Systematic designs of single metal compounds synthesized using ammonia fluoride-based complex as structure directing agents for energy storage.

Tailoring morphology and composition of metal organic frameworks (MOF) can improve energy storage by establishing high surface area, large porosity and multiple redox states. Structure directing agents (SDA) is functional of designing surface properties of electroactive materials. Ammonium fluoride has functional abilities for designing MOF derivatives with excellent energy storage abilities. Systematic design of MOF derivatives using ammonia fluoride-based complex as SDA can essentially create efficient electroactive materials. Metal species can also play significant roles on redox reactions, which are the main energy storage mechanism for battery-type electrodes. In this work, 2-methylimidazole, two novel SDAs of NH4BF4 and NH4HF2, and six metal species of Al, Mn, Co, Ni, Cu and Zn are coupled to synthesize MOF derivatives for energy storage. Metal species-dependent compositions including hydroxides, oxides, and hydroxide nitrates are observed. The nickel-based derivative (Ni-HBF) shows the highest specific capacitance (CF) of 698.0F/g at 20 mV/s, due to multiple redox states and advanced flower-like surface properties. The diffusion and capacitive-control contributions of MOF derivatives are also analyzed. The battery supercapacitor hybrid with Ni-HBF electrode shows a maximum energy density of 27.9 Wh/kg at 325 W/kg. The CF retention of 170.9% and Coulombic efficiency of 93.2% are achieved after 10,000 cycles.

Ligand Incorporating Sequence-dependent ZIF67 Derivatives as Active Material of Supercapacitor: Competition between Ammonia Fluoride and 2-Methylimidazole.

The zeolitic imidazolate framework 67 (ZIF67) derivative is a potential active material of supercapacitors (SC), owing to high specific surface area and porosity and possible formation of cobalt compounds. A novel ZIF67 derivative is synthesized using a one-step solution process with cobalt precursor 2-methylimidazole (2-Melm) and ammonia fluoride in our previous work. Due to its facile synthesis and excellent electrocapacitive behavior, it is crucial to understand the competition between ammonia fluoride and 2-Melm on forming derivatives with cobalt ions and to create more efficient ZIF67 derivatives for charge storage. In this work, several ZIF67 derivatives are designed using a one-step solution process with 2-Melm and ammonia fluoride incorporated in different sequences. The reaction durations for a single ligand and two ligands are controlled. The largest capacity of 176.33 mAh/g corresponding to the specific capacitance of 1057.99 F/g is achieved for the ZIF67 derivative electrode prepared by reacting ammonia fluoride and a cobalt precursor for 0.5 h and then incorporating 2-Melm for another 23.5 h of reaction (NM0.5). This derivative composed of highly conductive CoF2, NiF2, Co(OH)F, and Ni(OH)F presents high specific surface area and porosity. The relevant SC presents a maximum energy density of 19.5 Wh/kg at 430 W/kg, a capacity retention of 92%, and Coulombic efficiency of 96% in 10000 cycles.

Design of novel self-assembled MXene and ZIF67 derivative composites as efficient electroactive material of energy storage device.

High surface area and tunable pore size are beneficial for metal organic frameworks (MOFs) as electroactive material of energy storage devices. Novel ZIF67 derivative proposed in our previous work, nickel cobalt fluoride coupled with ammonia ions (NCNF), is synthesized using ammonia fluoride to solve poor electrical conductivity of MOFs. MXene is commonly incorporated in pseudo-capacitive materials to enhance electrical conductivity and energy storage ability. In this study, it is the first time to design MXene and NCNF composites (MXene/NCNF) with different MXene amounts via incorporating MXene in growing process of NCNF. MXene and NCNF are combined via self-assembly in a simple room temperature solution process. The optimized MXene/NCNF electrode shows a higher specific capacitance of 1020.0 F g-1 (170.0 mAh g-1) than that of NCNF electrode (574.2 F g-1 and 95.7 mAh g-1) at 20 mV s-1, due to excellent surface properties of MXene/NCNF with conductive network of MXene and high electrocapacitive performance of NCNF. A symmetric energy storage device composed of the optimized MXene/NCNF electrodes presents outstanding cycling stability with Coulombic efficiency of 100% during whole cycling process and a high capacitance retention of 99% after 6000 cycles. Excellent electrochemical performance and simple synthesis of MXene/NCNF open new blueprints for designing novel electrocapacitive materials for electrochemical applications.

Synthesizing nickel-based transition bimetallic oxide via nickel precursor-free hydrothermal synthesis for battery supercapacitor hybrid devices.

The Ni foam can act as the nickel ion source and the current collector for synthesizing Ni-based compounds using the hydrothermal synthesis especially in the acid condition. Using Ni foam as the Ni2+ source can grow materials on the substrate directly and uniformly since nickel ions are released from substrate thoroughly. Nickel-based bimetallic oxides are intensively investigated as battery-type materials for battery supercapacitor hybrid devices (BSHD) because of high electrical conductivities and abundant transition states for inducing multiple redox reactions. In this study, Mo, Mn, Al, and W precursors are simply added in Ni precursor-free acid solution for hydrothermal synthesis using Ni foam as the nickel ion source and the current collector to synthesize Ni-based bimetallic oxide electrodes for BSHD. The morphology of nickel-based bimetallic oxide prepared with and without incorporating the structure-directing agent is also carefully discussed. The highest specific capacitance (CF) of 1.80 F/cm2 corresponding to the capacity of 4.54 mAh/cm2 at 5 mA/cm2 is attained for the nickel molybdenum oxide (Ni-Mo oxide) electrode. The Ni-Mo oxide-based BSHD shows a potential window of 1.8 V, a CF value of 223.53 mF/cm2 corresponding to the capacity of 1.45 mAh/cm2 at 5 mA/cm2, the maximum energy density of 4.60 Wh/kg at the power density of 0.21 kW/kg, and the CF retention of 90% after 6000 times charging/discharging process.