Andrzej Lasia

Kinetics of hydrogen evolution on nickel electrodes

Abstract The mechanism and kinetics of the hydrogen evolution reaction (HER) was studied in 1 M NaOH on polycrystalline nickel electrodes using potential step charging, open circuit potential decay and ac impedance techniques. It was shown that the potential step charging technique cannot give correct kinetic results because of the experimental limitations. It was found that the experimental curves may be explained assuming the Volmer-Heyrovsky mechanism. New methods of data analysis for the open circuit potential relaxation and ac impedance are proposed.

Experimental study and modeling of impedance of the her on porous Ni electrodes

Abstract The electrochemical activity towards the hydrogen evolution reaction of pressed powder electrodes (NiZn and NiAl) was studied in alkaline solutions after leaching out the more active element. These electrodes displayed porous character, and electrochemical impedance spectroscopy was used to characterize surface porosity. The influence of overpotential, temperature, poisons, electrode composition and electrolyte concentration was studied and distinction criteria between faradaic and geometrical effects were formulated. Digital simulations of impedance values in different pore geometries were also carried out. The kinetic parameters of hydrogen evolution were determined. The main factor influencing the electrode activity seems to be the real surface area.

Study of the Kinetics of Hydrogen Evolution Reaction on Nickel‐Zinc Alloy Electrodes

The mechanism and kinetics of the hydrogen evolution reaction were studied in on nickel‐zinc alloy electrodes prepared by electrodeposition at controlled potential. A series of electrodes containing 70 to 28% Ni was prepared. Before the measurements, zinc was leached in alkaline solution. Using the ac impedance technique, it was found that the reaction proceeds via the Volmer‐Heyrovsky mechanism, and the kinetic parameters of the process were determined. With a decrease in the nickel content, the electrode becomes more active, and an increase in the real surface area is observed. The surface morphology was studied using SEM and optical microscopy.

Impedance of porous electrodes

Abstract Equations describing potential and current distribution in semi-infinite- and finite-length cylindrical porous electrodes, assuming Butler-Volmer equation for the pore walls, are presented. In the case of finite-length pores, for arbitrary values of transfer coefficients, only numerical solutions exist. Ac impedances of these electrodes were also simulated for different kinetic and geometric parameters. Complex nonlinear least-squares fit of the simulated impedances to the de Levies equation, which neglects the potential dependence of the charge transfer resistance, was studied and the approximation errors discussed. This procedure overestimates charge transfer resistances predicted by de Levies equation (for the same pore geometry), at most by a factor of 2. The double-layer capacitances obtained are also larger than those assumed. Errors of approximations are discussed in relation to earlier experimental work.

The structure, morphology and electrochemical impedance study of the hydrogen evolution reaction on the modified nickel electrodes

Abstract Composite layers of modified amorphous nickel were prepared by simultaneous electrodeposition of Ni and TiO2 on a Cu substrate from a solution containing TiO2 (anatase) particles suspended by stirring. Scanning electron microscopy, X-ray diffractometry, Auger spectroscopy and absorption spectroscopy, were used for physical and chemical characterization of the layers. Obtained deposits exhibit an amorphous structure of the Ni-P matrix in which the crystalline component, TiO2, is embedded. Additionally, the presence of non-stoichiometric oxide, Ti2O3, formed on a boundary of the TiO2 grain and nickel matrix in consequence of the reduction conditions during the electrodeposition, was revealed by auger electron spectroscopy (AES). The hydrogen evolution reaction (HER) was investigated on the Ni-P+TiO2 and compared with Ni-P electrode in 5 M KOH at 25°C using steady-state polarization and electrochemical impedance spectroscopy (EIS). In order to explain the electrochemical behaviour of the electrode materials, electrical equivalent circuits containing: (i) the constant-phase element (CPE), (ii) the porous electrode impedance, and (iii) two-CPE elements were compared and verified. The ac impedance behaviour of the electrodes may be well described using the two-CPE or porous electrode model in case of the Ni-P+TiO2, and a simple CPE model for the Ni-P. The results obtained from the EIS and steady-state measurements allowed for the determination of the mechanism and kinetics of the HER. It has been found that an increase in electrochemical activity of the Ni-P+TiO2 electrode is due to both the increase in the real surface area and the presence of titanium oxides TiO2 and Ti2O3, as compared with the Ni-P electrode.

Investigation of Hydrogen Adsorption and Absorption in Palladium Thin Films II. Cyclic Voltammetry

Hydrogen absorption into palladium thin films (10 monolayers and 100 μm) was studied using cyclic voltammetry. The behavior of thin layers prepared by electrodeposition is different from that of the bulk Pd. On very thin films hydrogen adsorption, absorption, and evolution processes are separated. Adsorption kinetics depends strongly on applied potential and layer thickness. A new method is proposed for the determination of the quantity of adsorbed hydrogen in the presence of the absorption process.

Studies of the Hydrogen Evolution Reaction on Ni‐P Electrodes

The hydrogen evolution reaction (HER) was studied on Ni-P electrodes containing 8 to 30 atomic percent P prepared by galvanostatic deposition. The electrodes were studied directly after preparation or after pretreatment by heating, leaching in HF solution, anodic oxidation, or potential cycling in the solution. The activity of these electrodes depended on the method of preparation and phosphorous content. The activity was higher for the materials deposited at lower temperatures and for those containing smaller amounts of phosphorous. The mechanism of the hydrogen evolution reaction was studied in 1 M NaOH, and the kinetic parameters were determined using steady-state polarization and electrochemical impedance spectroscopy techniques.

Kinetics of hydrogen evolution on Ni-Al alloy electrodes

The kinetics of the hydrogen evolution reaction have been studied on Ni-Al alloy electrodes. The electrodes after leaching aluminium in alkaline solution, are very active despite their large Tafel slopes. The SEM studies show a formation of deep pores on the electrode surface. The kinetics were studied using an ac impedance technique. It was found that the impedance plots may be explained assuming a fractal model. The logarithm of the charge transfer resistance is a linear function of the overpotential. Using a nonlinear least square approximation it was found that the reaction proceeds through the Volmer-Heyrovsky mechanism and the kinetic parameters were estimated.

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, η250 = 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.

General Model of Electrochemical Hydrogen Absorption into Metals

A general model for the hydrogen adsorption and hydrogen absorption into metals has been proposed. It includes reactions of hydrogen evolution M+H{sub 2}O+e=MH{sub ads}+OH{sup {minus}}; MH{sub ads}+H{sub 2}O+e=M+H{sub 2}+OH{sup {minus}}; and 2MH{sub ads}+2M+H{sub 2}; hydrogen absorption MH{sub ads}+MH{sub abs}; and hydrogen diffusion into metal. This problem leads to a system of differential equations which was solved using the differential algebraic equations method. Solutions were obtained for constant potential and constant current charging/discharging in the case of semi-infinite and finite length diffusion for planar, spherical, and cylindrical diffusion. Numerical solutions give new information about the reaction mechanism and may be useful in the determination of the kinetics of these processes.