Rethinking redox electrolyte design: Why diffusion layer ion exchange matters more than surface area

Abstract

Redox electrolyte-enhanced aqueous energy storage devices (RE-AESDs) offer a promising route to surpass the energy density of traditional supercapacitors, but their performance is often plagued by parasitic transport phenomena that lead to low efficiency. Conventional design paradigms that focus on maximizing electrode specific surface area (SSA)—a strategy inherited from supercapacitors—are physically insufficient if device operation is governed by diffusion kinetics. Here, we introduce and validate a quantitative physical diagnostic for the parasitic diffusive flux that undermines efficiency in these systems. By investigating the archetypal [Fe(CN)6]4−/[Fe(CN)6]3− system with carbon electrodes of vastly different SSAs (>1400-fold variation), we demonstrate that charge capacity is fundamentally decoupled from electrode geometry. Instead, performance is dictated by transport physics within the Nernst diffusion layer. Using multi-potential step measurements, we isolate a non-zero termination current (it) as a direct, quantitative measure of the parasitic flux, a key physical parameter previously discussed only qualitatively. This transport-centric model is further substantiated by power-law analysis of voltammetry data, which confirms diffusion-dominated kinetics (b ≈ 0.5). Finally, we link this quantified parasitic flux to long-term cycling degradation. These findings compel a paradigm shift in the RE-AESD design, from optimizing surface geometry to controlling the fundamental physics of ion transport and mitigating the parasitic fluxes quantified by our diagnostic method.