ABSTRACT
The hexacyano[3]radialene radical anion (1) is an attractive catholyte material for use in redox flow battery (RFB) applications. The substitution of cyano groups with ester moieties enhances solubility while maintaining redox reversibility and favorable redox potentials. Here we show that these ester-functionalized, hexasubstituted [3]radialene radical anions dimerize reversibly in water. The dimerization mode is dependent on the substitution pattern and can be switched in solution. Stimuli-responsive behavior is achieved by exploiting an unprecedented tristate switching mechanism, wherein the radical can be toggled between the free radical, a π-dimer, and a σ-dimer-each with dramatically different optical, magnetic, and redox properties-by changing the solvent environment, temperature, or salinity. The symmetric, triester-tricyano[3]radialene (3) forms a solvent-responsive, σ-dimer in water that converts to the radical anion with the addition of organic solvents or to a π-dimer in brine solutions. Diester-tetracyano[3]radialene (2) exists primarily as a π-dimer in aqueous solutions and a radical anion in organic solvents. The dimerization behavior of both 2 and 3 is temperature dependent in methanol solutions. Dimerization equilibrium has a direct impact on catholyte stability during galvanostatic charge-discharge cycling in static H-cells. Specifically, conditions that favor the free radical anion or π-dimer exhibit significantly enhanced cycling profiles.
ABSTRACT
Research studies on zinc metal-based batteries have attracted considerable attention as a candidate for post-lithium-ion batteries. Zinc is one of the few metal anodes that is compatible with aqueous and non-aqueous electrolytes, providing a large theoretical capacity of 820 mAh g-1. However, in aqueous electrolytes, the zinc metal anode suffers from hydrogen evolution reaction (HER), by which zinc is irreversibly consumed or corroded continually. Exact estimation of the corrosion rate has been a challenge in the development of Zn-based batteries. Measurement of the corrosion rate by conventional Tafel analysis meets serious problems because the cathodic current reflects deposition of Zn metal as well as HER, inhibiting exact measurement of the corrosion rate. Herein, we developed a chronocoulometric "deposition-rest-dissolution" method to quantify the corrosion rate without such interference from the deposition of Zn. The method was successfully applied to the quantification of the rate of chemical corrosion of Zn in aqueous electrolytes with various pH and concentration values. The "deposition-rest-dissolution" method and electrochemical impedance spectroscopy confirmed that saturated ZnSO4 (ca. 3.2 M) + 0.075 M Li2SO4 delivers the lowest corrosion rate compared to the other electrolytes, probably because the activity of water in such a concentrated electrolyte is low enough to suppress the kinetics of HER. Moreover, this method can be generally applied to determine the rate of chemical corrosion on various metal electrodes.
