Background introduction.

Supercapacitor is a promising electrochemical energy storage device, which is highly respected because of its high power density, reliability and long cycle life. Considering the increasing demand for miniaturized intelligent electronic products and electric vehicles, the volume energy density of supercapacitors is a more important parameter than the weight energy density for practical applications. In this case, the electrode material should have high inherent capacitance and high packing density. Due to high pseudo-capacitance and high density, self-supporting MXene thin films are expected to be used for compact energy storage. However, the slow ion transport caused by dense structures seriously hinders their rate performance.


Brief introduction of achievements.


In view of this, Professor Tao Ying of Tianjin University proposed a strategy to construct MXene-based flexible self-supporting thin film electrode with adjustable porous structure. Ti3C2Tx microgels decomposed by 3D structured hydrogels were reassembled with monolayer Ti3C2Tx nanowires with different mass ratios to form dense micro-scale 3D networks and macro-scale thin films (RAMX films). Through a good balance between density and porosity, the space utilization of the prepared films can be maximized, resulting in a high volume capacitance of 736F cm-3 at an ultra-high scanning rate of 2000 mV / cm-3. The prepared supercapacitor produces an excellent energy density of 40 Wh L kW L-1 at a power density of 0.83 kW L-1, and can maintain an energy density of 21 Wh L-1 even if the power density reaches 41.5 Wh L-1, which is the highest value reported so far for symmetrical supercapacitors based on aqueous electrolytes. More hopefully, the reassembled films can be used as electrodes for flexible supercapacitors, showing excellent flexibility and integration. The results of the related papers were published on Advanced Functional Materials on July 16, 2021 under the title of Reassembly of MXene Hydrogels into Flexible Films towards Compact andUltrafast Supercapacitors.


Full text guide.



Figure 1 shows:
The preparation diagram of Ti3C2Tx film, RAMX film (reassembled MXene film) and microgel film and the explanation of ion transport in different electrodes, in which Ti3C2Tx microgel was prepared by the decomposition of Ti3C2Tx hydrogel formed with the help of GO.

Figure 2 shows:
The morphology and structural characteristics of RAMX thin films. A) SEM images of Ti3C2Tx microgels. B) cross-sectional SEM images of RAMX-50% thin films and c) Ti3C2Tx thin films. D) XRD diagram of Ti3C2Tx film, RAMX film and microgel film. The nitrogen adsorption / desorption isotherms and pore size distribution of Ti3C2Tx, RAMX and microgel films. G) the relationship between the bulk density and specific surface area of the film and the content of Ti3C2Tx microgel.

Figure 3 shows:
Electrochemical properties of Ti3C2Tx film, RAMX film and microgel film. The CV curve collected at the scanning rate of a) 20 mV Smur1 and b) 2000 mV Smur1. C) the capacitance retention calculated according to the CV curve at a scanning rate in the range of 10 to 2000 mV Smurl. D) the weight and volume capacitance of the films with different microgel content at 2000 mV s-1. E) the relationship between the logarithm of peak current of anode (solid symbol) and cathode (hollow symbol) and the logarithm of scanning rate of Ti3C2Tx film and RAMX-50% film. F) Nyquist diagram collected at open circuit voltage.

Figure 4 shows:
The electrochemical performance of symmetrical supercapacitors made by pairing two identical RAMX-50% films. A) the CV curve compared with the device based on Ti3C2Tx thin film. B) the rate performance calculated from the GCD curve. C) cycle stability at 1000 mV smur1 scanning rate. The illustration shows the CV curves of the 1st and 20000 cycles. D) compared with the previously reported Ragone diagram of symmetrical supercapacitors based on MXene.

Figure 5 shows:
Manufacture and performance of flexible supercapacitors with PAM/H2SO4 electrolytes. A) schematic diagram of the device structure and b) corresponding digital photos. C) the digital photos of the flexible supercapacitor at different bending angles and d) the corresponding CV curve at the scanning rate of 20 mV. E) the GCD curves of two devices in series and in parallel at a current density of 2 A / g.

Summary.

The authors propose a unique assembly strategy to produce MXene thin film electrodes consisting of microgels and separate nanowires that balance porosity and density to provide excellent volume capacity and ultra-high rate performance when used in waterborne supercapacitors. The assembly process is essentially the integration of dispersed Ti3C2Tx microgels and monolayer nanowires in the film structure. The layered arrangement of monolayer nanowires forms a membrane structure with dense texture, and the well-dispersed microgel structure produced by decomposing 3D Ti3C2Tx hydrogel introduces more porosity in the whole process. When the proportion of microgel reaches 50%, the magnification ability of RAMX film is greatly improved, which is due to the introduction of mesopores to promote ion transport through the electrode at an acceptable density. Therefore, RAMX-50% thin films can provide ultra-high volume capacitance of 736F cm-3 at an ultra-high scanning rate of 2000 mV smurl. In addition, the assembled symmetrical supercapacitors show excellent cyclic stability, with a capacitance retention rate of 91.14% for more than 20000 cycles at 1000 mV Smurl, providing a high energy density of 40 Wh Lmurl at a power density of 0.83 kW Lmurl, and an energy density of 21 Wh Lmurl at a high power density of 41.5 kW Lmurl. In addition, the authors made a flexible supercapacitor that can withstand a variety of bending angles without attenuation of capacitance, and demonstrated the integration of devices for higher voltage or current output. This work may pave the way for the construction of flexible and compact next-generation electronic energy storage devices that require ultra-high power output.
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