(Invited) Extrinsic and Intrinsic Methods to Increase the Rate Performance of MnO2 Electrodes

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MnO2 is one of the most well studied pseudocapacitive material for aqueous supercapacitors owing to the surface and near surface-confined redox processes. However, due to its poor electronic conductivity, a large amount of conducting additive, needs to be added to achieve high-rate charge storage. Since the conducting additive does not contribute much to the specific capacitance, this leads to decrease in total energy density of the cell in terms of both mass and volume. In addition, polymeric binders are also necessary to fabricate rigid electrode with good contact.As an alternative to these typical conductive binders and polymer binders, we have studied the possibility of using RuO2 nanosheets as a novel inorganic binder with polymeric properties, high electronic conductivity and excellent pseudocapacitive properties. For example, by adding a small amount of RuO2 nanosheets to particulate MnO2, a large synergetic effect is observed leading to a high utilization of MnO2 pseudocapacitance.1 The optimized amount of RuO2 in the composite was 13 wt%, which is much lower than the amount generally used for conductive carbon additives. The size of nanosheet binders also affects the properties of the electrode. Using MnO2 nanosheets and RuO2 nanosheets with different lateral sizes, it has been shown that larger sized nanosheets affords better conductive binder properties through a better electronic conduction network structure.Asides from using conductive binders, heterogeneous doping can increase the intrinsic conductivity of MnO2. We have shown that Ir-doped Birnessite shows faster redox behavior than pristine MnO2 and this enhanced kinetics are reflected on the exfoliated nanosheets. These properties can be discussed based on the electronic interaction between Mn and Ir.The pseudocapacitive performance of MnO2 can also be manipulated by inducing cationic defects in Birnessite. Low temperature annealed MnO2 with higher number of defects shows higher specific capacitance at low rate, albeit at the loss of high-rate performance. This trade-off is discussed based grain-boundary resistance dominating the rate performance. Acknowledgments This work was supported in part by ISHIFUKU Metal Industry Co., Ltd. and a Japan Science and Technology Agency Program on Open Innovation Platform with Enterprises,Research Institute and Academia (JST OPERA JPMJOP1843).