High‐Capacity Hybrid Electrode for Seawater Batteries with Bio‐Waste Electrocatalyst

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.