Dynamics of the filling of a porous flow-through electrode, which is operating in a circulation mode, with metal

Abstract

A model, developed previously for describing the filling of a porous flow-through electrode (PFE) with a metallic deposit, is used to demonstrate that, in contradistinction to a straight-through mode, a decrease in the concentration of metal in a circulating solution (approximately by an order of magnitude in the course of the time period required for filling a critical cross-section of PFE with metal) leads to a change both in the direction of the spatial redistribution of metal inside the porous matrix and in the dynamics of the variation of its basic parameters. In a straight-through mode, the metal distribution inside a PFE is defined by the action of two opposite factors (development of a surface inside the working layer Lef at the expense of the growth of the diameter of fibers and, vice versa, its shrinking at the expense of a decrease in Lef with time and the expulsion of the process of deposition of metal in the direction of the front end of PFE) and is characterized by nonmonotonous dependences of the current efficiency and the outlet concentration of metal on time. The predominant tendency in the case of a circulation mode is different: Lef rises with time, which leads to a displacement of the process of the metal deposition in the direction of the rear end of PFE (at rear solution input), increase of maximal amount of deposit by approximately 1.4 times at L > Lef, as well as to dependences of the current efficiency and the ratio between the concentrations of metal at the outlet and inlet of PFE that are monotonously decreasing with time. At the expense of a continuous variation of the effective working surface area of PFE and the mass transfer coefficient with time, the circulation mode is characterized by a nonlinear dependence of the logarithm of the metal concentration in the circulating solution on the electrolysis duration. A comparison of indicators that are characterizing the dynamics of the filling of a PFE with metal in the course of galvanostatic and potentiostatic modes of electrolysis is performed. It is established that the application of a potentiostatic mode of electrolysis under the conditions that provide for the predominance of the target reaction is accompanied by a slight decrease in the maximum quantity of metal, but a very significant decrease (by 5–6 times) in the specific spendings of electric energy.