EDL structure of ionic liquid-MXene-based supercapacitor and hydrogen bond role on the interface: a molecular dynamics simulation investigation

As a new class of electrodes, MXenes have shown excellent performance in supercapacitors. At the same time, ionic liquid (IL) electrolytes with wider electrochemical windows are expected to substantially increase the supercapacitor capacitance. The combination of MXenes and ILs is promising for energy storage devices with a high energy density and power density. The studies have indicated that the surface terminations of MXenes and the functional groups of ILs, can both strongly influence the supercapacitor's performance. However, studies at the molecular level are still lacking. In this work, we performed molecular dynamics simulations to investigate the interfacial structures and their influence on the energy storage mechanism. The results show that the two ILs exhibit very different charging rates, though the charge densities are similar after charging equilibrium. The interfacial analysis reveals different electrical double-layer (EDL) structures, in which most cations stay perpendicular to the Ti3C2(OH)2 electrode when some cations shift to a vertical arrangement near the Ti3C2O2 electrode. Such structures have led to the higher capacitance of the Ti3C2(OH)2 electrode, even more than 2 times that of the Ti3C2O2 electrode as the potential difference ranges from 0 to 2 V. It was also found that hydrogen bonds between the -OH groups of HEMIm+ cations and terminations of the MXene play an important role in improving the capacitances by aggregating more HEMIm+ cations on the surface of the Ti3C2(OH)2 electrode. Our work provides clear mechanistic evidence that both terminations of the MXene electrodes and functional groups of the IL electrolytes affect the interfacial structures and the EDL formation, further leading to the different supercapacitor performance, which will be helpful in designing highly efficient energy-storage devices.