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

## TL;DR

This paper identifies metal impurity thresholds that trigger hydrogen gas buildup in sealed vanadium ion batteries, offering guidelines for safe and stable battery operation.

## Contribution

The study establishes impurity-specific thresholds for hydrogen evolution in sealed vanadium ion batteries, a novel approach for managing electrode purity.

## Key findings

- Noble metals strongly promote hydrogen evolution, while others are inert due to low solubility.
- Impurity thresholds were defined based on pressure buildup during battery cycling.
- The findings highlight key differences between sealed VIBs and conventional flow batteries.

## Abstract

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.