Neutral Red and Ferroin as Reversible and Rapid Redox Materials for Redox Flow Batteries.

Neutral red and ferroin are used as redox indicators (RINs) in potentiometric titrations. The rapid response and reversibility that are prerequisites for RINs are also desirable properties for the active materials in redox flow batteries (RFBs). This study describes the electrochemical properties of ferroin and neutral red as a redox pair. The rapid reaction rates of the RINs allow a cell to run at a rate of 4 C with 89 % capacity retention after the 100th  cycle. The diffusion coefficients, electrode reaction rates, and solubilities of the RINs were determined. The electron-transfer rate constants of ferroin and neutral red are 0.11 and 0.027 cm s-1 , respectively, which are greater than those of the components of all-vanadium and Zn/Br2 cells.

The smallest quaternary ammonium salts with ether groups for high-performance electrochemical double layer capacitors.

Electrochemical double layer capacitors (EDLCs) are energy storage devices that have been used for a wide range of electronic applications. In particular, the electrolyte is one of the important components, directly related to the capacitance and stability. Herein, we first report a series of the smallest quaternary ammonium salts (QASs), with ether groups on tails and tetrafluoroborate (BF4) as an anion, for use in EDLCs. To find the optimal structure, various QASs with different sized head groups and ether-containing tail groups were systematically compared. Comparing two nearly identical structures with and without ether groups, QASs with oxygen atoms showed improved capacitance, proving that ions with oxygen atoms move more easily than their counterparts at lower electric fields. Moreover, the ether containing QASs showed low activation energy values of conductivities, leading to smaller IR drops during the charge and discharge processes, resulting in an overall higher capacitance.

Ferrocene and cobaltocene derivatives for non-aqueous redox flow batteries.

Ferrocene and cobaltocene and their derivatives are studied as new redox materials for redox flow cells. Their high reaction rates and moderate solubility are attractive properties for their use as active materials. The cyclability experiments are carried out in a static cell; the results showed that these materials exhibit stable capacity retention and predictable discharge potentials, which agree with the potential values from the cyclic voltammograms. The diffusion coefficients of these materials are 2 to 7 times higher than those of other non-aqueous materials such as vanadium acetylacetonate, iron tris(2,2'-bipyridine) complexes, and an organic benzene derivative.

The effect of the ionic size of small quaternary ammonium BF4 salts on electrochemical double layer capacitors.

By varying the cation size of quaternary ammonium salts, approximately 10% higher capacitance was achieved with trimethylethylammonium BF4 and trimethylpropylammonium BF4 relative to tetraethylammonium BF4 using microporous activated carbon (AC) electrodes. The ions carried solvation shells in the bulk electrolytes, but became desolvated within the narrow AC pores when the electrochemical double-layer capacitor was charged to a high potential. The capacitance depended on the size of the cation rather than that of the BF4 anion because the anion is smaller than the quaternary ammonium ions. The capacitance was found to be proportional to the reciprocal radii of the neat cations. The effective radius of the asymmetric trimethylpropylammonium ion was estimated to be 0.314 nm based on the present results.

Electrochemical studies of molybdate-doped LiFePO4 as a cathode material in Li-ion batteries.

The use of molybdate as a new anionic dopant that replaces phosphate in LiFePO4 was studied. When a small amount of molybdate (0.5 mol%) was used as a dopant, the olivine structure was maintained, while the lattice volume increased by 0.4%. The expanded volume facilitates ionic transfer, because of which the capacity of doped LiFePO4 at high current discharge rates is higher than that of pure LiFePO4. The discharge value increased by 25.2% at a charge rate of 5 C when the material was doped with 0.5 mol% molybdate ions. The slight expansion of the lattice volume in the olivine structure facilitates a fast redox reaction by lowering the charge transfer resistance. The current values from cyclic voltammetry indicate that the oxidation (charge) process of the cathode material is more improved than the corresponding reduction (discharge) process. Increasing the level of doping beyond 0.5 mol% had no effect on the results. At some discharge rates, the discharge capacity became worse. Because molybdate is divalent while phosphate is trivalent, a large number of molybdate ions in the lattice can exert considerable stress on the structure.

Lithium-induced supramolecular hydrogel.

We describe a novel anisotropic supramolecular gel made of cyclodextrin-dye, in which physical gelation is completed by lithium salt. Rheological experiment reveals the elastic behaviors of the hydrogel, and high ionic conductivity represents a good mobility of ions inside the gel matrix.

Cation electrochemical stability in chloroaluminate ionic liquids.

The electrochemical stability of 10 organic cations, which can be used in ionic liquids (IL), was investigated as solutes in acetonitrile (ACN). The stability of three of the salts, BenMe2EtNCl (salt III), 1-butyl-2-methyl pyrrolidium chloride (salt VI), and its structural isomer, BuMe2ProNCl (salt VII), were also compared in chloroaluminate ILs. The chloroaluminate ILs of salts VI and VII are investigated for the first time. The NaCl-neutralized ILs of salts VI and VII have melting points of 43.2 and 3.7 degrees C, respectively. The benzyl-substituted cation, salt III, was more easily reduced in ACN or as the neutral chloroaluminate IL than the alkyl-substituted cation, salt VII, due to the better leaving ability of the benzyl group. Mass spectroscopy measurements before and after electrolysis on the benzyl-substituted solutions confirmed that reduction involves the loss of an alkyl group. In ACN, salt VI was found to be the most difficult to reduce (1 mA/cm2 at -2.09 V) due to its cyclic structure. However, in the chloroaluminate IL, the pyrrolidinium cation was more easily reduced than salt III or its isomer, salt VII, resulting in an insoluble black deposit. This is consistent with the mass spectrometry data, which do not show formation of low-molecular-weight products, as in the reduction of salts III and VII. The IL of salt VII was the most stable in the presence of sodium. Sodium ions could be reduced and reoxidized with a maximum Coulombic efficiency of 94.1% versus 87.2% for salt VI. Reduction of the pyrrolidinium cation produces insoluble products, most likely through opening of the cyclic ring, and an inferior medium for sodium ion reduction compared to the benzyl- and butyl-substituted cations, even though reduction of the cation occurs at a more negative potential in acetonitrile.