Understanding the chemical shifts of aqueous electrolyte species adsorbed in carbon nanopores

TL;DR

This study combines computational methods to analyze how various factors influence the NMR chemical shifts of electrolyte ions adsorbed in carbon nanopores, aiding interpretation of experimental data for energy storage applications.

Contribution

It introduces a multi-method approach to understand the factors affecting chemical shifts of ions in nanoporous carbons, highlighting ion-specific effects like hydration shells.

Findings

Hydration shells significantly influence chemical shifts for large ions.

Ion nature determines the relative importance of different factors.

Complementary computational methods improve interpretation of NMR data.

Abstract

Interfaces between aqueous electrolytes and nanoporous carbons are involved in a number of technological applications such as energy storage and capacitive deionization. The disordered nature of the carbon materials makes it challenging to characterize ion adsorption and relationships between materials properties and performance. Nuclear magnetic spectroscopy can be very helpful in that respect thanks to its nuclei specificity and ability to distinguish between ions in the bulk and in pores. Nevertheless, several factors can affect the measured chemical shifts making it difficult to interpret experimental results. We use complementary methods, namely density functional theory calculations, molecular dynamics simulations and a mesoscopic model, to investigate various factors affecting the chemical shifts of aqueous electrolyte species. We show that the relative importance of these factors depend on the ion nature. In particular, the hydration shell has a much more pronounced effect for large polarizable ions such as Rb$^+$ and Cs$^+$ compared to Li$^+$.