State of Charge and State of Health Assessment of Viologens in Aqueous‐Organic Redox‐Flow Electrolytes Using In Situ IR Spectroscopy and Multivariate Curve Resolution

Abstract

Aqueous‐organic redox flow batteries (RFBs) have gained considerable interest in recent years, given their potential for an economically viable energy storage at large scale. This, however, strongly depends on both the robustness of the underlying electrolyte chemistry against molecular decomposition reactions as well as the device's operation. With regard to this, the presented study focuses on the use of in situ IR spectroscopy in combination with a multivariate curve resolution approach to gain insight into both the molecular structures of the active materials present within the electrolyte as well as crucial electrolyte state parameters, represented by the electrolyte's state of charge (SOC) and state of health (SOH). To demonstrate the general applicability of the approach, methyl viologen (MV) and bis(3‐trimethylammonium)propyl viologen (BTMAPV) are chosen, as viologens are frequently used as negolytes in aqueous‐organic RFBs. The study's findings highlight the impact of in situ spectroscopy and spectral deconvolution tools on the precision of the obtainable SOC and SOH values. Furthermore, the study indicates the occurrence of multiple viologen dimers, which possibly influence the electrolyte lifetime and charging characteristics.

State and Prospects of Unbalanced, Compositionally Symmetric Flow Battery Cycling and Steady-State Amperometry Techniques for Electrolyte Stability Assessment: The Case of Methyl Viologen

“…Also, we assume K eq to be 1,300 M –1 , which was reported in the literature for low-concentrated, aqueous solutions of MV diiodide (at pH 7) . This value of K eq for mono-reduced MV differs from the equilibrium constant of 548 M –1 estimated recently in our group for the low-concentrated MV dichloride solution . However, applying either equilibrium constant results in nearly identical OCV cell calibration data (Figure a).…”

“…As a consequence, the deviation between the calibration curve and the experimental data originates from a fundamental property of the MV electrolyte. Since it is well known that partially reduced MV tends to form dimers, − eq needs to be modified for this special case as follows (Figure ) E = E 02 + R T n F ln ( 4 K normale normalq c 0 false( 1 − normalS normalO normalC false) 1 + 8 K e q c 0 S O C − 1 ) where E 02 is the reference potential of the reference electrolyte, considering reactions involving both the viologen mono-reduced cation, MV 1+ , and its dimer, MV dim 2+ , K eq is an equilibrium constant of dimerization of the MV 1+ , and c 0 is the total electron equivalent molar concentration of all MV species in the electrolyte. The derivation of eq is provided in the Supporting Information.…”

“…25 This value of K eq for mono-reduced MV differs from the equilibrium constant of 548 M −1 estimated recently in our group for the low-concentrated MV dichloride solution. 27 However, applying either equilibrium constant results in nearly identical OCV cell calibration data (Figure 2a). In addition to full eq 4, an approximation was performed by evaluating the equation for…”

Characterization of an Aqueous Flow Battery Utilizing a Hydroxylated Tetracationic Viologen and a Simple Cationic Ferrocene Derivative

“…[ 6 ] It was believed that the addition of a permanently charged moiety would impede the formation of stable dimers between the viologen radicals, that is, the main disadvantage related to MV, thus improving the overall negolyte performance. [ 11 ] However, it was recently proved that BTMAP‐Vi can still undergo dimerization, [ 13 ] though to a lesser extent than dicationic viologens. The latter, and thus MV, can form instead association complexes of higher order, for example, trimeric structures, complicating the chemistry of this negolyte.…”

“…The latter, and thus MV, can form instead association complexes of higher order, for example, trimeric structures, complicating the chemistry of this negolyte. [ 13 ]…”