Electrochemistry explores the charge transfer reaction across the interfaces, such as solid/liquid interface, liquid/liquid interface and also solid/solid interface. Most reported charge transfer behaviours across the “solid/solid” interface are involved liquid electrolyte. For example, in the lithium-ion batteries, the redox of cobalt-based active material is coupled by the heterogeneous intercalation/deintercalation of Li⁺ ions, which is existed in the electrolyte solution. In another words, the electron transfer occurs at the solid/solid interface while the ion transfer occurs at the solid/electrolyte solution.

We employ SECM to construct the solid-solid interface and to study the electron transfer phenomena therein. That means there is no liquid electrolyte environment. When the electron transfer occurs at the solid/solid interface, the counterion transfer will occur in the crystal due to the crystalline defects. For example, we cultured the NaCl microcrystals, which are doped with iron oxides, between the pair electrodes on a microchip. The multi-step electron transfer processes of iron from Fe(0) to Fe(VI) through Fe(II) and Fe(III) were observed.

The solid-state electrolyte solution just like an inorganic polymer. The electron transfer is wired through hopping between the neighboring redox lattice sites, coupling with the counterion transfer in the crystalline defects. The electrochemical behavior in the solid-state inorganic “polymer” electrolyte will be discussed in this presentation.