Synergetic Modulation on Solvation Structure and Electrode Interface Enables a Highly Reversible Zinc Anode for Zinc–Iron Flow Batteries

Zinc-based flow batteries hold great potential for grid-scale energy storage because of their high energy density, low cost, and high security. However, the inferior reversibility of Zn2+/Zn on porous carbon electrodes significantly deteriorates long-term zinc anode stability and, thus, impedes further technological advances for zinc-based flow batteries. Herein, we propose nicotinamide (NAM) as a cost-effective additive to neutral ZnCl2 anolyte, which realizes highly reversible zinc plating/striping reactions on carbon felt electrodes for zinc–iron flow batteries. Experimental characterization and theoretical calculation prove that the nicotinamide not only effectively reshapes the Zn2+ solvation structure by substituting two water molecules from the primary Zn2+-6H2O solvation shell but also is capable of adsorbing on deposited zinc layers to regulate Zn2+ diffusion toward the electrode interface and avoid an undesirable tip effect, thereby affording uniformly dendrite-free zinc deposition and significantly enhanced Zn plating/striping reversibility. Benefiting from NAM additives, the zinc–iron flow battery demonstrates a good combination of high power density (185 mW cm–2), long cycling stability (400 cycles, 120 h), enhanced resistance to self-discharge (98.9% capacity retention in 12 h), and preeminent battery efficiency (70% energy efficiency at 50 mA cm–2), which provides a new pathway to developing a robust zinc anode for advanced flow batteries.