Nanoconfined superionic water is a molecular superionic

TL;DR

This study demonstrates that nanoconfined water can be both molecular and superionic, exhibiting high ionic conductivity through a flexible hydrogen-bond network, with implications for designing new energy materials.

Contribution

The paper reveals how nanoconfined water can be both molecular and superionic, highlighting the role of low proton transfer barriers and flexible hydrogen bonds, which is a novel insight.

Findings

Nanoconfined water is both molecular and superionic.

Proton conduction occurs via concerted chain-like migrations.

Key factors include low transfer barriers and flexible hydrogen bonds.

Abstract

Superionic ice, where water molecules dissociate into a lattice of oxygen ions and a rapidly diffusing 'gas' of protons, represents an exotic state of matter with broad implications for planetary interiors and energy applications. Recently, a nanoconfined superionic state of water has been predicted which, in sharp contrast to bulk ice, is comprised of intact water molecules. Here, we apply machine learning and electronic structure simulations to establish how nanoconfined water can be both molecular and superionic. We also explore what insights this material offers for superionic materials and behavior more generally. Similar to bulk ice and other superionic materials, nanoconfined water conducts via concerted chain-like proton migrations which cause the rapid propagation of defects. However, unlike other molecular phases of water, its exceptional conductivity arises from the activation of the Grotthuss mechanism by (i) low barriers to proton transfer and (ii) a flexible hydrogen-bonded network. We propose that these are two key characteristics of fast ionic conduction in molecular superionics. The insights obtained here establish design principles for the discovery of other molecular superionic materials, with potential applications in energy storage and beyond.