Self-Supporting, Binder-Free, and Flexible Ti₃C₂T_x MXene-Based Supercapacitor Electrode with Improved Electrochemical Performance

MXenes have shown great potential for supercapacitor electrodes due to their unique characteristics, but simultaneously achieving high capacitance, rate capability, and cyclic stability along with good mechanical flexibility is exceptionally challenging. Here, highly enhanced capacitance, rate capability, and cyclic stability, as well as good mechanical flexibility for T₃C₂T_x MXene-based supercapacitor electrodes are simultaneously obtained by engineering the electrode structure, modifying the surface chemistry, and optimizing the fabrication process via an optimized integration approach. This approach combines and more importantly optimizes three methods that all require a calcination process: carbonizing in situ grown polymer ("C_polymer") on the MXene, alkali treatment ("A"), and template sacrificing ("P"); and the optimized processes lead to more abundant active sites, faster ion accessibility, better chemical stability, and good mechanical flexibility. The obtained P-MXene/C_polymer-A electrodes are binder-free and self-supporting and not only have good mechanical flexibility but also demonstrate much larger capacitances and better rate performance than the pristine MXene electrode. Specifically, the P-MXene/C_PAQ-A electrode (PAQ: quinone-amine polymer) achieves a high capacitance of 532.9 F g^-1 at 5 mV s^-1, together with superior rate performance and improved cyclic stability (97.1% capacitance retention after 40 000 cycles at 20 A g^-1) compared with the pristine MXene (79.6% retention) and P-MXene-A (77.3% retention) electrodes. In addition, it is discovered that carbonizing in situ grown polymers can variously remove the -F group and the removal effect can be accumulated with that by the alkali treatment.