H. Perrot

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.

Investigation of Hydrogen Adsorption-Absorption into Thin Palladium Films I. Theory

Models of insertion of hydrogen in metal films coating a gold electrode are presented in terms of faradaic impedance. They take into account adsorption of the protons from the solution on the electrode surface (two-step mechanism) or direct absorption in the metal (one-step mechanism) and diffusion in the metal. In addition, trapping of the hydrogen atoms and direct exchange of the trapped hydrogen with the solution are considered. The main feature discussed here is the change of the charge-transfer resistance with the thickness of the metal film. This surprising behavior is related to the exchange reaction of trapped hydrogen with the solution. The impedance diagrams are calculated for a palladium film both in the underpotential deposited (UPD) domain and the hydrogen evolution domain. In the UPD domain an impedance of the ion-blocking electrode is found. In the hydrogen evolution domain two capacitive loops are found for a film whose thickness is less than 1 micrometer.

Quartz Crystal Microbalance Investigation of Electrochemical Calcium Carbonate Scaling

Electrochemical calcareous scaling was investigated by using a quartz microbalance in order to devise an accelerated scaling test. The chronoamperometric and the chronoelectrogravimetric curves were simultaneously recorded during scaling in carbonically pure waters. The basic processes involved in scale gravimetric evaluation were surveyed: influence of mass transport on O 2 reduction, influence of electrode vibration, and calibration of the mass measurement. Then the validation of the quartz crystal microbalance (QCM) technique was carried out by changing some experimental conditions : flow rate, calcium concentration, and polarization potential. At the end foreign ions, commonly encountered in natural waters, were added to the synthetic waters. The scaling rate was determined by deriving the chronoelectrogravimetric curve. It was demonstrated that the QCM technique gives much more information on the scaling process than chronoamperometry and can be used as a practical scaling test.

Development of a Coupled SECM-EQCM Technique for the Study of Pitting Corrosion on Iron

The local breakdown of the iron passive layer and the resulting pitting corrosion were studied with a coupled scanning electrochemical microscopy/electrochemical quartz microbalance technique (SECM/EQCM). The initiation and the propagation of a single pit on iron deposited on the quartz gold electrode were controlled through the local production of chloride anion thanks to the silver chloride reduction at the tip of the scanning electrochemical microscope. The frequency response of the quartz was correlated to the pit evolution with respect to time. After the breakdown of the passive layer, the frequency changes were then directly linked to the generation of Fe(II) during the pit growth. It was also shown that in 0.1 M NaOH solution, the local amount of Cl - necessary to initiate the pit was about 0.15 M. In addition to the presence of the oxidized Fe 3 O 4 passive layer and corrosion products like Fe 2 O 3 5H 2 O, β-FeOOH, and Green Rust, Raman analysis evidenced the formation of Fe(OH) 2 inside the pit. Moreover, the influence of the pit size was also studied by varying the amount of chloride ions generated close to the iron substrate.

Study by EQCM on the voltammetric electrogeneration of poly(neutral red). The effect of the pH and the nature of cations and anions on the electrochemistry of the films

Generation of poly(neutral red) films has been studied by means of the simultaneous measurements of current–potential and mass–potential curves during cyclic voltammetry (CV) experiments. It has been proved that the presence of molecular oxygen in the solution increases the amount of polymer deposited on the electrode. Otherwise, using the mass/charge ratio it is possible to obtain quantitative information about the electrodeposition by different procedures. It is observed that this ratio decreases when the amount of polymer electrogenerated increases, except when the polymer is not reduced and oxidised after its electrogeneration. The study of poly(neutral red) by CV and quartz crystal microbalance in solutions without monomer allows to discern between the role of different charged species which are present in the solution: salt cations (Cs+, Na+ and K+), salt anions (NO3−, Cl−, I− and Br−) and hydrated protons that can compensate electrical charge within the film during electrochemical processes.

