The need for new and sustainable energy solutions that use abundant and readily available materials is becoming more significant than ever. Rechargeable seawater batteries (SWBs) use natural and abundant seawater as the source of active material, capitalizing on an almost infinite supply of Na cations. The open structure configuration of its cathode plays a pivotal role in the performance. Thus, the cathode current collector characteristics, such as its specific surface area, porosity, functionalization, and wettability, must be enhanced. Sustainable and scalable materials, such as waste carbon, are essential for these enhancements. Peanut shells, with a global annual volume of approximately 7.44 million tons, serve as excellent bio-waste precursors capable of producing activated carbon with a high surface area. In this work, an annealed commercially available carbon felt (HCF) both without and with a synthesized peanut shell electrocatalyst coating (PSCF) was studied in a full SWB cell. The cathodes were characterized through SEM, EDS, Hg porosimetry, N2 adsortion, and FTIR-ATR, with surface wettability assessment. The performance was evaluated through galvanostatic charge/discharge cycles, cyclic voltammetry, and impedance spectroscopy. The bio-waste electrode exhibited remarkable enhancements, showcasing a substantial capacitance increase by nearly 630% and reduced voltage gaps (averaging a 59% decrease) in a full-cell configuration when compared to HCF. Here, the enhanced electrochemical performance of the bio-waste electrode can be attributed to a mutual influence of a capacitive and electrochemical energy storage mechanisms, where the increased surface area and porosity (65% increase) facilitated the formation of a strong electric-double layer. Adding to this, the KOH activation process of peanut shells contributed to a heavily populated surface with oxygen-active functional groups which enhanced the oxygen evolution and reduction reactions activity efficiency. We were able to conclude that the application of a peanut shell electrocatalyst to a seawater battery is possible with promising performance. The bio-waste PSCF is a low-cost electrode, with a low carbon footprint and an easy and scalable manufacturing process. This work opens a new path for the integration of bio-waste in rechargeable seawater battery technology towards a more sustainable future.