen discharge on a Raney nickel composite-coated electrode

Abstract Raney nickel coated electrodes were prepared for hydrogen evolution cathodes in alkaline media using a new device to ensure efficient, more uniform agitation of the electroplating solution. The influence of parameters such as particle size and aging, presence of organic additives in the solution, quantity of codeposited Raney Ni particles and the codeposition time on the Tafel parameters for the hydrogen evolution reaction in 1 M KOH at 25°C during the electrode preparation are considered. The coating has a small Tafel slope and a low hydrogen overvoltage in 30 wt% KOH at 85°C.

Influence of anions on the passivity behavior of copper in alkaline solutions

Abstract The competitive effect of the inorganic anions OH-, Cl-, Br-, I-, ClO-4, NO-3, SO2-4, CO2-3 and phosphates on the behavior of copper in alkaline aqueous solutions at pH 12 was investigated. The nature of changes occurring on the copper surface was analyzed by potentiodynamic and potentiostatic methods in conjunction with different instrumental surface analysis techniques. The influence of these anions on the anodic dissolution of copper was classified into three distinct categories. In the first group, comprising NaCl, NaBr and NaI solutions, copper(I) salts form insoluble, nonprotective films on the metal surface and the copper is expected to dissolve via the formation of CuA( n - 1 )- n complexes. In the second group, comprising NaClO4, Na2SO4 and NaNO3 solutions, substantial dissolution of copper is caused by the formation of easily soluble Cu2+ salts, and copper (II) ions precipitate as hydroxides and oxides. In the third group, the hydroxide, phosphate and carbonate passivate the copper surface. In some solutions the phosphate and carbonate inhibit the anodic dissolution of copper. The aggressive nature of the anions investigated against the passivity of copper increases in the order SO2-4

Hydrogen Evolution Reaction in Alkaline Solution Catalytic Influence of Pt Supported on Graphite vs. Pt Inclusions in Graphite

Graphite powders with supported Pt [5 and 20 weight percent (w/o) Pt] and Pt inclusions (13 and 17 w/o) in graphite have been bonded with an inorganic polymer LaPO{sub 4} and used as cathodes for the hydrogen evolution reaction (HER) in 1 M KOH solution. Pt-supported electrodes [Pt(s)/C/LaPO{sub 4}] are more active than the electrodes containing Pt inclusions in graphite [Pt(i)/C/LaPO{sub 4}]. However, in high-current-density conditions (0.25 A/cm{sup 2}), Pt(i)/C/LaPO{sub 4} is stable while Pt(s)/C/LaPO{sub 4} disintegrates after 24 h. The ac impedance diagrams display two semicircles on the complex-plane plot for both Pt(s)/C/LaPO{sub 4} and Pt(i)/C/LaPO{sub 4} electrodes but only one for pure graphite. The same equivalent circuit with two constant-phase elements has been used to explain the behavior of these electrodes. For both materials, the HER proceeds via the Volmer-Heyrovsky mechanism. The difference in the electroactivity of these two materials is related to the effective amount of Pt available.