Ion exchange at the electrode/electrolyte interface studied by probe beam deflection techniques

Abstract

Probe beam deflection (PBD) techniques are a set of in situ electrochemical methods based on the measurement of refractive index gradients in front of the electrode/electrolyte interface. Here, the details of the techniques are described. First, the theory of potential step chronodeflectometry and pulse voltadeflectometry is discussed. Later, different numerical procedures used for data processing and/or simulation of cyclic voltadeflectometry experiments are described. Then, an overview is performed of the techniques use to study electrochemical systems. The systems described are limited to surface species in binary electrolytes, where a clear interpretation of the data is possible. Among the systems studied are: surface reactions on solid electrodes, electrochromic oxides, conducting polymers, redox polymers, double layer charging in nanostructured carbons and ion exchange in layer-by-layer self assembled multilayers. It is shown that PBD techniques have been able to ascertain the effect of different factors on ion exchange: anion size, pH, solvent, film composition, covalent modification, etc. The capability of the techniques to perform fast measurements was also used with success to find out kinetic effects on the ion exchange.

Charge neutralization process of mobile species developed during potentiodynamic conditions. Part 2: Simulation and fit of probe beam deflection experiments

“…These systems have deflection profiles with opposite directions because the variation of the concentration of the oxidized species dominates the response of the system Fe 3+ /Fe 2+ and the changes of on=oC FeðCNÞ 4À 6 control the signal of the couple FeðCNÞ 3À 6 =FeðCNÞ 4À 6 . As a consequence, the concentration profile of the species with the highest concentrative refractivity could be used to predict the sign of a deflection signal [25].…”

“…After doing this, a very good approximation of the distance Dx can be found when the set of values Dx is fitted to give a linear function that cuts the y-axes at (0.015 V) 1/2 . It is important to note that this method does not require the diffusion coefficients of species, but the increments of distance between the different curves of P m j oc j =ox À Á , to calculate Dx [23,25]. In the case of PBD, it is necessary to know the increments of distance between the different cyclic deflectograms of a set of experiments performed at the same scan rate.…”

“…Regarding PBD, it is an in situ electrochemical technique applied to study the mass fluxes in front of the electrode/electrolyte interface from which fast and localized measurements of the dynamic behavior of anionic and/or cationic fluxes can be obtained [3,5,8,[23][24][25]. It is based on measuring the deviation angle (H (t,x) ) of a probe laser beam that crosses a transparent medium with a non-homogeneous index of refraction [8,[23][24][25][26][27].…”

“…It is based on measuring the deviation angle (H (t,x) ) of a probe laser beam that crosses a transparent medium with a non-homogeneous index of refraction [8,[23][24][25][26][27]. This probe beam is aligned parallel to the electrode surface at a given beamelectrode distance (x).…”

“…In principle, those profiles could also be employed to calculate the concentration gradients produced during electrochemical experiments; however, they mostly lack the required precision or spatial resolution to achieve this. This explains why only interferometry [22] and probe beam deflection [8,[23][24][25] (PBD) have been used to monitor concentration gradients through refractive index gradients. On the other hand, spatially resolved UV-visible spectroelectrochemistry [10,19], confocal enhanced Raman microscopy [ ultramicroelectrodes [9] were implemented for measuring concentration profiles of different species within the diffusion layer.…”