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)2·6H2O (=M) and p-benzenedicarboxylic acid (=L) in M:L ratios of 1:1–4:1, 150–200 °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 °C; plates form at M:L = 1:1, ≥ 165 °C; nanoflowers are seen at M:L ≥ 2:1, 165 °C; globules form at M:L ≥ 2:1, ≥ 180 °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 γ-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?"