Hydrogen evolution reaction on Ni-Al electrodes

The hydrogen evolution reaction (h.e.r) was studied in alkaline solutions on two types of electrodes: (1) obtained by alloying Raney nickel without or with nickel and (ii) by pressing Raney nickel and nickel powders at room temperature. The obtained electrodes are usually very active for the h.e.r. The most active electrode was obtained by pressing Raney nickel with nickel powder (50 wt %). It was characterized by a large roughness factor, R 10 000 and a very low overpotential at the current density of 250 mA cm^–2, ₂₅₀ = 56 mV. The mechanism of the h.e.r. was studied using a.c. impedance measurements. The high electrode activity is connected with the increase in the intrinsic activity of the porous electrode surface.     

Electrochemical impedance spectroscopy of oxygen and hydrogen evolution on amorphous alloys in 1 M KOH

The mechanisms of oxygen and hydrogen evolution on amorphous alloys G 14 (Fe₆₀Co₂₀Si₁₀B₁₀) and G 16 (Co₅₀Ni₂₅Si₁₅B₁₀) in 1 M KOH at T = 298 K and 333 K were studied by electrochemical impedance spectroscopy (EIS). Comparative measurements were carried out on polycrystalline Pt electrodes. Impedance spectra in the frequency range 10^?3 Hz ≤ f ≤ 10⁴ Hz were analyzed to determine the kinetic behaviour of amorphous alloys by application of transfer function analysis, using non-linear fit routines. The EIS-data are interpreted in terms of consecutive reaction mechanisms for both oxygen and hydrogen evolution.     

Current efficiencies and regeneration of poisoned raney nickel in the electrohydrogenation of glucose to sorbitol

Poisoning of the Raney nickel cathode during the paired syntheses of sorbitol and gluconic acid has been studied. Sorbitol poisons the Raney nickel powder cathode and sorbitol current efficiencies decrease from 100% to <50%. Current losses of the poisoned Raney nickel were due to hydrogen evolution. Electrocatalytic activity was restored by washing the Raney nickel in a 17 wt% NaOH solution at 60°. This procedure, when performedin situ for the undivided packed bed flow reactor, increased sorbitol current efficiencies from 35–45% to 70–100%. The effects of glucose concentration and applied current on sorbitol and gluconic acid current efficiencies havealso been examined. High sorbitol current efficiencies were obtained with fresh Raney nickel when glucose concentrations were high and the current low. The high gluconic acid current efficiencies were independent of current and glucose concentration. To maintain high sorbitol current efficiencies as glucose was consumed, a systematic procedure for lowering the applied current was developed in which the current was decreased from 500 mA per 10 g of Raney nickel at 1.6M glucose to <100 mA per 10 g at 0.2M glucose.     

Selective hydrogenation of benzalacetone

Benzalacetone was hydrogenated using a Raney nickel catalyst, and various amounts of HCl, NaCl and H₃PO₄ in ethyl alcohol were added to test their inhibiting effect on catalytic activity. In the presence of HCl and H₃PO₄, Raney nickel lost its catalytic activity, and the rate of hydrogenation was decreased, but NaCl had no effect. While the hydrogenation of the ethylenic bond of benzalacetone was retarded, the decrease in hydrogen uptake by the carbonyl group was affected much more, and the amount of alcohol produced was markedly reduced.     

Oxidation of methanol on nickel—zinc catalysts

The activity of nickel—zinc and smooth nickel electrodes for methanol oxidation is investigated in KOH 1 M + CH₃OH 1 M at 60°C. Nickel—zinc alloys of different composition are easily obtained from electrodeposition. After their attack in KOH 1 M, the electrodes contain about 50% of zinc and they have the βNiZn alloy structure. The I(V) curves are compared. On smooth nickel the oxidation of methanol is strongly inhibited by the superficial oxides whereas on nickel—zinc electrodes superficial oxides are easier to reduce. The effect of adsorbed hydrogen and of mixed oxides is discussed.     

