A comprehensive review of the so-called standard model for potentiostatic current density transients due to multiple 3D nucleations with diffusion-controlled growth is reported in this chapter, along with a literature review where extensions of this model have been reported towards description of important situations where three-dimensional nucleation plays a central role, namely: the kinetics of nucleation and growth of active electrocatalytic materials on whose growing surfaces simultaneous faradaic reactions occur as well as its application in different relevant systems of great practical interest, in aqueous and non-aqueous media such as the so-called deep eutectic solvents, the electrodeposition of alloys and its application to describe the nucleation under conditions of forced convection as well.

This chapter presents aspects related to the electrochemical approach to hydrogenHydrogen technologies, considering key concepts that drive both the thermodynamicThermodynamic and kineticKinetic phenomena of the redoxRedox processes involved. Strategies to improve surface processes on various electrode materialsElectrode material are considered. The fundamental approach to the development of applied technologies illustrates the impact on the environmentEnvironment and energyEnergy, as well as the role of related physicochemicalPhysicochemical processes.

The mechanism and kinetics of zinc electrochemical nucleation and growth onto glassy carbon surface, from Zn(II) ions dissolved in the choline chloride (ChCl)–urea eutectic mixture, reline, at different temperatures, T , within the 303 to 363 K range, are reported for the first time. From the potentiodynamic study, the exchange current density, j 0 , and the energy transfer coefficient, α, of the Zn(II) DES + 2e − GCE/Zn(s) ↔ Zn (s) reaction were estimated as a function of T . It was found that while j 0 depends exponentially on T , α= 0.12 ± 0.02 remains almost constant. Furthermore, the activation energy, E * = (33 ± 0.2) kJmol −1 , of this reaction was assessed from the Arrhenius-type plot (ln j 0 vs. T −1 ). Analysis of the potentiostatic current density transients allowed to establish that the zinc electrodeposition mechanism occurs via the simultaneous presence of a Langmuir-type adsorption–desorption equilibrium, an instantaneous nucleation process with two-dimensional (2D) growth limited by the rate of lattice incorporation, and a diffusion-controlled three-dimensional nucleation and growth contribution (3D), which is particularly notorious at 348 K. This is the first time that a 2D-3D nucleation transition has been observed during the electrochemical deposition of metals from deep eutectic solvents (DES). From the 2D nucleation process contribution to the total current density, it was possible to determine the surface roughness factor and the electrochemically active surface area of the glassy carbon electrode (GCE). The time evolution of the Zn monolayer formation onto the GCE was also reported.

Contributing to the field: With this Special Collection, the aim is to transcend the barriers of geography and highlight the recent work of Latin American Electrochemists around the world.

The kinetics of the photoelectrochemical mineralization of p-nitrophenol (PNP) and p-methoxyphenol (PMP) on rutile:anatase TiO2 nanotube electrodes (% rutile: 0, 16, and 20) has been studied. Oxida...

Se determino la demanda quimica de oxigeno (DQO) utilizando un metodo electroquimico basado en electrolisis en condicion de capa fina. Se emplearon electrodos miniaturizados de grafito serigrafiado, modificados por electrodeposicion de PbO2-Bi, para la aplicacion de electrolisis exhaustiva en muestras con volumen en el orden de microlitros. Se establecio el protocolo de calibracion de la DQO en funcion de la carga electrica transferida durante electrolisis de soluciones patrones de compuestos organicos de interes. Se evaluo la respuesta del metodo frente al efecto de la concentracion del compuesto modelo usado como patron (ftalato acido de potasio), el potencial de electrodo aplicado, la concentracion de cloruros como interferencia tipica en la cuantificacion, compuestos organicos de diversa naturaleza, asi como la respuesta para un efluente industrial representativo. Se encontraron condiciones experimentales donde la desviacion asociada fue de 5 % en precision, y entre 1 y 6 % de error en cuanto a exactitud. El metodo propuesto es capaz de reproducir valores razonablemente exactos y precisos en comparacion con los resultados del metodo tradicional (oxidacion con H2SO4 y K2Cr2O7), demostrando que el proceso de oxidacion electroquimica en condicion de capa fina, es un metodo conveniente para determinar la DQO en soluciones acuosas.

