Ionic Liquid Electrolytes for Redox Flow Batteries

The development of efficient devices that store energy generated from intermittent resources (such as solar energy or wind energy) will play a crucial role for sustainable energy supply.  Redox flow batteries (RFBs) are considered to be one of the most promising energy storage devices owing to their capacity for storing very large amounts of energy, low cost and high flexibility of design.¹  RFBs store electrical energy by chemical changes to species dissolved in aqueous acidic media in two reaction compartments, which are separated by an ion-conducting membrane.¹  However, the use of aqueous systems significantly limits the energy and power density of the RFBs due to the narrow potential window of water.  A number of non-aqueous electrolytes based on ruthenium, vanadium, manganese and chromium have been reported in order to increase the cell voltage.²  Notably, a cell potential of 2.2 V was reported in a system that employed vanadium and acetonitrile electrolytes.³

The main problem of non-aqueous RFBs is the possibility that the organic solvents in the liquid electrolytes can evaporate during operation and result in poor long-term stability.  Moreover, organic solvents are highly flammable, which may pose a serious hazard.  In an effort to combat this problem, we propose room temperature ionic liquids (RTILs) as electrolytes in RFBs.  RTILs are thermally stable, non-flammable, conductive and, crucially exhibit very wide electrochemical windows of up to 6.0 V.⁴

In this poster, the electrochemical behavior of several RFB redox mediators are examined at glassy carbon (GC) electrodes using a RTIL as the electrolyte.  The RTIL was 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C₂C₁Im][Tf₂N], see inset of Figure 1C), and this RTIL was chosen because it has a low viscosity and a large electrochemical window.  Figure 1A shows the CVs recorded at GC in 10 mM V(acac)₃ in ([C₂C₁Im][Tf₂N], and this response is similar to the electrochemical behavior of V(acac)₃ in acetonitrile.³  Three oxidation and reduction processes were observed which were attributed to the redox process of V³⁺/V²⁺ (A₁/C₁), V³⁺/V⁴⁺ (A₂/C₂) and V⁴⁺/V⁵⁺(A₃/C₃).  Each process displayed an electrochemically quasi-reversible response.  The potential difference between A₁/C₁ and A₂/C₂ is 2.0 V and can reach upto 2.3 V for (A₁/C₁)/A₃/C₃).  Two redox processes were observed when Mn(acac)₃ in ([C₂C₁Im][Tf₂N] was studied (Figure 1B).  The oxidation/reduction of Mn³⁺ to Mn²⁺ (A₁/C₁) is electrochemically irreversible and its peak-to-peak separation is about 0.55 V at 50 mV s^-1, while the Mn³⁺/Mn⁴⁺(A₂/C₂) is quasi-reversible process with peak-to-peak separation of 0.1 V at 50 mV s^-1.  In the case of Cr electrochemistry in [C₂C₁Im][Tf₂N], electrochemical analysis indicated that the electro generated Cr⁴⁺ (A₃/C₃) is unstable in this RTIL, suggesting the presence of an additional chemical step during electron transfer (see Figure 1C, A₂/A₂ was assigned to the redox process of Cr³⁺/Cr²⁺ and A₁/C₁ was to Cr²⁺/Cr¹⁺).  Notably, a similar electrochemical response was also observed when acetonitrile was used as the electrolyte.⁵  The diffusion coefficients of V, Cr and Mn in [C₂C₁Im][Tf₂N] are 1.8 x 10^-7 cm² s^-1, 1.6 x 10^-7 cm² s^-1 and 1.5 x 10^-7 cm² s^-1 respectively.  Finally, the effect of ionic liquid composition (changing the anions) on the electrochemistry of each redox mediator will be presented.

References

1. A. Z. Weber, M. M. Mench, J. P. Meyers, P. N. Ross, J. T. Gostick and Q. H. Liu, J. Appl. Electrochem., 2011, 41, 1137-1164.

2. S. H. Shin, S. H. Yun and S. H. Moon, RSC Advances, 2013, 3, 9095-9116.

3. Q. H. Liu, A. E. S. Sleightholme, A. A. Shinkle, Y. D. Li and L. T. Thompson, Electrochem. Commun., 2009, 11, 2312-2315.

4. M. Armand, F. Endres, D. R. MacFarlane, H. Ohno and B. Scrosati, Nat. Mater., 2009, 8, 621-629.

5. Q. H. Liu, A. A. Shinkle, Y. D. Li, C. W. Monroe, L. T. Thompson and A. E. S. Sleightholme, Electrochem. Commun., 2010, 12, 1634-1637.