The ion adsorption at solid–water interfaces is the key underlying process for electrochemical energy storage, electrochemical separations, and electrocatalytic applications involving aqueous electrolytes. However, the impact of charged species including hydronium (H₃O⁺) and hydroxide (OH^-) on the ion adsorption and surface charge distribution are poorly understood, hindering the design and optimisation of devices with enhanced performance. It is challenging to achieve a microscopic level characterisation of mixed ion adsorption at the electrode-electrolyte interface due to the complex ion dynamics and the disordered structures of the porous carbon electrodes.

In this work, nuclear magnetic resonance (NMR) spectroscopy has been applied for the study of ion dynamics and charge storage mechanisms of aqueous supercapacitors. The unique power of NMR spectroscopy for the characterisation of electrode-electrolyte interfaces is that it can differentiate ions adsorbed inside the pores apart from the ions in the bulk counterpart. The element selective feature of NMR spectroscopy allows the separate observation of different chemical species in the electrolyte. Specifically, the cation and anion adsorption of LiTFSI (lithium bis(trifluoromethylsulphonyl)imide) aqueous electrolyte in activated carbon, a common electrode material for supercapacitors, are quantified by ⁷Li and ¹⁹F NMR. Apart from quantifying the electrolyte ions, the H₃O⁺ uptake by activated carbon is also measured through pH measurements, gathering a microscopic understanding of the mixed-ion adsorption at the carbon-electrolyte interfaces. Remarkably, the data suggests that H₃O⁺ plays a key role in maintaining the local charge neutrality within carbon nanopores due to the surface basicity of the activated carbon. To explore this further, the carbon surface was modified to be close to neutral through chemical oxidation. By comparison with the pristine activated carbon, the functionalized carbon exhibit changes in the ion adsorption, leading to enhanced electrochemical capacitance of the modified carbon materials. To gain insights into the enhanced capacitance and the role of H₃O⁺ in the charge storage mechanisms, operando NMR measurements were performed as a function of the electrolyte pH. These findings highlight the significance of H₃O⁺ in electrochemical systems involving aqueous electrolytes, guiding the design and development in the fields of energy storage, colloidal systems, multiphase catalysis, and beyond.