Dual role of Pb2+ in inhibiting hydrogen evolution and suppressing metal-ion-induced polarization in vanadium flow batteries

Flow batteries (FBs) have emerged as a compelling solution for large-scale, grid-level energy storage due to their decoupled power and energy capacities. However, their long-term electrochemical stability is critically undermined by trace metal impurities inadvertently introduced during electrolyte synthesis or system operation. These contaminants—particularly transition and multivalent metal ions—may catalyze the hydrogen evolution reaction (HER) at the negative electrode during charging, leading to severe parasitic losses, increased polarization, and capacity fade. Here, we systematically investigate the role of five representative impurity ions (Cu 2+ , Fe 2+ , Al 3+ , Zn 2+ , Ir 3+ ) in vanadium flow batteries (VFBs), revealing their dual role in exacerbating HER and selectively increasing the charge-transfer resistance of the V 3+ /V 2+ redox couple while sparing the V 5+ /V 4+ kinetics. This asymmetry results in a pronounced decline in voltage efficiency (VE), despite stable coulombic efficiency (CE), highlighting the insidious nature of impurity-driven degradation. To mitigate these effects, we demonstrate the efficacy of trace Pb 2+ additives as a selective HER suppressor. Electrochemical characterization, supported by density functional theory (DFT) calculations, shows that Pb 2+ preferentially anchors to defect sites on carbon electrodes—blocking HER-active centers while preserving vanadium redox activity. Remarkably, in multi-contaminated electrolytes, Pb 2+ outcompetes other metal ions for adsorption, reversing their deleterious effects and restoring electrochemical performance to near-pristine levels. These findings offer critical mechanistic insights and establish a practical strategy for impurity-resilient FB operation, advancing the path toward robust, large-scale energy storage deployment. Pb 2+ ions effectively suppress hydrogen evolution and counteract performance degradation caused by metal ion contaminants in vanadium flow batteries by preferentially adsorbing on electrode defects, preserving redox kinetics and efficiency. • Pb 2+ suppresses HER by competitively occupying active sites on carbon electrodes. • Pb 2+ reverses impurity-induced polarization in vanadium redox flow batteries. • DFT shows Pb 2+ blocks H + adsorption, effectively mitigating HER initiation. • Electrochemical performance is restored in multi-contaminated electrolytes with Pb 2+ .