Nanoionics for Energy Storage
Nanoionics is a study of ion transfer in interfaces at nanoscale regime. Manipulation of polymer molecular structure for enhanced ionic transport a plays primary role in designing and developing the optimized electrochemical devices.
EDLC formation mechanism in interfaces
Planar device architecture with highly confined ultra- thin solid polymer electrolyte film is considered as an essential approach to demonstrate electric double layer (EDL) formation mechanism at interfaces. Due to the synergistic effect of self-assembled polymer molecular structure and higher ionic conductivity, the Pt/PVA–KOH/Pt planar micro-supercapacitor (MSC) shows excellent volumetric capacitance behavior. Origin of the electromotive force (emf) and the effect on chemical potential gradient of dissolved ionic carriers in the interface have been investigated to understand the electrical double layer formation mechanism. This fundamental aspect of research is essential to understand the importance of confinement effect on the polymer molecular structure in rationally optimizing and developing not only the energy storage devices (EDL capacitor, thin-film battery, etc.) but also the EDL-based electronic devices.
Proposed ion transfer kinetics and electrostatic potential profile across the EDL region in the Pt/PVA–KOH/Pt planar MSC device (charging condition).
Charge separation and energy storage at electrode-polymer interface
In the confined poly(N‑vinyl imidazole) (PVI) structure, the hydrogen bonding facilitates long range as well as fast HO− ion migration in the PVI structure, which plays crucial role in determining charge separation and energy storage characteristics of the ITO/PVI−KOH/ITO planar MSC device. The hydroxide ion migration occurs in long range via breaking and forming of hydrogen bonds. Since hydrogen bonding in water is a three dimensional network, we have shown only representative hydrogen bonds for clarity. Previous studies have shown that OH− is a hypercoordinate species having four hydrogen bonds, HO−(H2O)4.35 Hence, we assumed that OH− ion in pristine state is a hypercoordinate ion with three hydrogen bonds and one coordination bond with K+(H2O)6 ion (Fig.8a) leading to form HO−(K+(H2O)6(H2O)3) species. In the charge separated state it transforms into a hypercoordinate hydroxide ion, HO-(H2O)4, as shown in charge separated state (Fig. b)
Proposed ionic transportation in the interfacial Poly(N‑vinyl imidazole) structure under (a) pristine state and (b) charge separated state (biasing condition).