Molecular Design Considerations for Azobenzene Anolytes

Abstract

Realization of high-density batteries requires the development of anolytes that display highly negative reduction potentials, solubility, and persistence in the charged state. Azobenzenes have garnered interest as potential anolytes for redox flow batteries. Here, we report the synthesis of a family of substituted azobenzene derivatives and evaluation of their solution-phase electrochemical properties. Systematic synthetic derivatization of this scaffold allows (1) access to anolytes of varying solubility, including intrinsically liquid derivatives that represent potential high-density charge carriers; (2) systematic variation of the reduction potential, and in some cases redox inventory, that provides azobenzenes with highly negative reduction potentials; and (3) control of the lifetime of the azobenzene radical anions that result from one-electron reduction. Electrokinetic experiments demonstrated that fast electron transfer occurs for all derivatives examined. Spectroscopic characterization of monoreduced azobenzene derivatives establishes that decomposition of the azobenzene radical anion proceeds via bimolecular disproportionation. Together, these results provide an experimental basis for the optimization of azobenzene anolytes for electrochemical storage applications, including redox flow batteries.

1

(a) Collection of molecular architectures that have been investigated as anolytes for nonaqueous redox flow batteries (NAORFBs) organized by one-electron reduction potential. (b) Peripheral substitution of azobenzenes has enabled application of these molecules as anolytes in aqueous and nonaqueous RFBs. (c) Here, we describe the impact of molecular structure on the electrochemistry, electrokinetics, solubility, and physical properties of functionalized azobenzenes.

2

(a) Summary of electrochemical data of azobenzenes 1a–1s. Cyclic voltammograms were measured using 0.005 M MeCN solutions (Fc⁺/Fc was used as a reference and 0.1 M TBAPF₆ was used as a supporting electrolyte). (b) Hammett plot generated from between E _X/H(1/2) vs. Hammett parameter (σ_p) for PhN = N–C₆H₄-4-X.

3

Development of liquid azobenzene toward achieving maximum concentration in NAORFBs.

4

(a) Plot of anodic and cathodic peak current versus square root of the scan rate. (b) Decay of the monoreduced product after bulk electrolysis at a constant potential of −1.54 V vs. Fc⁺/Fc in MeCN. (c) Absorbance vs. time for the decay of 1a′. (d) The linear correlation between inverse absorbance and time shows a second-order decay.