Aqueous electrolytes, which possess the advantages of nonflammability and high ionic conductivity for safe and sustainable energy storage systems, are restricted by their narrow potential windows due to water electrolysis. The recent study of high-voltage aqueous electrolytes has mainly focused on the molecular-level hydration structure of electrolyte salts, while the influence from subatomic-scale neutrons of the water solvent has never been considered. Here, for the first time, we report an electrochemical isotope effect in which the numerically increased neutrons in the water solvent extend the potential window of aqueous electrolytes. This effect is caused by the following factors: the lower zero-point energy of the deuterium compound, the smaller ion product, and the larger dehydration energy of heavy water. It is affected by ion species, electrolyte concentrations, and the ratio of deuterium to protium. Our finding provides the new insight into aqueous electrochemistry that the isotope in molecular water improves the performance of aqueous electrolytes.