Nonlinear diffusion Behavior for the Prussian blue electrode - II. Interpretation of variable diffusivity during the insertion/extraction processes

The apparent diffusivity (D-12) has been measured at the Prussian blue (PB) electrode. It was shown that D-12 varies with the applied potential (E) and there exists a minimum around the voltammetric peak potential. By applying the binary-solution model, which considers a partially reduced PB film as a binary mixture of PB and Everitt's salt (ES), the peak-shaped D-12-E behavior was successfully explained. According to the model, the concave-up D-12-E behavior is attributed to a corresponding concave-down excess energy-potential characteristic, which results in the maximum excess free energy and suppresses the random mixing of PB and ES. In addition, it is deduced that the coupled movement of K+ and e(-) into a PB lattice or out of an ES lattice results in a net binary diffusion of PB and ES. To be sure, the binary-solution model provides a common foundation for other insertion electrodes, as long as their redox behaviors are analogous to that of PB. (C) 2002 The Electrochemical Society. [DOI: 10.1149/1.1433472] All rights reserved.