Gerhard Kreysa

Electrocatalysis by amorphous metals of hydrogen and oxygen evolution in alkaline solution

Abstract Six commercially available and six especially designed amorphous metals have been investigated by steady-state polarization measurements and cyclic voltammetry to evaluate their suitability as electrocatalysts for hydrogen and oxygen evolution in alkaline solution. For hydrogen evolution Fe 60 Co 20 Si 10 B 10 provides lower overvoltages than platinum or nickel. The same alloy is also an effective electrocatalyst for oxygen evolution. Also, CO 50 Ni 25 Si 15 B 10 has been identified as a favourable oxygen electrocatalyst. Applying these materials in alkaline water electrolysis offers an energy saving of about 6% compared with Pt/Ni and of about 10% with respect to Ni/Ni.

Kinetic and thermodynamic analysis of hydrogen evolution at nickel electrodes

Abstract iR-corrected steady-state polarization curves for hydrogen evolution in alkaline solution at crystalline and amorphous nickel electrodes were measured at different temperatures, showing at higher c.d. the establishment of a limiting c.d. due to increasing Had coverage. The experimental behaviour is in perfect agreement with a complete kinetic model based on the Volmer—Tafel mechanisms. Symmetry factors and rate constants for forward and backward direction of both reactions were determined by a non-linear fitting procedure. Activation energies, enthalpic and entropic components of the symmetry factor, and thermodynamic quantities as equilibrium constant of the Tafel reaction, equilibrium potential of the Volmer reaction, enthalpy and entropy of adsorbed hydrogen atoms could be derived from these data providing new quantitative knowledge of elementary steps of hydrogen evolution at crystalline and amorphous nickel electrodes.

Modelling of gas evolving electrolysis cells. I. The gas voidage problem

A critical review of experimental gas voidage data for gas—liquid mixtures available in the literature yields the result that these data cannot be explained by known theories of the gas hold-up. Based on the empirical experience that bubble coalescence is hindered in electrolyte solutions, new equations are derived for the calculation of the gas voidage as a function of the superficial gas velocity by introducing a coalescence barrier model. Experimental investigations confirm the theoretical prediction of the existence of a limiting gas voidage which is a characteristic quantity of each gas—electrolyte combination.

A critical analysis of electrochemical nuclear fusion experiments

Abstract We have tried to confirm the results of the recent paper by Fleischmann and Pons in this Journal. Although we have, in principle, observed all the phenomena which they reported, additional check experiments have enabled us to explain all our results without assuming any nuclear fusion. Furthermore, we discuss some possible sources of error which should be eliminated carefully in future experiments.

The behaviour of NiCoP base amorphous alloys for water electrolysis in strongly alkaline solutions prepared through electroless deposition

Abstract The electrochemical behaviour of different binary NiP and CoP as well as ternary NiCoP amorphous metal alloys prepared by electroless deposition has been studied to test their suitability as electrocatalyst materials for water electrolysis. Exchange current densities and Tafel slopes for the hydrogen and oxygen evolution reactions were determined from quasi-steady state polarization curves obtained in 1 M KOH and 6 M KOH solutions at temperatures ranging from 30° to 90°C. In addition, the influence of thermal, chemical and electrochemical treatments on the polarization behaviour was studied and X-ray analysis and SEM were applied for the characterization of the sample material. The electrochemical behaviour of the electroless prepared amorphous metal alloys was found to be advantageous and very similar to that already reported for the conventionally prepared amorphous materials.

Kinetic behaviour of packed and fluidised bed electrodes

For the reaction of electrochemical quinone reduction at packed and fluidised bed electrodes an experimental parameter analysis of macrokinetic bed cd was carried out showing the influence of electrode potential, flow velocity, bed depth parallel to current flow direction, and electrolyte conductivity. A macrokinetic model of three-dimensional electrodes is established by introducing overpotential distribution within the electrode into the microkinetic rate equation. Theoretical calculations of macrokinetic bed cds yield reliable values as is shown by comparison with the experimental results. Both for packed and for fluidised bed electrodes analytical expressions are derived for penetration depth of diffusion limiting cd into bed electrodes.

Fundamental studies on a new concept of flue gas desulphurization

A new process for removal of sulphur dioxide from waste gases is proposed consisting of both electrochemical and catalytic sulphur dioxide oxidation. In the catalytic step a part of the sulphur dioxide is oxidized by oxygen on copper producing sulphuric acid and copper sulphate. The other part is oxidized electrochemically on graphite. The cathodic reaction of this electrolysis is used for recovering the copper dissolved in the catalytic step. The basic reactions of this process have been studied experimentally in detail. It has been shown that sulphur dioxide can be electrochemically oxidized on carbon electrodes to sulphuric acid with high current efficiency. The reaction rate of the electrochemical copper deposition is increased by dissolved sulphur dioxide in the electrolyte. The catalytic oxidation of sulphur dioxide on copper has been investigated for different sulphur dioxide concentrations and temperatures. The ratio of the reaction products, sulphuric acid and copper sulphate, varies over a wide range depending on the experimental conditions.

Indirect electrosynthesis ofp-methoxybenzaldehyde

High current efficiency and selectivity were obtained in the indirect electrochemical oxidation ofp-methoxytoluene top-methoxybenzaldehyde with Ce4+/Ce3+ as redox mediator system. Platinized titanium anodes can be used for oxidant regeneration. Selectivity ofp-methoxybenzaldehyde synthesis has been optimized up to 98% by Plackett-Burman and factorial design of experiments. The kinetics ofp-methoxytoluene oxidation by Ce4+ with CH2Cl2 as organic solvent has been found to be of mixed mass transfer and kinetic control.

Kinetic Investigations of the Primary Step of Electrochemical Coal Oxidation

The stress corrosion cracking of sensitized stainless steels due to thiosulfate and tetrathionate has been associated with an increased rate of anodic dissolution in the active-passive region of voltammetric scans, via the mutual sensitivity of both phenomena to a series of anionic inhibitors. The increased anodic dissolution is caused by the adsorption of the sulfur compound and its catalysis of metal oxidation. The inhibitors function, not by competitivel desorbing the corrodent, but by aiding the passivation process.

Potential and current distribution in porous electrodes under charge-transfer kinetic control

Abstract A mathematical model to represent porous electrodes assuming that the electrochemical reaction has a Tafel kinetic control is proposed. Expressions are developed to analyse the performance of the electrode and the effect of the geometric parameters on the effectiveness is discussed. The theoretical results are compared with experimental data of the oxygen evolution on thick hydrous nickel oxide electrodes under the condition that the resistivity of the pore material is greater than that of the electrolyte solution, ϱ m ⪢ ϱ s = 0.