Intercalating Ultrathin MoO_3 Nanobelts into MXene Film with Ultrahigh Volumetric Capacitance and Excellent Deformation for High-Energy-Density Devices

The restacking hindrance of MXene films restricts their development for high volumetric energy density of flexible supercapacitors toward applications in miniature,portable,wearable or implantable electronic devices.A valid solution is construction of rational heterojunction to achieve a synergistic property enhancement.The introduction of spacers such as graphene,CNTs,cellulose and the like demonstrates limited enhancement in rate capability.The combination of currently reported pseudocapacitive materials and MXene tends to express the potential capacitance of pseudocapacitive materials rather than MXene,leading to low volumetric capacitance.Therefore,it is necessary to exploit more ideal candidate materials to couple with MXene for fully expressing both potentials.Herein,for the first time,high electrochemically active materials of ultrathin MoO3 nanobelts are intercalated into MXene films.In the composites,MoO3 nanobelts not only act as pillaring components to prevent restacking of MXene nanosheets for fully expressing the MXene pseudocapacitance in acidic environment but also provide considerable pseudocapacitive contribution.As a result,the optimal M/MoO3 electrode not only achieves a breakthrough in volumetric capacitance(1817 F cm-3 and 545 F g-1),but also maintains good rate capability and excellent flexibility.Moreover,the corresponding symmetric supercapacitor likewise shows a remarkable energy density of 44.6 Wh L-1(13.4 Wh kg-1),rendering the flexible electrode a promising candidate for application in high-energy-density energy storage devices.

Concentrated hydrogel electrolyte for integrated supercapacitor with high capacitance at subzero temperature

Hydrogel electrolytes with anti-freezing properties are crucial for flexible quasi-solid-state supercapacitors operating at low temperatures.However,the electrolyte freezing and sluggish ion migration caused by the cold temperature inevitably damage the flexibility and electrochemical properties of supercapacitors.Herein,we introduce the concentrated electrolyte into a freezecasted poly(vinyl alcohol)hydrogel film not only reducing the freezing point of the electrolyte(−51.14℃)in gels for ensuring the flexibility,but also improving the ionic conductivity of the hydrogel electrolyte(5.92 mS cm^(−1)at−40℃)at low temperatures.As a proof,an all-in-one supercapacitor,synthesized by the one-step polymerization method,exhibits a good specific capacitance of 278.6 mF cm^(−2)at−40℃(accounting for 93.8%of the capacitance at room temperature),high rate performance(50%retention under the 100-fold increase in current densities),and long cycle life(88.9%retention after 8,000 cycles at−40℃),representing an excellent low-temperature performance.Our results provide a fresh insight into the hydrogel electrolyte design for flexible energy storage devices operating in the wide range of temperature and open up an exciting direction for improving all-in-one supercapacitors.