The chemical oxygen demand (COD) was determined using an electrochemical method based on electrolysis in a thin layer condition. Miniaturized electrodes of screen-printed graphite modified by PbO2-Bi electro-deposition were used for the application of exhaustive electrolysis in samples with volume in the order of microliters. The COD calibration protocol was established on the basis of the electrical charge transferred during electrolysis of standard solutions of organic compounds of interest. The response was evaluated with respect to the concentration of potassium hydrogen phthalate used as standard, the electrode potential, the concentration of chloride ions as typical interference, and the response of a typical industrial effluent. Experimental conditions were identified for high precision determinations with deviations of 5 % in all cases, and relative error between 1 and 6 %. The proposed method is capable of reproducing exact and accurate values of COD, when compared to the traditional method based in oxidation with H2SO4 and K2Cr2O7, demonstrating that electrochemical oxidation under thin layer condition, is a convenient method to determine the COD of aqueous solutions.

The electrochemical formation of copper phosphide phases on vitreous carbon electrodes from Cu(II) and hypophosphite acid aqueous solutions has been studied. It was found that the main component in the deposits was Cu, that their composition was independent of the electrodeposition potential, and that the P content increased with the proportion of hypophosphite with respect to Cu(II) in solution. Analysis of potentiostatic current transients recorded during deposition and the microscopic observation of the surface indicated that the mechanism of electrocrystallization occurs through copper electrodeposition, inducing the reduction of hypophosphite with concomitant formation of copper phosphide clusters followed by their aggregation, leading to the nucleation of three-dimensional centers with a growth rate controlled by the diffusion of electrodepositing species from the solution bulk, with generation also of byproducts of hypophosphite reduction. (C) 2020 Elsevier Ltd. All rights reserved.

Glucose solar light photoinduced oxidation on Bi2WO6 and the chemical kinetics conditions for concurrent photoelectrochemical H2 production are reported. The results show that the conversion of this organic compound is determined by their surface concentration according to the Langmuir-Hinshelwood mechanism. The performance of Bi2WO6 powder for the photocatalytic oxidation of glucose is higher than that observed with TiO2-based materials. Glucose degradation and mineralization rates are similar; therefore, stable intermediates are not formed during glucose oxidation. Photoluminescence studies indicate that glucose promotes electron injection into the valence band of semiconductor. The initial glucose concentration in combination with the electrode potential used, determines the H2 production. In fact, electrode potential of 0.9 V vs. SHE and 60 ppm of glucose defines kobs values equal to kK with maximum H2 evolution rate: 3.05 μmol h−1 cm−2. This value arises from the transformation of the 97% of the Faradaic current measured. The phenomenological conditions for renewable energy applications have been envisaged.

After introducing the basic aspects of the photoelectrochemical processes for the oxygen transfer reaction, we present the simplest kinetics models that have been built to represent the dynamics of light-induced redox reactions. Additionally, we discuss the basic chemical kinetics formalism used in photocatalysis and photoelectrochemistry, where interplay between the oxidation process and non-equilibrium adsorbed states is considered. We also comment about the experimental developments related to the treatment of wastewater with electricity or H2 generation. Finally, we discuss the need to measure reaction rates to determine and compare the physicochemical parameters describing the chemical interactions during photoelectrochemical processes, in order to understand these reactive systems and apply them to new challenges related to the environment and energy.

Scientists reflect on a year of civil unrest. Writing from Syria, Bolivia, Sudan, Iran, Chile, Ecuador, Lebanon, Venezuela, Hong Kong and Catalonia, correspondents tell of altered priorities, day-to-day challenges and hope in the dark times. Scientists from Syria, Bolivia, Sudan, Iran, Chile, Ecuador, Lebanon, Venezuela, Hong Kong and Catalonia reflect on a year of civil unrest.

This work summarizes progresses achieved in the physical chemistry aspects of the growth of anodic oxides under high-field conditions for the synthesis of semiconducting thin solid films and their implementation as photocatalytic materials. We discuss the scope and mechanisms for anodic oxide growth, describing the development of kinetic models and the correlations between theory and kinetic data, leading to fundamental information to characterize the primary processes occurring during the anodization of valve metals under high fields. The main features related to the widely used self-assembly of nanostructures by valve metal anodization are highlighted and briefly discussed. This is followed by general considerations of heterogeneous photocatalysis on these functional materials, considering the kinetics of the heterogeneous catalytic processes involved and the overall photoelectrochemical performance. High control of the characteristics of the materials obtained with the method described, combined with the possibility of electrochemically assisting photocatalysis, allows application of this technology to the treatment of wastewaters, energy conversion, and related fields.

