Kinetics and Mechanism of the Oxidation of Hexacyanoferrate(II) by Dicyanobis(2,2′-dipyridyl)iron(III) for Aqueous Dye-Sensitized Solar Cells

Abstract

Cost-effective techniques to building environmentally friendly dye-sensitized solar cells (DSSCs) are frequently investigated. As a result, ruthenium-based sensitizers and corrosive iodide/triiodide-based mediators are being replaced with nonharmful, relatively inexpensive, and effective alternatives. Chemistry, such as redox kinetics and mechanistic pathways, is therefore critical in determining the potential application of alternative substances. In this study, the kinetic insights of the electron transfer reaction between iron(III)/iron(II) based potential sensitizer/mediator pair in aqueous medium were investigated, and the reaction mechanism was proposed. Dicyanobis(2,2′-dipyridyl)iron(III); ([Fe^III(bpy)₂(CN)₂]⁺) oxidized hexacyanoferrate(II); ([Fe^II(CN)₆]⁴⁻) in aqueous medium. The reaction was electrochemically spontaneous and feasible. The kinetics of the reaction was probed under the pseudo-first-order condition by maintaining excess concentration of [Fe^II(CN)₆]⁴⁻ over [Fe^III(bpy)₂(CN)₂]⁺. The reaction was examined in the visible region by measuring the absorbance over time at constant ionic strength and room temperature. The reaction products were identified spectrophotometrically. A homemade instrumentation system was used to collect data at millisecond intervals due to the fast oxidation of [Fe^II(CN)₆]⁴⁻ by [Fe^III(bpy)₂(CN)₂]⁺ in water. The reaction proceeded in a defined sequence, starting with an observed overall zero order. This was succeeded by a general second order, identified due to the presence of various reactant-related species. The reaction parameters, including proton concentration, ionic strength, dielectric constant, and temperature, were optimized to determine the rate-determining step of the process. As a result, two rate laws were proposed for the redox reaction.