Minmin Hu

MXene, a new intercalation pseudocapacitive electrode material, possesses a high theoretical capacitance for supercapacitor application. However, limited accessible interlayer space and active sites are major challenges to achieve this high capacitance in practical application. In order to stimulate the electrochemical activity of MXene to a greater extent, herein, a method of hydrothermal treatment in NaOH solution with reducing reagent-citric acid is first proposed. After this treatment, the gravimetric capacitance of MXene exhibits a significant enhancement, about 250% of the original value, reaching 543 F g⁻¹ at 2 mV s⁻¹. This improved electrochemical performance is attributed to the tailoring of an interlayer structure and surface chemistry state. An expanded and homogenized interlayer space is created, which provides enough space for electrolyte ions storage. The –F terminations are replaced with O-containing groups, which enhances the hydrophilicity, facilitating the electrolyte’s accessibility to MXene’s surface, and makes MXene show stronger adsorption for electrolyte ion-H⁺, providing sufficient electrochemical active sites. The change in terminations further leads to the increase in Ti valence, which becomes more prone to reduction. This work establishes full knowledge of the rational MXene design for electrochemical energy storage applications.

Heteroatom doping is considered an effective method to substantially improve the electrochemical performance of Ti₃C₂T_x MXene for supercapacitors. Herein, a facile and controllable strategy, which combines heat treatment with phosphorous (P) doping by using sodium phosphinate (NaH₂PO₂) as a phosphorus source, is used to modify Ti₃C₂T_x. The intercalated ions from NaH₂PO₂ act as “pillars” to expand the interlayer space of MXene, which is conducive to electrolyte ion diffusion. On the other hand, P doping tailors the surface electronic state of MXene, optimizing electronic conductivity and reducing the free energy of H⁺ diffusion on the MXene surface. Meanwhile, P sites with lower electronegativity owning good electron donor characteristics are easy to share electrons with H⁺, which is beneficial to charge storage. Moreover, the adopted heat treatment replaces –F terminations with O-containing groups, which enhances the hydrophilicity and provides sufficient active sites. The change in surface functional groups increases the content of high valence-stated Ti with a high electrochemical activity that can accommodate more electrons during discharge. Synergistic modification of interlayer structure and chemical state improves the possibility of Ti₃C₂T_x for accommodating more H⁺ ions. Consequently, the modified electrode delivers a specific capacitance of 510 F g⁻¹ at 2 mV s⁻¹, and a capacitance retention of 90.2% at 20 A g⁻¹ after 10,000 cycles. The work provides a coordinated strategy for the rational design of high-capacitance Ti₃C₂T_x MXene electrodes.

MXene is a new intercalation pseudocapacitive electrode material for supercapacitor application. Intensifying fast ion diffusion is significantly essential for MXene to achieve excellent electrochemical performance. The expansion of interlayer void by traditional spontaneous species intercalation always leads to a slight increase in capacitance due to the existence of species sacrificing the smooth diffusion of electrolyte ions. Herein, an effective intercalation-deintercalation interlayer design strategy is proposed to help MXene achieve higher capacitance. Electrochemical cation intercalation leads to the expansion of interlayer space. After electrochemical cation extraction, intercalated cations are deintercalated mostly, leaving a small number of cations trapped in the interlayer silt and serving as pillars to maintain the interlayer space, offering an open, unobstructed interlayer space for better ion migration and storage. Also, a preferred surface with more −O terminations for redox reaction is created due to the reaction between cations and −OH terminations. As a result, the processed MXene delivers a much improved capacitance compared to that of the original Ti₃C₂T_x electrode (T stands for the surface termination groups, such as −OH, −F, and −O). This study demonstrates an improvement of electrochemical performance of MXene electrodes by controlling the interlayer structure and surface chemistry.