【Research Background】
The layered double hydroxide (LDH) composed of a positively charged metal hydroxide main layer and a charge compensating anion has a highly tunable chemical composition and structure. Due to its excellent Faraday redox activity, low cost and environment It is widely concerned as a potential electrode material for supercapacitors. However, due to their poor electrical conductivity, limited exposed active surface area, and irreversible face-to-face accumulation, their actual specific capacities are still far below their theoretical values. In recent years, various conductive substrates such as graphene, CNT and MXene have been introduced to enhance the electrical conductivity of LDH. MXenes, a rapidly rising star in the field of materials science, is a general term for the recently emerging 2D transition metal carbides and nitrides. It has attracted rapid attention in energy storage applications such as supercapacitors, lithium-ion, and lithium-sulfur batteries. MXene has excellent structural and physical properties, such as excellent electrical conductivity, high density, rich surface chemistry, and strong hydrophilicity, making it an ideal substitute for depositing LDH nanosheets to improve electrochemical performance.

[Achievement Profile]
Recently, Professor Qian Wang and Jun Yan of Harbin Engineering University have published an article titled Electrostatic self-assembly of MXene and edge-rich CoAl layered double hydroxide on molecular-scale with super high in the internationally renowned academic journal Journal of Energy Chemistry. Volumetric performances research paper. This study prepared a heterogeneous structure of MXene / CoAl-LDH on the molecular scale by electrostatically ordered assembly of single-layer MXene and edge-rich CoAl-LDH nanoflakes. Thanks to the unique structure between MXene and CoAl-LDH nanosheets, strong interfacial interactions and synergistic effects, electrical conductivity and exposed electrolyte access to active sites have been significantly enhanced.
[Picture and text guide]

Figure 1. (a) Schematic diagram of the preparation of MXene / CoAl-LDH heterostructure (be) AFM micrographs of single-layer CoAl-LDH and MXene nanosheets (f) Zeta potential (g ) Digital pictures of CoAl-LDH (left), MXene (middle) colloidal suspension and ML-80 (right). The first two samples showed significant Tyndall scattering effects under the laser.

Figure 2. TEM images of layered CoAl-LDH (a) and MXene (b, c) nanosheets. Cross-section SEM (d) and TEM (e, g, h) images of ML-80 (f) XRD images of MXene and ML-80 independent films

Figure 3. XPS measurement spectrum of ML-80 independent film

Figure 4. Electrochemical performance of ML-80 electrode

Figure 5. ML-80 // Electrochemical performance of MG-5 asymmetric supercapacitor

[Summary of this article]
In this paper, a simple and low-cost electrostatic ordered self-assembly method was used to prepare a novel MXene / CoAl-LDH interlayer heterostructure film at the molecular scale. By taking advantage of the synergistic effect between MXene and CoAl-LDH nanosheets, the conductivity and the surface area of electrolyte ions and the number of exposed electroactive sites are greatly increased. Due to the unique structure and strong interfacial interaction between MXene and CoAl-LDH, the ML-80 electrode exhibits excellent electrochemical performance. The ML-80 electrode shows an excellent volume capacity of 2472 C cm-3, the highest among previously reported electrode materials among aqueous electrolytes. More importantly, our prepared ML-80 // MG-5 ASC device can provide an energy density of 30.9 Wh kg-1, which is equivalent to the volume energy density of 85.4 Wh L-1, which can be compared with the energy reported by the ASC equipment previously Compared with density even beyond. In addition, the ASC device showed an excellent cycle stability of 94.4% after 30,000 cycles. The preparation of heterogeneous structures at the molecular scale paved the way for new composite materials based on 2D materials in the future for energy storage applications.

Literature link:
https://doi.org/10.1016/j.jechem.2019.10.023