Film electrode for the kinetic study of irreversible solid-to-solid reactions
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ORIGINAL PAPER
Film electrode for the kinetic study of irreversible solid-to-solid reactions Benlin Yao 1 & Yiqun Xiao 1 & Yanhong Jia 1 & Zhaokai Meng 1 & Hui Chen 1 & Guoan Ye 1 & Hui He 1 Received: 26 August 2020 / Revised: 14 September 2020 / Accepted: 17 September 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract In the last twenty years, direct electro-reduction of solid oxides in molten salt has been investigated extensively. Compared with thermodynamics, kinetics of this solid-to-solid reaction is far less concerned. In this work, Butler-Volmer model was adapted for cyclic voltammetric study of this reaction. Film electrode with three kinds of geometry was proposed to obtain kinetic parameters from the current-overpotential equations. AgCl reduction in 0.5 M KCl–0.5 M NaNO3 was chosen as a model system. The main predictions of planar electrode and cylindrical electrode (inward) were well demonstrated by the electrodes constructed in this work, and the characteristics of cylindrical electrode (outward) were revealed by experimental results from literature. With the obtained relations, kinetics of AgCl/Ag reaction was carefully examined. By in-situ formation of UO2 film on a planar surface in LiCl-KCl-UO2Cl2, the charge transfer coefficient and exchange current density for UO2/U reaction were obtained as 0.19 and 20 mA cm−2, respectively. With the model, electron transfer in an irreversible solid state reaction can now be quantitatively described. Keywords Butler-Volmer model . Three-phase interline . AgCl reduction . UO2 reduction . Molten salt electrolysis
Introduction The electrochemical study of solids can not only provide analytical information on the chemical composition, it is also valuable for their potential electrochemical applications [1]. In general, to maintain electroneutrality, the electroreduction of solids requires a charge compensating transfer of ions, i.e., expulsion of an anion from the solid to the electrolyte, or ingress of a cation from the electrolyte [1, 2]. Particularly, for a solid metal compound MX containing mobile Xn− ions, the reduction of MX to solid M is coupled with the release of Xn− ion, for example, the electro-reduction of metal oxides [3–7], metal sulfides [8–11], AgCl [12–17], etc. The electrochemical reduction of MX may proceed as MX þ ne− ¼ M þ Xn−
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* Hui He [email protected] 1
Department of Radiochemistry, China Institute of Atomic Energy, 102413 Beijing, People’s Republic of China
The reduction mechanism of the reaction is widely studied [5]. Compared with thermodynamics, kinetics seems to be far less concerned. There are several aspects to consider in the kinetic description of this reaction [2]: (1) transfer of electron, (2) transfer of ion, (3) conductivity of the solid phase, and (4) size and shape of the solid phase. To connect voltammetric response with the real reduction process, models which take one or more aspects into account are proposed: (1) thin layer model [14]; (2) IIRD (instantaneous ion released diffusion) model [
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