Isotopic Engineering of Water Reactivity for Stable Aqueous Iron‐Metal Batteries

ABSTRACT The development of aqueous iron‐metal batteries (AIBs) is critically hindered by the severe parasitic hydrogen evolution reaction (HER) at the anodes and the resulting structural degradation of the cathode. Moving beyond conventional additive‐based approaches, this work presents a kinetically targeted strategy to suppress HER and enhance overall cell stability through an isotope‐engineered deuterated water (D 2 O)‐based electrolyte. Leveraging the intrinsic differences in zero‐point energy between deuterium and hydrogen, D 2 O features a substantially higher activation energy barrier for water dissociation, effectively taming the reactivity of the problematic Fe anode. Concurrently, control experiments in Fe metal‐free configurations reveal that this isotope effect extends a vital secondary stabilization to the cathode host, establishing a cooperative, dual‐side protection mechanism. Consequently, the D 2 O‐based electrolyte enables a highly reversible iron anode with an average Coulombic efficiency of 99.6% and grants Fe||MoS 2 full cells a stable lifespan of 2000 cycles with an 87.6% capacity retention at 0.5 A g −1 . This work highlights the potential of isotopic modulation as a targeted, high‐efficacy strategy for stabilizing high‐performance aqueous batteries.