Bifunctional catalyst design for vanadium flow batteries: Oxygen vacancies for catalysis and metallic Bi for hydrogen evolution suppression

Electrode materials that simultaneously exhibit high catalytic activity and effective hydrogen evolution suppression is crucial for high-performance vanadium flow batteries. In this work, we design a bifunctional catalyst composed of metallic Bi, bismuth oxide, and nitrogen-doped amorphous carbon. Metallic Bi provides hydrogen evolution inhibition, while oxygen vacancies in bismuth oxide serve as active sites for enhanced catalytic activity. Meanwhile, nitrogen-doped carbon further facilitates catalysis and improves adhesion to the carbon felt substrate. The catalysts are prepared by hydrothermally converting a Bi-zeolitic imidazolate framework-8 precursor, followed by carbonization at various temperatures. Morphology, composition, electrochemical activity, and charge-discharge performance are systematically analyzed. It is found that: (i) The composite synthesized at 900 °C displayed uniform, finely dispersed particles, abundant oxygen vacancies, oxygen-containing functional groups, graphitic nitrogen, excellent hydrophilicity, and outstanding electrochemical performance; (ii) it significantly improves coulombic efficiency due to the hydrogen evolution suppression; (iii) it improves the energy efficiency by 8.5% compared to bare carbon felt electrodes at 100 mA cm −2 ; (ⅳ) it delivers marked enhancements in cell capacity, applied voltage, power output, high current density capability, and cycling life. Overall, this work proposes a novel electrochemical catalyst that achieves synergistic catalysis and hydrogen evolution suppression, enabling high-performance vanadium flow batteries.