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 Ni₃S₄-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 Ni₃S₄-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.