Flow Battery Electroanalysis 4: Investigation of Speciation and Correlated Electron Transfer Kinetics of the Cr(III/II) Redox Couple for Functional Flow Batteries

The iron-chromium redox flow battery (ICRFB) is promising for long duration energy storage due to its low cost and high elemental abundance. In this work, we address challenges relating to performance of the Cr(III/II) negative electrolyte, most notably sluggish electron-transfer kinetics and parasitic hydrogen evolution during the charge cycle. Specifically, we expanded on prior studies from the inorganic coordination chemistry literature demonstrating that Cr(III) chloride complexes undergo slow equilibration in aqueous solutions to form distributions of three species where the coordination environment around the metal center can be described generally as [CrClx(H2O)6−x](3−x)+ with 0 < x < 3, each yielding a distinct optical absorption signature. By correlating this speciation behavior with electro-analytical measurements over a range of chloride concentrations, we found the the reversibility of the Cr(III/II) redox feature improves dramatically as chloride concentration increases above 5 mol/L due to the increased concentration of tetraaquo chromium species Cr(H2O)4Cl2+. We used these insights to formulate a weakly acidic ICRFB electrolyte that relies on concentrated LiCl to drive favorable speciation for Cr-chloride at room temperature. This electrolyte exhibits markedly improved reversibility and evidence for diminished H2(g) production in analytical measurements; however, per-cycle capacity fade for this Cl-rich electrolyte still exceeded that of the conventional HCl-based ICRFB electrolyte by at least a factor of two. Despite these findings, insights into the relationship between speciation and electrochemical reversibility are potentially useful in informing the design of Cr-based aqueous electrolytes for energy storage and related applications.