Electrochemical Analysis of Chromium Acetylacetonate for Nonaqueous Flow Batteries

Although redox flow batteries (RFBs) provide the advantage of decoupled energy and power density useful for grid storage applications, current state-of-the-art technologies have yet to achieve widespread commercial success. Cost per unit energy storage capacity presents the largest barrier to this goal [1]. 

Wide voltage gaps between redox potentials, capacity for multiple redox reactions, and ability to serve as both negative and positive electrolyte (ability to disproportionate) make chromium acetylacetonate [Cr(acac)₃] attractive for RFB applications [2]. With multiple redox potentials separated by >3 V, as shown by the cyclic voltammogram in Figure 1, cell potentials can be raised significantly above state-of-the-art aqueous RFBs, which are limited within a ~1.5 V operating window. Employing the same Cr(acac)₃ solution in both electrolyte reservoirs delivers a truly symmetric single-metal RFB and can eliminate the need for costly ion-selective separators. Large operating potentials and inexpensive stack materials will help raise theoretical maximum energy density and mitigate cost concerns.

In spite of clear advantages, operation of nonaqueous Cr(acac)₃ RFBs display low voltage efficiency [2]. With the aim of elucidating a potential cause of this low efficiency, we investigated the electrochemical kinetics of nonaqueous chromium acetylacetonate on various electrodes, focusing on the redox couples centered at -2.1 & 1.2 V vs. Ag/Ag⁺ (shown in Figure 1). Initial linear-sweep voltammetry experiments on Au, Pt, and C microelectrodes depicted more complicated redox behavior than originally hypothesized. We will discuss the ramifications of this behavior with regard to reaction mechanisms, while also addressing how these observations can further guide RFB design for nonaqueous Cr(acac)₃ systems. 

Figure 1: Cyclic and normalized linear-sweep voltammograms of 0.01 M Cr(acac)₃/0.05 M TEABF₄ in acetonitrile, Pt WE (50um) & CE, 500 mVs^-1 and 1 mVs^-1 (insets), <1ppm O₂/H₂O, ambient temperature.

[1] S. Eckroad. EPRI Report 1014836. 2007.

[2] Q. Liu et al. Electrochem. Comm. 12 (2010): 1634.