T. Vitanov

Electrocatalytic activity of spinel related cobalties MxCo3−xO4 (M = Li, Ni, Cu) in the oxygen evolution reaction

Abstract A comparative study of the electrochemical and physicochemical behaviour of binary cobalt oxides with spinel structure M x Co 3− x O 4 (M = Li, Ni, Cu) was performed in order to elucidate the effect of the cation distribution in the crystal lattice on the electrocatalytic activity of the oxides studied in the oxygen evolution reaction in alkaline media. Teflon-bonded electrodes were studied whose true electrochemical surface area was estimated from the double layer capacitance. It was found that the activity of the spinels increases relative to that of Co 3 O 4 in the order: Co 3 O 4 x Co 3− x O 4 ⪡ Cu x Co 3− x O 4 x Co 3− x O 4 . A relation was found between the activity and the dopant content. The enhanced activity is determined by two inter-related factors: the cation distribution in the two types of spinel site (8a) and (16d) and the number and energetics of the active sites formed on the surface in the anodic peak potential range preceding the oxygen evolution potential. Tafel slopes between 2.3 RT / F and 2.3(2 RT /3 F ) at Co 3 O 4 , Cu x Co 3− x O 4 and Ni x Co 3− x O 4 electrodes and 2.3( RT /2 F ) at Li x Co 3− x O 4 electrodes were observed; the slopes decrease with increasing dopant content. The reaction order with respect to OH − concentration was nearly two. The data are consistent with two mechanisms: (i) Bockriss electrochemical oxide path with the second step, MOH + OH − → MO + H 2 O + e − , as rate determining for Co 3 O 4 , Cu x Co 3− x O 4 and Ni x Co 3− x O 4 and with the third step, 2MO → 2M + O 2 , as rate determining for Li x Co 3− x O 4 ; (ii) Krasilshchikovs path with the third step, MO − → MO + e − , as rate determining for the first three spinels and with the fourth step, 2MO → 2M + O 2 , as rate determining for the Li doped oxide (M = Co 4+ ). The decrease of the Tafel slopes with increasing dopant content is assumed to be due to the formation of active sites with larger charge transfer coefficients.

Tungsten carbide-based electrochemical sensors for hydrogen determination in gas mixtures

An amperometric study of gas-diffusion electrodes (GDE) catalysed by two types of tungsten carbide, WC(1) and WC(2), which differ considerably in their specific surface area (0.5 and 6 m2 g−1), was carried out. The H2–air gas mixture (H2 1–4%) measurements show that for this range of hydrogen concentration the hydrogen limiting diffusion current (id(H2)) may be attained so that a curve of limiting current density against hydrogen concentration can be obtained. The response and stability of the electrode performance were compared to those of platinum catalysed GDEs. The most promising for use in amperometric hydrogen sensing is the WC(1) catalyst of small specific surface area. Electrodes catalysed with this catalyst show inferior response time in comparison to electrodes catalysed with the other two catalysts (WC(2) and Pt) but their overall stability is much better.

The effect of the method of preparation on the catalytic activity of tungsten carbide for hydrogen evolution

Abstract The catalytic activity of tungsten carbide for the evolution of hydrogen in acid medium has been investigated. Specific activities were compared by taking current density values at a given potential in the straight line portion of Tafel curves. The real surface area of the electrodes was determined from capacitance measurements. It was shown that, depending on the starting material used, the activity of the carbides varied by a factor of up to three times. A relation was observed between catalyst activity and the average values of the Tafel slope. Thus, the results obtained confirm the assumption previously made that the different catalytic activity of tungsten carbides prepared from various starting materials for the anodic reaction is due to the different adsorption behaviour of the surface.

Adsorption of substances acting as brighteners in the electrolytic deposition of copper

The adsorption behaviours of polyethers of the polypropyleneglycol (P) type, derivatives of safronic dyes (B); sulpho-organic aliphatic compounds containing the disulphide group -S-S- (DS) and combinations of the three additives (KD) were investigated. For this purpose the dependence of the differential capacitance of a polycrystalline copper electrode on the potential was measured in 1 N H2SO4 solutions containing various concentrations of the additives. It was found that the adsorption activity of the substances investigated increases in the order P<B<DS. This order is directly related to changes in the axes of predominant orientation, internal stresses, deposition overvoltage and the brightness of the coatings in the presence of the various additives.

