Electrical energy storage on large-scale is very essential to the ultimate use of natural and clean renewable energy sources. Recently, research and development attempts on room-temperature seawater battery (SWB) have been rejuvenated, as SWBs are considered capable, low-cost substitutes to the existing Li-ion battery technology for large-scale uses. In this work, P2-type layered Na0.5Co0.5Mn0.5O2 oxide have been discovered for the first time as a new option of bifunctional cathode electrocatalyst material for aqueous SWB which admits two kinds of voltage profiles such as slope and flat voltage region corresponding to Na+ ion intercalation- deintercalation reaction and OER/ORR catalytic activities in comparison with carbon felt substrate and 20% Pt/C catalyst. Developed electrocatalyst for cathode of SWB demonstrates excellent activity and provides discharge capacity of ~ 30 mAh g–1 ascribed to Na+ ion intercalation - deintercalation reaction and cyclic voltammetric analysis corroborated with the redox peak at 3.31/3.04 V for Co3+/4+ redox species in comparison with carbon felt substrate. Further, OER and ORR catalytic activities of electrocatalyst were confirmed with full-cell seawater battery, exhibited less voltage difference 0.36 V and delivered hard carbon discharge capacity 183 mAh g–1 at 0.1 mA. Consequently, this cathode was demonstrated integrated voltage profile for seawater battery, shows potential to work on commercial scale.