Marine biomass (MB) offers an environmentally friendly and readily available carbon source from the ocean. However, the high concentration of alkali and alkaline earth metals (AAEMs) in MB typically reduces the carbon yield and inhibits micropore formation during heat treatment due to catalytic gasification. In this study, we successfully synthesized activated carbon (AC) with a high specific surface area (> 1,500 m2/ g) and significant mesopore content (60%, mean pore size: 3.4 nm) from MB by employing preheating, controlled acid purification, and CO₂ activation. The formation of mesopores in the MB-derived AC was driven by catalytic gasification induced by intrinsic and residual AAEMs during preheating and physical activation processes. We evaluated the potential of the MB-derived AC as an electrode material for electric doublelayer capacitors (EDLCs). The material demonstrated high specific capacitance values of 25.9 F/g and 29.4 F/g at 2.7 V and 3.3 V, respectively, during charge–discharge cycles. These high capacitance values at elevated voltages were attributed to the increased number of solvated ions (e.g., 1.93 mmol/g at 3.3 V) present in the mesopores. Fluorine-19 nuclear magnetic resonance (19F solid-state NMR) analysis revealed a substantial increase in solvated ion concentration within the mesopores of the MB-derived AC electrode at 3.3 V, demonstrating enhanced ion mobility and diffusion. These findings highlight the potential of MB-derived AC as a promising electrode material for high-voltage energy storage applications.

Marine biomass (MB) is gaining attention as a sustainable and eco-friendly carbon source within the carbon cycle, particularly in regions with extensive coastlines. However, the high content of alkali and alkaline earth metals (AAEMs) in MB poses challenges in producing functional carbon materials, like activated carbon (AC), with a high specific surface area (SSA). In this study, we employed a two-step CO2 activation process, coupled with acid treatment, to successfully convert MB into highly porous AC. Preheating followed by nitric acid washing reduced AAEM content from 22.4 to 2.5 wt%, and subsequent atmospheric CO2 activation produced AC with an SSA of 1700 m2/ g and mesopores of 3–5 nm. A further treatment with a mixed acid solution of nitric and acetic acids reduced impurities to below 1.0 wt%. A second pressurized CO2 activation at 1 MPa yielded AC with an SSA exceeding 2100 m2/ g, with mesopores accounting for more than 50% of the total pore volume. This method demonstrates an effective approach to producing high-performance AC from MB for advanced applications.