Ni-MOF Based Flexible Solid-State Supercapacitors in Aqueous and Non-Aqueous Electrolytes

Abstract

Supercapacitors are high power density devices which acts a bridge between the batteries and conventional capacitors. Several emerging materials like Metal Organic Frameworks (MOFs) have been developed as electrode material for energy storage. MOFs are highly porous materials containing metal centers and organic moieties called ligands [1].They are widely employed for energy storage applications owing to their structural and morphological tunability [2]. In our recent research work, Ni MOF is synthesized using microwave assisted solvothermal technique at different temperature and metal to ligand ratios. 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 [3] from room temperature to 150, 165, 180 and 200 °C to obtain light green colored samples. It is interesting to observe that different synthesis parameters lead to different morphologies thereby resulting in improved performance of Ni MOF. The structural and electrochemical characterizations reveal “why does a particular morphology exhibits better charge storage capacity?” The three electrode configuration experiments were conducted in aqueous electrolyte to evaluate the charge storage capacity. The results show that globules (∼1360 F g-1) perform better than nanoflower (∼850 F g-1), flakes (∼650 F g-1) and plates (∼450 F g-1) [4] respectively in 2M KOH at 0.5 A g-1. The reason is attributed to the low solution resistance (R sol) for globules, reduced charge transfer resistance (R ct) and the global and local n values of CPE.The synthesized material (Ni MOF) will be used as positive electrode material for flexible asymmetric supercapacitor. Recently, flexible state supercapacitors (FSSCs) have gained popularity due to their formability, flexibility and high energy density. Here, the objective is to increase energy density and power density by extending the operating potential window of the device. Hence, the aqueous electrolyte is substituted with PVDF based ionogels (ionic liquid gel polymer) to impart flexibility, bendability and high power density to FSSC. It is observed that the potential window improved remarkably from ~0.6 V (aqueous electrolytes) to ~5 V (ionic liquid). The reason is attributed to the absence of water, results in thermodynamic stability of the electrolyte. Also, the ionic liquid based FSSCs are non-flammable, non-toxic, thermally stable. In order to improve the conductivity and charge storage capacity some additives are incorporated in the gel polymer matrix. Herein, certain polar solvents are added to enhance the conductivity due to their high dielectric constant giving rise to faster ionic dissociation. MOF based flexible supercapacitors are fabricated and tested for their energy and power densities in conjunction with electrochemical and structural characteristics .