(Invited) Capacitive and Pseudocapacitive Intercalation of Aqueous Ions in Layered Materials (MXenes)

Abstract

The development of efficient aqueous electrochemical energy storage (EES) devices is an important sustainability issue to realize green electrical grids. Although current nonaqueous EES devices operate through the intercalation chemistry of host electrode materials, capacitive or pseudocapacitive intercalation has recently emerged as an alternative chemistry for advanced aqueous EES devices. In particular, metal carbide/nitride nanosheets termed MXene are a promising class of intercalation capacitor and pseudocapacitor electrode materials. However, general design strategies of MXenes for aqueous EES applications have not been established because of the limited understanding of the complex electrochemical properties of MXenes with aqueous electrolytes. In this talk, using experimental and theoretical analyses, we clarify the anomalous dielectric properties of aqueous ions confined between MXene nanosheets, leading to the enhanced charge storage capability of MXenes with aqueous electrolytes. Furthermore, we demonstrate that the change of water activity in aqueous electrolytes largely alters aqueous-ion intercalation chemistry, which also influences the charge storage capability of MXenes. References Y. Ando, M. Okubo, A. Yamada, M. Otani, Adv. Funct. Mater., 2020, 202000820.K. Kim, et al., M. Okubo & A. Yamada, Chem. Mater., 2019, 31, 5190-5196.A. Sugahara, et al., M. Okubo & A. Yamada, Nature Commun., 2019, 10, 850.M. Okubo, et al., & A. Yamada, Acc. Chem. Res., 2018, 51, 591-599.