Leaching of Raney nickel composite-coated electrodes

The changes in the morphology and the composition of Raney Ni composite-coated electrodes was investigated during leaching in concentrated NaOH aqueous solutions. The rate of hydrogen generated during leaching is increased by stirring the solution, raising the temperature and increasing the NaOH concentration to 6.09 M. The depletion of Al from the Raney Ni particles results in phase transformations and the metal/solution interfacial area increases with time. It is suggested that the electrodeposited nickel has a dominant influence on the hydrogen discharge since the electrocatalytic activity is practically independent of the leaching time.     

Thermodynamic parameters evolution versus plastic strain during HER on nickel in sulphuric acid

The dependence of the hydrogen evolution reaction (HER) on plastic strain has been studied on polycrystalline pre-strained nickel electrodes in H₂SO₄ 1 M using linear voltametry. A thermodynamic approach is proposed to determine the influence of temperature and plastic strain on HER characteristic parameters as symmetry coefficients, activation enthalpies, number of active sites, on the different steps of the HER mechanism. A direct correlation is established between crystallographic defects associated with dislocations emerging at the surface of nickel electrodes, in terms of density and distribution of defects, and the efficiency of electrochemical processes occurring during HER: hydrogen adsorption and desorption.     

Kinetics of hydrogen evolution on submicron size Co, Ni, Pd and Co–Ni alloy powder electrodes by d.c. polarization and a.c. impedance studies

Submicron size Co, Ni and Co–Ni alloy powders have been synthesized by the polyol method using the corresponding metal malonates and Pd powder by reduction of PdO _x in methanol. The kinetics of the hydrogen evolution reaction (HER) in 6 M KOH electrolyte have been studied on electrodes made from the pressed powders. The d.c. polarization measurements have resulted in a value close to 120 mV decade^–1 for the Tafel slope, suggesting that the HER follows the Volmer–Heyrovsky mechanism. The values of exchange current density (i _o) are in the range 1–10 mA cm^–2 for electrodes fabricated in the study. The a.c. impedance spectra measured at several potentials in the HER region showed a single semicircle in the Nyquist plots. Exchange current density (i _o) and energy transfer coefficient () have been calculated by employing a nonlinear least square-fitting program.     

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.     

Raney镍上松香加氢制备二氢和四氢枞酸的本征动力学

采用FYX-2G型高压搅拌釜,以Raney镍为催化剂,在温度423～453K、压力4.0～7.0MPa下进行松香催化加氢反应动力学的研究。利用改进搅拌器类型、提高搅拌速度、加入200#溶剂油和改变催化剂粒径的方法消除内外扩散,使反应处于化学动力学控制区,然后采集动力学数据,经对17种可能的反应机理模型进行筛选,结果表明,Raney镍催化枞酸加氢生成二氢和四氢枞酸是并行反应,最可几反应机理为:松香中的主要成分枞酸分子与催化剂表面上被吸附的氢原子进行反应,氢原子的吸附为控制步骤,生成二氢枞酸和四氢枞酸的活化能分别为47.18kJ.mol-1和106.35kJ.mol-1。     

A study of RuO2 as an electrocatalyst for hydrogen evolution in alkaline solution

The hydrogen evolution reaction (HER) on RuO₂ coated nickel electrodes was investigated by d.c. polarization studies and a.c. impedance measurements in alkaline solution. The electroactive RuO₂ coatings were prepared by thermal decomposition of a RuCl₃ solution at 450°C in air. The real surface area of the cathodes was estimated from cyclic voltammetric measurements and from the double layer capacitance (evaluated by impedance measurements). A mechanism for the HER has been proposed involving two steps (Volmer and Heyrovsky reactions). The rate constants of the forward and backward reactions of these two steps have been estimated by a nonlinear fitting procedure.     

