Heteroatom-Tuned Bi pz-Orbital Hybridization of Single-Atom Catalysts for High-Power Density Vanadium Flow Batteries

Highly active single-atom electrocatalysts can effectively enhance the power density of vanadium flow batteries by forming hybrid orbitals between metal atoms and heteroatoms. However, the mechanism through which hybrid orbitals boost the activity of vanadium ions remains unclear. Herein, we report the Bi single atoms electrocatalysts (XBi, X=N, P, S, B) with different heteroatoms coordination, while catalytic activity towards [V(H₂O)₆]³⁺/[V(H₂O)₆]²⁺ is proved to be NBi>PBi>SBi>BBi. The results demonstrated that PBi, SBi, and BBi display the electron cloud distributions of Bi atoms near the fermi level towards the planar direction (p_x, p_y), while NBi exhibits an axial orbital (p_z) distribution and a more pronounced Bi-N p-orbital hybridization due to the high electronegativity of N, thereby promoting vanadium ion dehydration. In consequence, a VFB single cell equipped with NBi decorated graphite felt (GF) electrode demonstrates the most superior battery performance among all fabricated electrodes, achieving a 7% increase in energy efficiency at a current density of 240 mA cm⁻² compared to the BBi-loaded counterpart. The hybrid orbital engineering strategy establishes a design paradigm for single-atom electrocatalysts in high-power density VFBs.