M. Szklarczyk

Ionic adsorption at the solid—solution interphase using three in situ methods

Abstract The adsorption of HSO4−, Cl− and I− from aqueous solutions of 1 M HClO4 was examined using radiotracer Fourier transform IR spectroscopy and ellipsometry. The dependence of the adsorption was measured as a function of time of equilibrium, concentration at constant potential, and at various potentials at constant concentration. For the HSO4− adsorption, some measurements were made as a function of temperature at constant potential. The dependence on time corresponded neither to the ln Γt-t nor Γt-ln t relations expected from previously deduced theoretical models. Up to ca. τ/4, θt was found to be proportional to the adsorption time. The dependence of adsorption on concentration at constant potential was carried out over an extended region, i.e. from 10−9 to 10−2 M. In the concentration range up to 10−5 M the behaviour followed the Henry isotherm and at concentrations up to 10−3 M the results followed a logarithmic law. (∂θ /∂ log c)E,T was about the same for HSO4− and Cl− but around two times higher for I−. The maximum coverage of HSO4− and Cl− was reached at ca. 0.25 and ca. 1 for I−. The dependence on potential at constant concentration was essentially linear until oxide formation for HSO4− and Cl−. I− results showed little variation with potential before oxide formation. The dependence of (θ)E,c as a function of temperature for HSO4− cannot be expressed as a simple law because of the low number of experiments successfully investigated; nevertheless, some approximate values of ΔG°, ΔH° and ΔS° were derived. An isotherm was derived in such account that the heterogeneity of the surface, ion—ion repulsion through a simple imaging model and ion—ion dispersive forces were considered. In the calculations, electrostriction effects on ion size were computed. Evidence for partial charge transfer was found: 0.2, 0.6 and 0.9 for HSO4−, Cl− and I− respectively. Indications of water displacement from the surface was found to be consistent with one displaced entity represented by a water dimer. The thermodynamic parameters derived for HSO4− were ΔG° = −32 kJ mol−1 (25°C), ΔH° = −57 kJ mol−1 and ΔS° = −78 J K−1 mol−1 at θss = 0.22. from these values, the Ptue5f8O bond was estimated to be ca. one-quarter of that expected for the isolated Ptue5f8O bond in the gas phase. The entropy was consistent with immobile adsorbed ions with hindered rotation.

Photoelectrocatalysis on silicon in solar light

Photoelectrochemical measurements have been carried out on p‐Si electrodes which contained islands of Pb, Cd, Au, Ni, Co, and Pt. These islands cause the photocurrent density of some of the Si systems to increase greatly (10×) in the middle region of the photocurrent potential curve. A plot of the change in potential observed for this region against logu2009i0 for the dark hydrogen evolution on the metals concerned is linear with a slope close to that expected for a rate determining step at the metal‐solution interface.

Sporadic observation of the fleischmann-pons heat effect

An examination has been made of the heat production at ten palladium electrodes, each prepared in a different way. Seven of these produced heat during D2 evolution in a D2-O2 electrolysis cell (no recombination attempted) which coincided precisely with the prediction of classical electrochemical theory, and thus eliminated the suspicion of heat through unintended D2-O2 recombination. Three electrodes clearly produced an excess heat of ~2-5 watts-cm -3 . The heat was observed for periods of 10-33 hrs. In one electrode the excess heat production shut off (after 33 hrs) with no apparent cause: it did not return in five days of further electrolysis. Fleischmann and Pons (1) reported the production of heat during the electrolysis of D2O at a Pd cathode and Pt anode in excess of that predicted by the electrochemical theory for the heat production in cells. Some of the measurements even indicated that the total energy out of the cell as heat was greater than that going in as electricity. The paper attracted attention because it was suggested that the excess heat was due to D-D fusion occurring in Pd at high overpotential. A principal difficulty, since the announcement in March 1989, has been that many electrochemically inexperienced investigators have tried, but failed, to reproduce the alleged heat (however, cf. Huggins (2), Landau (3), McKubre (4), Appleby and Srinivasan (5), Wadsworth (6)). In this note, we report the observation of this excess heat effect in three electrodes out of ten prepared and examined. The calorimeter used in the study was of the heat transfer type. The cell is a glass cylinder 10 cm. high and 5 cm in diameter. The cap is made of 1 cm thick Teflon with holes for the electrodes, the Joule heater and the temperature probe. The cell is placed inside another glass cylinder 12 cm. high and 7 cm. in diameter. For good heat transfer the space between the cylinders is filled with ethylene glycol. The assembly is immersed in a constant temperature bath (0.01°C accuracy) and the temperature difference between the bath and the cell is monitored with thermistor thermometers (Omega 700 series thermistors with 0.01°C resolution). The calorimeter was calibrated by passing different amount of direct current through the electric heater (resistance 275 Ω) in the solution of 0.1 M LiOD (made by dissolution of Li in D2O) and noting the potential drop across it. For each value of the electrical power put in, steady state temperature difference (monitored on a y-t recorder) was noted and plotted to yield a

