A new strategy for the morphology-controlled synthesis of Ni/Co MOFs for high-performance asymmetric supercapacitors

Ni & Co based Metal-Organic Frameworks (MOFs) with different Ni & Co ratios 1:1, 2:1, 1:2 - NCs were synthesized using Trimesic acid as linker by solvothermal process, and the same were abbreviated as NC(1:1), NC(2:1) and NC(1:2) respectively. All the samples were analysed by analytical techniques such as Powder X-ray diffraction (P-XRD), Fourier Transform-Infrared Spectroscopy (FT-IR), Brunauer-Emmett-Teller (BET) surface area, Scanning Electron Microscopy (SEM), and Transmission Electron Microscopy (TEM) techniques. Morphological variations in NCs were observed from SEM images as ultrathin nanoflakes (NC(1:1)) dandelion (NC(2:1)) and nano-spikes (NC(1:2)) due to the space coordination effect. Further, NCs were checked for their electrochemical performances in both three and two-electrode systems. Among NCs, NC(1:1) exhibited remarkable specific capacity of 172C g−1 @ 2.5 A g−1 in 1 M KOH solution. Asymmetric coin cell - ACC (CR-2032) constructed using NC(1:1) and commercially purchased AC in 1 M KOH solution, exhibited a high specific capacity of 92C g−1 @ 1 A g−1 with corresponding specific energy (SE) of 23 Wh kg−1 & specific power (SP) of 90 W kg−1. The coin cells indicated 20,000 cycles stability with 90 % specific capacity retention at 5000th cycle, however it was decreased to 47 % at the end of 20,000th cycle. The self-discharge of ACC was also investigated by estimating open circuit voltage decay and interestingly the decay was only 20.4 mV h−1 after 48 h. Two-coin cells, each charged to 0–1.8 V were connected in series and utilized for its potential flashlight application in powering 12 white LEDs for 2 min.