Improvement in Charge Storage Capability By Synergetic Effect between RuO2 Nanosheets and MnO2 Particles
Authors: Saito Ryota , Sato Yusuke , Takimoto Daisuke , Hideshima Sho , Sugimoto Wataru 
Published: December 2020

MnO2 shows fast redox behavior (Mn(Ⅲ)/Mn(Ⅳ)) and has a large overpotential for oxygen evolution reaction, hence has been used as an active material for aqueous electrochemical capacitors. MnO2 electrodes generally consists of MnO2 powder, conductive carbon and polymeric binders. However, conductivity additives and polymer binders are electrochemically non-active or poorly active for energy storage. The reduction in the amount of such additives and polymeric binder will lead to the improvement in the energy and power density. Conductive nanosheets such as MXene has been recently studied as an active binder, acting as a redox active conductive additive.1 Unfortunately, MXene has poor stability in aqueous electrolyte at high potentials due to irreversible oxidation and is not sufficient for use with MnO2 positive electrodes.2 In this work, RuO2 nanosheets with high electronic conductivity and specific capacitance is studied as an active and stable binder. 3

RuO2 nanosheets4 and MnO2 particles5 were synthesized according to literature. The MnO2-RuO2 nanosheet composite electrodes (MnO2(100-x)-RuO2(x); x=100, 80, 60, 50, 30, 20, 10, 0, x indicates the molar ratio of RuO2 ) were prepared by dropping dispersions of composite ink on a mirror-polished glassy carbon electrode. Electrochemical characterization was conducted by cyclic voltammetry using a three-electrode cell consisting of Pt mesh as a counter electrode and a Ag/AgCl (KCl sat.) reference electrode. 1.0 M Li2SO4 (25°C) was used as the electrolyte.
The morphologies of MnO2(50)-RuO2(50) and MnO2(80)-RuO2(20) were analyzed by SEM-EDX. The SEM images show the presence both of particles and nanosheets. The EDX mappings show a homogeneous distribution of ruthenium atoms, and the distribution of manganese atoms agrees with the location of the particles. Figure 1 show the specific capacitance of MnO2(100-x)-RuO2(x) as a function of RuO2 nanosheet additive at scan rates of 2 and 50 mV s-1. The specific capacitance of the MnO2 particle and the RuO2 nanosheet at 2 mV s-1 were 144 and 233 F (g-material)-1, respectively. By adding 20 mol% RuO2 nanosheet to the MnO2 particle, the specific capacitance increased 188 F (g-material)-1, which was 1.3 times higher than pure MnO2 particle. While the specific capacitance of MnO2 was 23 F (g-material)-1 at 50 mV s-1, by adding 20 mol% RuO2 nanosheet, the specific capacitance increased to 155 F (g-material)-1. This is ~6.5 times higher than pure MnO2. It should be noted that the specific capacitance of MnO2 can easily be improved by adding only 20 mol% RuO2 nanosheets. To clarify the reason of the improved specific capacitance, the specific capacitances per mass of MnO2 or RuO2 were calculated. Based on the results, it is concluded that RuO2 nanosheet acts as a conductive additive for MnO2 while MnO2 acts as a spacer to suppress re-stacking of RuO2 nanosheets, thereby affording high utilization of both MnO2 and RuO2 nanosheets.
This work was supported in part by a JST program on Open Innovation Platform with Enterprise Institute and Academia (OPERA JPMJOP 1843) and ISHIFUKU Metal Industry Co. Ltd.

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Figure 1