Gas Recombination in Sealed Ni–MHx Cells

This paper focuses on investigation of gas recombination in a positive-limited-sealed Ni–MH_x cell. The positive electrodes were prepared by electrochemical impregnation of fibrous nickel plaques. The metal hydride negative electrodes were made by pasting the mixture of rare-earth hydrogen storage alloy powders, conducting and binding agents on foamed nickel substrates. The measurement of the positive capacity at different charge times was used to estimate the partial current for oxygen evolution at the same time. The effects of charge rate, electrolyte saturation level and initial state of charge of the positive electrodes on the recombination were investigated in sealed Ni–MH_x cells. By determining the differential capacity of nickel hydroxide electrodes, an improved mathematical model was used to evaluate the gas recombination parameters during charge, overcharge, rest and discharge of the positive-limited-sealed Ni–MH_x cell. The gas recombination during rest, discharge and overdischarge was also examined. The oxygen recombination on the nickel hydroxide electrodes can be neglected due to the consumption of water when the nickel hydroxide electrodes were discharged. The longer overdischarge produced an increase in cell pressure for the sealed Ni–MH_x cell at an electrolyte unsaturated level and the evolving gas can be recombined by a following recharge operation. © 1997 SCI.     

Electrocatalytic properties of doped nickel boride based electrodes for the hydrogen evolution reaction

Amorphous nickel boride electrodes doped by small amounts of Rh, Ru, Co, Cr, Zn or Pt were studied for the hydrogen evolution reaction (HER) in 1 M NaOH solution at 70 °C. Steady-state galvanostatic measurements, a.c. impedance spectroscopy, X-ray diffraction and scanning electron microscopy were used. The properties of the pressed electrodes and the kinetic parameters of the HER were determined. The constant phase element model adequately describes the a.c. behaviour of these electrodes. It was found that the HER proceeds through the Volmer-Heyrovsky mechanism. An increase in catalytic properties was observed for materials doped with Rh, Ru and Co.     

Hydrogenation of heptaldehyde to heptyl alcohol with nickel catalysts

Hydrogenation of heptaldehyde to heptyl alcohol was studied with W2 Raney nickel catalyst, prepared in the laboratory, commercial Raney nickel catalyst and Rufert nickel catalyst by varying temperature, catalyst concentration, hydrogen pressure and reaction time. The products were analyzed by gas-liquid chromatography on SE-30 column. The optimum conditions found for quantitative conversion (99.6%) of heptaldehyde to heptyl alcohol were: temperature, 100°C, W2 Raney nickel catalyst concentration, 2% based on heptaldehyde (w/w), hydrogen pressure, 145 psig and reaction time, 1 h. IICT Communication No. 3085.     

Electrocatalytic activity of nickel black electrodes for the hydrogen evolution reaction in alkaline solutions

The electrocatalytic activity of nickel black electrodes for the hydrogen evolution reaction (HER) in alkaline solutions was studied. Three types of electrode were obtained through electrodeposition from different plating baths and their electrocatalytic properties were evaluated through slow potentiodynamic sweeps. The apparent and real exchange current densities and the Tafel slopes were calculated and compared to those of Raney nickel electrodes. The importance of the superficial topography on the apparent electrocatalytic characteristics was emphasized and some evidence was given to demonstrate that, when the electrode is previously aged in alkaline solution, the HER is produced on a thin nickel hydroxide overlayer.     

Composite electrodes for electrochemical supercapacitors

Manganese dioxide and Ag-doped manganese dioxide powders were prepared by a chemical precipitation method using KBH₄ as a reducing agent. The powders were studied by X-ray analysis, thermogravimetry, and electron microscopy. Composite electrodes for electrochemical supercapacitors (ES) were fabricated by impregnation of slurries of the precipitated powders and carbon black into porous nickel foam current collectors. In the composite electrodes, carbon black nanoparticles formed a secondary conductivity network within the nickel foam cells. Obtained composite electrodes, containing manganese dioxide and 20 wt% carbon black with total mass loading of 50 mg cm⁻², showed a capacitive behavior in the 0.5 M Na₂SO₄ solutions. The capacitive behavior of the composite electrodes can be improved by mixing of manganese dioxide and carbon black in solutions or using Ag-doped manganese dioxide powders. The highest specific capacitance (SC) of 150 F g⁻¹ was obtained at a scan rate of 2 mV s⁻¹. The electrodes showed good cycling behavior with no loss in SC during 1,000 cycles.     

