Rechargeable sodium seawater batteries (SWBs) are gaining the world leadership of high voltage energy storage devices for marine environments. With natural seawater as the source of active material, SWBs can be supplied infinitely with Na cations. Because of their open-structured cathode, the cathode material’s specific surface area, porosity and wettability need to be optimized to achieve a high-performance cell. In this work, activated textile polyacrylonitrile (PAN) fibers were used to produce an activated carbon felt with a facile manufacturing process. The easy and low-cost production of these fibers makes them excellent candidates for energy storage applications involving oxygen evolution and reduction reactions. The electrochemical performance results of the fabricated activated PAN fibers and of commercial carbon felts were measured and compared, being characterized through galvanostic charge discharge cycles, electrochemical impedance spectroscopy and cyclic voltammetries. A performance improvement was observed with PAN activated carbon felt as half cell with a capacitance increase (about 9000%), and as full cell with a smaller voltage gap (about 10%) and increased gravimetric capacitance (about 260%) when compared to the commercial carbon felt. The successful implementation of PAN activated carbon felts in an aqueous environment opens new paths toward high performance seawater battery’s cathodes.

New sustainable energy solutions are needed, and oceans—covering more than 70% of Earth's surface—can become a key asset in achieving this objective. Rechargeable seawater batteries (SWBs) use the almost infinite supply of Na ions present in natural seawater as the catholyte. Nevertheless, to obtain high performance, scalable and effective electrocatalysts must be added. The use of readily available carbon‐rich biowaste has become a widely advanced approach in scaling up new energy storage technologies. Furthermore, the tunable and porous characteristics inherent to bio‐waste precursors can enhance capacitive behavior, enabling hybrid performance. In this work, a peanut‐shell bio‐waste activated carbon—originally designed for supercapacitor electrodes, leveraging its high surface area and capacitive potential—is synthesized through an easy, affordable, and eco‐friendly process, and applied as an electrocatalyst in a SWB cathode. Notably, the bio‐waste coating on the immersed electrode exhibited remarkable enhancements over a thermally treated commercial carbon felt electrode. These improvements included a capacitance increase of nearly 166% and reduced voltage gaps (averaging a 59% decrease) in a full‐cell configuration. This enhanced electrochemical performance results from a synergy between capacitive effects and improved oxygen reaction efficiency, supporting sustainable supercapacitor‐like capacitive electrodes in SWB technology.