Nickel(II)-1,4,8,11-Tetraazacyclotetradecane As a Single Electrolyte Component for Non-Aqueous Redox Flow Batteries

Energy storage systems (ESSs) are required for stable electricity supply from power plants and efficient usage of generated electricity. ESSs are also needed to accommodate fluctuations in renewable energy generation. Redox flow batteries (RFBs) are considered as one of the reliable choices for ESSs.

 Since 1970s, RFB technologies have been developed mainly on the basis of aqueous electroytes, which show a critical drawback; limited working voltage (< 1.23 V) due to a narrow electrochemical stability window of water.¹ This constraint can, however, be relieved if electrochemical stability window is widened by deliberately controlling the composition of non-aqueous electrolyte systems.

 Redox couple is the core component in non-aqueous electrolyte systems, which should satisfy at least the following requirements; (i) high working voltage, (ii) high solubility in non-aqueous solvents, (iii) stability for high couloumbic efficiency and cycle retention, and (iv) facile molecular diffusion in solutions. It must be additionally beneficial if it can be used for both catholyte and anolyte. When such a single electrolyte component is used for RFBs, the loss of redox couple by side reactions can be greatly suppressed.

 A metal-multidentate complex, (nickel(II)-1,4,8,11-tetracyclotetradecane, Ni(cyclam)²⁺) has been tested if or not it satisfies the above requirements.

 The nickel complex is reduced and oxidized at -1.78 V and 0.74 V (vs. Fc/Fc⁺), respectively (Fig. 1). Hence, this redox couple can be used as a single electrolyte component for non-aqueous RFBs. The expected working voltage is as high as 2.52 V, which is much larger than those for conventional aqueous RFBs. The maximum solubility is ~0.4 M at room temperature. This value is again higher than those for the conventional metal-ligand complexes (0.1 M).²  A much enlarged energy density is thus expected with this RFB since it shows larger values for both cell voltage and capacity. This redox couple also exhibits a stable cycle  performances. As seen in Fig. 1, the current profiles are not changed even after 50 cycles.

 Galvanostatic charge-discharge cycling is carried out in a non-flowing static cell. The coulombic efficiency is ~ 75% after 3 cycles.

References

1. M. Skyllas-Kazacos, M.H. Chakrabarti, S.A. Hajimolana, F.S. Mjalli, and M. Saleem, J. Electrochem. Soc., 158, R55 (2011).

2. W. Wang, Q. Luo, B. Li, X. Wei, L. Li, and Z. Yang, Adv. Funct. Mater., 23, 970 (2013).

Fig. 1. The 1^st and 50^th cyclic voltammograms of Ni(cyclam)²⁺