Understanding Sorption of Aqueous Electrolytes in Porous Carbon by NMR spectroscopy

Ion adsorption at solid–water interfaces is the key process that underlies for many electrochemical processes including energy storage, electrochemical separations, and electrocatalytic applications involving aqueous electrolytes. However, the impact of the hydronium (H3O+) and hydroxide (OH–) ions on the ion adsorption and surface charge distributions are poorly understood, and many fundamental studies of electrochemical double layer capacitors focus on non-aqueous electrolytes to avoid addressing the role of the surface functional groups and the pH in altering ion uptake. This is particularly true for the most widely used materials – namely microporous carbons – where it is especially challenging to achieve a microscopic level characterisation of mixed ion adsorption at their electrode-electrolyte interfaces due to the complex ion dynamics; this is in part due to the disordered and hierarchical structures of the porous carbon electrodes. This work addresses these challenges starting with pH measurements to quantify the adsorbed H3O+ ion concentrations, revealing the basic nature of the activated carbon YP-50F, a common electrode material for supercapacitors and ion separation. Solid-state NMR spectroscopy is then used to study the uptake of the lithium bis(trifluoromethanesulfonyl)-imide (LiTFSI) aqueous electrolyte in this activated carbon across the full pH range. Analysis of the NMR data shows the importance of including the fast ion-exchange processes involving ions moving in and out of the carbon pores to obtain a more accurate quantification of the adsorbed ions. Under acidic conditions, more anions are adsorbed in the carbon pores than Li cations, charge compensation occurring via the uptake of protons. Under neutral and basic conditions, when the carbon’s surface charge is close to zero, Li+ and TFSI– ions exhibit similar but lower affinities towards the carbon pores. Our experimental approach and evidence of interfacial H3O+ adsorption will lead to advances in the fields of energy storage, colloidal systems, multiphase catalysis, and beyond, providing a methodology to relate local structure to performance.