Electrochemical Impedance Spectroscopy in a Droplet of Solution for the Investigation of Liquid/Solid Interface.

The local electrochemical behavior of a solid-liquid interface can be studied by electrochemical impedance spectroscopy (EIS). The investigated surface area can be delimited by adding a drop of solution, which forms an interface between the liquid drop and the working electrode, and performing the measurements inside. The size of the drop must be sufficiently small for a simultaneous wettability characterization (from the contact angle measurement) and appropriately large so that wettability is not influenced by the presence of the working and the counter electrode inserted in the droplet. In this work, we showed that EIS measurements can be performed in a solution droplet of 2 to 4 µL, although the electrochemical cell lacks the usual geometry. For our measurements, we studied a model system consisting of a KCl aqueous solution of [Fe(CN)6]3-/4- redox couple at a Pt electrode. All the results were compared with those obtained for a bulk configuration. The sessile drop configuration and the EIS response were modeled using finite element method for different electrode sizes and configurations to account for electrochemical kinetics and both current and potential distributions.

Alternating current measurements in scanning electrochemical microscopy, part 2: detection of adsorbates.

A scanning electrochemical microscope (SECM) in ac mode is used for the characterisation of the adsorption process during the hydrogen evolution reaction (HER) in sulfuric acid solution. It is shown that this technique allows quantitative analysis of the adsorption process, and measurements of the differential capacitance with the frequency as parameter are obtained. The time constant for relaxation of adsorbed hydrogen (H(ads)) is approximately 2 Hz, and analysis of the Nyquist plot allows direct evaluation of the charge involved. In addition, the direct comparison of the usual electrochemical impedance data and ac-SECM results obtained simultaneously permits characterisation of processes occurring at the surface and in solution.

Alternating-current measurements in scanning electrochemical microscopy, part 1: principle and theory.

The development of the scanning electrochemical microscope in ac mode is presented from both experimental and theoretical point of views. The experiments are performed with the ferri/ferrocyanide redox mediator as model system. Based on analysis of the frequency-dependent collection efficiency, diffusion between the probe and the substrate is investigated, and analysis of time constants allows evaluation of the size of the sensing area under investigation. The experimental results are in good agreement with numerical simulations.

Progress in scanning electrochemical microscopy by coupling with electrochemical impedance and quartz crystal microbalance.

Scanning electrochemical microscopy (SECM) is a powerful technique for performing quantitative measurements at a local scale. This paper covers the development of combinations of SECM with electrochemical impedance spectroscopy (EIS) and electrochemical quartz crystal microbalance (EQCM). Basic aspects are described and potential applications reported by several research groups are covered. The unique advantages of the coupled techniques--with additional information being obtained from each coupling--are also discussed.

Performances and limits of a parallel oscillator for electrochemical quartz crystal microbalances.

This paper describes a driving circuit for an electrochemical quartz crystal microbalance (EQCM) adapted to a wide range of applications. The oscillator is a Miller-type parallel oscillator using an operational transconductance amplifier (OTA). A theoretical study of the oscillating circuit led to the analytical expression of the microbalance frequency as well as to an overestimation of the error on the mass measurement. The reliability of the EQCM was then experimentally verified through electrochemical copper deposition and dissolution. The limit of operation of the EQCM was also investigated, both analytically and experimentally. This work shows that parallel oscillators using few electronic components allow a very reliable EQCM to be obtained for mass measurements on metallic films, even if they are highly damped.