A review of the high-concentration processing, densification, and applications of graphene oxide and graphene

Abstract

Dense graphene assemblies, composed of tightly stacked graphene sheets, have outstanding chemical stability and excellent mechanical, thermal, and electrical properties. They also do not have the problems of low density, low mechanical strength, poor electrical conductivity, or poor thermal conductivity found in porous graphene aerogels, making them ideal materials for future portable electronic and smart devices. We summarize work on high-concentration graphene oxide (GO) and graphene dispersions prepared by mechanical dispersion, evaporation concentration, centrifugal concentration, and liquid phase exfoliation, as well as two-dimensional (2D) dense graphene-based films and three-dimensional (3D) dense graphene-based structures prepared by vacuum-assisted filtration, interfacial self-assembly, and press-forming, and evaluate the advantages and disadvantages of each method. The applications of dense graphene-based assemblies in energy storage, thermal management, and electromagnetic interference (EMI) shielding are summarized. Finally, their challenges and prospects in future research are outlined. This review provides a reference for exploring and developing their large-scale, cost-effective manufacture and use.

1. Introduction

Graphene is a 2D flexible nanomaterial composed of sp2-bonded carbon atoms. It has a large specific surface area, good chemical stability, and outstanding mechanical, thermal and electrical properties[1–4]. Since the discovery of graphene in 2004, researchers have conducted numerous in-depth studies on its structure, properties and applications, deeming it a promising new material for the future[5]. So far, graphene shows promising application prospects in sensors[6–9], energy storage[10–12], catalysis[13–15], thermal management[16–18], and adsorption[19–21], but it still faces some challenges. For example, graphene aerogels with 3D porous structures have the advantages of low density, high specific surface area and excellent elasticity. However, their poor mechanical, electrical and thermal properties limit their applications in miniaturized, integrated and intelligent devices. The strategy of densification is a simple and effective solution to enhance the mechanical strength, electrical conductivity and thermal conductivity of graphene assemblies[22,23].

To date, there have been numerous studies on dense graphene assemblies, such as 2D films, 3D monoliths and 3D microlattices. Dense graphene materials are typically formed under external forces such as pressure[24,25], surface tension[26–28] and intermolecular forces[29]. Specifically, 2D films can be formed by pressing[30,31], self-assembly[32,33], filtration[34] and other methods. 3D graphene monoliths and microlattices can be formed through pressing and self-assembly[25,35,36]. The tightly stacked graphene sheets in dense graphene assemblies facilitate electron migration and heat transfer[37]. Dense graphene assemblies exhibit excellent mechanical strength, thermal conductivity and electrical conductivity compared to porous graphene aerogels, showing great potential in energy storage[38–40], thermal management[41,42] and EMI shielding[43,44].

Graphene has poor solvent compatibility and dispersibility due to its chemical inertness[45], leading to poor processability. Moreover, there are strong π-π interactions and van der Waals forces between graphene sheets, which make them prone to stacking and agglomeration.