Effect of preparation conditions on the structure and catalytic activity of carbon-supported platinum for the electrooxidation of methanol

Carbon-supported platinum catalysts (Pt/C) were prepared by treatment of PtO2/C under different conditions: (a) heating at 380°C in air, argon and hydrogen; (b) electrochemical reduction in H2SO4; and (c) reduction with NaBH4. The effect of the preparation conditions on the structure and the catalytic activity of the catalysts for the electrooxidation of CH3OH in acid media was studied. The highest activity was achieved for the catalyst treated in air. The activity is determined by the crystal faces exposed at the particle surface as well as particle size and the partial oxidation of the carbon support.

The effect of method of preparation on the catalytic activity of tungsten carbide

Abstract The catalytic activity of tungsten carbide for the oxidation of hydrogen in an acid medium has been investigated on sedimented powder electrodes. Specific activities were compared by taking current density values at a given potential in the straight line portion of Tafel curves. The real surface area of the electrodes was determined from capacitance measurements. It was shown that, depending on the starting material employed, the activity of the carbides varied by half an order of magnitude. This observation is discussed in terms of differences in surface properties. No effect on activity was observed when changing the carburization atmosphere from carbon monoxide to a mixture of carbon monoxide and hydrogen or argon.

The effect of method of preparation on the corrosion resistance and catalytic activity during corrosion of tungsten carbide I. Corrosion resistance of tungsten carbide in sulfuric acid

Abstract The corrosion resistance in 4.5N and 9.9N H 2 SO 4 of tungsten carbide synthesized under different conditions has been investigated. The amount of oxides in both electrolytes increases linearly with time, suggesting that during the corrosion process the catalyst surface is not passivated. The corrosion rate depends on the method of preparation of the tungsten carbide. Under identical preparation conditions, carbides synthesized from H 2 WO 4 displayed the lower corrosion resistance. The corrosion resistance of carbides synthesized from identical starting materials depends on the carburation temperature: carbides synthesized at elevated temperatures display higher corrosion resistance. The different corrosion rates are explained by the variations in the specific surfaces, the grain size, and the different extent of segregation of the tungsten carbide particles, respectively. It is shown that the corrosion rate in 4.5N H 2 SO 4 is twice that in 9.9N H 2 SO 4 . This observation is discussed in terms of differences in the rate of the anodic process and the lower solubility of the corrosion process depolarizer (oxygen) in highly concentrated solutions.

Tungsten carbide cathodes for electrolysis of sulphuric acid solutions

Abstract Cathodes for the electrolysis of sulphuric acid solutions were prepared from a catalyst (WC), binding agent (PTFE), and pore-forming additive (Na 2 SO 4 ). It is shown that the best characteristics are obtained with thin electrodes ( d = 0.25−0.3 mm). Current densities up to 200 mA cm −2 at −200 mV vs HE reference in the same solutions are attained. After 4000 h operation at 200 mA cm −2 the electrodes gave no evidence of increased polarization.

Adsorption of halide ions Cl−, Br− and I− on the basal face of a cadmium single crystal

A qualitative and quantitative study of the adsorption of halide ions on the electrolytically grown (0001) face of a cadmium single crystal is described. It is found that adsorption increases in the order Cl− < Br− < I−. Comparison with the results for other electrodes shows an increase in the adsorption in the sequence Cd(0001) < Cdpoly < Ag(100) < Ai(111). An attempt is made to explain this sequence in terms of the hydrophilicity of the electrode investigated.

Effect of pyrolysis temperature of the catalytic activity of active carbon + cobalt phthalocyanine in sulfur dioxide oxidation by oxygen

Structural changes during the pyrolysis of the catalyst ‘active carbon+cobalt phthalocyanine’ (Ac+CoPc) in the temperature range 500–800°C are investigated using infrared spectroscopy, XRD and DTG analysis. The decomposition products contain organic residues of the phthalocyanine rings and separate phases of CO3O4 and β-Co. The effect of pyrolysis temperature on: (i) the chemical stability of Co in the pyrolyzed CoPc in 0.5M H2 SO4; (ii) the electrocatalytic activity for oxidation of SO2 by oxygen; and (iii) the partial electrochemical reactions of SO2 oxidation and oxygen reduction, has been studied. The rate of electrochemical oxidation of SO2 is affected only by the active centres formed on pyrolysis of the chelate rings. The presence of Co at lower pyrolysis temperatures decreases the oxygen diffusion limitations. It is shown that the rate of SO2 oxidation by oxygen is limited by mass transport of oxygen.