Supercapacitive Characteristics and Charge-Discharge Mechanism of Mn-Ni Oxide Solid Solution Electrodes

Abstract

In recent years, electrochemical capacitors, also called supercapacitors, have attracted attention because of their high power density and long cycle life compared with secondary batteries. Manganese oxide having a high theoretical capacity has been actively studied as a typical electrode material for redox capacitors. Zuo et al. reported that an aqueous asymmetric capacitor combining a divalent Mn-Ni oxide solid solution (Mn-Ni-O) electrode and an activated carbon electrode exhibited an operating voltage up to 2.4 V.1 However, in the paper, essential parameters such as specific capacitance and energy density based on mass of electrode materials are not shown, and it is also unclear whether the synthesis condition and microstructure of Mn-Ni-O are optimal. For further improvement of supercapacitive performance of the Mn-based oxide electrodes, these issues need to be clarified. In this study, the synthesis conditions of Mn-Ni-O were examined, and the potential window was successfully expanded to an upper limit of 1.5 V vs. Ag/AgCl in LiCl aqueous solution. This led to achievement of the cell voltage of 2.5 V in an asymmetric capacitor with an activated carbon (AC) electrode. Moreover, relationship between the oxidation number and potential of divalent Mn-Ni-O electrode was estimated and discussed to clarify the charge-discharge mechanism. Mn-Ni-O was synthesized by hydrothermal method and subsequent heat treatment similar to those described in the literature.1 Carbon was introduced to the active material to improve the conductivity. These were deposited on a Ti mesh substrate to prepare an Mn-Ni-O/C electrode. The synthesis conditions such as temperature and concentration of constituents in precursor solution were optimized. The resultant electrodes were characterized by SEM, EDX, XRD, and ICP. The electrochemical characteristics were evaluated by cyclic voltammetry (CV) in a 1 M LiCl aqueous solution at room temperature, using an AC electrode as a counter electrode and an Ag/AgCl/saturated KCl electrode as a reference electrode. An aqueous asymmetric capacitor was assembled using an Mn-Ni-O/C electrode as the cathode and an AC electrode as the anode. Its capacitive performance was investigated by galvanostatic charge-discharge (GCD) tests. In order to obtain an insight for charge-discharge mechanism of the electrode, average oxidation numbers of Mn were estimated by XPS after holding the electrode at various potentials of -0.2, 0, 0.5, 1.0, and 1.5 V. For an optimized Mn-Ni-O/C electrode, it was found that oxide solid solution with composition of Mn0.75Ni0.25O was deposited relatively uniformly on the Ti mesh substrate. Fig. 1 shows CV curves of the Mn-Ni-O/C electrodes. It was found that the optimized electrode had a potential window up to 1.5 V vs. Ag/AgCl and a specific capacitance of 363 F g-1 (based on mass of Mn-Ni-O). This electrode was synthesized with decreasing the conventional concentration of NH4F in precursor solution from 0.2 M as described in the literature1 to 0.1 M, resulting in significant improvement in the specific capacitance. This can be ascribed to an increase in the utilization ratio of the active material by forming further uniform, dense and thin layer on the Ti mesh substrate. As shown in Fig. 2, an aqueous asymmetric capacitor cell with the optimized Mn-Ni-O/C and AC electrodes exhibited an upper limit voltage of 2.5 V in GCD tests. The energy density and average power density (based on total mass of Mn-Ni-O and AC) can be calculated as 23.3 Wh kg-1 and 12.7 kW kg-1, respectively. The average oxidation numbers of Mn at different electrode potentials were shown in Fig. 3. It was indicated for the first time that the oxidation number of Mn in the electrode changed nearly from divalent to tetravalent during charge and discharge. Considering such a large available range of the oxidation number compared with conventional MnO2-based electrodes, divalent MnO-based electrode materials can be very important candidates for further increasing the capacitance and energy density of supercapacitors. Acknowledgement This work was supported by JSPS Grants-in Aid for Scientific Research (C) JP19K05596. References 1) W. Zuo et al., Adv. Mater., 29, 1703463 (2017). Figure 1