context: The electrochemical performance of the 400-KOH-Ti3C2 anode was measured in a three-electrode system with the 2 M H2SO4 solution serving as the electrolyte. To determine the potential range of the 400-KOH-Ti3C2 electrode in this acid electrolyte, the CV curves of the 400-KOH-Ti3C2 electrode were collected in various potential windows of −0.7 to 0.3 V, −0.6 to 0.3 V, −0.5 to 0.3 V and −0.4 to 0.3 V (vs. Ag/AgCl) at a scan rate of 5 mV s−1 (Fig. S6†). It is found that the potential window of the 400-KOH-Ti3C2 electrode can be stably extended to −0.6 V (vs. Ag/AgCl) without the decomposition of the electrolyte. Thus, the potential range of the 400-KOH-Ti3C2 electrode is fixed at −0.6–0.3 V (vs. Ag/AgCl) in 2 M H2SO4 solution. To further evaluate the stable potential window of the 400-KOH-Ti3C2 electrode, we collected CV curves at various scan rates in the potential window of −0.6–0.3 V (vs. Ag/AgCl) as shown in Fig. 2a. The shape of CV curves can be well retained even at a high scan rate of 200 mV s−1, indicating good rate capability of the 400-KOH-Ti3C2 electrode. Except for CV measurements, the galvanostatic discharge–charge profiles of the 400-KOH-Ti3C2 electrode at various current densities were recorded as displayed in Fig. 2b. The discharge–charge curves exhibit a triangular symmetric shape with an inappreciable deviation, suggesting high reversibility and prominent pseudocapacitive behavior. Based on the above discharge–charge curves, the specific capacitance of the 400-KOH-Ti3C2 electrode is calculated to be 334.5 F g−1 at 1 A g−1. Even at 100 A g−1, the 400-KOH-Ti3C2 electrode still retains a specific capacitance of 93.3 F g−1 (Fig. 2c). Such high specific capacitance of the 400-KOH-Ti3C2 electrode should be attributed to the large interlayer distance, which provides more accessible active sites for the electrolyte ionic diffusion and electrochemical reaction. To illustrate this view, EIS measurements were performed at a potential of −0.1 V (vs. Ag/AgCl) for the 400-KOH-Ti3C2 electrode. The corresponding Nyquist plot of the 400-KOH-Ti3C2 electrode (Fig. 2d) shows a negligible semicircle and a quite small charge transfer resistance (Rct) of 1.2 Ω, which is obviously lower than that of original Ti3C2Tx (1.8 Ω, Fig. S7†), revealing a fast ion transport and good electronic conductivity for the 400-KOH-Ti3C2 electrode. The Nyquist plots are fitted with the equivalent circuit model shown in Fig. S8.† In addition, the 400-KOH-Ti3C2 electrode shows a stable cycle performance at 4 A g−1, maintaining a capacitance retention of about 87% over 40000 cycles and almost 100% coulombic efficiency. Such excellent cycling stability is comparable to that of most recently reported electrode materials for supercapacitors (see Table S1†). Furthermore, the SEM images of the 400-KOH-Ti3C2 electrode after 5000 cycles show that the layered structure of 400-KOH-Ti3C2 is well retained after cycling (Fig. S9†). It is noteworthy that the capacitance of the 400-KOH-Ti3C2 electrode decreases significantly in the first few hundred cycles, which is likely related to the phase transition of the electrode material.
answer: 400-KOH-Ti3C2 