Research Progress on Preparations and Electrochemical Applications of Functional Coal-Based Carbon Materials

Coal is a fossil resource that is abundant and widely distributed, and it plays an extremely important role in the economic development and social progress of different countries. However, conventional coal utilization methods (such as direct combustion for power generation and heating, and metallurgy) produce large amounts of CO₂, sulfides, and nitrides, seriously threatening ecology, environment, and human-being health.It is thus necessary to improve the utilization methods of coal and promote the green and low-carbon utilization of coal resource. Functional coal–based carbon materials exhibit a superior performance in the field of electrochemical energy/matter conversion and storage due to their unique physicochemical properties, such as high electrical conductivity, large specific surface area, controllable pore structure, and rich surface functional groups. This is of great significance for achieving high value-added utilization of coal and carbon neutrality. In recent years, researchers have made significant progress in the preparation of functional coal-based carbon materials and their electrochemical applications. However, there are a few systematic reviews summarizing this rapidly developing field, and the relationship between the structural design and synthesis of functional coal-based carbon materials and the optimization of their electrochemical properties has not been comprehensively studied yet. This review deeply analyzes the latest research progress on functional coal-based carbon materials in the field of electrochemistry. Firstly, the structural features and chemical element composition of the different coal raw materials are summarized, and the applicable processing methods are described via analyzing the structural characteristics and applicability of representative coal raw materials. Subsequently, the process flows of commonly used methods for preparing functional coal-based carbon materials (i.e., liquid-phase, solid-phase, gas-phase, direct-carbonization, and coupling) are analyzed. The effect of preparation method on the macrogeometric structure, mesoscopic surface physicochemical properties, and microscopic active site electronic structure of coal is discussed, and the regulatory mechanisms of preparation methods on the structure and composition of functional coal-based carbon materials is revealed. Afterwards, an in-depth investigation on the applications of functional coal-based carbon materials in mainstream electrochemical fields such as fuel cells, electrocatalytic CO₂ reduction, supercapacitors, and lithium/sodium-ion batteries is represented. The intrinsic correlation among material preparation, structural regulation, and performance optimization is given via studying the impact of the structure and composition of functional coal-based carbon materials on their electrochemical performance. Finally, this review looks forward to the future development directions of functional coal-based carbon materials, proposes strategies for optimizing the preparation methods and for improving material performance, and points out the key challenges for the industrialization of functional coal-based carbon materials. This review can provide theoretical guidance and technical references for the designs and electrochemical applications of functional coal-based carbon materials, promoting the clean and efficient utilization of coal resource and the rapid development of electrochemical technologies.

The main methods for preparing functional coal-based carbon materials include liquid-phase, solid-phase, gas-phase, direct-carbonization, and coupling methods. The liquid-phase method involves mixing coal raw materials, activators, and dopants with solvents, followed by heat treatment to prepare porous coal-based carbon materials doped with heteroatoms. The advantage of this method is its ability to precisely modulate the materials' morphological structure and surface physicochemical properties, as well as to load highly active metal species onto the coal support. The solid-phase method utilizes mechanochemistry (such as ball mills, mortars, etc.) to mix coal raw materials, dopants, and activators, which can prepare functional coal-based carbon materials with large specific surface areas, high doping contents of heteroatoms, and uniform structures. The gas-phase method utilizes gases to synergistically regulate the morphology of coal and types and contents of doped atoms, enabling atomic-level modification of the coal surface. The direct-carbonization method converts coal into functional coal-based carbon materials with an large number of defects and closed pore structures through high-temperature carbonization, which is suitable for anode materials in lithium/sodium ion batteries. Coupling method integrates the advantages of different methods to achieve multidimensional and precise modulation of the structure, composition, and performance of the functional coal–based carbon materials.

