A Multimodal In-Situ Liquid Phase TEM and Electrochemical Study on Boric Acid as Electrolyte Additive for Alkaline Zinc-Air Batteries.

Rechargeable aqueous zinc-air batteries are critically hindered by dendrite formation, parasitic hydrogen evolution (HER), and corrosion at the zinc anode. Here, we employ a multimodal characterization strategy, bridging nanoscale visualization with macroscale information, to understand the effects of boric acid (BA) as an additive in alkaline media. In-situ liquid phase TEM (LPTEM) observations demonstrate that BA additive suppresses HER and promotes a uniform, compact plating with highly reversible stripping. This contrasts sharply with the base electrolyte, which shows vigorous bubbling, nonuniform plating, and incomplete stripping. This nanoscale mechanism translates to the macroscale, where BA induces a transformation from dendritic structures to the desirable, compact "boulder-like" deposit. Macroscale electrochemical analysis quantifies this effect, revealing that 130 mM BA suppresses the corrosion current density by an order of magnitude significantly inhibiting HER. Further investigations combining XRD, ICP-OES suggest that BA is likely to exist as the tetrahydoxyborate anion, which functions as a non-consumed, non-incorporated surface-active agent. Finally the cycling of ZnO-PTFE composite anode in BA containing electrolyte over 75 cycles at a current density of 120 mAh/gZn at 30% DOD with 100% columbic efficiency shows the promise of BA as a practical electrolyte additive.