Boosting Energy Density of an All Solid-State Asymmetric Supercapacitor Based on rGO/Ni(OH)2 and Activated Carbon Via Model Guided Design
Authors: Huang Man Chen , Chen Hsun-Yi 
Published: February 2020

Supercapacitors are one of the promising energy storage devices because of their long life span and high power density, which leads to their advantageous application in regenerative braking system of electric vehicles (EVs). The major challenge that supercapacitors facing is their relatively lower energy density. There are two possible routes to overcome this shortcoming: first, increasing the capacitance by selecting electrode materials with a higher specific area, such as activated carbon (AC); second, enlarging the operable potential window by choosing two electrodes of different materials, forming the so called asymmetric supercapacitors (ASCs). Despite ASCs do enlarge the operable voltage window, most of the aqueous ASCs still suffer from the decomposition of water, limiting the maximum operation voltage to around 1.6 V. Capacitors employing aqueous electrolyte are also prone to leakage, leading to safety concerns. The gel polymer electrolyte (GPE), on the other hand, has higher electrochemical stability compared to aqueous ones. Their solid-state characteristics prevent the electrochemical system from leakage, which is advantageous for supercapacitors employed in portable electronics or EVs. In this work, we aim to construct an asymmetric supercapacitor, where rGO/Ni(OH)2 and AC are used as electrode materials and a LiClO4–based gel polymer is employed as the electrolyte, to achieve enhanced energy density.
Cyclic voltammetry is first conducted on rGO/Ni(OH)2 and AC half-cells with GPEs to identify the operational voltage window, and the former is selected as the positive electrode and the latter as the negative electrode to optimize the utilization of electrical double layer capacitance (EDLC) of the system. With limited redox reactions involved, the cell can not only have longer cycle life but also endure higher current densities. The assembled ASC is further examined by galvanostatic charge-discharge (GCD) experiments. GCD examination is conducted with an upper voltage limit of 3 V and under a current density of 2 mA/cm2, and the results are shown in Figure 1. Figure 1 (a) shows charge/discharge curves of the first five and the last five of the recorded 10,000 cycles of our ASC, and the highly similar voltage-time behavior after the first cycle indicates high electrochemical reversibility of this ASC. Figure 1 (b) shows that the coulombic efficiency remains close to 100% after the initial activation cycles. The specific capacitance, energy density and power density of this ASC system are measured to be 2 F/g, 61.6Wh/kg, and 1800 W/kg, respectively.
By exerting excess faradaic reaction of the rGO/Ni(OH)2 electrode, this ASC system can potentially have enhanced specific capacitance. We propose to conduct multi-physics simulations to better match the two electrodes when faradaic reaction is involved, as well as optimize the operation window of the ASC. COMSOL software is employed to simulate charge/discharge profiles under pseudo capacitive conditions, with the physical parameters measured by experiments. With the simulation guided design of the ASC, we anticipate to systematically enhance the functionality of this electrochemical device for future application such as in EVs.

Figure 1