Spectroelectrochemistry of Vanadium Acetylacetonate and Chromium Acetylacetonate for Symmetric Nonaqueous Flow Batteries

Abstract

Chromium acetylacetonate, or Cr(acac) 3 , is a promising active species for high-energy-density symmetric redox flow batteries because the neutral complex supports multiple charge-transfer reactions with widely separated redox potentials. Voltammetric and spectroelectrochemical measurements were performed to probe the mechanism of the first electrochemical disproportionation of Cr(acac) 3 -i.e., the cell reaction associated with the two redox couples immediately adjacent to the equilibrium potential of a freshly prepared nonaqueous Cr(acac) 3 solution. Substantially different limiting currents are observed for the positive and negative half-reactions, suggesting that at least one deviates from the similar outer-sphere single-electron transfer mechanisms proposed earlier. Spectroelectrochemical chronoamperometry suggests ligand dissociation in the negative reaction, and consequent structural reorganization of the Cr(acac) 3 complex. Vanadium acetylacetonate was investigated for comparison, and no ligand dissociation was observed. A negative half-reaction mechanism consistent with the voltammetric and spectroelectrochemical data is proposed, and used to rationalize observations of charge/discharge behavior in cycling Cr (acac) As researchers have identified the economic benefits that largescale energy storage delivers to the grid, research into redox flow batteries (RFBs) has expanded.1 In efforts to increase energy density or decrease active-species loading, researchers have targeted nonaqueous RFB chemistries, which can produce operating potentials outside the stability window of water.2-8 Several nonaqueous chemistries are based on transition-metal-centered coordination complexes. 2,3,[6][7][8][9] Chromium acetylacetonate, or Cr(acac) 3 , appears especially promising as an RFB active species: it has five accessible redox states, and there is a wide voltage separation between the first reduced and first oxidized states of the neutral complex. The Cr(acac) 3 complex has also been used for catalysis 10 and as a standard for atomic absorption spectra.11 Whereas vanadium acetylacetonate, or V(acac) 3 , appears to support outer-sphere electron transfer for both single-electron addition and withdrawal in certain nonaqueous electrolytes, and to undergo both charge exchanges with relatively facile kinetics, 12 the voltammetric response of Cr(acac) 3 is harder to interpret. Experimental efforts to clarify the electrochemistry of Cr(acac) 3 have been challenging because the reaction mechanism apparently varies with the choices of solvent, supporting electrolyte, and working electrode. 13-17The literature describing nonaqueous Cr(acac) 3 electrochemistry does not provide a consensus about the mechanisms for either the first reduction or first oxidation of the neutral complex. The electrochemical response during reduction depends heavily upon solvent, with different behavior observed in dimethylsulfoxide, 13,15,18,19 N,N-dimethylformamide, 15 dichloromethane, 14 acetonitrile, 15,17,20 and others. 15 Various mechanisms have...

A High-Current, Stable Nonaqueous Organic Redox Flow Battery

“…For example, the flow cell using this separator can operate at currents as high as 50 mA cm –2 while still delivering an EE of ∼70% and a material utilization ratio of ∼80% (Figures d and S1f). Such operational current densities are around 2 orders of magnitude higher than those of many nonaqueous flow batteries developed by other research teams ,, and are comparable to or even better than those for neutral electrolyte-based aqueous flow batteries. , …”

Multi-redox Molecule for High-Energy Redox Flow Batteries

“…Considering the high applied current density of 20 mA cm À2 , the observed efficiency indicates rapid kinetics in the electrochemical reaction, which outperforms other non-aqueous flow cells and is even comparable with those of some aqueous systems. 15,18,19,24,48 The fast electrochemical reactions are attributed to the observed high kinetic rate constants (k 0 ) and diffusion coefficients (D) of the DMPZ and FL, as demonstrated in Figure 2. Stable cycling was also achieved for the flow cell without noticeable capacity fading, with retention of z98% of the initial capacity (2.43 A hr L À1 ) after 30 cycles and capacity retention of z90% was obtained after 80 cycles in the dilute condition (Figure S4).…”

Multi-Redox Molecule for High-Energy Redox Flow Batteries

“…Considering the high applied current density of 20 mA cm À2 , the observed efficiency indicates rapid kinetics in the electrochemical reaction, which outperforms other non-aqueous flow cells and is even comparable with those of some aqueous systems. 15,18,19,24,48 The fast electrochemical reactions are attributed to the observed high kinetic rate constants (k 0 ) and diffusion coefficients (D) of the DMPZ and FL, as demonstrated in Figure 2. Stable cycling was also achieved for the flow cell without noticeable capacity fading, with retention of z98% of the initial capacity (2.43 A hr L À1 ) after 30 cycles and capacity retention of z90% was obtained after 80 cycles in the dilute condition ( Figure S4).…”