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 [1,2]. Recently, a nanoconfined superionic state of water has been predicted [3,4], which exists at far milder temperatures than conventional superionic ices and at pressures similar to those created naturally in Van der Waals materials [5]. Interestingly, in sharp contrast to bulk ice, this phase is comprised of intact water molecules. This molecular superionic behaviour has possible applications in a range of electrochemical and electrocatalytic applications. However, at present, we lack the design principles necessary to design other materials with these properties.

In this talk, I will use 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 states in general. Similar to bulk superionic ice and other superionic materials [6], nanoconfined water conducts via concerted chain-like proton migrations, which cause the rapid propagation of defects [7]. However, unlike other molecular phases of water, its exceptional conductivity arises from: (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.

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