The progress of high-performance miniature supercapacitors plays an important role in the development and application of miniature electronic devices. Graphene is considered to be the preferred choice of micro supercapacitor electrode materials due to its advantages such as high theoretical specific surface area, good conductivity, and excellent mechanical properties. However, the characteristics of graphene's two-dimensional structure make it easy to stack and agglomerate, hinder its effective contact with the electrolyte, and inhibit the material's electrochemical energy storage performance. Therefore, there is an urgent need to develop an efficient method to adjust the graphene-based electrode structure, alleviate the problem of stacking agglomeration, and enhance the electrochemical energy storage performance of graphene-based microcapacitors.

In response to this problem, Li Lei's research group at the School of Materials Science and Engineering of Xi'an Jiaotong University developed an efficient method to prepare graphene-carbon nanotube composite ink, and then prepared miniature supercapacitors through ink direct writing technology. In the device electrode material, the addition of carbon nanotubes can directly control the electrode structure and suppress the stacking and agglomeration of graphene. In this paper, the effect of carbon nanotube content in the electrode on its electrochemical energy storage performance was studied in detail. The study found that as the content of carbon nanotubes increases, the surface capacity of the micro supercapacitor increases first and then decreases. When the content of carbon nanotubes is 5%, the device has the optimal surface capacity, that is, at 0.05 mA/cm-2, the surface capacity reaches 9.81 mF/cm-2, and when the current density increases to 0.40 mA/cm-2 Time, it remains at 8.05 mF/cm-2.

The device achieves a high areal energy density of 1.36 µWh cm–2 at a power density of 0.026 mW cm–2. At the same time, the device also exhibits excellent mechanical properties and cycle stability. Under different bending strain conditions, there is no obvious change in the surface capacity; charge and discharge cycles are performed at a current density of 0.10 mA/cm-2, and the capacity retention rate is still 95.5% after 10,000 cycles.

The graphene-based electrode structure control method developed in this work provides a reference for developing high-performance graphene-based micro-supercapacitors and solving the stacking reunion problem of other two-dimensional materials. Related results were published in Advanced Functional Materials (DOI: 10.1002/adfm.201907284).

Supercapacitors, especially electric double layer capacitors, are a type of electrical storage device that quickly stores and releases charge on the electrode surface. Since electrodes with a high specific surface area can increase the storage capacity (capacitance) of the supercapacitor, the electrode material of the supercapacitor needs to have a high specific surface area. The most commonly used method to increase the specific surface area of the electrode is to prepare the electrode material into a porous structure. However, if the porosity of the electrode material is too high, the electrode density will be reduced, and application-level problems will occur:

(1) The low density makes the electrode material fluffy and increases the volume of the super capacitor, which is not conducive to application in miniature or portable electronic devices;

(2) The excess pores in the electrode will be filled with liquid electrolyte, without energy storage capacity, but the quality of the device is increased. It is also unfavorable for the application in portable electronic equipment.

Therefore, how to control the specific surface area and density of the electrode material, while retaining the high specific surface area of the electrode material while increasing its density as much as possible has become a challenge for supercapacitor research.

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