A. Czerwiński

The absorption of hydrogen and deuterium in thin palladium electrodes - Part I: acidic solutions

Abstract A comparative study of hydrogen and deuterium sorption in Pd from basic solutions (0.1 M NaOH, NaOD, LiOH and LiOD) has been performed, using electrodes obtained by electrochemical deposition of palladium on gold. The amount of absorbed hydrogen or deuterium has been found to depend on the electrode potential. The shape of H(D)/Pd vs. E plots and the rate of hydrogen (or deuterium) absorption are strongly influenced by the composition of the solution. Lithium appears to have a marked influence on the α to β phase transition. The maximum H(D)/Pd ratios were about 0.8 for all studied solutions. The greater isotopic effect has been found during absorption and desorption in lithium solutions. 1. 1. The amount of absorbed hydrogen-deuterium in the bulk of palladium depends on the electrode potential. 2. 2. The shape of the plots of absorbed hydrogen amount from basic solutions vs. absorption potential strongly depends on composition of solution and significantly differs from the shape of the same plots in acidic solution (2). The results show that alkali metal cations markedly influence hydrogen and deuterium sorption processes. 3. 3. Lithium ions seem to affect the α-β transition more than sodium ions. 4. 4. The isotopic effect is much stronger during oxidation of absorbed hydrogen than during absorption. This effect is greater in lithium solutions. 5. 5. No significant influence of solution composition on the maximum values of the H(D)/Pd ratios has been found. 6. 6. The charging curves in all the solutions studied are not reversible, as was the case in acidic solutions. This means that the sorption or desorption causes irreversible changes in the Pd lattice.

Kinetics of carbon dioxide adsorption on a platinum electrode

Summary The kinetics of CO2 adsorption on a platinized electrode were investigated by radiometric method. It was shown that the CO2 adsorption is due to surface reaction with hydrogen adsorbed on the electrode. The rate of adsorption increases with the electrode potential in the range of 0.05–0.15 V and follows the secondorder reaction. In the range of 0.20–0.25 V the rate of adsorption decreases according to the first-order reaction. The COOH radical is postulated as a product of CO2+Hads reaction. In the range of potentials 0.05–0.20 V the maximum surface concentration of adsorbed species is ca. 9.6×1014 molecules cm−2. The bulk concentration of CO2 and temperature do not influence the surface concentration. The apparent heat of activation of CO2+Hads reaction are 50.0 kJ mol−1 at 0.05 V and 38.5 kJ mol−1 at 0.20 V, respectively.

Electrochemical preparation and characterization of electrodes modified with mixed hexacyanoferrates of nickel and palladium.

Mixed nickel/palladium hexacyanoferrates have been prepared both as thin films and bulk precipitates (powders) attached to electrode surfaces. The mixed material does not seem to be a simple mixture of hexacyanoferrates of nickel and palladium, and it shows unique voltammetric and electrochromic characteristics when compared with the respective single-metal hexacyanoferrates. Electrodeposition of a mixed film is achieved by potential cycling in the solution for modification containing nickel(II), palladium(II) and hexacyanoferrate(III). It comes from elemental analysis that, in general, the stoichiometric ratios of nickel to palladium in mixed metal hexacyanoferrate films reflect relative concentrations of Pd(II) and Ni(II) in the solutions for modification. In the case of the films that have been electrodeposited from the solutions containing palladium ions in amounts lower or comparable with those of nickel ions, the mechanism of film growth seems to involve formation of nickel hexacyanoferrate during negative potential scans followed by simultaneous insertion of palladium ions as countercations into the system. In such cases, palladium ions tend to substitute potassium countercations at interstitial positions in the electrodeposited nickel hexacyanoferrate microstructures. We have determined the following stoichiometric formula, K1.74−2yPdIIyNiII1.13[FeII(CN)6] (where y<0.72) for such films. At higher molar fractions of palladium in solutions for modification, the formation of a mixed phase of nickel/palladium hexacyanoferrate (in which both nickel(II) and palladium(II) are nitrogen-coordinated within the cyanometallate lattice) is expected. This seems to be more probable than simple codeposition of separate palladium hexacyanoferrate and nickel hexacyanoferrate microstructures during the film growth. Mixed (composite) nickel/palladium hexacyanoferrate films show long-term stability as well as promising charge storage and transport capabilities during voltammetric potential cycling. Well-defined and reversible cyclic voltammetric responses have been obtained in lithium, sodium and potassium electrolytes.

