INTERPRETATION OF VARIABLE DIFFUSIVITY OBSERVED AT THE PRUSSIAN BLUE ELECTRODE DURING THE INSERTION/EXTRACTION PROCESSES

摘要

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 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 diffusion of K~+ and e~- into a PB lattice or out of an ES lattice will result 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.