Recent progress on the preparation of three-dimensional vertically aligned graphene and its applications insupercapacitors

Abstract

<p indent=0mm>As the key parts of the electrode, materials and structure design are the most important factors to determine the properties of supercapacitors. The structure design of the electrode plays a critical role in deciding its reaction kinetics, ion transportation, and consequently the electrochemical performance of energy storage system. Graphene, as a building block of sp² carbon materials, is characterized by large surface area, excellent electron transfer and great chemical stability, allowing it as the most promising material for energy storage and the results from many laboratories confirm its potential to change today’s energy-storage landscape. Typically, graphene-based materials are randomly oriented with respect to the current collectors in a conventional stacked geometry in supercapacitors. However, increasing the mass loading or film thickness seriously declines the charge storage capability, including specific capacitance and rate capability. This is mainly due to the moderate electrical conductivity, slow ion diffusion, and poor mechanical stability. Control of structure and morphology is key for carbon-based electrodes to allow the effective permeation of the electrolyte to establish electrical double layers in supercapacitors. Three-dimensional vertically aligned graphene (3DVAG) on the current collector is believed to provide an ideal structure for supercapacitors electrode capable of high energy density. 3DVAG electrodes have attracted much research interest owing to their excellent reaction kinetics and mass-transfer capability. The unique properties such as large surface area, vertical open channels and low pore tortuosity, can enhance the ion/electron transfer and increase mass loading of active materials, and thereby ultimately improves the rate capability and energy density of the electrode material. Compared with the traditional graphene-based electrode, 3DVAG electrode has short ion diffusion pathways, better electrochemical performance and also inherits the excellent cycle stability of graphene-based electrodes. At present, the main preparation methods of 3DVAG include directional freezing, plasma-enhanced chemical vapor deposition, KOH assisted hydrothermal and others (e.g.,<italic> </italic>rolling and cutting rGO, electric field deposition, magnetic field alignment). Through manipulating the preparation processes, 3DVAG materials with highly ordered microstructure, tight pore density and high conductivity will be obtained. In this contribution, we review the ongoing research progress in preparation of 3DVAG and its applications in supercapacitors. Firstly, the main preparation methods, the mechanism and the influence of process parameters on the texture of 3DVAG are emphasized. Secondly, the application of 3DVAG materials in the field of supercapacitors including double electric layer capacitors (EDLCs), the related composites for pseudocapacitors (SCs) and hybrid supercapacitors (HSCs) are highlighted. Finally, the challenges and opportunities of further development and application of 3DVAG materials are discussed. The purpose of this review is to introduce a new type of graphene structure with vertically aligned microtexture, providing ideas for the effective utilization of graphene, and with its good vertical channels, it provides a feasible solution for development of high mass loading and high energy density electrode materials.