Non-Monotonic Capacitance Change of Layered Ti3C2Tx MXene Film Structures under Increasing Compressive Stress

The electronic devices contain electrochemical components in their construction, leading to a need for a proper understanding of the behavior of energy storage materials under varying mechanical and temperature conditions. Supercapacitors based on Titanium Carbide (MXene) exhibit a non-monotonic capacitance behavior, which increases up to an applied pressure. However, further increases in pressure cause the capacitance to drop within the pressure range of less than 10 MPa. This behavior is intrinsic to the MXene film due to its charge storage mechanism. The surface activity is susceptible to change under pressure-induced functionality dissociation and inter-flake space shrinkage of the inherent layered structure. This research not only advances our comprehension of the mechanical-electrochemical coupling of layered MXene films under pressure, but also provides us with invaluable design ideas for flexible energy storage systems and electronics that utilize MXenes.

The demand for flexible energy storage devices has increased due to the growth of 5G and smart cities. Electrochemical capacitors are incredibly useful tools, known for their remarkably quick responsiveness, exceptional power density and dependability. Ti₃C₂T_x or titanium carbide is a promising material for these devices and falls under the category of MXenes, which are distinct from other pseudocapacitive materials like MoS₂ or MnO₂ as they have electrical conductivity and can form freestanding films through vacuum-assisted filtration or 3D shapes when printed using specialized techniques. The performance evaluation of MXenes is important under varying conditions of temperature and external loading (stresses) for a proper feasibility analysis of its use as an energy storage medium.

To this account,  Professor Qing Zhang, Mr. Ran Ning, Mr. Jinxin Cao from the Anhui University together with Mr. Qingrui Song,  Professor Jiaxin Ye from the Hefei University of Technology and Professor Bingqing Wei, the Director of the Center for Fuel Cells and Batteries from the University of Delaware conducted experiments to assess how squeezing thin films made of a material called Ti₃C₂T_x MXene affects their ability to store energy. They first made these films and tested their properties. Then they put pressure on the films and tested their ability to store energy again. They found that applying pressure to the films improved their energy storage capacity, but only up to a certain point. Beyond that point, the capacity started to decrease. The authors also measured the thickness of the films while applying pressure to better understand how this affected their energy storage properties. The research work was published in the peer-reviewed Journal Advanced Functional Materials.

MXene is a 2D material that has shown potential in forming a compact layered feature. . Utilizing three distinct alkaline electrolytes,  the capacitance of MXene under pressure was found to increase, which is typically attributed to enhanced electrical conductivity, improved surface wettability, and accelerated ion diffusion kinetics. However, a non-monotonic capacitance change was observed, with the capacitance increasing up to a certain pressure (P-Cmax) before decreasing. The authors suggest that this is due to an increase in active sites accessible to electrolytic ions as pressure increases, followed by a decrease as pressure continues to increase. The electrochemical properties of MXene films were found to be significantly driven by a surface-controlled pseudocapacitance.

The authors showed that the capacitance of layered Ti₃C₂T_x MXene materials first increased and then decreased under a pressure of less than 10 MPa. This behavior has not been seen before in other similar materials. The decrease in capacitance was due to the breakdown of surface properties and limited movement of ions in the material. The pressure that gave the highest capacitance depended on the type of electrolyte used. The study gives us a better understanding of how pressure affects the properties and performance of MXenes and could help design flexible electronics like pressure sensors and energy-storage devices for different applications.

In a nutshell, the research team investigated the cause of non-monotonic change in capacitance under increasing compressive stress of layered MXene film structure. The MXene film’s capacitance initially increases with pressure up to a certain pressure (P-Cmax) but drops beyond that due to pressure-induced dissociation of the layered structure. MXenes have electrical conductivity and can form freestanding films or versatile shapes. Squeezing thin Ti₃C₂T_x MXene films improves their energy storage capacity up to a certain point. However, beyond that point, the capacity decreases due to the breakdown of surface properties and limited ion movement. The study paves the way for the development of future energy storage devices based on MXene electrodes. The pressure range of 0 to 10 MPa is usually encountered in the environment and this variation in pressure can cause a change in the charge storage capability of the substrate material. The results of this study are useful for designing flexible electronics like pressure sensors and energy-storage devices.