The electrode reactions of Fe(CN) 63− and Fe(CN) 64− ions studied at temperatures below the melting point of stoichiometric electrolytes

Abstract

The electrode reactions of Fe(CN) 6 3− and Fe(CN) 6 4− ions have been studied at temperatures below the melting point of stoichiometric electrolytes. Tetramethylammonium cation hydrates: (CH 3) 4NOH· nH 2O( n = 5, 7.5, 10) and (CH 3) 4NF·4H 2O were selected as electrolytes. The redox active ion with higher electric charge, Fe(CN) 6 4−, is more stable at temperatures below as well as above the melting point of these electrolytes. However, the temperature dependence of the redox potential of the Fe(CN) 6 3−/Fe(CN) 6 4− couple is more pronounced in these conditions. In the limited temperature range below the electrolyte melting point the kinetics of the reaction are controlled by the rate of reactant transport towards the electrode surface. The apparent diffusion coefficient of redox active ions does not change substantially at temperatures around the electrolyte melting point. The activation energy of reactant transport is twice as large in frozen than in liquid electrolyte. It has been concluded that the motion of the redox active ions is restricted to a limited volume—the intergrain space of the electrolyte. This conclusion is supported by results of experiments performed in a cell filled with chemically inert beads and liquid electrolyte.