Confinement-Controlled Water Engenders Unusually High Electrochemical Capacitance

Abstract

The electrodynamics of nanoconfined water have been shown to change dramatically compared to bulk water, opening room for safe electrochemical systems. We demonstrate a nanofluidic "water-only" battery that exploits anomalously high electrolytic properties of pure water at firm confinement. The device consists of a membrane electrode assembly of carbon-based nanomaterials, forming continuously interconnected water-filled nanochannels between the separator and electrodes. The efficiency of the cell in the 1−100 nm pore size range shows a maximum energy density at 3 nm, challenging the region of the current metal-ion batteries. Our results establish the electrodynamic fundamentals of nanoconfined water and pave the way for low-cost and inherently safe energy storage solutions that are much needed in the renewable energy sector.

Wet Etching of Silicon in Planar Nanochannels

“…It has been widely accepted that under nanoconfinement, water is actually a system consisting of a bulk phase water in the center of the confinement, an interfacial ice-like and well-ordered immobile water layer (referred to as the hydration layer or the adsorbed water phase, with a typical thickness of 1.5 nm), resulted from the hydration force and van der Waals force near the confinement surface, − and an intermediate phase where water molecules loosely couple in the direction perpendicular to the surface in between. Because of the different water structures within the hydration layer and the intermediate phase layer, the corresponding water properties such as water viscosity, dielectric constant, proton mobility, and chemical activity can be drastically different from the bulk water. − Therefore, in our case, it is reasonable to assume that the presence of these two unique water layers near the nanochannel wall would increase the activation energy for the KOH–silicon etching reaction and thus lead to lower etching rates in smaller nanochannels. On the other hand, as the nanochannel height increases, the effect of the hydration/intermediate layer will gradually decrease, and the etching rate would saturate by reaching a rate plateau.…”