The adsorption of carbon oxides on a palladium electrode from acidic solution

The adsorption of carbon oxides from sulfuric acid solution on a palladium electrode was investigated with electrochemical and radiometric methods. It was found that adsorption of CO takes place in the 0.0–0.8 V range. During the CO adsorption the anodic (below 0.15 V vs. SHE) and cathodic (above 0. 15 V vs. SHE) currents are passing through the solution | electrode interface. Mechanisms for the CO adsorption and the final product structure have been proposed. A small amount of adsorbed CO2 on the surface of the palladium electrode has been observed. A strong correlation between the decrease in the real surface of the palladium electrode and the currents of sorbed hydrogen during its oxidation has been found. 1. Carbon monoxide adsorption on palladium electrode from acidic solution (0.5 M H2SO4) occurs at the potentials below 0.9 V (vs. SHE) 2. The main products of CO adsorption on Pd at potentials where no hydrogen absorption occurs are CO species that seem to be mostly bridge bonded to Pd, also a more reduced form of CO adsorption product should exist on the Pd surface. A certain amount of CO species linearly bound is also possible [13–21] 3. The CO adsorption products at potentials where hydrogen absorption occurs, reflect experimental conditions strongly. Probably they are in a more reduced form in comparison with the adsorption products at more positive potentials 4. A decrease of the real palladium surface during CO adsorption and cyclic electrode polarization is observed. 5. Strong correlation between the decrease of the real surface of the palladium electrode and the currents for the sorbed hydrogen oxidation has been found. One of the oxidation peaks related to the sorbed hydrogen oxidation (the smaller at positive potentials) probably corresponds to the oxidation of adsorbed hydrogen. 6. A small amount (almost negligible) of carbon dioxide is adsorbed on the palladium electrode.

Electrochemical behavior of palladium–gold alloys

Abstract The behavior of Pd–Au alloys, prepared by electrochemical codeposition, has been studied in acidic solutions (1 M H 2 SO 4 ) using mainly the cyclic voltammetry technique. Morphology of the alloy surface and bulk compositions were examined by SEM/EDAX method. Various types of voltametric behavior during potential cycling in the oxygen region are presented. The influence of hydrogen absorption on electrochemical properties of surface oxides is demonstrated. The problem of the nature of oxygen electrochemisorbed on Pd–Au alloys is discussed.

Electrochemical behavior of manganese dioxide on a gold electrode

The oxidation of Mn2+ to MnO2 and subsequent reduction of MnO2 on a gold electrode were investigated in H2SO4 solutions. Various experimental conditions, e.g. electrolyte composition, potential scan rate, temperature, and stirring were investigated with the purpose of optimization of the manganese dioxide electrodeposition process. The electrochemical properties of electrodeposited MnO2 on Au electrode have been investigated using rotating disc electrode (RDE) and the voltammetry techniques. Scanning electron microscope method was also used to study the structure of the deposit. Not mentioned in previous literature, one additional reduction peak was observed on cyclic voltammograms at the potential of ca. 450 mV. The appearance of this peak strongly depends on the sulfuric acid concentration. A characteristic cross-over on the cathodic branch was observed during cyclic voltammetry, the effect not considered before in the literature. The presence of this loop depends on the number of sweep segments, surface activation, electrolyte composition (pH, Mn2+ concentration), electrode rotation speed and on the temperature. Obtained results were supported with thin layer electrode (TLE) with Nafion® as separator.

The influence of carbon monoxide on hydrogen absorption by thin films of Palladium

Abstract (1) Adsorbed carbon monoxide inhibits the hydrogen absorption and desorption reactions in a thin-layer palladium electrode. A“blocking” effect of adsorbed hydrogen inside the palladium by adsorbed carbon monoxide is observed. (2) During CO adsorption at 0.00 V an anodic current appears, probably due to the oxidation of adsorbed hydrogen during the exchange with carbon monoxide molecules. (3) In addition to “bridged” and “linearly” bonded CO molecules as the predominant surface species, another adsorption product may exist on the palladium surface. (4) Electrodes constructed by depositing a thin layer of Pd on gold allow the ratio between absorbed and adsorbed hydrogen to be decreased. This creates new possibilities for electrochemical studies of this metal.

Electrochemical behavior of lead dioxide deposited on reticulated vitreous carbon (RVC)

The electrochemical performance of lead dioxide deposited on reticulated vitreous carbon (RVC) has been investigated in basic and acidic solutions (0.1 M NaOH, 0.1 M Na2BB4OO7 and 0.5 M H2SSO4)). For comparison, pure lead and lead dioxide deposited on platinized RVC (Pt/ RVC) were also included in the study. Our results indicate that the behavior of RVC covered with lead dioxide (without platinum) resembles that of lead dioxide generated electrochemically on metallic lead.

Electrosorption of carbon dioxide on PdPt alloys

The adsorption of carbon dioxide on PdPt alloy electrodes in acidic solution has been studied. The influence of both bulk and surface composition of the alloy on CO2 adsorption is presented. The results obtained suggest that only hydrogen interacting with platinum atoms is active in reaction with CO2. At the same time these platinum atoms are inactive in the hydrogen absorption/desorption process. The latter reaction probably proceeds only with participation of palladium atoms, unblocked by products of adsorption of CO2.

The comparative study of CO2 + Hads reaction on platinum electrode in H2O and D2O☆

The observed isotopic effect in CO2 + Hads reaction showed that at 0.05 V the rate determining step is formation whereas at 0.2 V the reorientation of adsorbed intermediate: is probably the slowest step of reaction. The oxidation of adsorbed product is slower in D2SO4 than H2SO4 solution like the surface oxidation of platinum. The rate determining step of COOHads oxidation is a reaction with OHads.