Reversible Surface Two-Electron Transfer Reactions in Square Wave Voltcoulommetry: Application to the Study of the Reduction of Polyoxometalate [PMo₁₂O₄₀]^3– Immobilized at a Boron Doped Diamond Electrode

Reversible surface two-electrons transfer reactions (stepwise processes) are analyzed using square wave voltcoulommetry (SWVC), which is a variety of square wave techniques based on the measurement of the transferred charge. Such reversible surface redox processes are exhibited by many two-redox center and multicenter biomolecules (proteins, enzymes, ...) and inorganic molecules like polyoxometalates (POMs), which have very interesting applications, mainly as electrocatalysts. Because of the stationary character of the response obtained, the key parameters that govern the cooperativity degree of the two reversible electron transfers (ETs) are the difference between their formal potentials, Δ<i>E</i>⁰, and the square wave amplitude, |<i>E</i>_SW|, whose combined effect sets the two peaks → one peak transition in the response. Working curves based on the variation of the peak parameters (peak potentials, half-peak widths, and peak heights) with Δ<i>E</i>⁰ and |<i>E</i>_SW| are given, from which the formal potentials and the total surface excess can be accurately determined. SWVC has been applied to the study of the reduction of polyoxometalate [PMo₁₂O₄₀]^3– adsorbed at a boron doped diamond electrode (BDD), for which three stable and well-defined reversible charge peaks, corresponding to three cooperative EE processes, are obtained in the interval (0.6, −0.2) V by using low square wave frequencies. From the analysis of these peaks, the values of the total surface excess and the formal potentials of the six ETs have been obtained in aqueous media for two electrolytes: HClO₄ and LiClO₄