MXene for supercapacitor application which shows outstanding proton-induced pseudocapacitance in acidic aqueous electrolytes. [14,15] High electronic conductivity (up to 15 000 S cm −1), high packing density (up to 4 g cm −3), along with high pseudocapacitance endow Ti 3 C 2 T x ultrahigh volumetric capacitance (≈1500 F cm −3), which gives Ti 3 C 2 T x incomparable advantages over other electrode materials for supercapacitors. [16] However, Ti 3 C 2 T x electrodes suffer from long ion transport pathways due to the stacking nature of 2D materials, leading to ultra-low rate performance in a thick electrode. When used as a power supply for electronic devices where high areal energy densities at high rates are required, MXene electrodes need to be thick enough to ensure high charge storage capability. In this context, the ion transport issue becomes more critical because the low rate performance will deteriorate with the increase of film thickness. [17] Numerous efforts have been made to alleviate the restacking issue of Ti 3 C 2 T x film electrodes. Typical strategies include interlayer insertion of graphene, carbon nanotube or other nanomaterials, pillared structure design, template sacrifice method, vertical alignment, and etching holes. [17-27] However, by most of the reported approaches, the rate performances are increased at the expense of volumetric capacitance because of the introduction of inactive materials, excess spacing, or active materials with lower volumetric capacitance. For example, ≈15% decrease

MXenes are a class of two-dimensional (2D) transition metal carbides, nitrides and carbonitrides that have shown promise for high-rate pseudocapacitive energy storage. However, the effects that irreversible oxidation have on the surface chemistry and electrochemical properties of MXenes are still not understood. Here we report on a controlled anodic oxidation method which improves the rate performance of titanium carbide MXene (Ti3C2Tx, Tx refers to -F, =O, -Cl and -OH) electrodes in acidic electrolytes. The capacitance retention at 2000 mV/s (with respect to the lowest scan rate of 5 mV/s) increases gradually from 38% to 66% by tuning the degree of anodic oxidation. At the same time, a loss in the redox behavior of Ti3C2 is evident at high anodic potentials after oxidation. Several analysis methods were employed to reveal that preserving the structure and surface chemistry while simultaneously introducing defects, without compromising electrochemically active sites, are key factors for improving the rate performance of Ti3C2Tx. This study demonstrates improvement of the electrochemical performance of MXene electrodes by controlling the surface chemistry and transition metal stoichiometry.