Hydrangea-like NiCo-based Bimetal-organic Frameworks, and their Pros and Cons as Supercapacitor Electrode Materials in Aqueous Electrolytes

Hydrangea-like NiCo-based bimetal-organic frameworks (NiCo-MOF) are synthesized in DMF-EtOH solution via a solvothermal method, using 4,4 '-biphenyldicarboxylic acid as a ligand. NiCo-MOF having a highest capacity of 1056.6 F center dot g(-1) at 0.5 A center dot g(-1) and 457.7 F center dot g(-1) even at 10 A center dot g(-1) is achieved at a Ni/Co/BPDC molar ratio of 1:1:1, a temperature of 170 degrees C and a reaction time of 12 hours. It exhibits secondary 3D microsphere structures assembled by primary 2D nanosheet structures, good crystalline structure and good thermal stability below 350 degrees C in air. All the electrochemical data show that NiCo-MOF has the pros and cons as supercapacitor electrode materials in aqueous electrolytes. On the one hand, NiCo-MOF has a high capacity even at a high current density, low internal resistance, charge-transfer resistance and ion diffusion impendence, owing to the ordered coordination structure, 2D nanosheet structure and 3D assembled microsphere structure of NiCo-MOF. On the other hand, the cycling stability and rate capability are not ideal enough due to the hydrolysis of coordination bonds in aqueous electrolytes, especially, in alkaline solution. The good dispersion and high electrochemical activity of metal ions bring a high capacity for NiCo-MOF, but they result in the poor stability of NiCo-MOF. In the future work, finding a suitable organic electrolyte is an effective way to enhance the cycling stability of NiCo-MOF as well as deriving more stable skeleton materials from NiCo-MOF.