Unburned Carbon in Coal Fly Ash: Exploring Its Potential for Electrochemical Energy Storage in Supercapacitors

Coal fly ash (CFA) is one of the most widely generated industrial solid wastes globally, yet its valorization remains limited, particularly regarding its unburned carbon (UC) fraction. UC is typically considered an undesirable impurity that restricts CFA utilization in construction applications and often requires costly beneficiation or disposal. Despite being carbon rich, low cost, and abundantly available, the use of UC as a functional carbon precursor for electrochemical energy storage, especially in supercapacitors, remains largely unexplored. Only a limited number of studies have reported UC‐derived carbons as supercapacitor electrodes, with a single well‐documented example demonstrating encouraging electrochemical performance. This scarcity highlights a significant knowledge gap and an underutilized opportunity for sustainable carbon material development. This review examines the origin, physicochemical properties, and separation of UC from CFA, with emphasis on its conversion into electrochemically active carbon through carbonization, activation, functionalization, and composite formation. While biomass‐derived and commercial carbons are extensively studied for supercapacitor applications, investigations on UC remain fragmented despite favorable structural characteristics. Available evidence suggests UC‐derived carbons can achieve competitive surface areas and capacitive performance. This review positions UC from CFA as a promising carbon resource for sustainable electrochemical energy storage applications worldwide in future development.