Marine macroalgae biomass as a sustainable resource for energy storage devices: progress, challenges, and future perspectives.

The rapidly increasing demand for sustainable, high-performance energy-storage systems has driven strong interest in renewable, low-cost carbon resources. Among these resources, marine macroalgae represent a distinctive biomass platform because of their rapid growth, abundance, carbohydrate-rich composition, and intrinsic heteroatom-containing functionalities. The present review provides a focused and critical assessment of marine macroalgae as precursors for advanced energy-storage materials with particular emphasis on the relationships among precursor chemistry, processing pathways, carbon structure, surface functionality, and electrochemical performance. A systematic analysis is presented of how differences among brown, red, and green macroalgae influence carbon yield, mineral/ash content, heteroatom retention, pore development, and the properties of the resulting electrode materials. Key structural-engineering strategies, including controlled carbonization, physical and chemical activation, intrinsic heteroatom self-doping, morphology regulation, and hybridization with transition-metal oxides/chalcogenides and conductive components, are critically discussed in the present work. The roles of these structural and compositional features in regulating ion transport, electronic conductivity, surface wettability, redox activity, and charge-storage mechanisms are linked to performance in electric double-layer capacitors, pseudocapacitors, and rechargeable batteries, including lithium-, sodium-, and zinc-ion systems. A further focus of this assessment is the translation of laboratory-scale performance toward practical applications by examining feedstock variability, pretreatment, processing energy, cost, environmental implications, scalability, and precursor standardization. Finally, the discussion identifies key research gaps and future directions toward rational feedstock selection, greener processing, multifunctional hybrid architectures, and device-level optimization. Overall, the present work establishes marine macroalgae as a versatile and sustainable platform for developing next-generation energy-storage materials while highlighting the scientific and practical challenges that must be addressed for scalable implementation.