การใช้สารประกอบเชิงซ้อนของแอมโมเนียมและฟลูออไรด์เพื่อเป็นสารก่อโครงสร้างสำหรับการสังเคราะห์สารประกอบเลเยอร์ดับเบิ้ลไฮดรอกไซด์และแมกซีนสำหรับระบบกักเก็บพลังงาน

Researchers are striving hard to build efficient energy storage systems/devices with improved performance and reliability. Developing energy storage devices with high energy and power density is the main goal at present. Designing battery supercapacitor hybrid (BSH) is highly desirable to obtain high energy and power densities as well as excellent cycling ability. It has been witnessed that two-dimensional (2D) materials such as layered double hydroxide (LDH) and Mxene with high-aspect-ratio structure, extended lateral dimension and atomic/molecular thickness were extensively studied in the past years among the materials research community [1-3]. It was reported in our previous studies that ammonia based fluoride complex such as NH₄F, NH₄BF₄ and NH₄HF₂ can be used to design unique properties of 2D materials by controlling the pH value of precursor solutions [4-6]. The low pH environment is favorable for co-precipitation and tunable interlayer spaces of 2D materials such as LDH and Mxene. On the other hand, a series of novel ZIF67 derivatives have been synthesized using a facile one-step co-precipitation process with cobalt salt, 2-methylimidazole (2-Melm) and NH₄F as precursors [7-9]. By designing active materials with excellent surface properties such as high specific surface area and electrical conductivity, the relevant energy storage ability can be improved. The competition between NH₄F and 2-Melm combining with cobalt on forming ZIF67 derivatives was investigated. Numerous products such as CoF₂, NiF₂, Co(OH)F, Ni(OH)F, CoOOH, NiOOH and β-Ni(OH)₂ were generated [10]. It was found that the adding sequences of NH₄F and 2-Melm have large impacts on the pH value and concentration of fluorine ions. The LDH was intended to form at the low pH environment. The highest capacity of 176.33 mAh/g corresponding to the specific capacitance of 1057.99 F/g was achieved for this LDH derived from ZIF67. Also, the relevant BSH presents a maximum energy density of 19.5 Wh/kg at 430 W/kg, as well as a capacity retention of 92% and Coulombic efficiency of 96% after 10000 cycles of the charge/discharge process [10]. Also, a MXene-based composite with the ZIF67 derivative synthesized using NH₄F (NCNF) was proposed to solve poor electrical conductivity of the ZIF67 derivative and prevent MXene from stacking. The optimized MXene/NCNF electrode shows a higher specific capacitance of 1020.0 F/g (170.0 mAh/g) than that of NCNF electrode (574.2 F/g and 95.7 mAh/g) at 20 mV/s, due to excellent surface properties of MXene/NCNF, conductive network formation of MXene and high electrocapacitive property of NCNF [11]. It has been verified that incorporating NH₄F for synthesizing LDH and MXene/ZIF67 derivative composites can largely enhance their energy storage abilities.

The theoretical capacity of LDH is hard to attain owing to its poor conductivity, serious agglomeration and structural defects. [12-14] To solve these issues, the synthetic methods should be modified to refine the physical features of LDH. Co-precipitation is the most common method for the preparation of LDHs in which inorganic salts are co-precipitated in an alkaline medium at constant or increasing pH. Also, the inorganic salts are able to diffuse into interlayers of LDH during synthesizing process to dominate the interspacing of LDH. On the other hand, Mxene nanosheets tend to aggregate and self-accumulate in the presence of interlayer hydrogen bonding and weak van der Waals forces. This phenomenon enhances the impedance and reduces active sites during the charge/discharge process. To solve these issues, MXene-based composites were widely proposed to weaken the MXene stacking phenomenon and improve the oxidation resistance. Liu et al. proposed a hierarchical MXene@ZIF67 core-shell/yolk-shell cobalt hydroxide composite using in situ gradient etching of cubic ZIF67 loaded MXene. The interlayer cobalt hydroxide ensured efficient electron transfer by preventing self-aggregation of MXene sheets [15]. Guo et al. in situ anchored Ni-doped ZIF-67 (Ni-Co-ZIF-67) on negatively charged MXene surface via chemical bonds to supply more available active sites for the enhancement of capacitance. The prepared composite of MXene/NiCo double hydroxide (NiCoZDH) electrode shows a larger specific capacitance of 877 F/g than MXene/NiCo-ZIF-67 (460 F/g), NiCoZDH (354 F/g), NiCo-ZIF-67 (130 F/g) and MXene (15 F/g) [16].

Therefore, to synthesize efficient LDH and MXene-based composites as active material of energy storage, it is important to seek for functionalized additives to play as pH and interspacing regulators. This design is especially important for the energy storage systems with multi-ion electrolyte. Three structure directing agents of NH₄F, NH₄BF₄ and NH₄HF₂ will be proposed to control the pH value and plays as the electrochemical active anions inserting in interlayers of LDH based on ZIF67. The participation of NH₄F, NH₄BF₄ and NH₄HF₂ can regular the interspacing and reduce the agglomeration. Also, the structural defects can be partially reduced by controlling the growth of structures. The defects can also play as the reaction sites for energy storage, but too many defects could reduce the conductivity. Well controlling the amounts of defects could enhance the energy storage ability. In addition, multiple metals will also be incorporated in the LDH and MXene-based composites. The intrinsic properties such as size and affinity are different for metals. These varieties will influence the growth of LDH and MXene-based composites in different systems with NH₄F, NH₄BF₄ or NH₄HF₂. Suitable parameters controlling is favorable for designing efficient active materials as dependence of metal species and structure directing agent types.