Enhanced oxygen reduction reaction using polyoxometalates as mediators in near neutral electrolytes for zinc-air batteries

Abstract

The quest for effective and sustainable energy storage solutions has led to the exploration of devices like Metal-air batteries, whose performance heavily depends on the efficiency of electrocatalysts in oxygen reduction reactions (ORR) at the cathode. Zinc-Air batteries (ZABs) are a great option, since its components are low cost, safe and eco-friendly. Despite significant efforts, creating cost-effective electrocatalysts with high activity for ORR remains a major challenge. Furthermore, most of the research in the ORR field is carried out in highly alkaline electrolytes, which allow for favorable ORR kinetics but lead to corrosion and low durability of the catalysts and the cells. This work explores the possibility to use polyoxometalate clusters as homogeneous (i.e. in solution) catalysts/mediators in near-neutral pH, evidencing an alternative path to facilitate ORR. Several cobalt containing POMs with Keggin (K₅H[Co²⁺W₁₂O₄₀], K₈[Co²⁺Co²⁺(H₂O)W₁₁Co²⁺O₄₀]·15H₂O, K₇ [Co³⁺Co²⁺(H₂O)W₁₁O₃₉]·14H₂O), Dawson (K₁₂[Co²⁺⁽H₂O)H₂P₂W₁₂O₄₈]) or Superlacunary (K₁₂Li₁₆Co²⁺₂[Co²⁺₄(H₂O)₁₆P₈W₄₈O₁₈₄]) nanocluster, were used in solution showing significant activity on the ORR. The unique existence of redox processes for both Co and W moieties within the cluster suggests the possibility of bifunctional electrocatalysis, and redox mediation with a potential impact in aqueous rechargeable metal-air batteries operating in near – neutral media. The electrochemical action of easily reduced soluble POM clusters on ORR shows an increased activity when the Co-substituted Keggin POMs are dissolved at pH 5. Keggin based structures containing Co offer optimal kinetic parameters, as compared with POMs based on Dawson structures, with the possible involvement of the tetrahedral Co present in the former.