A systematic approach for selecting suitable morphologies for supercapacitor applications through morphological stability map-A case of Ni-MOF

Nickel metal organic frameworks (Ni MOFs) were synthesized employing microwave technique for supercapacitor applications using Ni(NO3)(2)center dot 6H(2)O (=M) and p-benzenedicarboxylic acid (=L) in M:L ratios of 1:1-4:1, 150-200 degrees C. Flakes, plates, nanoflowers, and globules are obtained and arranged in M:L-Temperature space to yield morphological stability map. Flakes appear at all M:L, 150 degrees C; plates form at M:L = 1:1, >= 165 degrees C; nanoflowers are seen at M:L >= 2:1, 165 degrees C; globules form at M:L >= 2:1, >= 180 degrees C. Cyclic voltammograms from these morphologies within 0 and + 0.6 V vs SCE at 10-100 mV s(-1) show redox peaks corresponding to partial diffusion control, substantiated by galvanostatic charge-discharge (GCD) curves. From GCD, globules at M:L = 3:1 exhibit the highest specific capacitance (Csp) of 1361-600 F g(-1) at 0.5-5.0 A g(-1). This is attributed to their smallest size, presence of monoclinic and gamma-NiOOH phases. Further, globules exhibit the lowest charge transfer resistance as estimated from the Nyquist plots showing two incomplete depressed semicircles. This work presents a systematic approach to select suitable Ni MOF morphologies for supercapacitor applications using morphological stability map. Also, it answers the question: "why does a particular morphology exhibit superior charge storage performance?"