(Digital Presentation) Vanadium Intercalated Cobalt Trimesic MOF Composite for Enhanced Solid-State Asymmetric Supercapacitor Storage
Authors: Mondal Monojit , Goswami Dipak Kumar , Sett Avik Kanti , Bhattacharyya Tarun Kanti 
Published: July 2022

Metal organic framework (MOF) is a supramolecular coordination material composed of metal ions coordination, nodes, and organic ligands. MOFs were used as active materials for chemical capacitors to realize the high electrochemical performance, providing adequate redox sites and accelerating ion diffusion rate. Using MOF as a template to fabricate a novel platform can deliver large specific surface areas and abundant oxidation-active sites to augment electrical conductivity. The electrode material based on a MOF has a unique morphology that can significantly shorten charge transport paths to alleviate the electron transport and diffusion, thus strengthening the catalytic performance of the electrode materials. Also, the incorporation of vanadium in the intermediate leads to a broad impact on the morphology and the electrochemical energy storage of Cobalt-Trimesic matrix worked as a base for fabricating the composite. The DMF is a solvent to prepare the vanadium cobalt oxide composite derived using the MOF matrix. Trimesic acid is used as an initial organic linker precursor to initiate the formation of MOF of cobalt in DMF solvent. Then the vanadium ion is integrated into the same material matrix. In this study, the impact of organic system precursors and the integration of annealing play a pivotal role in alleviating the enhanced electrochemical storage response. After analyzing through XRD, the corresponding MOF crystal peaks are prominently affirmed. The high intensified peaks in the obtained patterns symbolize the oxide phase of cobalt vanadium and confirm the formation derived from MOF. The FTIR spectroscopy investigation depicted the proper presence of cobalt, vanadium, carbon, and nitrogen. The observed FESEM morphology illustrated the dissimilar structure and involved in the charge storage likewise. The morphology significantly impacts surface-dependent charge storage with having porosity introduced. High-temperature annealing also consists of the crystal orientation of the bi-metallic MOF matrix and creates porosity on the effective surface area.

The Co-MOF-V oxide composite is thoroughly investigated individually in three-electrode arrangements for analyzing the electrochemical charge storage in 2MKOH represented in Fig.1 (c-d). Firstly, the oxide is scrutinized as working positive electrodes and measured in the potential window of -0.2 to 0.6 V w.r.t the saturated calomel reference electrode. The accomplished specific capacitance value from the cyclic voltammetry plot is 549 F/g at 2mV/s scan rate. Furthermore, the charge-discharge analysis of the composite delineated an admirable discharge time at 3mA/cm2 current density. It portrayed 349 F/g specific capacitance, leading to higher specific energy and specific power of 31.02Wh/kg and 159.53W/kg at the value of the same current density. The Nyquist plot corroborates that the electrode resistance, electrode-electrolyte interfacing resistance, and the value of diffuse layer resistance are significantly less. This study can bestow a greater pathway of ion transportation from the electrolyte to active electrode material that abetted to conversate superior storage using complete morphology. The transition metal possesses a variational oxidation state that directly contributes to this MOF composite's pseudocapacitive charge storage process. Alternatively, comparable analysis is directed over anode material that is lignocellulose derived activated porous carbon. It is depicted in Fig.1(e-f) in the potential window of -1 to 0v w.r.t the Ag/AgCl electrode in 2M KOH. The gained specific capacitance from the cyclic voltammetry analysis is 426 F/g at a 2mV/s scan rate. Besides, the charge-discharge study of the same carbon depicted admirable discharge time at 0.5A/g current density and portrayed 565 F/g specific capacitance. The specific energy and power of 78.47Wh/kg and 282W/kg at the same current density. Also, the integrated porous system with the high active surface area, which significantly comes from the MOF matrix, provides an added phase to strengthen the amount of storage like EDLC formation.

Furthermore, combining both of those active materials, the asymmetric supercapacitor storage is made up [Fig.1 (g-h)]. That system works upon 0 to 1.4V windows. The attained areal capacitance is 0.54 F/cm2, and volumetric capacitance is 0.68 F/cm3 in the current density of 4mA/cm2. The Ragone plot shows its perfect positioning in the region of asymmetric storage. This ASC system delineated coulombic efficiency 79% of capacitive retention 75% after the 10,000 long cycle stability study. From the electrochemical impedance spectroscopy analysis, the obtained interfacing and electrode resistance diffuse layer resistance is found significantly lesser and upwards.

Figure 1