Electrochemical quartz crystal microbalance studies of anion and pH effects on water fluxes accompanying redox switching of Prussian blue

Abstract

A kinetic study, using cyclic voltammetry and the electrochemical quartz crystal microbalance (EQCM), is made of the PB switching process in 0.1 and 0.5 M potassium supporting electrolytes at different potential scan rates. The first cyclic potential scan in the supporting electrolytes we studied is very different from all subsequent potential scans, which are all quite similar. During the first potential scan in 0.5 M K 2SO 4 solution at 0.100 V s −1, very little water transfers between the film and the solution. After the first potential cycle, the average mole ratio of water/potassium is approx. −0.5 at 0.010 V s −1 and is less than −0.5 at 0.100 V s −1. Our results demonstrate that water transfer is decoupled from K +-transfer during both the oxidation and reduction of Prussian blue. EQCM measurements were then carried in 0.1 M K 2SO 4 (pH 2.7) and 0.2 M potassium hydrogen phthalate (pH 3.8), potassium p-toluenesulfonate (pH 7.1) and a potassium borate buffer (pH 7.7) to investigate the possibility of hydrated proton and/or anions transfers during the redox switching process. The molar mass of the transferring species depends markedly on the electrolyte solution used, the fraction of PB that is oxidized and the potential scan rate. The molar mass was calculated assuming only K + and water transfer occurs. At pH 2.7, the marked change in the molar mass as compared to 0.1 and 0.5 M K 2SO 4 supporting electrolytes was rationalized by assuming that hydrated proton transfer takes place. In all the other solutions, the change in the molar mass indicated that anion transfer competes with K +-transfer.