The nucleation of Ag onto vitreous carbon from aqueous 3 M NaCl or 0.6 M NaClO4 and deep eutectic solvent (DES) 1:2 M mixture of choline chloride:urea solutions containing Ag+, has been studied analyzing the chronoamperometric response to single potential steps. From the coordinates of the maxima observed in the current responses, the nucleation frequencies A (s− 1) and number densities of nucleation sites N0 (cm− 2) were obtained from the standard model of nucleation with diffusion-controlled three-dimensional growth. Analysis of the overpotential dependence of nucleation frequencies using the classical electrochemical nucleation theory allowed to calculate the Gibbs free energy of nucleation ΔG˜nc and critical nucleus size nc as well as the exchange current density j0, transfer coefficient α and surface tension σ of silver nuclei. The kinetics of Ag+ reduction is two orders of magnitude slower in DES compared to both aqueous systems studied, and values of α ≪ 0.5 where found in both aqueous and DES media, indicating either that the intermediate state for metal ion reduction is located close to the initial state, i.e., the solvated or complexed metal ion in solution, or that the metal ion is specifically adsorbed on the surface and the symmetry factor involved requires an alternative electron transfer formalism. The low ΔG˜nc and nc values observed indicate that the discharge of a single Ag ion on the surface already becomes a supercritical nucleus, involving a very low Gibbs energy barrier, characteristic of a non-activated process.

Venezuela’s researchers strive to work amid the breakdown of democracy, often without water or power, says Benjamin Scharifker.

The nucleation rate and the number density of nucleation sites, as well as the contact angle of silver clusters growing under diffusion control, have been determined with single-step potentiostatic experiments, from the electrical current and optical transmittance during electrocrystallization of silver on optically transparent indium tin oxide electrodes. It was found that silver grows at overpotentials as crystallites with an aspect corresponding to an effective contact angle with the indium tin oxide surface of ca. 13°. Comparison with results obtained from ex situ atomic force microscopy analysis suggests that electrodeposited nuclei undergo conformational relaxation at open circuit, leading to significantly higher equilibrium contact angles. The results obtained indicate that while the contact angles measured during growth at overpotentials have no bearing on the values of nucleation rates determined from the electrical and optical responses to potential steps, they lead to lower number densities of sites, in relation to values corresponding to the growth of hemispherical centers. Implications of the results obtained for the clustering of silver atoms leading to the electrocrystallization process are also discussed.

After reviewing the chemical aspects of the environmental applications of heterogeneous photocatalysis under solar irradiation based on TiO2 and Bi2WO6. The mechanism for free radicals formation, strategies for improving energy conversion processes based on the solid materials and the modification of the catalyst surface by the adsorption of specific molecules are discussed. Aspects associated with the growing research on photocatalytic systems based on the simplest Aurivillius oxide: Bi2WO6 are studied, considering the key concepts that define the solar to chemical energy conversion and finally, various perspectives about environmental applications using the photoelectrocatalytic technology based on nanomaterials are exposed.

The electrochemical detection of ethanol, acetaldehyde and acetylcholine using a nickel nanowire array electrode was studied. For each compound, the sensitivities, detection limits and coefficients of determination obtained by the traditional direct current (DC) amperometric technique using the nanowire array electrode were compared with the results obtained using a flat electrode, and for the three compounds the nanowire array electrode showed improved performance. The amperometric responses displayed current fluctuations with root mean square amplitudes linearly related to the concentration of each compound in solution, and from their analysis the alternating current (AC) sensitivities and coefficients of determination of the three compounds were determined. Although the AC responses were less sensitive than the corresponding DC responses, the AC strategy allows determining the concentration of the compounds without requiring the independent measurement of the background currents flowing through the electrode in the absence of the compounds under analysis. (C) 2014 Elsevier B.V. All rights reserved.

After discussing the general aspects of photocatalytic oxidation of organic compounds in aqueous solutions and the application of the rotating disk theory to (photo)catalytic phenomena, we report the oxidation of p-nitrophenol (PNP) on illuminated phosphorus modified TiO2 (P-TiO2) and TiO2 photocatalysts under controlled hydrodynamic conditions. We found that the performance of the oxygen transfer reaction can be described considering mass transport in solution and surface reaction according to Langmuir-Hinshelwood kinetics. From analysis of the dependence of the reaction rate on the rotation rate, we obtained the apparent organic-photocatalyst adduct interaction constant K, the rate constant k of the surface reaction between adsorbed PNP and hydroxyl radicals, and the diffusion coefficient D of PNP in solution. The results showed that degradation of PNP using P-TiO2 and UV light was readily possible, with reaction half-lives significantly lower than those obtained using pristine TiO2 photocatalyst. (C) 2014 Elsevier B.V. All rights reserved.