Two-Dimensional Pseudocapacitive Nanomaterials for High-Energy- and High-Power-Oriented Applications of Supercapacitors

Abstract

ConspectusSupercapacitors (SCs) are electrochemical energy storage devices that can fill the gap between batteries and electrolytic capacitors. However, the widespread applications of commercialized carbon-based SCs are limited by their energy density, arising from their physical charge storage mechanism, which is by far lower than that of batteries. Moreover, the high-powered applications of SCs are also limited by their kinetics, which are slower than those of electrolytic capacitor due to the diffusion and distribution of ions onto the tortuous porous surface. Therefore, the energy and power performance of SCs need to be improved to open or further extend their practical applications.Since all atoms of two-dimensional (2D) nanomaterials are located on the surface, the design of surface structure is critical to determining the bulk electrochemical properties. Such a surface-oriented property of 2D nanomaterials is well fitted to control the surface charge storage mechanism of SCs, thereby discovering emerging capacitive materials through the rational design of surface chemistry and multiscale structures. This Account discusses our recent progress on 2D pseudocapacitive materials for high-energy- and high-power-oriented SCs applications and provides our perspective into the rational design of the microstructure, multiscale architecture, and surface chemistry. Examples of 2D nanomaterials include heteroatom-doped graphene, black phosphorus, transition-metal dichalcogenides, and transition-metal carbide/nitrides (MXene). We also highlight the in-depth spectroelectrochemical and computational analyses that can correlate the structures and chemistries of 2D nanomaterials with their charge storage/transport/transfer behaviors.In this Account, our design concept of 2D nanomaterials is based on two aspects of charge storage capability and kinetics that can determine the thermodynamic (capacitance) and kinetic (rate) performances. First, chemical strategies, such as atomic incorporation, surface functionalization/coordination, and hybridization of 2D nanomaterials, will be provided and correlated with the population of redox storage sites and interaction between sites and ions. The charge storage capacitance can be improved by controlling these factors for high-energy-oriented applications. Second, we will address key factors such as charge-transfer kinetics, ion-transporting pathways, and percolated electron transport for high-power-oriented applications. Several approaches such as multiscale architecture, hybridization with electronically conductive materials, pore orientation, and an expanded interlayer space will be introduced to improve the kinetic performance of 2D nanomaterials. Finally, we will provide our perspective on technical impediments and future research directions of 2D nanomaterials for practical energy- and power-oriented applications of SCs.

Topics & Concepts