Unveiling the energy storage of supercapacitors containing water-in-salt electrolytes confined in MXenes by molecular dynamics simulations†

In recent decades, the interest in sustainable energy production solutions has surged, driven by the need to control and mitigate the growing impacts of anthropogenic global warming. This increasing focus has emphasized the necessity for effective energy storage solutions. Batteries and supercapacitors are the most prominent and widely utilized energy storage devices. In this context, highly concentrated aqueous electrolytes, known as “Water-in-Salt Electrolytes” (WiSE), offer an efficient, safe, and environmentally friendly alternative to organic solvent electrolytes in energy storage devices. WiSEs are considered excellent electrolyte candidates for supercapacitors and can be combined with various types of electrodes, such as transition metal carbides with two-dimensional (2D) structures, known as MXenes. Here, we present a detailed computational study of WiSEs confined in planar and porous Ti₃C₂F₂ electrodes, utilizing molecular dynamics (MD) simulations with an extension of the constant potential method (CPM_χ), which accounts for the different electronegativities of heterogeneous electrodes. Data analysis with unsupervised learning techniques for planar electrodes revealed that, regardless of the type of WiSE and electrode, cations maintain their solvation spheres when attracted to the negative electrode or repelled by the positive electrode. The behavior of water molecules within the layers closest to the electrode appears to be more sensitive to the chemical nature of the electrode. In porous Ti₃C₂F₂ electrodes with a pore size of 12.6 Å, the NaClO₄ electrolyte demonstrates greater charge accumulation compared to other WiSEs. Furthermore, during the simulation, the charging mechanism of porous electrodes with sodium WiSEs evolves from an initial counter-ion adsorption mechanism to a co-ion desorption mechanism over the simulation time.