Protonated nickel 2-methylimidazole framework as an advanced electrode material for high-performance hybrid supercapacitor

Metal-organic frameworks (MOFs) with high conductivity have proven to be an exciting electrode material for energy storage devices. However, most of the MOFs exhibit a low electrical conductivity, which limits their use in supercapacitors. To overcome this issue herein, a simple acid treatment method was adopted to obtain nanoflower-like nickel 2-methylimidazole framework (Ni-MOF) to improve the electrical conductivity without disrupting its framework. The sample treated with a solution of sulfuric acid (H₂SO₄) at optimal pH 2 (Ni-MOF-2), exhibited improved surface texture with excellent electrochemical characteristics. The Ni-MOF-2 sample displayed a high specific capacity (C_s) of 467 C/g at 1 A/g in aqueous 6 M potassium hydroxide (KOH) electrolyte than that of other samples. This is mainly due to enhanced proton conduction in Ni-MOF-2 after acid treatment. In addition, a hybrid supercapacitor (HSC) device was fabricated using battery-type Ni-MOF-2 as a positive electrode and heteroatom-enriched activated carbon (O, N, S@AC) as a negative electrode. The fabricated HSC exhibited a maximum specific capacity (C_s) of 38 mAh/g with high specific energy (E_s) 39 Wh/kg and maximum specific power (P_s) of 11,079 W/kg. Moreover, the HSC displayed excellent cyclic stability of ~87% for 10,000 continuous galvanostatic charge/discharge (GCD) cycles.