On the Dielectric Breakdown of Water: An Electrochemical Approach

The dielectric breakdown of water under static fields has been studied by current-potential relation for six metals. The relations are quasi-linear up to a current density of a few A-cm{sup {minus} 2}. The limiting current continues for a few volts to a few hundred volts, depending on the metal. A glow develops at the electrode and becomes continuous at the end of the plateau, where the current density increases irregularly (breakdown). The breakdown potential does not depend on the field in the water. It occurs at about the same current density for most of the metals. When electrolytes are added, the cell potential at the breakdown is decreased. Adsorbed layers and organic coatings increase the breakdown potential. Electrical energy storage in water is increased by {approximately} 10 times by coatings. The breakdown potential decreases with increase of log of rate constant for hydrogen evolution on the various electrodes and with the corresponding work function. The cell potentials for breakdown correspond to fields in the dielectric below that needed to dissociate it.

In Situ STM Studies of Lead Electrodeposition on Graphite Substrate

Le depot electrolytique de plomb sur une electrode composee dun monocristal de graphite est etudie in situ par microscopie tunnel a balayage. Le reseau atomique du depot de plomb est observe et compare avec celui obtenu a lair

Eight chemical explanations of the Fleischmann-Pons effect

Abstract Eight possible explanations for the heat produced in the Fleischmann-Pons effect are examined with the various conservative assumptions concerning the quantities used. No individual explanation is sufficient to explain the heat produced. All of them together can only explain heat as much as 3 W cm −3 .

IN SITU STM STUDIES OF POLYCRYSTALLINE PLATINUM ELECTRODES

Abstract In situ scanning tunneling microscopy (STM) was applied in a 10−2M NaClO4 solution to study topographical changes of a platinum electrode surface due to its oxidation and reduction. Surface morphology depends on the electrode potential. There are two stages in surface reduction: the fast electrochemical charge transfer and the slow crystallographic reconstruction. Reconstruction during surface reduction tends to proceed in the direction of the topography obtained during the annealing process. Methods of avoiding Faradaic current flowing through the STM tip are discussed in detail.

Substrate effects on photoelectrochemical kinetics in hydrogen production

Abstract The surface of p-Si has been treated in a number of ways, and the rate of the photoelectrochemical reduction of water to hydrogen measured on each type of surface. Correspondingly, in-situ ellipsometric determinations of thickness and refractive index of the surface films were made; corresponding XPS, ISS and SIMS studies were also carried out. The photoelectrochemical activities of the surfaces, as measured by the positive shift on the potential axis of the mid-current point of the photocurrent/potential curve, differed greatly. In treatments with HF, the chemical structure of the surface remains that of SiO2; in treatments with aquaregia and HF, the surface becomes SiO. After hydrogen evolution, SiOH bonds appear. The degree of dependence of the photoelectrochemical activity on the surface characteristics indicates that a reaction at the semiconductor/solution interface controls the overall (consecutive) photoelectrochemical reaction. The increase in rate with change of surface structure depends on the following factors in increasing order of importance: the presence of band gap surface states, the (established) jump in the order of magnitude of conductance of SiOx at x = 1.8, and the (argued) increasing availability of ue5f8Siue5f8 bonds during increasing reduction of the surface, thus causing an increase in the rate constant of a rate-determining proton transfer.

On photoelectrocatalysis of hydrogen and oxygen evolution

Abstract In most previous treatments of photoelectrochemical kinetics, the focus of attention has been on the hole-electron pair formation by light, transport to the surface and various losses arising from hole-electron recombination. In surfaces of SiO x (2 > x > 0.8), the photoelectrochemical onset potential varies by ca 0.8 V. When the variation of the photoelectrochemical onset potential on metal-decorated p-type semiconductor is plotted against the exchange current density for the dark evolution of hydrogen a slope close to 2RT/F is found. For oxygen evolution on metal decorated n-type semiconductor, there is a dependence of the onset potential on the metal-oxygen bond strength in alkali but zero dependence in acid solution. Work-function dependences are present: there is no correlation with effects of hydrogen on the work-function. The formation of band-gap surface states, recombination and effects of the surface treatment upon work-function are compared with the data but found inconsistent with it. If it is assumed that the surface treatment and metal deposition have a strong effect on the charge-transfer rate constants, several correlations can be rationalized. Implications are drawn regarding the role to be played by electrocatalysis in photoelectrochemistry.

Influence of metal additions on silicon during hydrogen evolution

Le depot dilots de metal sur des surfaces de semiconducteurs influe fortement sur le degagement photoelectrochimique dhydrogene. La presence du metal entraine un deplacement des courbes photocourant-potentiel du cote negatif ou positif, suivant e metal. Explication des phenomenes