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González Sánchez, Joaquín

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González Sánchez, Joaquín
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Universidad de Murcia. Departamento de Química Física
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    A reasoned general explanation about the concepts of diffusion and reaction layers
    (Springer Nature, 2023-10-25) Laborda Ochando, Eduardo; González Sánchez, Joaquín; Molina Gómez, Ángela; Química Física; Facultad de Química
    The understanding of the implication of the transport of matter is fundamental in the teaching of heterogeneous charge transfers, which are the central element of the electrochemical science and technologies. Thus, even in the absence of intrinsic kinetic complications, the overall rate of such events is frequently conditioned by the mass transport and also by the occurrence of homogeneous coupled chemical reactions. Upon comprehending and accounting for these two effects, the emergent and widely-used concepts of the diffusion and reaction layers will be discussed and disentangled under potential-controlled conditions. First, some basic concepts about the transport of charged species in solution are recalled, in order to establish the experimental conditions under which diffusion-only transport can be considered. Under these premises, a fast electron transfer will be considered to introduce the concept of "diffusion layer", discussing its relationship with the time of the experiment and with the characteristics of the diffusive field that is strongly influenced by the geometry and size of the electrode considered. Next, fast electron transfer reactions ‘conditioned’ by homogeneous chemical reactions undergone by the reactant and/or the product of the redox couple will be analyzed. A historical introduction to the concept of "reaction layer" is carried out, pointing out its value in processes with chemical regeneration of the electroactive species and its non-negligible interplay with the mode of diffusion.
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    Investigating comproportionation in multielectron transfers via UV–visible spectroelectrochemistry: the electroreduction of anthraquinone-2-sulfonate in aqueous media
    (ACS Publications, 2022-08-22) Romay García, Luis; González Sánchez, Joaquín; Molina Gómez, Ángela; Laborda Ochando, Eduardo; Química Física; Facultad de Química
    UV–vis spectroelectrochemistry is assessed as a tool for the diagnosis and quantitative in situ investigation of the incidence of comproportionation in multielectron transfer processes. Thus, the sensitivity of the limiting current chronoabsorptometric signals related to the different redox states to the comproportionation kinetics is studied theoretically for different working modes (normal and parallel light beam arrangements) and mass transport regimes (from semi-infinite to thin layer diffusion). The theoretical results are applied to the spectroelectrochemical study of the two-electron reduction of the anthraquinone-2-sulfonate in alkaline aqueous solution, tuning the thermodynamic favorability of the comproportionation reaction through the electrolyte cation. The quantitative analysis of the experimental results reveals the occurrence of comproportionation in the three media examined, showing different kinetics depending on the cationic species in solution.
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    Voltammetric kinetic studies of electrode reactions: guidelines for detailed understanding of their fundamentals
    (American Chemical Society, 2022-12-27) González Sánchez, Joaquín; Laborda Ochando, Eduardo; Molina Gómez, Ángela; Química Física; Facultad de Química
    Theoretical and practical foundations of basic electrochemical concepts of heterogeneous charge transfer reactions that underline electrochemical processes are presented for their detailed study by undergraduate and postgraduate students. Several simple methods for calculating key variables, such as the half-wave potential, limiting current, and those implied in the kinetics of the process, are explained, discussed, and put in practice through simulations making use of an Excel document. The current–potential response of electron transfer processes of any kinetics (i.e., any degree of reversibility) are deduced and compared for electrodes of different size, geometry, and dynamics, namely: static macroelectrodes in chronoamperometry and normal pulse voltammetry, and static ultramicroelectrodes and rotating disc electrodes in steady state voltammetry. In all cases, a universal, normalized current–potential response is obtained in the case of reversible (fast) electrode reactions, whereas this is not possible for nonreversible processes. For this last situation, different widely used protocols for the determination of the kinetic parameters (the mass-transport corrected Tafel analysis and the Koutecký–Levich plot) are deduced, proposing learning activities that highlight the foundations and limitations of such protocols, as well as the influence of the mass transport conditions. Discussions on the implementation of this framework and on the benefits and difficulties found are also presented.
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