CoSn Alloy Catalysts With Optimized <i>d</i> ‑Band Center for Chloride‐Resistant ORR in Harsh Marine Environment

Seawater batteries (SWBs) that rely on the dissolved oxygen reduction reaction (ORR) on the cathode are a crucial choice for the long‐term power supply of underwater equipment. However, chloride ions (Cl − ) poisoning and the lean‐oxygen condition in seawater severely suppress the ORR kinetics, leading to rapid activity decay and poor durability of catalysts. Herein, we present a Cl − ‐resilient ORR catalyst of CoSn alloy anchored on nitrogen‐doped carbon substrate (CoSn@NC). The introduction of p ‐block Sn atoms effectively tunes the d ‐band center of Co, weakening the adsorption of competitive Cl − while simultaneously optimizing the binding strength with oxygenated intermediates. This dual regulation mitigates chloride‐induced surface poisoning even in a lean‐oxygen environment. The obtained CoSn@NC exhibits superior ORR activity and durability in natural seawater, delivering the higher half‐wave potential of 0.72 V and enhanced kinetic current density compared with commercial Pt/C. Furthermore, a SWB prototype employing CoSn@NC as cathode demonstrates excellent rate capability and a stable 1.49 V operating voltage sustained for over 130 h. Promisingly, we demonstrate stable SWB operation beneath the sea‐ice layer in the Bohai Bay, which confirms that CoSn@NC can sustain ORR under the Cl − corrosion and low temperature, thereby giving guidance for the design of practical SWBs in a harsh marine environment.