Defining Metal-Impurity Thresholds for Hydrogen Evolution in Sealed Vanadium Ion Batteries

The vanadium ion battery (VIB) has emerged as a next-generation aqueous energy storage system offering high safety, long cycle life, and scalability. Unlike redox flow batteries, the VIB employs a fully sealed architecture without external pumps, making suppression of the hydrogen evolution reaction (HER) a critical requirement. Here, we systematically investigate the role of metal ion impurities on HER in sealed VIBs. Representative impurities originating from vanadium mining, refining, and handling were introduced into the vanadium liquid electrode at concentrations up to 500 mg L^-1 and internal pressure changes during cycling were monitored as a sensitive indicator of gas evolution. Distinct categories of impurity behavior were identified, ranging from strongly HER-promoting noble metals to species whose apparent inertness arises from limited solubility. From these results, impurity-specific threshold ranges were established, defining the onset concentrations at which HER leads to irreversible pressure build-up. These thresholds provide practical guidance for setting vanadium liquid electrode purity specifications and highlight the inherent differences between sealed VIBs and conventional redox flow batteries. Collectively, this work bridges fundamental understanding of impurity-HER interactions with industrial requirements for liquid electrode management, enabling long-term stability and safe operation of sealed VIB systems.