High-Capacitance Mechanism for Ti₃C₂T_x MXene by in Situ Electrochemical Raman Spectroscopy Investigation.

Abstract

MXenes represent an emerging family of conductive two-dimensional materials. Their representative, Ti₃C₂T_x, has been recognized as an outstanding member in the field of electrochemical energy storage. However, an in-depth understanding of fundamental processes responsible for the superior capacitance of Ti₃C₂T_x MXene in acidic electrolytes is lacking. Here, to understand the mechanism of capacitance in Ti₃C₂T_x MXene, we studied electrochemically the charge/discharge processes of Ti₃C₂T_x electrodes in sulfate ion-containing aqueous electrolytes with three different cations, coupled with in situ Raman spectroscopy. It is demonstrated that hydronium in the H₂SO₄ electrolyte bonds with the terminal O in the negative electrode upon discharging while debonding occurs upon charging. Correspondingly, the reversible bonding/debonding changes the valence state of Ti element in the MXene, giving rise to the pseudocapacitance in the acidic electrolyte. In stark contrast, only electric double layer capacitance is recognized in the other electrolytes of (NH₄)₂SO₄ or MgSO₄. The charge storage ways also differ: ion exchange dominates in H₂SO₄, while counterion adsorption in the rest. Hydronium that is characterized by smaller hydration radius and less charge is the most mobile among the three cations, facilitating it more kinetically accommodated on the deep adsorption sites between the MXene layers. The two key factors, i.e., surface functional group-involved bonding/debonding-induced pseudocapacitance, and ion exchange-featured charge storage, simultaneously contribute to the superior capacitance of Ti₃C₂T_x MXene in acidic electrolytes.