Evaluating reaction kinetics between solid booster and dissolved active species in redox‐mediated flow batteries using scanning electrochemical microscopy

Redox-mediated flow batteries boost energy density by utilizing dissolved redox species as charge carriers for solid charge-storage materials. This strategy strongly depends on the thermodynamics and kinetics between the solid booster and dissolved redox species. Conventional electrochemical methods often convolute intrinsic reactivity with mass transport effects, introducing complexity in determining limiting steps. We propose a strategy that confines solid boosters within recessed microelectrodes and employs scanning electrochemical microscopy (SECM) to estimate reaction kinetics between booster and dissolved active redox species. Confining the solid booster in the recessed microelectrode overcomes mass transport limitations of dissolved redox species and enables controlled polarization of the booster material, allowing deconvolution of key rate-determining factors. As an initial model system, Prussian blue-ferricyanide/ferrocyanide [Fe(CN)6]3−/4− was used as solid booster and dissolved redox active species, respectively. The methodology was further explored for copper hexacyanoferrate with N,N,N-2,2,6,6-heptamethylpiperidinyl oxy-4-ammonium chloride and nickel hydroxide with [Fe(CN)6]3−/4− and extended to Mn-based Prussian blue analogues in combination with organic redox species. Our results demonstrate that SECM coupled with the proposed recessed microelectrode strategy provides a powerful platform to disentangle interfacial kinetics and guide the rational design of solid booster-dissolved redox species and electrolytes for high-performance redox-mediated flow batteries.