Nuclear Magnetic Resonance Studies of Aqueous Electrolyte Ion Adsorption in Microporous Carbon Electrodes

Bragg, Ryan and Griffin, John (2025) Nuclear Magnetic Resonance Studies of Aqueous Electrolyte Ion Adsorption in Microporous Carbon Electrodes. PhD thesis, Lancaster University.

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Abstract

To improve processes that rely on guest-surface interactions within microporous carbonaceous materials (e.g., electrocatalysis, gas sorption, and energy storage) it is critical to understand the behaviour of adsorbed species within these materials. A particular application of interest is electric double-layer capacitors (or supercapacitors), where charge is stored through the non-faradaic electrosorption of ions at the electrolyte-surface interface. However, despite extensive research, the adsorption mechanisms in these complex nanoscale structures remain unclear. This is especially true for aqueous electrolytes, where limited prior study and the challenges of pH effects further complicate our understanding. In this thesis, solid-state nuclear magnetic resonance (NMR) spectroscopy is used to observe and quantify aqueous adsorbate partitioning behaviour driven by spontaneous physisorption within the micropores. The solvation properties of the electrolyte ions are shown to influence the ionophilicity/ionophobicity of the adsorbate-carbon system, with ionophilic and ionophobic systems exhibiting distinct behaviour concerning the electrolyte loading volume. Additionally, the diameter of micropores is shown to influence spontaneous electrolyte partitioning behaviour and disrupt ion solvation. In situ NMR spectroscopy experiments conducted on a working supercapacitor with microporous carbon electrodes and aqueous sodium sulphate or aqueous sodium bis(trifluoromethane)sulphonimide electrolytes reveals that spontaneous electrolyte partitioning behaviour affects the charge-balancing mechanism. The findings suggest that spontaneously ionophilic systems favour charge-balancing by counter-ion adsorption under an applied voltage, and spontaneously ionophobic systems favour a co-ion ejection mechanism under an applied potential. Finally, planewave-based electronic density functional theory (DFT) simulations were conducted on periodic graphene systems, with magnetic shielding parameters calculated using the gauge-including projector augmented wave (GIPAW) method. The complexities arising from the unique electronic structure of graphene are discussed, and the associated convergence challenges in determining magnetic shielding values are identified. Converged nucleus-independent chemical shift (NICS) values and 13C chemical shifts are presented for both a pristine periodic graphene system and a Stone-Wales defected system. The work outlined in this thesis provides molecular-level insight into the role of electrolyte properties on spontaneous physisorption behaviour and charged electrosorption behaviour within microporous carbon electrodes.