Lewis‐Basic Metal Sulfide Electrode Interfaces Unlock High Power Density Alkaline All‐Iron Redox Flow Batteries

Alkaline all‐iron redox flow batteries (AIRFBs) are promising for safe, low‐cost grid‐scale energy storage using Earth‐abundant elements. However, insufficient energy efficiency and power density limit practical deployment, while the interfacial kinetic origin of these losses remains unclear. Here, we identify sluggish outer‐sphere electron‐transfer kinetics of Fe‐chelate anolytes as a key kinetic bottleneck governing polarization in alkaline AIRFBs and further link this bottleneck to limited outer Helmholtz plane (OHP)‐region accessibility and weak interfacial electronic coupling. To overcome this limitation, we develop Lewis‐basic sulfide interfaces that regulate the local Fe‐chelate/electrode environment. These regulated interfaces convert inefficient diffusion encounters into productive sulfide‐assisted outer‐sphere electron‐transfer events, thereby accelerating anolyte redox kinetics and suppressing polarization. An alkaline AIRFB assembled with a Ni 3 S 4 ‐modified electrode delivers an energy efficiency of 80.65% at 200 mA cm −2 and a peak power density of 501.2 mW cm −2 , compared with 65.69% and 185.5 mW cm −2 for the pristine cell. The Ni 3 S 4 ‐based cell further maintains an average Coulombic efficiency of 99.9%, an average energy efficiency of 77.8%, and stable operation over 3000 cycles at 200 mA cm −2 . This work reveals an interfacial kinetic bottleneck in alkaline AIRFBs and establishes Lewis‐basic interface engineering as a strategy for high‐power and low‐cost long‐duration energy storage.