Die wasserstoffspeicherkapazität des Raney-Nickels

The electrochemically chargeable and dischargeable hydrogen capacity of the Raney-nickel catalyst was investigated at constant state of oxidation and constant aluminium concentration as a function of pretreatment. The present extensive BET measurements indicate that the amount of hydrogen sorbate is exactly proportional to the BET surface within the limit of measuring and calculating accuracy; quantitatively the relation is 0,00072 Ah/m², corresponding to 1 H-atom per 1 Ni-atom of the inner surface. Thus the reversible part of the hydrogen sorption capacity of Raney nickel may be explained by sorption of hydrogen at the nickel BET surface, and subsequent anodic oxidation of this hydrogen coating layer under discharge.

Résumé

La capacité relative à l'hydrogène dissous dans le nickel Raney est déterminée en fonction du traitement antérieur du catalyseur, à concentrations constantes d'oxygène et d'aluminium. Les mesures des surfaces par la méthode BET montrent que la concentration de l'hydrogène est proportionnelle à la surface du catalyseur. Le nickel Raney contient approximativement 0,00072 Ah/m², c'est à-dire 1 atome H par atome Ni en surface. Pour cette raison la capacité electrochimique de l'hydrogène du nickel Raney peut s'interprêter par l'adsorption de l'hydrogène sur la surface du catalyseur pendant la charge et par l'oxydation de cet hydrogène pendant la décharge.     

Effect of mesh size on the performance of Teflon bonded oxygen-evolving electrodes

Teflon bonded and porous NiCo₂O₄ electrodes were prepared on nickel current collectors of different mesh sizes. It was shown that the oxygen evolution performance of the Teflon bonded electrodes is affected more significantly by the size of the nickel mesh than that of the porous electrodes. A simple theoretical calculation of the ohmic losses showed that for large mesh openings (20 mesh), the Teflon bonded catalyst in the centre of the opening cannot be efficiently utilized because of high ohmic resistances. Provided smaller hole size meshes (eg. 100 mesh) are used, Teflon bonded electrodes perform significantly better than porous electrodes.     

Activation of porous Ni cathodes towards hydrogen evolution by electrodeposition of Ir nuclei

Porous Ni electrodes were modified by electrodeposition of Ir nuclei from H₂IrCl₆ solutions at 70 °C, with the aim of activating them towards the hydrogen evolution reaction and comparing their performance with those of porous Ni electrodes activated by spontaneous deposition of noble metals (Ir and Ru). The current efficiency of Ir deposition was found to be very low (1% or lower, decreasing upon increasing deposition current density). Ir deposits characterised by SEM-EDX and XRD consisted of nanocrystals decorating the Ni dendrites forming the porous layers. Ir electrodeposition led to a strong activation of the hydrogen evolution reaction from aqueous 1 M NaOH. The electrocatalytic activity of the cathodes was independent of the Ir deposition charge above the minimum explored value of 1  C cm^?2. This charge is estimated to correspond to the deposition of ca. 2.5 10^?8 Ir moles cm^?2. The kinetic parameters for hydrogen evolution were similar for porous Ni electrodes modified by either spontaneous deposition (studied in a previous work) or electrodeposition of Ir.     

A simple and novel method for preparing Ni(OH)2 Part I: Structural studies and voltammetric response

Ni/Ni(OH)₂ electrodes were prepared by in situ precipitation of nickel hydroxide into porous nickel substrates. The thermal decomposition of urea in solutions containing Ni²⁺ was used for the synthesis of the active material. The compound prepared by this method was a poorly crystallized (II)-type phase. The electrochemical behaviour of Ni/Ni(OH)₂ electrodes thus prepared was studied by cyclic voltamperommetry and the efficiency of the in situ precipitation was followed by the measurement of the cathodic charge obtained from the j-E response. The amount of incorporated material depends on the urea concentration and the electrochemical response of the electrodes was improved by increasing the number of impregnation cycles.