Fabrication, characterization, and applications of hard carbons: A comprehensive review

Hard carbon, an amorphous carbon material, has gained significant attention as a sustainable and high-performance energy storage solution due to its unique structure, characterized by disordered graphene layers, hierarchical porosity, and rich surface functionalities. This review provides a comprehensive analysis of its synthesis, structural and electrochemical characteristics, and applications, with a focus on its potential as a next-generation anode material for sodium-ion, potassium-ion, and lithium-ion batteries. It explores fundamental ion storage mechanisms such as adsorption, intercalation, and pore-filling, which contribute to its high reversible capacity and cycling stability. Various synthesis methods, including feedstock selection, one-step and two-step synthesis strategies, and advanced fabrication techniques, are critically examined to understand their impact on material properties. State-of-the-art analytical techniques are reviewed to elucidate porosity, disorder, and surface functionality, linking them to electrochemical performance. Beyond batteries, this review highlights hard carbon's potential in supercapacitors, hydrogen storage, and fuel cell catalysis while addressing challenges and opportunities in large-scale production and performance enhancement for commercial applications. By bridging synthesis, structure, and functionality, this work underscores the versatility of hard carbon as a promising material for sustainable energy storage and advanced applications.