Functional coal-based carbon materials have attracted widespread attention in electrochemical energy fields such as fuel cells, electrocatalytic CO₂ reduction, supercapacitors, and lithium/sodium-ion batteries due to their structural designability and cost advantages. For fuel cells, the functional coal-based carbon materials can significantly improve the efficiency and stability of fuel cells due to their better mass/charge transfer efficiency and stronger corrosion resistance, compared to commercial platinum carbon materials. In the field of electrocatalytic CO₂ reduction, functional coal-based carbon materials can enhance the local concentration of CO₂/intermediates around their active sites via leveraging the size effects of their geometric structures (e.g., spatial confinement effect). In addition, the functional coal-based carbon materials can also optimize the adsorption strength and thermodynamic conversion energy barriers of CO₂/intermediates via regulating the local electron density of the active sites, thereby achieving a highly selective conversion of CO₂ into valuable chemicals such as CO and ethylene. In supercapacitors, the functional coal-based carbon materials with large specific surface areas, hierarchical porous structures, and rich surface functional groups can improve electrolyte transfer rates and provide sufficient adsorption sites for ions, thereby achieving high double-layer capacitance and pseudocapacitance. For lithium/sodium-ion batteries, the large interlayer spacing, rich defects and closed pore structures of functional coal-based carbon materials enable them to optimize the transfer and adsorption processes of lithium/sodium ions, thereby improving the storage capacity and rate performance of batteries.

The structural adjustability of functional coal-based carbon materials enables to adopt different preparation methods according to the actual application scenarios to obtain the suitable functional coal-based carbon materials, greatly promoting the development of electrochemical energy/matter conversion and storage technologies. Moreover, developing functional coal-based carbon materials can achieve the non-fuel utilization of coal and reduce environmental pollution generated during coal utilization, which is in line with the concept of green sustainable development. Also, the combination of functional coal-based carbon materials and electrochemical technologies can extend the coal industry chain and enhance the added value of coal, thus promoting the transformation and upgrading of the coal industry.

Functional coal-based carbon materials demonstrate an enormous application potential in the field of electrochemical energy/matter conversion and storage due to their abundant raw materials, low cost, and adjustable structure. This review systematically summarizes the research progress on the preparation methods and electrochemical applications of functional coal-based carbon materials, and explores the intrinsic relationships between material structure and electrochemical performance. Based on the existing research status of the preparations and electrochemical applications of functional coal-based carbon materials, we propose some prospects for their future research directions, i.e., 1) Simple and environmental-friendly physical activation methods need to be developed, such as supercritical CO₂-assisted activation and microwave carbonization, which achieve controllable pore structure construction through radical reactions and interfacial energy regulation. Low-energy technologies such as photothermal-driven carbonization and plasma pulse activation should be developed to utilize non-equilibrium thermodynamics to regulate the degree of graphitization and defect quantity of carbon layers, thereby reducing carbon emissions from the preparation process. In addition, machine learning can be also used to optimize preparation parameters (such as temperature field, pressure field, atmosphere composition, etc.) to achieve accurate prediction and targeted control of materials’ morphology, structure, and chemical composition, 2) The structure–property relationship and performance improvement mechanism of materials need to be analyzed. In-situ characterization techniques (such as electrochemical-synchrotron radiation coupling technology and in-situ aberration-corrected electron microscopy) can analyze the material structure and composition during electrochemical processes, clarify the mechanisms of material performance degradation under complex conditions, and develop modification strategies. In addition, it is also necessary to develop kilowatt-scale industrial electrochemical devices suitable for functional coal-based carbon materials to promote the practical application of coal-based carbon materials and 3) Some precise coal quality analysis methods and standardized coal processing methods need to be establised and the continuous, large-scale production equipment needs to be developed, thereby addressing the impact of coal raw materials on product structure and performance consistency. For instance, establishing a rapid analysis of coal quality (i.e., combining near-infrared spectroscopy with Raman spectroscopy)-three-dimensional structural simulation (i.e., Material Studio)-process parameter feedback system ensures that the structural parameters of coal raw materials from different production areas remain within a certain range. The design of a modular mobile doping-activation-pyrolysis integrated system is expected to enable the continuous conversion of coal raw materials into functional coal-based carbon materials, thus addressing the impact of conventional batch preparation process on the product performance. These technologies can contribute to the large-scale production and application of high-performance functional coal-based carbon materials.