Synergistic Single‑Atom Catalysis and Electrolyte Additive Engineering Enables High‑Performance Aqueous Zn-Se Batteries.

Aqueous Zn─Se batteries are limited by the sluggish solid‑solid conversion of Se/ZnSe, large voltage hysteresis, and rapid capacity decay. Here we show that integrating an iron single‑atom catalytic host (Fe SAs@PNC) with a guanidinium iodide (GuI) electrolyte additive in a 3 M ZnSO4 electrolyte overcomes these limitations. The atomically dispersed Fe─N4 sites provide strong Se affinity and a hierarchical porous structure (550.4 m2 g- 1, 56.47 wt.% Se loading). Density functional theory calculations reveal that the Fe─N4 sites reduce the rate‑determining barrier for the selenium reduction reaction from 0.74 eV on pristine N4C to a thermodynamically favorable process (-0.426 eV) under the operating electrochemical potential. The GuI additive exhibits a stronger coordination with Zn2+ than H2O and chemisorbs strongly on the Zn (002) facet. More critically, the iodide anion weakens the Zn─Se bond in the discharge product ZnSe, as evidenced by bond elongation from 2.335 Å to 2.482 Å and a decrease in the crystal orbital Hamilton population from -1.451 to -1.018 eV. Consequently, the Fe SAs@PNC/Se cathode delivers a high capacity of 665.7 mAh g- 1 after 100 cycles at 0.2 A g- 1 (86.75% retention) and maintains 499.6 mAh g- 1 over 1000 cycles at 1 A g- 1 (85.57% retention), with a flat discharge plateau and a polarization as low as 0.30 V, demonstrating a highly reversible Se/ZnSe conversion.