Regulation of Ion Transport in Prussian Blue MOF Films by a Ru-Complex Primer Nanolayer on an ITO Electrode and Its Energy Storage Application

The control of ion and electron transport (ET) by external stimuli in redox-active crystals of Prussian blue (PB), a well-known nanoporous metal–organic framework (MOF), is key to the development of PB-based electronic and energy devices. To achieve ion storage and release in PB nanopores, thin layers of redox-active Ru complexes were inserted as a primer layer between an indium tin oxide (ITO) electrode and a PB film for electrochemical interfacing. On unmodified ITO electrodes, PB films can be oxidized to Berlin Green (BG) and reduced to Prussian White (PW) via two-step processes. However, the Ru-complex-modified heterolayer films ITO||(Ru-complex)3/(PB) exhibited only one redox process, i.e., either PB/PW or BG/PB, due to the ET rectification between the PB and the Ru complex, which depends on the redox potential of the Ru complex. Electrochemical quartz crystal microbalance (EQCM) measurements indicated that the storage/release of K+ ions in the PB nanopores is gated by the redox potential of the Ru-complex primer layer. The slower rate of ion transport relative to ET leads to a large hysteresis loop on EQCM, which can be exploited to develop ion storage memory devices. Additionally, a K+ ion rocking-chair-type asymmetric redox capacitor was constructed by combining two ITO||(Ru-complex)3/(PB) films. The electrochemical interfacing of redox-active PB MOF crystals on the heterojunction electrode surface can thus be expected to open new possibilities for charge storage via the regulation of ion transport in the PB-based devices.