Development of MXene-MOF synergized electrode materials for efficient energy storage applications

Conventionally supercapacitors (SCs) tend to possess high power density and low energy density, whereas batteries have high energy density but low power density. Therefore, it is imperative to design SC electrodes with energy density comparable to batteries if not equal, while maintaining their characteristic high values of power density. The goal is to shrink the gap between SCs and batteries, as well as improvement in capacity retention is also of vital. It is therefore very important to study and evaluate new composite materials that have not been studied much. Identification of relevant material, synthesis route and stability of the active materials are the key questions that need to be thoroughly addressed. MXene (2D titanium carbide) is widely studied in recent time for a variety of applications. This family of 2d materials offers tremendous properties such as hydrophilicity, metallic conductivity as well as high redox activity when employed in energy storage applications. However, MXene possess excellent electrochemical properties, yet there are some structural defects such as restacking and sheet crushing that limit their stability which is of paramount importance for an energy storage device. Material design that could prevent the sheets from restacking and cease the process of sheet crushing is very important. It is therefore concluded that if gaps between MXene sheets should be filled with an appropriate material that could fit in the spacing between the flakes without harming the nature of actual material. Metal organic frameworks (MOFs) a class of highly crystalline materials, with porous structure and large surface area which is widely studied for SCs applications. Pristine MOFs suffer from compromised values of specific capacities and rate capability when used as electrode therefor using them in a composite form for electrochemical applications is preferable. Herein, a relatively novel strategy is proposed to design composite material between MXene, MOFs and MOF derived structures. It is also worth mentioning that synthesis strategy proposed here involves no external binder instead all the active materials are directly grown in an in-situ growth method on the substrate to achieve a direct electrode growth. MOFs and MXene flakes are blended, in a hydrothermal synthesis route and subsequently grown over the Ni foam (NF). This way ion/electron diffusion pathways could be shortened, and electroactive sites could be enhanced. Consequently, MXene flakes were prevented from restacking that improved overall stability of the composite, whereas high crystallinity and surface area of MOFs helped in overall improvement of rich active sites. Three different electrode materials and devices were fabricated and assembled. MXene-CoS, MXene-FeCu MOF and MXene-Fe MOF were individually tested in three electrode system as well as assembled in a two electrode supercapacitor device. As prepared electrodes and devices manifested excellent rate capability and cyclic performances. Moreover, all the electrodes showed superb values of energy and power density that are crucial to evaluate the performance of any energy storage device. The proposed method offers a generic protocol to design and fabricate MXeneMOF synergized composite with excellent electrochemical energy storage properties as well as extraordinary cyclic stability. Moreover, this study also offers an endless potential to be explored for future electrochemical energy storage devices especially for commercial purposes.