Redox Mediated Neutral Electrolyte Solution for High Performing Ni(OH)2-MnO2-rGO Supercapacitor Electrode

Abstract

The most ascertain key to resolving the ever-increasing global energy crisis is the enrichment of electrochemical energy storage technologies. Besides, towards achieving the goals of SDG-7 (including zero-emission vehicles, modern use of electricity, and green grid storage, etc.), the development of heavy-duty supercapacitors (SC) is now a center of focus for scientists. Among the various strategies and efforts, reduced graphene oxide (rGO) is the most studied carbonaceous material for the fabrication of EDLC based SC [1]. But, one of the major problems regarding rGO based electrode materials for SC is its uncontrolled stacking layer formation property which is not usually favorable for electrical charge storage [2]. Although the pristine rGO, prepared via chemical vapor deposition is highly conductive with a large surface area, a chemically synthesized rGO possesses much lower electrical conductivity with a lower surface area [3]. Usually, rGO prepared by the modified Hummer method contains so many unwanted functional groups that hinder the actual conducting properties of rGO and significantly affect the surface area. Another reason is the random stacking of rGO interlayers. This work demonstrated the approach to resolve the electrical conductivity problem by the successful incorporation of α-Ni(OH)₂ and γ-MnO₂ on and in between the hydrothermally synthesized rGO interlayers by a simple gel formation method. The crystalline phase of the metal oxides in the Ni(OH)₂-MnO₂-rGO is characterized by X-ray Diffraction (XRD) and found to be α-Ni(OH)₂ and γ-MnO₂. Initially, KMnO₄ abruptly reacted with glycerine and produced hydrated manganese oxide in the presence of GO and NiNO₃ solution. The hydrothermal reaction was carried out at 180°C for 24 h to convert this material into Ni(OH)₂-MnO₂-rGO. The incorporation of Ni(OH)₂ and MnO₂ unzipped the stacked layers of rGO sheets and facilitated a way to enhance the interlayer distance of rGO sheets [3,4]. Besides, the incorporation of single or multiple metal oxides or hydroxides with the pure rGO layers always provides extra surface area and induces pseudocapacitance with a suitable electrolyte system. Due to the synergistic effects of Ni(OH)₂ and MnO₂, the overall electrochemical performance of as-synthesized hybrid has shown high performance as SC electrodes [4]. Now, the architecting of proper electrode materials is not the only challenge remaining towards fabricating a high-performing SC device. Instead, the equal focus on electrolyte architecting is also a must issue of achieving maximum efficiency from the electrode material.

Usually, aqueous electrolytes (Na₂SO_4, H₂SO₄ KOH) with rGO based hybrids are always a safe choice for the fast-charging and discharging features because of their higher electronic conductivity and lesser solution resistance. However, the limited working window of aqueous electrolytes limits the energy density of an energy storage device. Although organic electrolytes are commonly used in commercial energy storage devices, organic liquids are still under consideration due to their high cost, low conductivity, high volatility, and extremely flammable nature, making them unsafe for high-voltage and long-term operation. So, towards this solution, an adequately designed electrolytic system is required to achieve maximum capacitance with high energy density and long cyclic stability of a hybrid SC device. Towards this solution, a redox-mediated (Na₂SO₄ and K₄FeCN₆) electrolyte solution was used for the first time with the Ni(OH)₂-MnO₂-rGO electrode as shown in fig. 1. During the charging-discharging process, K₄FeCN₆ introduces a reversible redox (FeCN_6­^4- to FeCN_6­^3- ) reaction in the hybrid electrode and electrolyte interfaces. During this time, the adsorption and desorption of FeCN_6­^4- and FeCN_6­^3- took place on the electrode surface [5]. Thus, the redox-mediated aqueous media facilitated different faradic reactions to introduce more pseudocapacitance without hampering the material stability. Hence, a high specific capacitance of 870 Fg^-1 was found at 1 Ag^-1 current density with a high energy density of 30.2 Whkg^-1 by using symmetrical two-electrode setup.  Besides, even after 5,000 cycles, high capacitance retention of 92 % was found with maintaining 95% Coulombic efficiency. The total electrochemical evaluation of Ni(OH)₂-MnO₂-rGO with different electrolytes is presented in table 1. It is expected that this work will contribute to developing a high-performing and low-cost SC device that can be used in smart and sustainable hybrid vehicles.