Investigation of Hydrogen Adsorption and Absorption in Palladium Thin Films III. Impedance Spectroscopy

The electrochemical impedance of palladium films of various thicknesses, deposited on a gold electrode, was measured in the potential range where hydrogen absorbs in palladium. The kinetic and diffusion parameters of the whole electrochemical process were determined. One of the main results was the observation of a maximum on the curves of charge-transfer resistance vs. electrode potential. At this maximum the charge-transfer resistance decreased with the increase of film thickness. This finding is in agreement with the model proposed in Part I, where such a behavior is found to be related to the trapping of the hydrogen atoms in a sublayer just under the electrode surface and a direct exchange of the trapped hydrogen with the electrolyte.

An electrochemical impedance and ac-electrogravimetry study of PNR films in aqueous salt media

Abstract Electrochemical impedance spectra and ac-eletrogravimetry of poly(neutral red) films are studied in different aqueous media. The dependence of these experimental data on the nature of alkaline cation and monovalent anion present in the solution and on the pH is analysed. During the electrochemical processes, it is observed that all three species—the salt cation, the salt anion and the hydronium ion—participate to balance the electrical charge within the film. It is also possible to conclude that the participation of anions takes place faster than hydronium ions participation. Besides, the relative participation of these species is related to the pH of the solution.

Validation of antibody-based recognition by piezoelectric transducers through electroacoustic admittance analysis.

The development of immunosensors based on piezoelectric transducers is widely investigated due to their attractive potentialities. The quartz crystal microbalance (QCM) may give a direct response signal which characterizes the binding event between a sensitive layer, immobilized onto the surface transducer, and the analysis to be detected. However, for small biomolecules, such as some antigens, it is quite difficult to obtain an observable signal. This is mainly due to the lack of sensitivity of the commonly used QCM (5 to 10 MHz quartz crystal). Moreover, the mass estimated with the QCM response through the Sauerbrey equation and the mass which can be measured thanks to other analytical techniques, in our case an enzymatic assay, are different: the deposited mass is generally overestimated by the QCM. To validate QCM mass measurements and, therefore antigens recognition, the interactions of acoustic shear waves with a biolayer were investigated during enzyme adsorption onto the microbalance gold electrode or during the antibody/antigen binding. Electroacoustic admittance was measured around the resonance frequency of a 27 MHz quartz resonator in parallel with microbalance measurements. The parameters which characterize the quartz microbalance equivalent circuit were compared with the classical microbalance frequency. The mass overestimation, given by the microbalance, could be explained either by modification of the rheological properties of the sensitive layers and/or by an inadequacy of the assay performed.

Immunodetection by quartz crystal microbalance

Biodetection is one of the most important challenges for the twenty-first century: many fields are concerned, mainly environmental and medical. The quartz crystal microbalance (QCM) may offer great possibilities for this purpose: a direct response signal, which characterizes the binding event between a sensitive layer, immobilized onto the surface transducer, and the analyte to be detected, can be obtained. However, for the detection of small biomolecules such as antigens, it is quite difficult to obtain an observable signal that corresponds directly to the binding event. In general, this is owing to the lack of mass sensitivity of the commonly used QCM, with 5-to 10-MHz quartz crystals. For improving this mass sensitivity, a 27-MHz quartz resonator was developed and incorporated in a flow-through microcell. Two biospecies, IgG rabbit and peroxidase enzyme, were studied with this ultrasensitive QCM in terms of specificity, detection limit, and calibration curve.

Influence of the Interfacial pH on Electrochemical CaCO3 Precipitation

The influence of the pH at the electrode/solution interface on CaCO 3 precipitation by means of electrochemistry was studied both theoretically and experimentally in pure calcocarbonic aqueous solutions. This interfacial pH was measured at the electrode by using a pH sensor-grid assembly in a submerged impinging cell. A model which takes into account the electrochemical oxygen reduction and the chemical dissociation of hydrogen carbonate and carbonic acid is proposed to calculate the pH at the electrode/ solution interface. It can reproduce the data using values of adjustable parameters obtained from experiments. Hence, the local concentrations of OH - , HCO - 3, CO 2 - 3, Ca 2 + , and the supersaturation coefficient, β, were determined at the electrode. These data were used to interpret CaCO 3 precipitation on a gold electrode.