Research progress on carbon materials derived from low-rank coal

Abstract

Low-rank coal is considered a high-quality precursor for carbon materials due to its abundant reserves, rich polycyclic aromatic hydrocarbons, high carbon content, and low cost. However, differences in ash content, microstructure, and interface among different low-rank coals lead to difficulties in effectively controlling the structure and performance of coal-based carbon materials. In recent years, researchers have proposed effective methods for regulating the microstructure and surface/interface of low-rank coal-based carbon materials. This review focuses on the differentiated strategies for preparing activated carbon, capacitive carbon, hard carbon, graphite, and nanocarbon materials from low-rank coal. It further discusses the effects of coal type and process on the microstructure, interface characteristics, and functional group types of coal-based carbon materials. Additionally, the applications of coal-based carbon materials in adsorption, supercapacitors, and alkali metal batteries are introduced. Finally, future research directions and challenges for low-rank coal-based carbon materials are prospected.

1. Introduction

Coal has supported the energy demands of human society and ensured energy security. Under the 'dual carbon' goals, clean and efficient utilization and materialization of coal have become important pathways for its future transformation. Coal is mainly formed from various amorphous, degraded plant remains. Over time, with increased pressure and heat, the degree of metamorphism of these remains gradually increases, forming peat, lignite, sub-bituminous coal, bituminous coal, and anthracite with different degrees of metamorphism (Fig. 1a). Although coals of different ranks differ in structure, their constituent elements are C, H, O, N, and S, and they contain abundant aromatic and aliphatic organic matter interconnected by aliphatic and ether bonds. Moreover, the molecular structure of coal is naturally similar to that of carbon materials, and it is rich in intermediate crystalline structures between amorphous carbon and graphite, known as 'turbostratic structure', which can be converted into various coal-based carbon materials through processing. Therefore, coal is not only a fossil fuel but also a high-quality precursor for carbon materials [1,2].

A wide variety of carbon materials can be prepared from coal, and related research is extensive. Carbon fibers [5], carbon nanotubes [6], carbon nanosheets [3], fullerenes [7], graphene [8], porous carbon spheres [9], carbon aerogels [10], carbon foams [11], and porous graphite spheres [12–14] have been reported using coals of different ranks, such as lignite, bituminous coal, and anthracite [1–4]. Among them, anthracite has long been used to prepare capacitive carbon, porous carbon, graphene, and other carbon materials [15], and reviews have been published, but anthracite has surface inertness and is difficult to activate for pore formation. Therefore, low-rank coal has attracted increasing attention and research due to its excellent aromatic and aliphatic structure tunability, high volatile content, and ease of pore formation, but systematic summaries and reviews are rare. Coal-based carbon materials not only break the disciplinary boundaries between coal chemistry and carbon materials but also achieve good profits in industry. Therefore, it is necessary to review the research on preparing carbon materials from low-rank coal in a specific field.