Metal Organic Frameworks (MOFs) are highly porous materials constituting of metal centers and organic moieties called ligands. They are widely employed for energy storage applications owing to their structural and morphological tunability. In this work, nickel MOF (Ni MOF) are studied for supercapacitor applications. Ni MOF is synthesized by varying metal to ligand ratio (M:L) as 1:1, 2:1, 3:1 and 4:1. The solution is heated by microwave irradiation technique from room temperature to 150, 165, 180 and 200 °C to obtain light green colored samples. The obtained samples are denoted in the present study as 'NMOF p @ q'. Here, p: 1,2,3 and 4 correspond to M:L ratios of 1:1, 2:1, 3:1, and 4:1, respectively; and q: 150, 165, 180 and 200°C. These samples possess monoclinic phase of nickel MOF and exist in only four types of morphologies namely flake, plates, nanoflowers and globules for the chosen M:L ratio and temperature. Interestingly, the nanoflowers and globules consists of several 2D entities (sheets and worm-like structures) assembled to form 3D structures resulting in open porosity. These morphologies can be arranged on a Metal: Ligand-Temperature space to yield a morphological (stability) map as shown in Fig. 1. At 150 °C all the Ni MOFs appear as only flakes (Fig. 1(a,e,i,m)). Similarly, at M:L = 1:1 the stacked plates observed at ≥165 °C (Fig. 1(n,o,p)) are only a variant of the flakes obtained at 150 °C (Fig. 1(m)). It is interesting to see this increasing circularity is pronounced in the third dimension also upon increase in the M:L ratio beyond 1:1. At M:L ≥ 2:1 irregular flakes and nanoflowers can be seen at 150 (Fig. 1 a,e,i) and 165 °C (Fig. 1 b,f,j), respectively. However, upon further increase in the temperature globular morphology can be observed (Figure 1 c,d,g,h,k,l). The higher circularity of the obtained morphologies is a possible result of the surface energy modification.

Various morphologies mentioned above render different electrochemical characteristics and charge storage performance, with globular morphology being the best. The electrochemical characteristics of Ni MOF deposited on Nickel foil are determined by cyclic voltammetry (CV) conducted in 2.0 M KOH between +0.0 to +0.6 V vs SCE at several scan rates between 10-100 mV s^-1. Broadly, two redox peaks (cathodic:∼ +0.2 V; anodic:∼ +0.35 V) are observed and arise due to the intercalation and deintercalation of OH^-, as the charge storage means, and resulting in the reversible oxidation states of Ni²⁺/Ni³⁺. Power law analysis (peak current: i_p=aν^b; ν: scan rate; a,b: constants) yields b≈0.5 suggesting that this charge storage is diffusion-controlled. Electrochemical charge storage by galvanostatic charge/discharge tests indicates that the plates, flakes, nanoflowers and globules exhibit specific capacitance (C_sp) of ∼450, ∼650, ∼800 and ∼1300 F g^-1 respectively at 0.5 A g^-1. The globular morphologies NMOF 3 @ 180 and NMOF 3 @ 200 exhibit C_sp of ∼750 and ∼800 F g^-1 at 5 A g^-1, which are ∼86% higher than those from plates and flakes. The enhanced capacitance is ascribed to the open porosity offered by the 2D entities on these globules. Furthermore, Nyquist plots reveal two distorted semi-circles one each attributed to the electrochemical phenomena occurring at the nominal electrode/electrolyte interface (global) and those in the pores (local). The equivalent circuit used to model the Nyquist plots consists of two series-connected blocks of constant phase element (CPE) in parallel with charge transfer resistance (R_ct) which are further connected with the solution resistance. The global R_ct for plates, flakes, nanoflowers and globules are ∼10 Ω, ∼9 Ω, ∼6 Ω, ∼3 Ω, respectively. The lowest R_ct in globules implies facile charge transfer between electrode and electrolyte possibly due to the active sites provided by the 2D entities within globules. The superior performance of the globules is also attributed to the global and local n values of CPE. The electrochemical charge storage performance of the synthesized morphologies will be discussed in conjunction with electrochemical and structural characteristics.