Seawater Zinc-Air Batteries: Axial Chlorine Coordination May Bypass Chloride Poisoning

A newly proposed synthesis strategy claims to stabilise single-atom catalysts against chloride poisoning, potentially improving the viability of seawater zinc-air batteries. Historically, the development of marine-based energy storage has been obstructed by the aggressive nature of the electrolyte itself. Seawater contains high concentrations of chloride ions, which rapidly degrade conventional catalysts and stifle the oxygen reduction reaction (ORR) required for efficient power generation.

Optimising Seawater Zinc-Air Batteries

The research focuses on constructing heteroatom axially coordinated Fe-N₄ single-atom materials. By introducing specific elements—chlorine (Cl) and sulphur (S)—into the catalyst structure, the team aimed to protect the active iron sites. X-ray absorption spectroscopy confirmed that these materials maintain a five-coordinated square pyramidal structure, a specific geometric arrangement intended to maximise stability.

A distinct divergence emerges when comparing the chlorine-coordinated variant (Cl-Fe-N₄) against its sulphur-coordinated counterpart (S-Fe-N₄). Conventional material science often equates a larger electrochemical active surface area with superior performance; essentially, more space for reactions usually yields better results. However, the data here presents a contradiction. The S-Fe-N₄ material possessed a larger surface area, yet the Cl-Fe-N₄ variant achieved a higher limiting current density of 5.8 mA cm^-2. This indicates that the quality of the active site matters more than the sheer quantity of surface area. The chlorine coordination appears to enhance the intrinsic ORR activity, overriding the limitations of a smaller surface area.

Density functional theory (DFT) calculations offer a theoretical explanation for this performance. The data suggests that introducing heteroatoms in the axial direction regulates the electron centre of the single iron atom. This regulation increases the electron density at the iron sites, which may facilitate the reduction of adsorbed intermediates. Consequently, the Cl-Fe-N₄ assembled batteries demonstrated a power density of 187.7 mW cm^-2. Furthermore, the material showed stronger resistance to chloride poisoning, a common failure point in marine environments.

While the laboratory results indicate enhanced stability and improved power density, these findings represent a controlled environment. The transition from a small-scale cell to a functional marine battery involves variables not fully captured in chronoamperometry tests. The study demonstrates that axial coordination is a viable path for catalyst design, but the long-term durability of these sites under the chaotic conditions of open seawater remains to be fully proven.